Glossary
Glossary
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A-Index
A daily index of geomagnetic activity for a specific station or network of stations derived as the average of
the 8, 3-hourly K indices. The A-Index varies from 0 to 400.
AP-Index
A 3-hour equivalent amplitude of magnetic activity based on KP-Index data from a planetary network of
magnetic observatories. NOAA’s SWPC estimates the value of the AP index by measuring the
geomagnetic field in near real time at magnetometer stations around the world. The real-time station
indices are used to estimate the official KP & AP based on historical data.
Accelerating Power
In a generator, the difference between the mechanical input power & the electrical output power
(assuming no losses). For a generator’s torque angle to change, the generator must have accelerating
power.
Access
The contracted right to use an electrical system to transfer electrical energy.
Accumulated Inadvertent
The accumulation over time of inadvertent energy. The accumulated inadvertent account of a BA
represents the amount of excess energy a BA has either supplied to the interconnection or absorbed from
the interconnection. BAs monitor their accumulated inadvertent to ensure the accounts do not grow too
large.
Active Power (MW)
The component of the complex power (MVA) that performs the work. The common unit of active power is
the MW & the symbol is “P”. Active power is sometimes called real power.
Active Power Transmission Losses
The power consumed due to resistive characteristics of transmission elements. Calculated as I2R.
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Actual Net Interchange (NIA)
Algebraic sum of actual MW transfers across all Tie Lines & includes pseudo-ties. BAs directly connected
via asynchronous ties to another Interconnection may include or exclude MW transfers on those tie lines
in their actual interchange, provided they are implemented in the same manner for net interchange
schedule.
Actual Load
The actual MW drawn by a load from the power system. The actual load will be different from the rated or
nominal load if the load’s voltage or frequency vary from nominal values.
Adequacy
The ability of the electric system to supply the aggregate electrical demand and energy requirements of
the end-use customers at all times, taking into account scheduled and reasonably expected unscheduled
outages of system elements. (NERC Glossary of Terms)
Adequate Regulating Margin
The minimum on-line capacity that can be increased or decreased to allow the system to respond to all
reasonable demand changes in order to be incompliance with the CPS.
Adjacent Balancing Authority
A Balancing Authority whose Balancing Authority Area is interconnected with another Balancing Authority
Area either directly or via a multi-party agreement or transmission tariff. (NERC Glossary of Terms)
Adverse Reliability Impact
The impact of an event that results in Bulk Electric System instability or Cascading. (NERC Glossary of
Terms)
After the Fact (ATF)
A time classification assigned to an RFI when the submittal time is greater than one hour after the start
time of the RFI. (NERC Glossary of Terms)
AGC Pulses
The AGC system will send signals to selected (regulating) generators to adjust their set-points. These
signals are sent via telecommunication equipment. The signals are often called AGC pulses.
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Aggregator
An entity responsible for planning, scheduling, accounting, billing, & settlement for energy deliveries from
the aggregator’s choice of sellers &/or buyers. Aggregators seek to bring together customers or
generators so they can buy or sell power in bulk, making a profit on the transaction.
Air-Gap
The air space between the rotor & stator of a motor or generator.
Alternating Current (AC)
A time varying electric current that changes periodically in magnitude & direction. In power systems, the
changes follow the pattern of a sine wave having a frequency of 60 cycles per second in North America.
AC is also used to refer to voltage, which follows a similar sine wave pattern.
Alternative Interpersonal Communication
Any Interpersonal Communication that is able to serve as a substitute for, and does not utilize the same
infrastructure (medium) as, Interpersonal Communication used for day-today operation. (NERC Glossary
of Terms)
Alternator
A rotating machine that develops alternating voltage & current outputs.
Aluminum Cable Steel Reinforced (ACSR)
A common type of transmission line conductor that comes in many different sizes & designs. The outer
aluminum strands form the conductor & while the inner steel strands provide strength.
Ambient Conditions
Common, prevailing, & uncontrolled atmospheric conditions at a particular location, either indoors or out.
The term is often used to describe the temperature, humidity, & airflow or wind that equipment or systems
are exposed to.
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Amortisseur Winding
A machine winding consisting of a number of conducting bars attached between the magnetic poles of
the machine’s rotor. Amortisseur windings may be used as starter windings or to help dampen power
oscillations.
Ampere (A)
The unit of measurement of electrical current produced in a circuit by 1 volt acting through a resistance of
1 Ohm.
Amplitude
The value or magnitude that a waveform has at a specific point in time.
Analog Electronics
Electronic circuitry in which the magnitudes of quantities are used in a continuous manner to perform
functions. For example, input current values may be amplified & used to perform work in an analog
circuit. Analog is different than digital in the sense that analog circuits use voltage & current magnitudes
continuously while digital circuits use voltage & current to determine the state (on or off) of elements.
Ancillary Service
Those services that are necessary to support the transmission of capacity and energy from resources to
loads while maintaining reliable operation of the Transmission Service Provider's transmission system in
accordance with good utility practice. (From FERC order 888-A.) (NERC Glossary of Terms)
Angle Instability
The opposite of angle stability. When a power system loses angle stability it enters a period of angle
instability. An angle unstable system has lost the magnetic bound that holds sections of an
interconnected AC power system in synchronism with 1 another.
Angle Stability
An angle stable power system is 1 in which all elements of the power system are bound together via
magnetic forces. For example, an angle stable generator’s internal magnetic field rotates in synchronism
(in-step) with the magnetic field of the 3 power system to which it connects.
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Anti-Aliasing Filter
An analog filter installed at a metering point to remove the high frequency components of the signal over
the AGC sample period. (NERC Glossary of Terms)
Apparent Power
The product of the voltage & current phasors. Apparent power comprises both active & reactive power,
usually expressed in kVA or MVA. Another term for apparent power is complex power.
Arc
The discharge of current through the air or in a gas.
Arcing Horns
An enhancement to a disconnect switch to increase the switch’s current interrupting capability.
Area Control Error (ACE)
The instantaneous difference between a Balancing Authority’s net actual and sch eduled interchange,
taking into account the effects of Frequency Bias, correction for meter error, and Automatic Time Error
Correction (ATEC), if operating in the ATEC mode. ATEC is only applicable to Balancing Authorities in the
Western Interconnection. (NERC Glossary of Terms)
Area Interchange Methodology
The Area Interchange methodology is characterized by determination of incremental transfer capability
via simulation, from which Total Transfer Capability (TTC) can be mathematically derived. Capacity
Benefit Margin, Transmission Reliability Margin, and Existing Transmission Commitments are subtracted
from the TTC, and Postbacks and counterflows are added, to derive Available Transfer Capability. Under
the Area Interchange Methodology, TTC results are generally reported on an area to area basis. (NERC
Glossary of Terms)
Armature
The component of an electrical machine in which the voltage is induced. In a synchronous generator the
armature is usually the stator. The armature winding is wrapped about the armature.
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Arranged Interchange
The state where a Request for Interchange (initial or revised) has been submitted for approval. (NERC
Glossary of Terms)
Arrested Frequency
Point of maximum frequency excursion in the 1ST swing of the frequency excursion between time zero
(point “A”) & time zero plus 20 seconds. Also called the frequency nadir.
Arresting Period Frequency Response
Combination of load damping & the initial primary control response acting together to limit the duration &
magnitude of frequency change during the arresting period.
Asynchronous
To be out-of-step with a reference. An induction generator is an asynchronous generator as its rotor does
not rotate in synchronism with the power system.
Attaining Balancing Authority
A Balancing Authority bringing generation or load into its effective control boundaries through a Dynamic
Transfer from the Native Balancing Authority. (NERC Glossary of Terms)
Aurora
Phenomenon associated with geomagnetic activity that is visible in the night sky. Aurorae occur within a
band of latitudes known as the auroral oval, the location of which is dependent on geomagnetic activity.
Aurorae are a result of collisions between atmospheric gases & charged particles (mostly electrons)
guided by the geomagnetic field from the Earth’s magnetotail. Each gas (oxygen, nitrogen, etc.) emits a
particular color depending on the energy of the particles & atmospheric composition. Auroral altitude
range is 80 to 1,000 kM. Aurorae in the Northern Hemisphere are called Aurora Borealis while aurorae in
the Southern Hemisphere are called Aurora Australis.
Aurora Oval
Elliptical band around each geo-magnetic pole ranging from about 75° magnetic latitude at local noon to
about 67° at midnight under average conditions. These locations experience the maximum occurrence of
aurorae. The aurora widens to both higher & lower latitudes during magnetic storms.
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Auto-Load
A feature on some generators that automatically moves the generator to a target load (MW) level.
Automatic Generation Control (AGC)
Equipment that automatically adjusts generation in a Balancing Authority Area from a central location to
maintain the Balancing Authority’s interchange schedule plus Frequency Bias. AGC may also
accommodate automatic inadvertent payback and time error correction. (NERC Glossary of Terms)
Automatic Operating Systems
SPS, RAS, or other operating systems installed on electric systems that require no intervention on the
part of system operators.
Automatic Reclosing
Device that automatically recloses a CB after it has been tripped by a protective relay. Automatic
reclosing is applied on both distribution & transmission overhead lines. Since many faults clear
automatically, automatic reclosing improves service continuity. Automatic reclosing applied in less than 1
second is called high speed automatic reclosing. NERC includes both the reclosing device and its
associated control circuitry in its definition of automatic reclosing.
Automatic Time Error Correction
A frequency control automatic action that a BA uses to offset its frequency contribution to support the
Interconnection’s scheduled frequency.
Auto Transformer
A power transformer with a single coil for each electrical phase, as opposed to a conventional
transformer, which has 2 coils per phase. In an auto-transformer, the entire coil acts as the primary
winding while a portion of the same coil acts as the secondary winding.
Auxiliary Relay
A relay whose function is to supplement the actions of other types of relays. For example, a lockout relay
is an auxiliary relay with numerous contacts that each can perform an action. ANSI has numbers
assigned to 2 types of auxiliary relays; 86 (lock-out) & 94 (tripping).
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Availability
A measure of time a generating unit, transmission line, or other facility is capable of providing service,
whether or not it actually is in service. Typically, this measure is expressed as a percent available for the
period under consideration.
Available Resource
The sum of existing generating capacity, plus new units scheduled for service, plus the net of equivalent
firm capacity purchases & sales, less existing capacity unavailable due to planned outages.
Available Flowgate Capability (AFC)
A measure of the flow capability remaining on a Flowgate for further commercial activity over and above
already committed uses. It is defined as TFC less Existing Transmission Commitments (ETC), less a
Capacity Benefit Margin, less a Transmission Reliability Margin, plus Postbacks, and plus counterflows.
(NERC Glossary of Terms)
Available Margin
The difference between available resources & net internal demand, expressed as a percent of available
resources. This is the capacity available to cover random factors such as forced outages of generating
equipment, demand forecast errors, weather extremes, & capacity service schedule slippages.
Available Transfer Capability (ATC)
A measure of the transfer capability remaining in the physical transmission network for further commercial
activity over and above already committed uses. It is defined as Total Transfer Capability less Existing
Transmission Commitments (including retail customer service), less a Capacity Benefit Margin, less a
Transmission Reliability Margin, plus Postbacks, plus counterflows. (NERC Glossary of Terms)
Average Demand
The electric energy delivered over any interval of time as determined by dividing the total energy by the
units of time in the interval.
Avoided Cost
The cost the utility would incur but for the existence of an independent generator or other energy service
option. Avoided cost rates have been used as the power purchase price utilities offer independent
suppliers.
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Back EMF
When AC current flows through a conductor an alternating magnetic field is created. This alternating
magnetic field induces a voltage in the conductor. The induced voltage is called the conductor’s back
EMF.
Back-to-Back HVDC
An HVDC system in which AC is converted to DC & then immediately back to AC. The HVDC
transmission path is very short, normally consisting of a short section (a few feet) of buswork.
Backup Power
Power provided by contract to a customer when that customer’s normal source of power is not available.
Balancing Area ACE Limit (BAAL)
A generation control standard in which the allowable ACE bounds vary as a function of the
Interconnection frequency error. The BAAL Standard is part of BAL-001-2 & replaces CPS2.
Balancing Authority (BA)
The responsible entity that integrates resource plans ahead of time, maintains load-interchangegeneration balance within a Balancing Authority Area, and supports Interconnection frequency in real
time. (NERC Glossary of Terms)
Balancing Authority Area
The collection of generation, transmission, and loads within the metered boundaries of the Balancing
Authority. The Balancing Authority maintains load-resource balance within this area. (NERC Glossary of
Terms)
Bandwidth
The acceptable range of a quantity. For example, if it is desired to hold the voltage at a 345 kV bus
between 355 & 360 kV then the bandwidth is 355 - 360 kV or 5 kV.
Banking
Energy delivered or received by a utility with the intent that it will be returned in kind (as energy) in the
future.
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Base Load
The minimum amount of electric power delivered or required over a given period at a constant rate.
(NERC Glossary of Terms)
Base Load Capacity
Capacity used to serve an essentially constant level of customer demand. Base load generating units
typically operate whenever they are available, & they generally have a capacity factor that is above 60%.
Base Load Generating Units
Electric generating units that produce energy at a constant rate, usually at a low cost relative to other
generating units available to the system. Baseload units are used to meet some or all of a given region’s
continuous energy demand on a seasonal or daily basis, including at minimum load levels, & tend to
operate non-stop except for maintenance or forced outages.
BES Cyber Asset
A Cyber Asset that if rendered unavailable, degraded, or misused would, within 15 minutes of its required
operation, misoperation, or non-operation, adversely impact one or more Facilities, systems, or
equipment, which, if destroyed, degraded, or otherwise rendered unavailable when needed, would affect
the reliable operation of the Bulk Electric System. Redundancy of affected Facilities, systems, and
equipment shall not be considered when determining adverse impact. Each BES Cyber Asset is included
in one or more BES Cyber Systems. (A Cyber Asset is not a BES Cyber Asset if, for 30 consecutive
calendar days or less, it is directly connected to a network within an ESP, a Cyber Asset within an ESP,
or to a BES Cyber Asset, and it is used for data transfer, vulnerability assessment, maintenance, or
troubleshooting purposes.) (NERC Glossary of Terms)
BES Cyber System
One or more BES Cyber Assets logically grouped by a responsible entity to perform one or more reliability
tasks for a functional entity. (NERC Glossary of Terms)
BES Cyber System Information
Information about the BES Cyber System that could be used to gain unauthorized access or pose a
security threat to the BES Cyber System. BES Cyber System Information does not include individual
pieces of information that by themselves do not pose a threat or could not be used to allow unauthorized
access to BES Cyber Systems, such as, but not limited to, device names, individual IP addresses without
context, ESP names, or policy statements. Examples of BES Cyber System Information may include, but
are not limited to, security procedures or security information about BES Cyber Systems, Physical Access
Control Systems, and Electronic Access Control or Monitoring Systems that is not publicly available and
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could be used to allow unauthorized access or unauthorized distribution; collections of network
addresses; and network topology of the BES Cyber System. (NERC Glossary of Terms)
Bilateral Agreement
A written statement signed by 2 parties that specifies the terms for exchanging energy.
Bilateral Payback
Inadvertent interchange accumulations paid back via an interchange schedule with another BA.
Billing Demand
The demand upon which customer billing is based as specified in a rate schedule or contract. Billing
demand may be based on the contract year, a contract minimum, or a previous maximum &, therefore
does not necessarily coincide with the actual measured demand of the billing period.
Bipolar HVDC
An HVDC system that uses 2 conductors (poles). 1 pole is energized with a positive voltage & the other a
negative voltage. The voltage between the poles may be 1,000 kV or greater.
Blackstart Capability Plan (BCP)
A documented procedure for a generating unit or station to go from a shutdown condition to an operating
condition delivering electric power without assistance from the electric system. This procedure is only a
portion of an overall system restoration plan. (NERC Glossary of Terms)
Blackstart Resource (BSR)
A generating unit(s) and its associated set of equipment which has the ability to be started without
support from the System or is designed to remain energized without connection to the remainder of the
System, with the ability to energize a bus, meeting the Transmission Operator’s restoration plan needs for
real and reactive power capability, frequency and voltage control, and that has been included in the
Transmission Operator’s restoration plan. (NERC Glossary of Terms)
Blackstart Unit (BSU)
A generating unit that has the ability to go from a shutdown condition to an operating condition & start
delivering power without assistance from the power system the generator normally connects to.
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Blade
The component of a steam turbine that is acted upon by the flow of steam. Blades in steam turbines are
also referred to as buckets. Similarly, in gas, or combustion turbines, the blades are the components
acted upon by the flow of the high pressure, high temperature gases produced in the combustor. In both
steam & combustion turbines, the blades are arranged in multiple stages of varying diameter, with many
blades per stage. Modern wind turbines, in contrast, typically utilize only 3 long blades. The purpose of
the blades is to extract energy from the motion of the propelling fluid (steam, combustion gases, or air) &
convert it into rotational form by direct coupling to a common spinning shaft that is in turn used to drive a
generator.
Block Dispatch
A set of dispatch rules such that given a specific amount of load to serve, an approximate generation
dispatch can be determined. To accomplish this, the capacity of a given generator is segmented into
loadable “blocks,” each of which is grouped and ordered relative to other blocks (based on
characteristics including, but not limited to, efficiency, run of river or fuel supply considerations, and/or
“must -run” status). (NERC Glossary of Terms)
Blocked Governor
A governor control system that has somehow been blocked or prevented from responding to a frequency
deviation. There are many ways to block a governor.
Boiler
A device for generating steam for power, processing, or heating purposes; or hot water for heating
purposes or hot water supply. Heat from an external combustion source is transmitted to a fluid
contained within the tubes found in the boiler shell. This fluid is delivered to an end-use at a desired
pressure, temperature, & quantity.
Boiler Follow
A mode of operation of a fossil unit’s turbine/boiler control system in which the unit’s turbine is allowed to
immediately respond to a load (MW) change request while the boiler response “follows” with the resulting
temperature & pressure swings.
Broker
A third party who establishes a transaction between a seller & a purchaser. A Broker does not take title
to the capacity or energy.
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British Thermal Unit (BTU)
The measurement of heat released by burning any material. The amount of energy necessary to raise
the temperature of 1 pound of water by 1 degree Fahrenheit from 58.5 to 59.5 degrees Fahrenheit under
standard pressure of 30 inches of mercury at or near its point of maximum density.
Brush
A sliding contact, usually made of carbon, located between the excitation current source & the rotor field
winding leads of a synchronous generator.
Bulk Electric System (BES)
Unless modified by the lists shown below, all Transmission Elements operated at 100 kV or higher and
Real Power and Reactive Power resources connected at 100 kV or higher. This does not include facilities
used in the local distribution of electric energy.
Inclusions:




I1 - Transformers with the primary terminal and at least one secondary terminal operated at 100
kV or higher unless excluded by application of Exclusion E1 or E3.
I2 - Generating resource(s) including the generator terminals through the high-side of the step-up
transformer(s) connected at a voltage of 100 kV or above with:
a) Gross individual nameplate rating greater than 20 MVA. Or,
b) Gross plant/facility aggregate nameplate rating greater than 75 MVA.
I3 - Blackstart Resources identified in the Transmission Operator’s restoration plan.
I4 - Dispersed power producing resources that aggregate to a total capacity greater than 75 MVA
(gross nameplate rating), and that are connected through a system designed primarily for
delivering such capacity to a common point of connection at a voltage of 100 kV or above.
Thus, the facilities designated as BES are:

a) The individual resources, and
b) The system designed primarily for delivering capacity from the point where those
resources aggregate to greater than 75 MVA to a common point of connection at a
voltage of 100 kV or above.
I5 - Static or dynamic devices (excluding generators) dedicated to supplying or absorbing
Reactive Power that are connected at 100 kV or higher, or through a dedicated transformer with a
high-side voltage of 100 kV or higher, or through a transformer that is designated in Inclusion I1
unless excluded by application of Exclusion E4.
Exclusions:

E1 - Radial systems: A group of contiguous transmission Elements that emanates from a single
point of connection of 100 kV or higher and:
a) Only serves Load. Or,
b) Only includes generation resources, not identified in Inclusions I2, I3, or I4, with an
aggregate capacity less than or equal to 75 MVA (gross nameplate rating). Or,
c) Where the radial system serves Load and includes generation resources, not identified in
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Inclusions I2, I3 or I4, with an aggregate capacity of nonretail generation less than or
equal to 75 MVA (gross nameplate rating).
Note 1 – A normally open switching device between radial systems, as depicted on prints or one-line
diagrams for example, does not affect this exclusion.
Note 2 – The presence of a contiguous loop, operated at a voltage level of 50 kV or less, between
configurations being considered as radial systems, does not affect this exclusion.



E2 - A generating unit or multiple generating units on the customer’s side of the retail meter that
serve all or part of the retail Load with electric energy if: (i) the net capacity provided to the BES
does not exceed 75 MVA, and (ii) standby, back-up, and maintenance power services are
provided to the generating unit or multiple generating units or to the retail Load by a Balancing
Authority, or provided pursuant to a binding obligation with a Generator Owner or Generator
Operator, or under terms approved by the applicable regulatory authority.
E3 - Local networks (LN): A group of contiguous transmission Elements operated at less than 300
kV that distribute power to Load rather than transfer bulk power across the interconnected
system. LN’s emanate from multiple points of connection at 100 kV or higher to improve the level
of service to retail customers and not to accommodate bulk power transfer across the
interconnected system. The LN is characterized by all of the following:
a) Limits on connected generation: The LN and its underlying Elements do not include
generation resources identified in Inclusions I2, I3, or I4 and do not have an aggregate
capacity of non-retail generation greater than 75 MVA (gross nameplate rating);
b) Real Power flows only into the LN and the LN does not transfer energy originating
outside the LN for delivery through the LN; and
c) Not part of a Flowgate or transfer path: The LN does not contain any part of a permanent
Flowgate in the Eastern Interconnection, a major transfer path within the Western
Interconnection, or a comparable monitored Facility in the ERCOT or Quebec
Interconnections, and is not a monitored Facility included in an Interconnection Reliability
Operating Limit (IROL).
E4 - Reactive Power devices installed for the sole benefit of a retail customer(s).
Note - Elements may be included or excluded on a case-bycase basis through the Rules of Procedure
exception process. (NERC Glossary of Terms)
Bulk Power System (BPS)
A) facilities and control systems necessary for operating an interconnected electric energy transmission
network (or any portion thereof); and (B) electric energy from generation facilities needed to maintain
transmission system reliability. The term does not include facilities used in the local distribution of electric
energy.
Bundled Load
The combined or aggregate usage of customers who purchase electric service as a single service rather
than multiple services, including transmission service, which may be purchased separately.
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Burden
Operation of the Bulk Electric System that violates or is expected to violate a System Operating Limit or
Interconnection Reliability Operating Limit in the Interconnection, or that violates any other NERC,
Regional Reliability Organization, or local operating reliability standards or criteria. (NERC Glossary of
Terms)
Bus
An abbreviation of the word busbar. A bus is a location in an electrical network where 1 or more
elements are connected together. CBs & disconnect switches are connected to buses.
Bus Tie Breaker
A circuit breaker that is positioned to connect two individual substation bus configurations. (NERC
Glossary of Terms)
Business Practices
Those business rules contained in the Transmission Service Provider’s applicable tariff, rules, or
procedures; associated Regional Reliability Organization or regional entity business practices; or NAESB
Business Practices. (NERC Glossary of Terms)
Capacitance (C)
The property of an electrical circuit that opposes voltage changes by storing energy in its electric field.
The symbol for capacitance is “C” & the unit is the Farad. All energized equipment has a natural
capacitance.
Capacitive Load
A load that supplies lagging reactive power to the system.
Capacitive Reactance (XC)
The opposition that capacitance provides to AC current. The capacitive reactance (XC) in a 60 HZ circuit
is:
XC 
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Capacitively Coupled Voltage Transformer (CCVT)
An instrument transformer that is similar in function to a potential transformer (PT). A capacitive voltage
divider circuit is used in a CCVT to reduce power system voltage magnitudes to usable (120 Volt) levels.
Capacitive Voltage Transformers (CVTs), Coupling Capacitor Potential Devices (CCPDs), & Bushing
Potential Devices (BPDs) are similar in design & function to a CCVT.
Capacitor
A device intentionally designed to act as a capacitor & store energy in its electric field. Shunt capacitors
are very common & act as a source of reactive power to the system. Series capacitors are less common
devices that are used to reduce the inductive reactance of a transmission path.
Capacitor Bank
A capacitor is an electrical device that provides reactive power to the system & is often used to
compensate for reactive load & help support system voltage. A bank is a collection of 1 or more
capacitors at a single location.
Capacity
The rated continuous load-carrying ability, expressed in MW or MVA of generation, transmission, or other
electrical equipment.
Capacity Benefit Margin (CBM)
The amount of firm transmission transfer capability preserved by the transmission provider for LoadServing Entities (LSEs), whose loads are located on that Transmission Service Provider’s system, to
enable access by the LSEs to generation from interconnected systems to meet generation reliability
requirements. Preservation of CBM for an LSE allows that entity to reduce its installed generating
capacity below that which may otherwise have been necessary without interconnections to meet its
generation reliability requirements. The transmission transfer capability preserved as CBM is intended to
be used by the LSE only in times of emergency generation deficiencies. (NERC Glossary of Terms)
Capacity Emergency
A capacity emergency exists when a Balancing Authority Area’s operating capacity, plus firm purchases
from other systems, to the extent available or limited by transfer capability, is inadequate to meet its
demand plus its regulating requirements. (NERC Glossary of Terms)
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Capacity Factor
The ratio of the total energy generated by a generating unit for a specified period to the maximum
possible energy it could have generated if operated at the maximum capacity rating for the same
specified period, expressed as a percent.
Capacity Margin
The difference between net capacity resources & net internal demand expressed as a percent of net
capacity resources.
Capacity Market
A market where LSEs purchase generating capacity (including adequate reserves) to cover their peak
loads.
Cascading
The uncontrolled successive loss of system elements triggered by an incident at any location. Cascading
results in widespread electric service interruption that cannot be restrained from sequentially spreading
beyond an area predetermined by studies. (NERC Glossary of Terms)
Cavitation
Noise or vibration causing damage to a hydro unit’s turbine blades as a result of bubbles that form in the
water as it goes through the turbine which causes a loss in capacity, head loss, efficiency loss, & the
cavity or bubble collapses when they pass into higher regions of pressure.
Centrifugal Ballhead Governor
A mechanical governor that uses rotating flyweights to sense shaft speed. A very common governor due
to its durability & accuracy.
Charge
An atomic force. An electron has a negative charge. A proton has a positive charge. Like charges repel
1 another while opposite charges attract 1 another. Current is the flow of charge.
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Charging
The MVAr production of a transmission line. For example, a 100-mile long 345 kV line creates
approximately 75 MVAr.
Charging Current
The leading current that flows into the natural capacitance of a transmission line when it is first energized.
Chromosphere
Layer of the Sun’s atmosphere above the photosphere & beneath the corona.
CIP Exceptional Circumstance
A situation that involves or threatens to involve one or more of the following, or similar, conditions that
impact safety or BES reliability: a risk of injury or death; a natural disaster; civil unrest; an imminent or
existing hardware, software, or equipment failure; a Cyber Security Incident requiring emergency
assistance; a response by emergency services; the enactment of a mutual assistance agreement; or an
impediment of large scale workforce availability. (NERC Glossary of Terms)
CIP Senior Manager
A single senior management official with overall authority and responsibility for leading and managing
implementation of and continuing adherence to the requirements within the NERC CIP Standards, CIP002 through CIP-011. (NERC Glossary of Terms)
Circuit
A conductor or a system of conductors through which electric current flows.
Circuit Breaker (CB)
A piece of power system equipment that is used to disconnect other equipment from the power system.
CBs are grouped according to their insulating medium. For example, Air CB (ACB), Gas CBs (GCB), Oil
CBs (OCB), etc.
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Circuit Switcher (CS)
An enhanced disconnect switch that is similar in function to a CB. A CS includes both an interrupting
device (typically SF6 gas based) to enhance its current interrupting ability & a disconnect switch for
electrical isolation.
Clock Hour
The 60-minute period ending at :00. All surveys, measurements, and reports are based on Clock Hour
periods unless specifically noted. (NERC Glossary of Terms)
Cogeneration
Production of electricity from steam, heat, or other forms of energy produced as a by-product of another
process. (NERC Glossary of Terms)
Coincidental Demand
The sum of 2 or more demands that occur in the same time interval.
Cold Load Pick-Up
The increase in a load’s magnitude during the initial period after it is energized. The cold load pick-up
consists of a short-term (possibly several seconds) in-rush component & a longer-term (possibly many
minutes) loss of load diversity component.
Combined Cycle
An electric generating technology in which electricity & process steam are produced from otherwise lost
waste heat exiting from 1 or more combustion turbines. The exiting heat is routed to a conventional boiler
or to a heat recovery steam generator for use by a steam turbine in the production of electricity. This
process increases the overall efficiency of the electric generating unit.
Combustion Turbine Generator (CT)
A type of generator in which a fuel (oil, gas, etc.) is ignited & the combustion products are used to drive a
turbine.
Comision Federal de Electricidad (CFE)
A Mexican governmental entity that generates, transmits, distributes & sells electricity to more than 34.2
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million customers, representing more than 100 million people annually. CFE interconnects to ERCOT via
3 HVDC ties & to WECC via AC transmission lines at the California border just south of San Diego.
Comma Separated Values (CSV)
Refers to a file containing the values in a table as a series of ASCII text lines organized so that each
column value is separated by a comma from the next column’s value & each row starts a new line. A
CSV is sometimes referred to as a “flat file”.
Commission
Normally refers to the Federal Energy Regulatory Commission (FERC).
Commonly or Jointly Owned Units (COU/JOU)
These terms may be used interchangeably to refer to a unit in which 2 or more Generation Owners share.
Commutation
The process of turning off 1 valve & turning on another in an HVDC converter. In a 12-pulse converter
there are 12 commutations per cycle of the AC supply voltage.
Complex Power (S)
The vector sum of the MW & MVAr power. The common unit for complex power is MVA & the symbol is
“S”. Complex Power is sometimes called apparent power.
Compliance Monitor
The entity that monitors, reviews, and ensures compliance of responsible entities with reliability
standards. (NERC Glossary of Terms)
Complimentary Currents
When a subsynchronous current flows in the stator winding of a synchronous machine it will induce 2
complimentary currents in the rotor of the machine. These complimentary currents will have frequencies
of 60  the frequency of the subsynchronous stator current.
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Composite Confirmed Interchange
The energy profile (including non-default ramp) throughout a given time period, based on the aggregate
of all Confirmed Interchange occurring in that time period. (NERC Glossary of Terms)
Composite Protection System
The total complement of protection systems that function collectively to protect an element. Backup
protection provided by a different element’s protection systems is excluded. (NERC Glossary of Terms)
Compressor
A machine that increases the pressure of a gas (typically air) or vapor.
Condenser
In a steam turbine generating station, the condenser is a type of heat exchanger that cools the steam
exiting the turbine to the point where it condenses into water, thereby recovering the high quality feed
water for reuse. The cooling is accomplished using separate cooling water. Surface condensers use a
shell & tube assembly wherein the cooling water is circulated in the tubes, & the steam & condensate are
contained in the tank-like housing, or shell, that surrounds & encloses the tubes.
Conductor
A material with a low resistance to current flow. A conductor is the opposite of an insulator.
Confirmed Interchange
The state where no party has denied and all required parties have approved the Arranged Interchange.
(NERC Glossary of Terms)
Congestion
The condition that exists when market participants seek to dispatch generation in a pattern that would
result in power flows that cannot be physically accommodated by the transmission system. Although the
transmission system will not normally be operated in an overload condition, it may be described as
congested based on requested/desired schedules.
Congestion Contract
A financial instrument that provides a hedge against congestion price differences between zones.
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Congestion Management Report
A report that the Interchange Distribution Calculator issues when a Reliability Coordinator initiates the
Transmission Loading Relief procedure. This report identifies the transactions and native and network
load curtailments that must be initiated to achieve the loading relief requested by the initiating Reliability
Coordinator. (NERC Glossary of Terms)
Congestion Pricing
A system for pricing transmission service based in part on the instantaneous demand for transmission
system capacity.
Consequential Load Loss
All Load that is no longer served by the Transmission system as a result of Transmission Facilities being
removed from service by a Protection System operation designed to isolate the fault. (NERC Glossary of
Terms)
Constant Frequency Control (CFC)
An operating mode of an AGC system. While in CFC an AGC system will determine the ACE value by
considering only the frequency error.
Constant Net Interchange (CNI) Control
An operating mode of an AGC system. While in CNI control an AGC system will determine the ACE
value by considering only the interchange error.
Constrained Facility
A transmission facility (line, transformer, breaker, etc.) that is approaching, is at, or is beyond its System
Operating Limit or Interconnection Reliability Operating Limit. (NERC Glossary of Terms)
Constraint
A limitation placed on interchange transactions that flow over a constrained facility.
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Contiguous
To belong to a common area. Adjoining. A contiguous BA is self-contained. A non-contiguous BA may
have pieces spread over an entire Interconnection. Pseudo Ties are used to link the different areas of a
non-contiguous BA.
Contingency
The unexpected failure or outage of a system component, such as a generator, transmission line, circuit
breaker, switch or other electrical element. (NERC Glossary of Terms)
Contingency Analysis (CA)
The process by which the effects of contingencies are identified. CA software is used to identify N-1
violations.
Contingency Reserve
The provision of capacity deployed by the Balancing Authority to meet the Disturbance Control Standard
(DCS) and other NERC and Regional Reliability Organization contingency requirements. (NERC
Glossary of Terms)
Continuous Rating
The rating as defined by the equipment owner that specifies the level of electrical loading, usually
expressed in MW or other appropriate units that a system, facility, or element can support or withstand
indefinitely without loss of equipment life.
Contract Path
An agreed upon electrical path for the continuous flow of electrical power between the parties of an
Interchange Transaction. (NERC Glossary of Terms)
Contractually Adjacent Balancing Authorities
Two BAs that are part of the same RTO.
Control Center
One or more facilities hosting operating personnel that monitor and control the Bulk Electric System
(BES) in realtime to perform the reliability tasks, including their associated data centers, of: 1) a Reliability
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Coordinator, 2) a Balancing Authority, 3) a Transmission Operator for transmission Facilities at two or
more locations, or 4) a Generator Operator for generation Facilities at two or more locations. (NERC
Glossary of Terms)
Control Performance Standard (CPS)
The reliability standard that sets the limits of a Balancing Authority’s Area Control Error over a specified
time period. (NERC Glossary of Terms)
Control System
A collection of electrical & mechanical components designed to produce a series of outputs based on a
series of measured inputs. Common control systems related to power are excitation control & governor
control systems.
Cooling Tower
A structure & associated equipment intended to facilitate the evaporative cooling of water by contact with
air. In steam turbine generating stations, cooling water is routed through the cooling tower for cooling
after having absorbed heat in the condenser.
Coordinated Control
A mode of operation of a fossil unit’s turbine/boiler control system in which the operation of the boiler &
the turbine systems are coordinated. The coordination balances the pressure & temperature limits of the
boiler system with the desire for a turbine to immediately respond to load (MW) change requests.
Core
The material used within a transformer. A transformer’s core is formed of a magnetic material such as
iron. The purpose of the core is to confine the magnetic field to a target area.
Corrective Action Plan
A list of actions and an associated timetable for implementation to remedy a specific problem. (NERC
Glossary of Terms)
Corona
(1) Condition that occurs on energized equipment when the surface potential (voltage) is so large that the
dielectric strength of the surrounding air breaks down (ionization occurs). Symptoms of corona are a
visible ring of light & a hissing sound. Corona is undesirable as energy losses & electrical interference
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are a consequence. Corona rings are used to reduce the gradient of the electric field & reduce the
likelihood of corona occurring.
(2) Outermost layer of the Sun’s atmosphere, characterized by low densities & high temperatures.
Coronal Mass Ejection (CME)
Sun’s corona is formed by strong magnetic fields. Occasionally the solar atmosphere can violently
release tongues of gas & magnetic fields called coronal mass ejections (CME). A large CME can contain
10,000,000,000 tons of matter that can be accelerated to several million miles per hour in a spectacular
explosion. Solar material streaks out through space, impacting any planet or spacecraft in its path.
CMEs are often associated with solar flares but may occur independently. If the CME strikes the Earth’s
magnetic field, it can result in a GMD. A CME carries with it the magnetic field orientation of the Sun,
which is either a North or South orientation. If the CME magnetic field has a southward orientation,
GMDs are expected.
Cosine
Cosine of either of the unknown angles of a right triangle is the ratio of the side adjacent to the unknown
angle to the hypotenuse.
Cranking Path
A portion of the electric system that can be isolated and then energized to deliver electric power from a
generation source to enable the startup of one or more other generating units. (NERC Glossary of
Terms)
Cranking Power
Power delivered to the next generator to restart after a black-start unit has been started.
Critical Assets
Facilities, systems, and equipment which, if destroyed, degraded, or otherwise rendered unavailable,
would affect the reliability or operability of the Bulk Electric System. (NERC Glossary of Terms)
Critical Cyber Assets (CCA)
Cyber Assets essential to the reliable operation of Critical Assets. (NERC Glossary of Terms)
Current (I)
The rate of flow of electrons in an electrical conductor. Symbol is “I” & unit is the ampere, or amp.
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Current Transformer (CT)
A low power transformer (an instrument transformer) used to reduce normally high power system current
magnitudes to lower values ( 5 amps). A CT has a low number of turns on the primary winding & a high
number of turns on the secondary winding. The low magnitude secondary currents of a CT are typically
input to protective relays, meters, etc.
Curtailability
The right of a TSP to interrupt all or part of a transmission service due to constraints that reduce the
capability of the transmission network to provide that transmission service. Transmission service is to be
curtailed only in cases where system reliability is threatened or emergency conditions exist.
Curtailment
A reduction in the scheduled capacity or energy delivery of an Interchange Transaction. (NERC Glossary
of Terms)
Customer
The term applies to a customer for transmission, capacity or energy services (a PSE, Generator Owner,
LSE, or end-use customer).
Cut-Out
A switching device typically found in the distribution system. Cut-outs often include a fuse so the device
serves the dual purpose of providing electrical isolation & protection.
Cyber Assets
Programmable electronic devices, including the hardware, software, and data in those devices. (NERC
Glossary of Terms)
Cyber Security Incident
A malicious act or suspicious event that:


Compromises, or was an attempt to compromise, the Electronic Security Perimeter or Physical
Security Perimeter or,
Disrupts, or was an attempt to disrupt, the operation of a BES Cyber System.
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(NERC Glossary of Terms)
Cycle
A complete sequence of a repeating waveform. For example, the magnitude of a sine wave changes as
the degrees progress from 0 to 360. After 360 the sine wave begins to repeat itself. 1 cycle of a sinewave is therefore 360.
Damping
Forces or control system actions that reduce the magnitude of oscillations. Damping is provided by
various natural phenomena & by various electrical equipment. Damping can either be positive (reduce
oscillations) or negative (amplify oscillations).
Deadband
The distance about a target value within which a control system will not respond. For example, a
governor control system may have a deadband of 0.036 HZ. Unless the frequency deviation exceeds
0.036 HZ, the governor will not respond.
Defense-In-Depth
The defense-in-depth strategy for reliability standards development recognizes that each requirement in a
NERC Reliability Standard has a role in preventing system failures, & that these roles are complementary
& reinforcing. Reliability standards should not be viewed as a body of unrelated requirements, but rather
should be viewed as part of a portfolio of requirements designed to achieve an overall defense-in-depth
strategy & comport with the quality objectives of a reliability standard.
Delayed Fault Clearing
Fault clearing consistent with correct operation of a breaker failure protection system and its associated
breakers, or of a backup protection system with an intentional time delay. (NERC Glossary of Terms)
Delta Connection
A 3 electrical connection in which the 3 phases are connected in series with 1 another. The 3 phases
form a closed triangle. Transformer windings are often connected in a Delta configuration.
Demand
1. The rate at which electric energy is delivered to or by a system or part of a system, generally
expressed in kilowatts or megawatts, at a given instant or averaged over any designated interval of time.
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2. The rate at which energy is being used by the customer.
(NERC Glossary of Terms)
Demand-Side Management (DSM)
All activities or programs undertaken by any applicable entity to achieve a reduction in Demand. (NERC
Glossary of Terms)
Derating (Generator)
Reduction in a generating unit’s net dependable capacity.
Deregulation
Elimination of some or all government regulations from a previously regulated industry or sector of an
industry.
Deviation
Variance from the scheduled value. For example, if the scheduled value of voltage is 142 kV & the actual
value is 138 kV then the voltage deviation is -4 kV.
Device Numbers
ANSI has adopted a numbering system for labeling various types of protective equipment. Often used
numbers include:
ANSI Number
Device
Function
11
Multi-function relay
Used to denote a protective relay with multiple functions.
21
Distance relay
Operates when the ratio of voltage divided by current falls
below a value.
25
Synchronizing Relay
Operates when the phase angle across a CB is outside
allowable limits.
27
Undervoltage relay
Operates when voltage falls below a value.
49
Thermal Relay
Operates when the temperature exceeds a value.
50
Instantaneous
Overcurrent
Operates with no time delay when current rises above a
value.
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ANSI Number
Device
Function
51
Time Overcurrent
Operates with a time-delay when the current rises above a
value.
52
CB
The device that clears the fault current.
59
Overvoltage relay
Operates when voltage exceeds a value.
63
Pressure Relay
Operates when the oil pressure exceeds a value.
67
Directional Overcurrent
Operates if current is above a value & flowing in the proper
direction.
78
Out-of-step
Operates if the phase angle exceeds a preset value.
79
Reclosing relay
Initiates an automatic closing of a CB following a relay
operation.
81
Frequency Relay
Operates if frequency goes above or below a value.
86
Lockout Relay
Auxiliary relay that locks-out after operation.
87
Differential relay
Operates when there is a difference in the current entering &
leaving the protected zone.
94
Tripping Relay
Auxiliary tripping relay.
Dial-up Connectivity
A data communication link that is established when the communication equipment dials a phone number
and negotiates a connection with the equipment on the other end of the link. (NERC Glossary of Terms)
Dielectric
Insulating material used to separate & insulate. Capacitors use a dielectric between its 2 conducting
plates.
Differential Relay (87)
A protective relay that responds to the difference between the currents entering & leaving the protected
zone of the relay. Differential relays are typically applied to transformers, substation buses, & generators.
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Digital Electronics
Electronic circuitry that operates on data in the form of digits. Most digital circuits use the binary system
in which digital components are either turned on (a data value of “1”) or off (a data value of “0”) based on
circuit voltage magnitudes. Digital electronics have replaced analog electronics in most, but not all,
applications.
Direct Control Load Management (DCLM)
Demand-Side Management that is under the direct control of the system operator. DCLM may control the
electric supply to individual appliances or equipment on customer premises. DCLM as defined here does
not include Interruptible Demand. (NERC Glossary of Terms)
Direct Current (DC)
Electric current that is steady & does not change in either magnitude or direction with time. DC is also
used to refer to voltage &, more generally, to smaller or special purpose power supply systems utilizing
direct current either converted from AC, from a DC generator, from batteries, or from other sources such
as solar cells.
Disconnect Switch
A mechanical device that is used to isolate equipment from energized parts of the power system.
Disconnect switches are not rated for interrupting current unless additional arc interrupting components
are added to the disconnect.
Dispatch Order
A set of dispatch rules such that given a specific amount of load to serve, an approximate generation
dispatch can be determined. To accomplish this, each generator is ranked by priority. (NERC Glossary of
Terms)
Dispatchable Generation
Generation available physically or contractually to respond to changes in system demand or to respond to
transmission security constraints.
Distance Relay (21)
A distance relay is similar to an impedance relay with a key difference. Impedance relays activate if the
ratio of the measured voltage divided by the current falls below a pickup point. A distance relay also
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monitors the angle between the voltage & current. Distance relays are inherently directional & more
selective than impedance relays.
Distribution
For electricity, the function of distributing electric power using low voltage lines to retail customers.
Distribution Facilities
The facilities owned or controlled by a TO & used to provide wholesale distribution service.
Distribution Factor (DF)
The portion of an Interchange Transaction, typically expressed in per unit that flows across a transmission
facility (Flowgate). (NERC Glossary of Terms)
Distribution Lines
Conductors used to distribute power to the utilities customers. Distribution lines may be 3 or 1.
Distribution Network
The portion of an electric system that is dedicated to delivering electric energy to an end user. The
distribution network consists primarily of low voltage lines & transformers that “transport” electricity from
the bulk power system to retail customers.
Distribution Provider (DP)
Provides and operates the “wires” between the transmission system and the end-use customer. For those
end-use customers who are served at transmission voltages, the Transmission Owner also serves as the
Distribution Provider. Thus, the Distribution Provider is not defined by a specific voltage, but rather as
performing the Distribution function at any voltage. (NERC Glossary of Terms)
Disturbance
1. An unplanned event that produces an abnormal system condition. 2. Any perturbation to the electric
system. 3. The unexpected change in ACE that is caused by the sudden failure of generation or
interruption of load. (NERC Glossary of Terms)
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Disturbance Control Standard (DCS)
The reliability standard that sets the time limit following a Disturbance within which a Balancing Authority
must return its Area Control Error to within a specified range. (NERC Glossary of Terms)
Disturbance Monitoring Equipment (DME)
Devices capable of monitoring and recording system data pertaining to a Disturbance. Such devices
include the following categories of recorders:



Sequence of event recorders which record equipment response to the event
Fault recorders, which record actual waveform data replicating the system primary voltages and
currents. This may include protective relays.
Dynamic Disturbance Recorders (DDRs), which record incidents that portray power system
behavior during dynamic events such as low-frequency (0.1 Hz – 3 Hz) oscillations and abnormal
frequency or voltage excursions
(NERC Glossary of Terms)
Diversity Factor
The ratio of the sum of the coincident maximum demands of 2 or more loads to their non-coincident
maximum demand for the same period.
Droop
A characteristic of a governor control system that requires a decrease in generator shaft speed to
produce an increase in the generator’s MW output. There are 2 types of droop; permanent & transient:
Permanent Droop
Permanent droop is the droop used to enable all generators (with active governors) to share in frequency
regulation & to ensure a MW response in proportion to unit size. Desired values of permanent droop are
in the neighborhood of 5%. The 5% droop means that a 5% frequency change will result in the unit’s
governor moving the fuel (steam, water, gas, etc.) valves across their full range.
Transient Droop
A feature implemented in some governor control systems to ensure generators do not enter into power
oscillations following load changes. Power oscillations may occur due to the inherent time delay between
a request for a load change by the governor & the ability of a generator to actually deliver the load
change. This natural time delay could lead to excessive control action by the governor. A condition of
oscillation called “hunting” could result. If a generator is on isochronous control, transient droop is a
necessity. The transient droop function or “compensation” dampens a governor’s initial response
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following a speed change. The effect is temporary as opposed to a permanent droop function, which is
permanent.
Droop Curve
A graphical method of representing the performance of a governor. The horizontal axis is typically
generator output while the vertical axis is system frequency. When a governor with a % droop is plotted
on such a curve the plot droops from left to right with increasing generator output.
Dynamic Rating
The process that allows a system element rating to vary with the changing environmental conditions in
which the element is located.
Dynamic Reactive Reserve
Reactive power held in reserve in fast responding sources. Generators & static var compensators are
possible sources of dynamic reactive reserve.
Dynamic Schedule
A time-varying energy transfer that is updated in Real-time and included in the Scheduled Net
Interchange (NIS) term in the same manner as an Interchange Schedule in the affected Balancing
Authorities’ control ACE equations (or alternate control processes). (NERC Glossary of Terms)
Dynamic Transfer
The provision of the real-time monitoring, telemetering, computer software, hardware, communications,
engineering, energy accounting (including inadvertent interchange), and administration required to
electronically move all or a portion of the real energy services associated with a generator or load out of
one Balancing Authority Area into another. (NERC Glossary of Terms)
Dynamic Transfer Signal
The electronic signal used to implement a pseudo-tie or dynamic schedule using either a metered value
or a calculated value.
Earth Surface Potentials (ESP)
Quasi-DC voltages induced in the Earth’s surface during GMD events. ESP magnitude varies by location
on the Earth’s surface, which causes geomagnetically induced currents (GICs) to flow in the Earth’s
surface.
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Economic Dispatch
The allocation of demand to individual generating units on line to effect the most economical production of
electricity. (NERC Glossry of Terms)
Economy Energy
Electrical energy produced & supplied from a more economical source in 1 system & substituted for that
being produced or capable of being produced by a less economical source in another system.
Electric Field
The invisible lines of force that surround an energized piece of equipment. An electric field is created
when a conductor is energized by a voltage. Energy is stored in an electric field.
Electric System Losses
Total electric energy losses in the electric system. The losses consist of transmission, transformation, &
distribution losses between supply sources & delivery points. Electric energy is lost primarily due to
heating of transmission & distribution elements.
Electric Utility
Person, agency, authority, or other legal entity that owns or operates facilities for the generation,
transmission, distribution, or sale of electric energy primarily for use by the public, & is defined as a utility
under the statutes & rules by which it is regulated. An electric utility can be investor owned, cooperatively
owned, or government owned (by a federal agency, crown corporation, state, provincial government,
municipal government, & public power district).
Electrical Circuit
An electrical circuit consists of a voltage source, a conducting path to a load, a load, & a return path from
the load to the voltage source. All 4 of these ingredients are necessary for current to flow in the electrical
circuit.
Electric Reliability Organization (ERO)
The Energy Policy Act of 2005 required the creation of an independent ERO to be certified by FERC &
tasked with developing & enforcing mandatory reliability standards applying to the bulk power system.
NERC currently fills the role of ERO.
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Electrical Distance
The impedance of a transmission line is a measure of the electrical distance. For example, if a line has a
100  impedance then 50  is ½ the line’s electrical length. Impedance relays are often called distance
relays in reference to the use of impedance as a measure of electrical distance.
Electrical Energy
The generation or use of electric power by a device over a period of time, expressed in kilowatthours
(kWh), megawatthours (MWh), or gigawatthours (GWh). (NERC Glossary of Terms)
Electrodes
The connection to ground of an HVDC converter. The grounding electrode provides a voltage reference
& may be part of the current return path.
Electro Hydraulic Control (EHC)
A form of a control system in which electrical devices are typically used to measure a quantity magnitude
& hydraulics used to perform a control action. For example, a governor control system may consist of a
simple electrical machine that measures the shaft’s speed of rotation (frequency) & a hydraulic system
that changes the positions of fuel valves.
Electrojet
An auroral electrojet is a current that flows in the Earth’s ionosphere in the auroral zone. An equatorial
electrojet is a current layer in the ionosphere above the equator.
Electromagnet
Temporary magnet created by passing electric current through a coil. The coil is typically wound about a
magnetic core.
Electromagnetic Induction
The creation of a voltage in a conductor due to a relative movement between the conductor & a magnetic
field. Electromagnetic induction is the basic principle of operation of transformers & generators.
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Electromotive Force (EMF)
The voltage produced by a generator is called an electromotive force or EMF. The symbol “E” is often
used to indicate an EMF.
Electronic Access Control or Monitoring Systems (EACMS)
Cyber Assets that perform electronic access control or electronic access monitoring of the Electronic
Security Perimeter(s) or BES Cyber Systems. This includes Intermediate Systems. (NERC Glossary of
Terms)
Electronic Access Point (EAP)
A Cyber Asset interface on an Electronic Security Perimeter that allows routable communication between
Cyber Assets outside an Electronic Security Perimeter and Cyber Assets inside an Electronic Security
Perimeter. (NERC Glossary of Terms)
Electronic Security Perimeter (ESP)
The logical border surrounding a network to which BES Cyber Systems are connected using a routable
protocol. (NERC Glossary of Terms)
Element
Any electrical device with terminals that may be connected to other electrical devices such as a
generator, transformer, circuit breaker, bus section, or transmission line. An element may be comprised of
one or more components. (NERC Glossary of Terms)
Emergency or BES Emergency
Any abnormal system condition that requires automatic or immediate manual action to prevent or limit the
failure of transmission facilities or generation supply that could adversely affect the reliability of the Bulk
Electric System. (NERC Glossary of Terms)
Emergency Assistance (EA)
Energy &/or capacity provided to a utility to assist that utility during a capacity emergency.
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Emergency Energy
Electrical energy purchased by a member system whenever an event on that system causes insufficient
operating capability to cover its own demand requirement.
Emergency Rating
The rating as defined by the equipment owner that specifies the level of electrical loading or output,
usually expressed in megawatts (MW) or Mvar or other appropriate units, that a system, facility, or
element can support, produce, or withstand for a finite period. The rating assumes acceptable loss of
equipment life or other physical or safety limitations for the equipment involved. (NERC Glossary of
Terms)
Emergency Response Rate
The rate of load change that a generating unit can achieve under emergency conditions, such as loss of a
unit, expressed in MW per minute.
Emergency Voltage Limits
The operating voltage range on the interconnected systems that is acceptable for the time, sufficient for
system adjustments to be made following a facility outage or system disturbance.
End-Use Customer
The party served by an LSE (energy) & DP (wire service).
Energy
The power used over a period of time. Electrical energy is measured in watt-hours, kWh, or MWh.
Energy Conversion
The conversion of energy from 1 form to another. For example, a coal fired steam turbine/generator
converts the coal’s chemical energy to thermal energy by burning the coal. The thermal energy is then
converted to mechanical energy by heating water & turning the turbine with steam. The mechanical
energy of the turbine is then converted to electrical energy via electromagnetic induction.
Energy Deficient Entity
An LSE who foresees or is experiencing an energy emergency situation.
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Energy Emergency
A condition when a Load-Serving Entity has exhausted all other options and can no longer provide its
customers’ expected energy requirements. (NERC Glossary of Terms)
Energy Emergency Alert (EEA)
NERC Reliability Standard EOP-002 prescribes the use of an energy emergency alert (EEA) procedure
when an LSE is unable to meet its customers’ expected energy requirements. These energy
emergencies are declared by the LSE’s RC, & are categorized by level of severity, i.e., EEA1, 2, or 3,
with level 3 being the most severe.
Energy Exchange
Transaction whereby the receiver accepts delivery of energy for a supplier’s account & returns energy
later at times, rates, & in amounts as mutually agreed.
Energy Imbalance Service
Provides energy correction for any hourly mismatch between a transmission customer’s energy supply &
the demand served.
Energy “In-Kind” Payback
Inadvertent interchange accumulated during “on-peak” hours shall only be paid back during “on-peak”
hours. Inadvertent interchange accumulated during “off-peak” hours shall only be paid back during “offpeak” hours.
Energy Management System (EMS)
An EMS system is a computer system used by an energy company. The EMS includes subfunctions of
SCADA & AGC.
Energy Only Market
A market for electric energy that pays resources only for delivered energy & ancillary services, & does not
pay for installed capacity.
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Envelope
The imaginary boundary that surrounds the fundamental frequency oscillations waveshape. The
frequency of lower frequency oscillations can be determined by noting the frequency of the oscillation
envelope.
Epsilon 1 (ε1I)
Epsilon 1 is the 1-minute average of the root mean square (RMS) targeted frequency error for each
Interconnection, as recommended by the NERC Resources Subcommittee & approved by the NERC
Operating Committee. Epsilon 1 values for each Interconnection are unique. Epsilon 1 is used in the
calculations for CPS1 & BAAL.
Epsilon 10 (ε10)
Epsilon 10 is the 10-minute average of the root mean square (RMS) targeted frequency error for each
Interconnection, as recommended by the NERC Resources Subcommittee & approved by the NERC
Operating Committee. Epsilon 10 values for each Interconnection are unique. Epsilon 10 is used in the
calculation for the L10 bound.
Equal Area Criterion
A method of determining the angle stability or instability of a simple power system. The Equal Area
Criterion states that the decelerating area of a power-angle curve must be at least as large as the
accelerating area for the power system to be angle stable.
Equipment
An electrical device with terminals that may be connected to other electrical devices.
Equipment Rating
The maximum and minimum voltage, current, frequency, real and reactive power flows on individual
equipment under steady state, short-circuit and transient conditions, as permitted or assigned by the
equipment owner. (NERC Glossary of Terms)
ERCOT
Acronym for the Electrical Reliability Council of Texas. The ERCOT ISO operates the Texas
Interconnection.
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E-Tag
Electronic Tagging, or E-Tag, is used to schedule an interchange transaction in wholesale electricity
markets. NERC &/or Regional Entities collect all E-Tag data in near real-time to assist RCs in identifying
transactions to be curtailed to relieve overload when transmission constraints occur.
Excitation System
A generator control system used to control the production of reactive power. The excitation system’s
main components are the voltage regulator & the exciter.
Exciter
The DC power source for an excitation system.
Exciting Current
The current drawn by a transformer to magnetize its core & supply the core losses. The exciting current
is typically 1-2% of the transformer’s full load current.
Exempt Wholesale Generator (EWG)
Wholesale generators created under the 1992 Energy Policy Act that are exempt from certain financial &
legal restrictions stipulated in the Public Utilities Holding Company Act of 1935.
Existing Transmission Commitments (ETC)
Committed uses of a Transmission Service Provider’s Transmission system considered when determining
ATC or AFC. (NERC Glossary of Terms)
External Routable Connectivity
The ability to access a BES Cyber System from a Cyber Asset that is outside of its associated Electronic
Security Perimeter via a bi-directional routable protocol connection. (NERC Glossary of Terms)
Extinction Advance Angle ()
In an HVDC converter operating as an inverter, the angle by which the valve firing is advanced from its
normal voltage based commutation point. The Greek lower case letter gamma ( ) is the symbol for the
extinction advance angle. The extinction advance angle is used to control the magnitude of the
converter’s (when operated as a inverter) output voltage.
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Facilities Study
An engineering study conducted by the TSP to determine the required modifications to the TSP’s
transmission system, including the cost & scheduled completion date for such modifications, that will be
required to provide the requested transmission service.
Facility
A set of electrical equipment that operates as a single Bulk Electric System Element (e.g., a line, a
generator, a shunt compensator, transformer, etc.) (NERC Glossary of Terms)
Facility Rating
The maximum or minimum voltage, current, frequency, or real or reactive power flow through a facility
that does not violate the applicable equipment rating of any equipment comprising the facility. (NERC
Glossary of Terms)
Farad (F)
The unit of capacitance. Symbol is “F”.
Fast Valving
A method of reducing the accelerating energy in a steam turbine/generator. Fast valving involves the
rapid adjustment of turbine valves when a generator starts to accelerate. Fast valving may assist in
maintaining the angle stability of a power system.
Fault
An event occurring on an electric system such as a short circuit, a broken wire, or an intermittent
connection. (NERC Glossary of Terms)
Federal Energy Regulatory Commission (FERC)
A U.S. Federal agency that, among other responsibilities, regulates the transmission & wholesale sales of
electricity in interstate commerce.
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Feedback Loop
A control loop in which current control actions are influenced by the responses to previous control actions.
In a feedback loop (such as in a generator’s excitation system) the response of the controlled variable is
constantly monitored to determine what new control actions should be taken.
Ferranti Rise
A phenomena in which a transmission line, with 1 end closed & 1 end open, is exposed to its highest
voltage magnitude at the open-end of the line. The Ferranti Rise Effect is due to the absorption of a
leading charging current when a transmission line is energized but open-ended.
Ferroresonance
A resonance condition in which a portion of the inductance is provided by an iron-core inductance (Ferro
is Latin for iron.). Iron-core inductances change magnitude when the iron is saturated. Ferroresonance is
possible in the transmission or distribution system but is much more common in the distribution system.
Field Winding
The winding wrapped about the rotor of a synchronous machine. DC excitation current is fed to the field
winding to produce the rotor’s magnetic field.
Filament
Mass of gas suspended over the Sun’s chromosphere by magnetic fields & seen as dark ribbons
threaded over the solar disk. A filament on the edge of the Sun seen in emission against the dark sky is
called a prominence.
Filter
A combination of capacitors, inductors, & resistors used to encourage or block the flow of a specific
frequency or band of frequencies of energy.
Firm Capacity
Capacity that is as firm as the seller’s native load unless modified by contract. Associated energy may or
may not be taken at option of purchaser. Supporting reserve is carried by the seller.
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Firm Demand
That portion of the Demand that a power supplier is obligated to provide except when system reliability is
threatened or during emergency conditions. (NERC Glossary of Terms)
Firm Transmission Service
The highest quality (priority) service offered to customers under a filed rate schedule that anticipates no
planned interruption. (NERC Glossary of Terms)
Flare
A sudden eruption of energy from the sun’s surface lasting minutes to hours, from which radiation &
particles are emitted.
Flashover
An electrical discharge through air around or over the surface of insulation, between objects of different
potential, caused by placing a voltage across the air space that results in the ionization of the air space.
(NERC Glossary of Terms)
Flat Frequency Control
An alternative name for constant frequency control.
Flat Tie-line Control
An alternative name for constant net interchange control.
Flowgate
1.) A portion of the Transmission system through which the Interchange Distribution Calculator calculates
the power flow from Interchange Transactions. 2.) A mathematical construct, comprised of one or more
monitored transmission Facilities and optionally one or more contingency Facilities, used to analyze the
impact of power flows upon the Bulk Electric System. (NERC Glossary of Terms)
Flowgate Methodology
The Flowgate methodology is characterized by identification of key Facilities as Flowgates. Total
Flowgate Capabilities are determined based on Facility Ratings and voltage and stability limits. The
impacts of Existing Transmission Commitments (ETCs) are determined by simulation. The impacts of
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ETC, Capacity Benefit Margin (CBM) and Transmission Reliability Margin (TRM) are subtracted from the
Total Flowgate Capability, and Postbacks and counterflows are added, to determine the Available
Flowgate Capability (AFC) value for that Flowgate. AFCs can be used to determine Available Transfer
Capability (ATC). (NERC Glossary of Terms)
Flyweights
The portion of a centrifugal ballhead governor that rotates.
Force Majeure
A superior force, act of God or unexpected & disruptive event, which may serve to relieve a party from a
contract or obligation.
Forced Outage
1. The removal from service availability of a generating unit, transmission line, or other facility for
emergency reasons. 2. The condition in which the equipment is unavailable due to unanticipated failure.
(NERC Glossary of Terms)
Forced Outage Rate
An indication of the percentage of time a generating unit, transmission line or other energy facility is
forced out of service for emergency reasons. This percentage excludes the time during which the unit
experiences a planned service shutdown (for example, scheduled maintenance). This number can be
used to describe a facility’s reliability. A weighted average of forced outage rates for all generating
facilities in a utility’s system may be used to describe system reliability. Forced Outage Hours + InService Hours x 100 = Forced Outage Rate.
Forecast
Predicted demand for power. A forecast may be short term (for example, 15 minutes) for system
operation purposes, long-term (5 to 20 years) for generation planning purposes, or for any range in
between. A forecast may include peak demand, energy, reactive power, or demand profile. A forecast
may be made for total system demand, transmission loading, substation/feeder loading, individual
customer demand, or appliance demand.
Fourier Analysis
A scientific process in which the various frequency components (harmonics) of a waveform are identified.
For example, a waveform may have a fundamental frequency of 60 HZ but also contain 3 rd & 5th harmonic
components. Fourier analysis is used to identify the order of the harmonic components.
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FRCC
Acronym for the Florida Reliability Coordinating Council. FRCC is 1 of the eight NERC Regional Entities.
Frequency (F)
The rate at which a repeating waveform repeats itself. Frequency is measured in cycles per second or in
Hertz (HZ). The symbol if “F”.
Frequency Bias
A value, usually expressed in megawatts per 0.1 Hertz (MW/0.1 Hz), associated with a Balancing
Authority Area that approximates the Balancing Authority Area’s response to Interconnection frequency
error. (NERC Glossary of Terms)
Frequency Bias Setting (B)
A number, either fixed or variable, usually expressed in MW/0.1 Hz, included in a Balancing Authority’s
Area Control Error equation to account for the Balancing Authority’s inverse Frequency Response
contribution to the Interconnection, and discourage response withdrawal through secondary control
systems. (NERC Glossary of Terms)
Frequency Deviation
A change in Interconnection frequency. (NERC Glossary of Terms)
Frequency Error (F)
The difference between the actual and scheduled frequency. (FA – FS ) (NERC Glossary of Terms)
Frequency Regulation
The ability of a Balancing Authority to help the Interconnection maintain Scheduled Frequency. This
assistance can include both turbine governor response and Automatic Generation Control. (NERC
Glossary of Terms)
Frequency Response (Equipment)
The ability of a system or elements of the system to react or respond to a change in system frequency.
(NERC Glossary of Terms)
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Frequency Response (System)
The sum of the change in demand, plus the change in generation, divided by the change in frequency,
expressed in megawatts per 0.1 Hertz (MW/0.1 Hz). (NERC Glossary of Terms)
Frequency Response Characteristic (FRC or β)
The MW response of the power system (or a section of the power system) to a frequency deviation. The
FRC is typically stated in terms of MW per 0.1 HZ. For example, a BA may have an FRC of 200 MW/0.1
HZ. This value of FRC indicates that for a frequency deviation of 0.1 HZ this BA would respond with 200
MW. The FRC of a system varies with changing system conditions.
Frequency Response Measure (FRM)
The median of all the Frequency Response observations reported annually by Balancing Authorities or
Frequency Response Sharing Groups for frequency events specified by the ERO. This will be calculated
as MW/0.1Hz. (NERC Glossary of Terms)
Frequency Response Obligation (FRO)
The Balancing Authority’s share of the required Frequency Response needed for the reliable operation of
an Interconnection. This will be calculated as MW/0.1Hz. (NERC Glossary of Terms)
Frequency Response Sharing Group (FRSG)
A group whose members consist of two or more Balancing Authorities that collectively maintain, allocate,
and supply operating resources required to jointly meet the sum of the Frequency Response Obligations
of its members. (NERC Glossary of Terms)
Frequency Swings
Constant changes in frequency from its nominal or steady-state value.
Function
A set of related reliability tasks.
Functional Entity
The term used in the Functional Model which applies to a class of entity that carries out the tasks within a
function.
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Fundamental Frequency
The base frequency for a system. For example, the fundamental frequency of North American power
systems is 60 HZ while a large portion of the world uses 50 HZ as the fundamental frequency.
Gallery
A passageway within a water dam created to allow inspection of the dam’s structure.
Gate/Grid Control
The means of controlling a mercury arc valve (MAV) or a thyrister valve. A pulse of current or voltage is
applied to the grid of a MAV or the gate of a thyrister. The pulse will turn the valve on if it is forward
biased. Gate/grid control is typically only used to turn a valve on.
Generating Unit Capability
The MW production rating of a generator.
Generation
The process of producing electrical energy from other forms of energy; also, the amount of electric power
produced, usually expressed in kW or MW.
Generation Capability Import Requirement
The amount of generation capability from external sources identified by a Load-Serving Entity (LSE) or
Resource Planner (RP) to meet its generation reliability or resource adequacy requirements as an
alternative to internal resources. (NERC Glossary of Terms)
Generation Control
The process by which the generation supply is adjusted to both maintain system frequency & keep a
close match between the actual tie-line flows & the scheduled tie-line flows.
Generator
Generally, an electromechanical device used to convert mechanical power to electrical power.
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Generator-to-Load Distribution Factor (GLDF)
The algebraic sum of a Generator Shift Factor and a Load Shift Factor to determine the total impact of an
Interchange Transaction on an identified transmission facility or Flowgate. (NERC Glossary of Terms)
Generator Operator (GOP)
The entity that operates generating unit(s) and performs the functions of supplying energy and
Interconnected Operations Services. (NERC Glossary of Terms)
Generator Owner (GO)
Entity that owns and maintains generating units. (NERC Glossary of Terms)
Generator Runback
The intentional rapid reduction of the output level of an generating unit or an entire generating station,
either manually or automatically via plant controls, due to any of a variety of problems in the plant that
limit the plant’s capacity to generate power, or problems on the transmission system external to the plant
which limit the capability of the system to accept the plant’s power output.
Generator Shift Factor (GSF)
A factor to be applied to a generator’s expected change in output to determine the amount of flow
contribution that change in output will impose on an identified transmission facility or Flowgate. (NERC
Glossary of Terms)
Geomagnetic Disturbance (GMD)
A worldwide disturbance of the Earth’s magnetic field. Also called solar magnetic disturbance (SMD) &
geomagnetic storm. GMDs occur 1 to 4 days after a flare or other eruption (such as a CME) on the Sun.
Geomagnetic Induced Currents (GIC)
Quasi-DC currents induced in the Earth’s surface as a result of GMDs & the ESPs produced. GICs enter
the power system via the grounded neutrals of equipment. High GIC levels can thermally damage
transformers, lead to increases in Mvar usage, & create high harmonic levels that can further harm the
power system. The extra high voltage (EHV) portion of the grid (≥ 345 kV) is especially vulnerable to
GICs due to its low impedance & common usage of grounded autotransformers.
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Geomagnetic Storm
See Geomagnetic Disturbance.
Good Utility Practice
Any of the practices, methods & acts engaged in or approved by a significant portion of the electric utility
industry during the relevant time period, or any of the practices, methods & acts which, in the exercise of
reasonable judgment in light of the facts known at the time the decision was made, could have been
expected to accomplish the desired result at a reasonable cost consistent with good business practices,
reliability, safety & expedition. Good Utility Practice is not intended to be limited to the optimum practice,
method, or act to the exclusion of all others, but rather to be acceptable practices, methods, or acts
generally accepted in the region.
Governor
The electronic, digital or mechanical device that implements primary frequency response of generating
units or other system elements.
Governor Characteristic Curve
A graphical method of representing the performance of a governor / generator combination. The
horizontal axis is typically generator output while the vertical axis is system frequency. When a governor
with a % droop is plotted on such a curve the plot droops from left to right with increasing generator
output.
Greek Alphabet
Upper & Lower case letters from the Greek alphabet are typically used by electrical engineers to
designate angles & represent variables. The following Greek letters are commonly used:

alpha

delta
ε
epsilon

mu

Phi

beta

theta

pie

omega

gamma

lambda

rho

Omega
Grid
An electrical transmission &/or distribution network.
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Gross Generation
The output power (in MW) at the stator terminals of a generator.
Ground
A conducting connection between an electrical circuit or device & the earth. A ground may be intentional,
as in the case of a safety ground, or accidental, which may result in high overcurrents.
Half-Cycle Saturation
A magnetic saturation of a transformer’s core due to the presence of a DC current in the transformer
windings. The operating point of the transformer on its saturation curve is shifted such that for a portion
of ½ of each cycle the transformer saturates.
Harmonics
Integer multiples of the fundamental frequency. If the fundamental frequency is 60 HZ then the 2 nd
Harmonic has a frequency of 120 HZ, 3rd Harmonic 180 HZ, etc.
Head
Vertical change in elevation, expressed in either feet or meters, between the head water level & the
tailwater level of a hydro-electric facility.
Heat Rate
An expression for the efficiency of a thermal power plant. The heat rate is the amount of heat (measured
in British Thermal Units or BTU) that is required to produce a kWh or electrical output. The lower the heat
rate, the more efficient the power plant.
Heat Tracing
The application of a heat source to pipes, lines, & other equipment which, in order to function properly,
must be kept from freezing. Heat tracing typically takes the form of a heating element running parallel
with & in direct contact with piping.
Henry (H)
The unit of inductance. The symbol for a Henry is “H”.
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Hertz (HZ)
Hertz is the unit of frequency equal to 1 cycle per second.
High Side Winding (HS)
The high voltage winding of the transformer.
High Voltage Direct Current (HVDC)
In electric power systems, the term HVDC (high voltage DC) is used to describe a transmission-level
facility that interconnects between 2 portions of a power system, 2 different power systems, or 2 different
Interconnections. The HVDC Tie consists of: (1) a converter station to convert 3 phase AC power to DC;
(2) a DC connection to a second converter station; & (3) a second converter station that reconverts the
DC power back to 3-phase AC. The DC connection between the 2 converter stations may be either a
long HVDC transmission line or, in the case of “back-to-back” converters at the same location, a simple
set of bus bars. The power flow in DC ties is not free-flowing as it is in AC lines, but rather is controlled
precisely by control systems on the converters. Unlike AC lines, DC ties can interconnect between
asynchronous interconnections such as ERCOT, the Eastern Interconnection, & the Western
Interconnection because concerns about frequency, phase angle, & voltage differences are rendered
immaterial by the AC-to-DC-to-AC conversion process.
Host Balancing Authority
1. A Balancing Authority that confirms and implements Interchange Transactions for a Purchasing Selling
Entity that operates generation or serves customers directly within the Balancing Authority’s metered
boundaries. 2. The Balancing Authority within whose metered boundaries a jointly owned unit is
physically located. (NERC Glossary of Terms)
Hourly Value
Data measured on a Clock Hour basis. (NERC Glossary of Terms)
HVDC
Acronym for High Voltage Direct Current. The term HVDC is commonly used when the DC voltage is
above 100 kV.
HVDC Converter
An arrangement of equipment designed & operated to convert between AC & DC power. A converter can
be operated as an inverter (DC to AC) or a rectifier (AC to DC).
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HVDC Modulation
A feature added to the controls of an HVDC system. HVDC modulation modulates (adjusts) the power
flow into an HVDC converter in order to dampen power oscillations in the AC supply system. HVDC
modulation assists with damping AC system power oscillations.
Hydraulics
The use of fluid forces to perform work. For example, hydraulics are often used in governor control
systems to develop the large forces required to move steam or water valves.
Hypotenuse
The side of a right triangle which is opposite the 90 angle.
IEEE
Acronym for the Institute of Electrical & Electronic Engineers. The IEEE is an international standards
organization that publishes guidelines for, among other areas, power systems.
Igneous Rock
Rock that was created by volcanic activity.
Ignition Delay Angle ()
In an HVDC converter operating as a rectifier, the angle by which the valve firing is delayed from its
normal voltage based commutation point. The Greek lower case letter alpha () is the symbol for the
ignition delay angle. The ignition delay angle is used to control the magnitude of the converter’s (when
operated as a rectifier) output voltage.
Imbalance
A condition where the generation & interchange schedules do not match demand.
Impedance (Z)
The total opposition to the current flow in an electrical circuit. The symbol for the impedance is “Z”. The
impedance includes the resistance (R), capacitance (C), & the inductance (L).
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Impedance Relay
A protective relay that activates if the ratio of the measured voltage divided by the current (the
impedance) falls below a pickup point. Impedance relays were first developed in the 1920’s. Impedance
relays have largely been replaced by distance relays. Distance relays monitor voltage, current, & the
angle between voltage & current.
Implemented Interchange
The state where the Balancing Authority enters the Confirmed Interchange into its Area Control Error
equation. (NERC Glossary of Terms)
Impulse Turbine
A water turbine in which high velocity water is directed through nozzles at the turbine buckets. A Pelton
Wheel is an example of an impulse turbine.
Inadvertent Energy
When inadvertent interchange exists for a period of time, inadvertent energy will be accumulated.
Inadvertent Energy Balancing
A BA’s accounting of its inadvertent interchange, which is the accumulated difference between actual &
scheduled interchange.
Inadvertent Energy Payback
When the inadvertent energy that a BA accumulates exceeds a specified value, the BA should arrange
for an inadvertent payback.
Inadvertent Interchange
The difference between the Balancing Authority’s Net Actual Interchange and Net Scheduled
Interchange. (IA - IS) (NERC Glossary of Terms)
Incremental Cost (λ)
The cost associated with producing an additional MWh of energy from a generating unit. Incremental
cost is typically stated in $/MWh or Mills/kWh. The Greek letter Lambda (λ) is often used as the symbol
for incremental cost.
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Incremental Heat Rate
The amount of additional heat that must be added to a thermal generating unit at a given loading to
produce an additional unit of output. It is usually expressed in British thermal units per kilowatt hour
(BTU/kWh) of output.
Incremental Losses
The increase in losses due to an increase in power flow. For example, assume the power flow on a
transmission line is initially 100 MW. If the power flow is increased to 101 MW there will be incremental
energy losses associated with the 1 MW increase in power flow. The percentage of the incremental loss
increases with increasing levels of power flow.
Independent Power Producer (IPP)
Any entity that owns or operates an electricity generating facility that is not included in an electric utility’s
rate base. This term includes, but is not limited to, cogenerators and small power producers and all other
nonutility electricity producers, such as exempt wholesale generators, who sell electricity. (NERC
Glossary of Terms)
Independent System Operator (ISO)
An organization responsible for the reliable operation of the power grid in a particular region & for
providing open access transmission access to all market participants on a nondiscriminatory basis. ISOs
in the U.S. include the California ISO, ISO New England, the New York ISO, PJM, the Midwest ISO, &
ERCOT. These ISOs dispatch generation in their respective geographic territories.
Indirect Demand-Side Management
Programs such as conservation, improvements in efficiency of electrical energy use, rate incentives,
rebates, & other similar activities to influence the customer’s demand.
Inductance (L)
The property of an electrical circuit that opposes a change in current flow. The symbol for inductance is
the letter “L” & the unit is the Henry (symbol “H”).
Induction Machine
An AC machine that can be operated as a generator or as a motor. When operated as a generator the
induction machine’s rotor is driven at a speed greater than synchronous speed. When operated as a
motor the induction machine’s rotor is driven at a speed less than synchronous speed. Induction
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generators are rarely used by large scale power generators. Induction motors are the most common type
of AC motor. Induction machines absorb reactive power (always a lagging load) & cannot be used to
produce reactive power as a synchronous machine can.
Inductive Load
A load that absorbs lagging reactive power from the system.
Inductive Reactance (XL)
The opposition that inductance provides to AC current. The inductive reactance (X L) in a 60 HZ circuit is:
X L  2fL
Inertia
The property of an object that resists changes to the motion of the object. For example, the inertia of a
rotating object resists changes to the object’s speed of rotation. The inertia of a rotating object is a
function of its mass, diameter, & speed of rotation.
Inertial Energy
Energy stored within a rotating mass. For example, a spinning generator contains inertial energy. The
terms inertial energy, stored energy, & rotational energy are often used interchangeably to refer to the
energy stored in the rotating elements (motors & generators) of the power system.
In-Rush Current
The sudden rush of current when a transformer or a motor is first energized. The peak magnitudes of the
in-rush current last only a few cycles but can reach levels more than 10 times the device’s full load
current.
Installed Reserve
The difference between a utility’s expected annual peak MW generation capability & their annual peak
MW load. Installed reserves are a rough approximation of a utility's spare or reserve generation.
Instantaneous Demand
The rate of energy delivered at a given instant.
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Instrument Transformers (IT)
A low power transformer classification. Instrument transformers (IT) include current transformers (CTs),
potential transformers (PTs) & capacitive devices.
Insulator
A material with high impedance to current flow. An insulator is the opposite of a conductor.
Integrated Demand
The average of the instantaneous demands (MW) over the demand interval (usually hours).
Integration
In the terms for dynamic schedule and pseudo-tie above means the value could be mathematically
calculated or determined mechanically with a metering device.
Interactive Remote Access
User-initiated access by a person employing a remote access client or other remote access technology
using a routable protocol. Remote access originates from a Cyber Asset that is not an Intermediate
System and not located within any of the Responsible Entity’s Electronic Security Perimeter(s) or at a
defined Electronic Access Point (EAP). Remote access may be initiated from: 1) Cyber Assets used or
owned by the Responsible Entity, 2) Cyber Assets used or owned by employees, and 3) Cyber Assets
used or owned by vendors, contractors, or consultants. Interactive remote access does not include
system-to-system process communications. (NERC Glossary of Terms)
Inter Area Mode
A power oscillation mode (frequency) in which a large section of an interconnected power system
oscillates with respect to another large section of the same interconnection. The Inter-area mode ranges
from 0.05 to 0.5 HZ.
Inter Control Center Communications Protocol (ICCP)
ICCP is a communications protocol used by utility organizations throughout the world to provide data
exchange over wide area networks (WANs).
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Interchange
Energy transfers that cross Balancing Authority boundaries. (NERC Glossary of Terms)
Interchange Arrangement
The process of finding a seller & buyer for an interchange transaction, plus reserving transmission
services.
Interchange Authority (IA)
The responsible entity that authorizes implementation of valid and balanced Interchange Schedules
between Balancing Authority Areas, and ensures communication of Interchange information for reliability
assessment purposes. (NERC Glossary of Terms)
Interchange Block Accounting
Energy accounting that assumes a beginning & ending ramp time of zero minutes. For accounting
purposes, this moves the energy associated with the starting & ending ramps into the adjacent starting &
ending clock time of the Interchange.
Interchange Confirmation
Agreement of the terms of the interchange schedule prior to its implementation.
Interchange Coordinator (IC)
The responsible entity that authorizes implementation of valid & balanced Interchange schedules
between BA Areas, & ensures communication of interchange information for reliability assessment
purposes.
Interchange Distribution Calculator (IDC)
The mechanism used by Reliability Coordinators in the Eastern Interconnection to calculate the
distribution of Interchange Transactions over specific Flowgates. It includes a database of all Interchange
Transactions and a matrix of the Distribution Factors for the Eastern Interconnection. (NERC Glossary of
Terms)
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Interchange Implementation
The physical initiation of the interchange schedule by entering it into the BA’s energy management
system or by approving a schedule that has been electronically transferred into the energy management
system.
Interchange Meter Error (IME)
The meter error correction factor, which represents the difference between the integrated hourly average
of the actual net interchange (NIA) & the cumulative hourly net interchange energy measurement (in
MWH).
Interchange Schedule
An agreed-upon Interchange Transaction size (megawatts), start and end time, beginning and ending
ramp times and rate, and type required for delivery and receipt of power and energy between the Source
and Sink Balancing Authorities involved in the transaction. (NERC Glossary of Terms)
Interchange Transaction
An agreement to transfer energy from a seller to a buyer that crosses one or more Balancing Authority
Area boundaries. (NERC Glossary of Terms)
Interchange Transaction Cancellation
The complete withdrawal of an interchange transaction by a purchasing-selling entity prior to the start
time of the transaction.
Interchange Transaction Curtailment
The complete or partial interruption of an interchange transaction that has started or “holding” of a new
interchange transaction that has not yet started by a TSP, RC, or BA to maintain operating security.
Interchange Transaction Tag
The details of an Interchange Transaction required for its physical implementation. (NERC Glossary of
Terms)
Interchange Transaction Termination
The complete interruption of an interchange transaction by a PSE after the start time of the transaction.
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Interconnect
When 2 power systems tie together with AC transmission, the transmission line is referred to as an
interconnect.
Interconnected Operations Service (IOS)
A service (exclusive of basic energy and transmission services) that is required to support the reliable
operation of interconnected Bulk Electric Systems. (NERC Glossary of Terms)
Interconnected System
A system consisting of 2 or more individual electric systems that normally operate in synchronism & have
connecting tie lines.
Interconnection
When capitalized, any one of the four major electric system networks in North America: Eastern, Western,
ERCOT and Quebec. (NERC Glossary of Terms)
Interconnection Reliability Operating Limit (IROL)
A System Operating Limit that, if violated, could lead to instability, uncontrolled separation, or Cascading
outages that adversely impact the reliability of the Bulk Electric System. (NERC Glossary of Terms)
Interconnection Reliability Operating Limit T V
The maximum time that an IROL can be violated before the risk to the Interconnection or other RC areas
becomes greater than acceptable. Each IROL’s TV shall be less than or equal to 30 minutes.
Interconnection Time Monitor
An entity that monitors time error & initiates & terminates time error corrections.
Interface
The specific set of transmission elements between 2 areas or between 2 areas comprising 1 or more
electrical systems.
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Intermediate Balancing Authority
A Balancing Authority on the scheduling path of an Interchange Transaction other than the Source
Balancing Authority and Sink Balancing Authority. (NERC Glossary of Terms)
Intermediate System
A Cyber Asset or collection of Cyber Assets performing access control to restrict Interactive Remote
Access to only authorized users. The Intermediate System must not be located inside the Electronic
Security Perimeter. (NERC Glossary of Terms)
Interpersonal Communication
Any medium that allows two or more individuals to interact, consult, or exchange information. (NERC
Glossary of Terms)
Inter-regional Security Network (ISN)
The telecommunications & data system used to share operating information, including security data,
among the data recipients.
Interruptible Load or Interruptible Demand
Demand that the end-use customer makes available to its Load-Serving Entity via contract or agreement
for curtailment. (NERC Glossary of Terms)
Interruptible Responsive Reserve
Interruptible load controlled by high-set underfrequency relaying.
Interruption
A reduction in non-firm transmission service due to economic reasons.
Intra-Area Mode
A power oscillation mode (frequency) in which a pocket of generation in a power system oscillates with
respect to another pocket of generation in the same power system. The intra-area mode ranges from 0.4
to 1 HZ.
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Intra-Plant Mode
A power oscillation mode (frequency) in which several generators in a multi-unit power station oscillate
with respect to 1 another. The intra-plant mode ranges from 1.5 to 3 HZ.
Inverter
An HVDC converter operated to convert DC power to AC power.
Ionization
Ionization occurs when an atom (or group of atoms) receives enough energy to split into 1 or more free
electrons & a positive ion. Ionization is a special case of charging. The visible corona effect that often
surrounds energized equipment is due to ionization of the air by the strong electric field surrounding the
energized conductor.
Ionosphere
Region of the Earth’s upper atmosphere containing free electrons & ions produced by ionization of the
atmosphere by solar radiation.
Island
An electrically isolated portion of an Interconnection. An islanded system maintains its own frequency.
Islands are frequently formed after major disturbances or during restoration following a major disturbance.
Isochronous Governor Control
A governor operated with a 0% droop. When in isochronous control, a governor will try to maintain 60
HZ. Isochronous control may be used during a system restoration.
Isolated
To be electrically separated from the remainder of the Interconnection. An isolated system does not have
transmission ties to the rest of the Interconnection. An isolated system is an electrical island.
Joint Control
Automatic Generation Control of jointly owned units by two or more Balancing Authorities. (NERC
Glossary of Terms)
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Jointly Owned Unit (JOU)
This term refers to a unit in which 2 or more entities share ownership.
Journal Bearing
The bearing that supports the actual weight of a generator.
K-Index
Earth’s geomagnetic field can be measured with instruments called magnetometers. Data from many
magnetometers allow observers to track the current state of geomagnetic conditions. Magnetometer data
are often recorded in the form of 3-hourly indices (units of nano-Tesla) that provide a quantitative
measure of the level of geomagnetic activity. One such index is called the K-Index. K-Index values
range from 0 to 9 & are directly related to the amount of fluctuation (relative to a quiet day) in the
geomagnetic field over a 3-hour interval. A K-Index is tied to a specific observatory location.
K-Index Alert
K-Index Alerts are an increased level from K-Index Warnings and are issued by SWPC for K-Index from 4
to 9 based on increasing likelihood of GMD effects using data from the ACE satellite which monitors the
solar wind.
K-Index Warning
K-Index Warnings are issued by SWPC for any period with expected K-Index ≥ 4.
KP-Index
KP (K-Planetary) index is used to indicate the severity of a planetary magnetic disturbance. K P is an
index based on the average of weighted K-Indices at multiple ground magnetic field observatories & was
first implemented in 1932. KP is based on the range of the magnetic field variation within 3-hour intervals.
KP values range from 0 (very quiet) to 9 (severely disturbed).
Kilo Var (KVAr)
Unit of reactive power equal to 1,000 Var.
Kilo Volt (kV)
Unit of electrical potential equal to 1,000 Volts.
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Kilo Volt Amperes (kVA)
Unit of complex power equal to 1,000 volt amperes. Here, complex power is in contrast to active power.
On AC systems the voltage & current will not be in phase if reactive power is being transmitted.
Kilo Watt Hour (kWh)
Unit of energy equaling 1 thousand watt hours, or 1 kilowatt used over 1 hour. This is the normal quantity
used for metering & billing electricity customers. The retail price for a kWh varies from approximately 4
cents to 20 cents. At a 100% conversion efficiency, 1 kWh is equivalent to about 4 fluid ounces of
gasoline, 3/16 pound of liquid petroleum, 3 cubic feet of natural gas, or ¼ pound of coal.
Kirchhoff’s Current Law
A basic electrical law that states that the sum of all the currents at any point in an electrical circuit equal
zero amps.
Kirchhoff’s Voltage Law
A basic electrical law that states that the sum of all the voltages around any closed electrical circuit equal
zero volts.
L Sub-Ten (L10)
The bandwidth that ACE is bounded in order to comply with CPS2. An ACE value (±) whose width is
proportional to a BA’s size.
Lagging
Term used when comparing voltage & current waves. The wave that is heading positive & crosses zero
last is the lagging wave. In a lagging load the current wave lags the voltage wave.
Lambda ()
The incremental cost of generation. Lambda is commonly expressed in $/MWh or Mils/kWh. The symbol
for lambda is “” (the Greek letter lambda).
Leading
Term used when comparing voltage & current waves. The wave that is heading positive & crosses zero
first is the leading wave. In a leading load the current wave leads the voltage wave.
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Leap Second
A Leap Second is a second of time added to coordinated universal time (UTC) to make it agree with
astronomical time to within 0.9 seconds. Historically, Leap Seconds are implemented as needed on June
30th or December 31st.
Lightning Arrester
A piece of equipment that is designed to protect the power system from high voltages. Lightning
arresters activate when transient over voltages (TOVs) occur & harmlessly shunt the voltage surge to
ground.
Limiting Element
The element that is: 1.) Either operating at its appropriate rating, or 2.) Would be following the limiting
contingency. Thus, the Limiting Element establishes a system limit. (NERC Glossary of Terms)
Line Trip
Refers to the automatic opening of a transmission line by its CBs. These openings or “trips” are
controlled by protective relays & are designed to protect the power system during faulted conditions.
Load
An end-use device or customer that receives power from the electric system. (NERC Glossary of Terms)
Load Cycle
The normal pattern of demand over a specified time period associated with a device or circuit.
Load Damping
The damping effect of the load to a change in frequency due to the physical aspects of the load such as
the inertia of motors & the physical load to which they are connected.
Load Duration Curve
A non-chronological, graphical summary of demand levels with corresponding time durations using a
curve, which plots demand magnitude (power) on 1 axis & percent of time that the magnitude occurs on
the other axis.
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Load Factor
A measure of the degree of uniformity of demand over a period of time, usually 1 year, equivalent to the
ratio of average demand to peak demand expressed as a percentage. Load factor is calculated by
dividing the total energy provided by a system during the period by the product of the peak demand
during the period & the number of hours in the period.
Load Following
Commitment of energy based resources (generation or energy schedules) to match the forecast load
level for a given period. This is a form of course control for moment-to-moment resource/load matching.
Load Overshoot
A short-term increase in load magnitude due to an increase in the customer’s voltage. Load overshoot
results from downstream tap changers boosting the customer’s voltage prior to the upstream tap changer
responding.
Load Reference Set-Point
In governor control systems this setting determines the position of the controlled valve when the
frequency is at the scheduled value. From a system operations perspective, the load reference set-point
is the MW a generator will produce when the frequency is 60 HZ.
Load Rejection
The rejection of load by a generator. If a generator suddenly loses its transmission path, it has
undergone a load rejection. The generator will speed up until its mechanical power input can be removed
or the unit tripped.
Load/Frequency Relationship
The relationship between frequency deviations & the load magnitude. In general, the load magnitude
varies with the frequency. If the frequency rises the load magnitude rises & vice versa.
Load Serving Entity (LSE)
Secures energy and transmission service (and related Interconnected Operations Services) to serve the
electrical demand and energy requirements of its end-use customers. (NERC Glossary of Terms)
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Load Shedding
The process of deliberately removing (either manually or automatically) pre-selected customer demand
from a power system in response to an abnormal condition, to maintain the integrity of the system &
minimize overall customer outages.
Load Shift Factor (LSF)
A factor to be applied to a load’s expected change in demand to determine the amount of flow
contribution that change in demand will impose on an identified transmission facility or monitored
Flowgate. (NERC Glossary of Terms)
Local Mode
A power oscillation mode (frequency) in which a generator oscillates with respect to the remainder of the
power system. The local mode ranges from 0.8 to 2 HZ.
Lockout
A state of a transmission line following CBs operations where the condition detected by the protective
relaying was not eliminated by temporarily opening & reclosing the line. In this state, the CBs cannot
generally be reclosed without resetting a lockout device.
Logistics
The handling of the details of an operation.
Long-Term Firm Point-To-Point Transmission Service
Firm point-to-point transmission service under Part II of the Tariff with a term of 1 year or more.
Long Term Transmission Planning Horizon
Transmission planning period that covers years six through ten or beyond when required to
accommodate any known longer lead time projects that may take longer than ten years to complete.
(NERC Glossary of Terms)
Loop Flow
See unscheduled power flow.
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Loss of Load Diversity
An increase in the total load that occurs due to a majority of the customer’s load drawing power from the
system at the same time. During normal system operations only a percentage of the customer’ total load
is drawing power at any 1 time. When load diversity is lost a larger percentage of the customer load
draws power simultaneously.
Loss of Load Expectation (LOLE)
The expected number of days in the year when the daily peak demand exceeds the available generating
capacity. The LOLE is obtained by calculating the probability of daily peak demand exceeding the
available capacity for each day & adding these probabilities for all the days in the year. The index is
referred to as Hourly Loss-of-Load-Expectation if hourly demands are used in the calculations instead of
daily peak demands. LOLE also is commonly referred to as Loss-of-load-probability.
Loss of Synchronism
The loss of the magnetic bond between elements of a power system. Loss of synchronism & out-of-step
refer to the same concept.
Losses
The energy losses in the power system. The total system losses consist of the transmission,
transformation, & distribution system losses.
Low Side Winding (LS)
The low voltage winding of the transformer.
Magnetic Field
The invisible lines of force between the north & south poles of a magnet. A magnetic field is created
when current flows through a conductor. Energy is stored in a magnetic field.
Magnetism
A property of matter associated with moving charges. A material may be a permanent magnet or it may
acquire magnetic characteristics due to current flow through the material.
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Magnetosphere
The magnetic cavity surrounding a magnetized body. The magnetosphere is carved out of the passing
solar wind, which impedes the direct entry of solar wind plasma into the cavity. The Earth has a
magnetosphere.
Magnetotail
Extension of the Earth’s magnetosphere in the opposite direction of the Sun as a result of the
magnetosphere’s interaction with the solar wind.
Margin
The difference between net capacity resources & net internal demand. Margin is usually expressed in
MW.
Market Assessment
The evaluation & verification of the commercial details of arranged Interchange (required purchase, sale,
& transmission service arrangements) during initial request for interchange & the evaluation & verification
of the commercial details of a market adjustment to confirmed & implemented Interchange.
Market Clearing Price
The price at a location at which supply equals demand - all demand at or above this price has been
satisfied, & all supply at or below this price has been purchased.
Market Operator
An entity that is responsible for the implementation of an organized market & submits market adjustments
based on market outcomes. A Market Operator must be registered in the Electric Industry Registry
(currently TSIN) in order to submit market adjustments.
Market Participant
An entity participating in the energy marketplace by buying/selling transmission rights, energy, or ancillary
services into, out of, or through an ISO-controlled grid.
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Marketer
An entity that has the authority to take title to electrical power generated by itself or another entity & remarket that power at market-based rates.
Mechanical Energy
Energy of a mechanical nature. For example, a rotating mass possesses mechanical energy.
Mechanical Input Power
Power input of a mechanical nature. For example, a steam or water turbine input mechanical power to
the rotor of a generator.
Mega Watt hour (MWh)
1 million watt hours.
Mercury Arc Valve (MAV)
A high power switch that utilizes older “tube” based technologies. MAVs were commonly used in HVDC
converters & other power converter applications. MAVs have largely been replaced by thyristers.
Metered Value
A measured electrical quantity that may be collected by telemetering, SCADA, or other means.
Metering
A device for measuring a quantity. For example, meters are used to measure power flows, voltages,
current, frequency, etc.
Microprocessor
An arithmetic, logic, & control unit all contained on 1 integrated circuit chip. 1 microprocessor contains
thousands of transistors.
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Mill
A unit of currency equal to 1/10 of a cent.
Minimum Vegetation Clearance Distance (MVCD)
The calculated minimum distance stated in feet (meters) to prevent flash-over between conductors and
vegetation, for various altitudes and operating voltages. (NERC Glossary of Terms)
Misoperation
The failure of a composite protection system to operate as intended for protection purposes. Any of the
following is a misoperation:
1. Failure to trip - during fault - a failure of a composite protection system to operate for a fault
condition for which it is designed. The failure of a protection system component is not a
misoperation as long as the performance of the composite protection system is correct
2. Failure to trip - other than fault - a failure of a composite protection system to operate for a nonfault condition for which it is designed, such as a power swing, undervoltage, overexcitation, or
loss of excitation. The failure of a protection system component is not a misoperation as long as
the performance of the composite protection system is correct.
3. Slow trip - during fault - a composite protection system operation that is slower than required for a
fault condition if the duration of its operating time resulted in the operation of at least 1 other
element’s composite protection system.
4. Slow trip - other than fault - a composite protection system operation that is slower than required
for a non-fault condition, such as a power swing, undervoltage, overexcitation, or loss of
excitation, if the duration of its operating time resulted in the operation of at least 1 other
element’s composite protection system.
5. Unnecessary trip - during fault - an unnecessary composite protection system operation for a fault
condition on another element.
6. Unnecessary trip - other than fault - an unnecessary composite protection system operation for a
non-fault condition. A composite protection system operation that is caused by personnel
during on-site maintenance, testing, inspection, construction, or commissioning activities
is not a misoperation. (NERC Glossary of Terms)
Mode
A specific oscillation frequency. For example, a steam/turbine generator’s shaft has specific frequencies
at which it is susceptible to SSR. These frequencies are called modes.
Monopolar HVDC
An HVDC system that uses 1 conductor energized with either a positive or negative voltage & a current
return path.
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Motor Load
A simplified grouping of all spinning type load. Motor load includes air conditioner compressors, motor
drives, etc. Total load is composed of motor load plus non-motor load.
MRO
Acronym for the Midwest Reliability Organization. MRO is 1 of the eight NERC REs.
Must-Run Generation
Generation designated to operate at a specific level & not available for dispatch.
Native Balancing Authority
A Balancing Authority from which a portion of its physically interconnected generation and/or load is
transferred from its effective control boundaries to the Attaining Balancing Authority through a Dynamic
Transfer. (NERC Glossary of Terms)
Native Load (NL)
The end-use customers that the Load-Serving Entity is obligated to serve. (NERC Glossary of Terms)
Natural Frequency
Every mechanical device has a natural frequency of oscillation. For example, when a force is applied to a
bridge the bridge will oscillate at its natural frequency. Electrical circuits also have a natural frequency of
oscillation. An electrical circuit’s natural frequency is determined by its resistance, inductance, &
capacitance.
Near Term Transmission Planning Horizon
The transmission planning period that covers Year One through five. (NERC Glossary of Terms)
NERC
An acronym for the North American Electric Reliability Corporation. The purpose of NERC is to enhance
the reliability of the interconnected power systems of North America. NERC manages the development of
Reliability Standards that provide mandatory requirements to ensure reliable operation of the power
system.
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Net Actual Interchange (NIA)
The algebraic sum of all metered interchange over all interconnections between two physically Adjacent
Balancing Authority Areas. (NERC Glossary of Terms)
Net Capacity
The maximum capacity (or effective rating), modified for ambient limitations, that a generating unit, power
plant, or electric system can sustain over a specified period, less the capacity used to supply the demand
of station service or needs.
Net Capacity Resource
The total owned capacity, plus capacity available from independent power producers, plus the net of total
capacity purchases & sales, less the sum of inoperable capacity, & less planned outages.
Net Dependable Capacity
The maximum capacity a unit can sustain over a specified period modified for seasonal limitations &
reduced by the capacity required for station service.
Net Energy for Load
Net Balancing Authority Area generation, plus energy received from other Balancing Authority Areas, less
energy delivered to Balancing Authority Areas through interchange. It includes Balancing Authority Area
losses but excludes energy required for storage at energy storage facilities. (NERC Glossary of Terms)
Net Generation
The net power available from a generator to be fed to the power system. Net generation is equal to gross
generation minus the generator’s internal power usage (station service).
Net Interchange Schedule (NIS)
The algebraic sum of all Interchange Schedules with each Adjacent Balancing Authority. (NERC
Glossary of Terms)
Net Internal Demand
Total of all end-use customer demand & electric system losses within specified metered boundaries, less
direct control management & interruptible demand.
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Net Schedule
The algebraic sum of all Interchange Schedules across a given path or between Balancing Authorities for
a given period or instant in time. (NERC Glossary of Terms)
Network Customer
An entity receiving transmission service pursuant to the terms of the TSP’s Network Integration
Transmission service.
Network Integration Transmission Service
Service that allows an electric transmission customer to integrate, plan, economically dispatch and
regulate its network reserves in a manner comparable to that in which the Transmission Owner serves
Native Load customers. (NERC Glossary of Terms)
Network Resource
Any designated generating resource owned, or purchased or leased by a network customer under the
network integration transmission service tariff. Network resources do not include any resource, or any
portion thereof, that is committed for sale to third parties or otherwise cannot be called upon to meet the
network customer’s network load on a non-interruptible basis.
Nodal Market
Prices are assessed at points (i.e., nodes) where electricity enters or leaves the grid. Transmission lines
throughout the grid may be subject to congestion rents, which means generators may receive different
prices based on how they contribute to or relieve congestion on the grid. For example, ERCOT
transitioned from a zonal to a nodal market on December 1, 2010. Their nodal market calculates
transmission costs from the point of generation from roughly 4,000 delivery points. Nodal pricing is
intended to provide a more detailed & accurate picture of transmission & generation than zonal pricing.
ERCOT’s nodal system reduces the time interval for which the market-clearing price is calculated to five
minutes (from fifteen minutes in their former zonal market).
No Load Tap Changer (NLTC)
A tap changer that is designed to change the turns ratio only when the transformer has no current flow
across its windings. The term offload tap changer (OLTC) is also used to refer to this type tap changer.
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NOAA
The acronym for the National Oceanic & Atmospheric Administration of the U.S. Government. NOAA is
the source of the K-Indices for geomagnetic activity.
Nominal
The design or rated value. Not necessarily the value that is intended or that occurs. For example, the
nominal voltage for a piece of equipment would be the design or rated voltage but in operation the
equipment may be operated at a different value of voltage.
Nominal Load
The rated or nameplate load. For example, 100 MW of customer load may be fed from a utility bus. This
load will draw 100 MW if the voltage & frequency at the bus are at nominal values. If voltage or frequency
should vary then the actual load will be different than the nominal load.
Non-Coincident Demand
The sum of 2 or more demands that occur in different demand intervals.
Non-Consequential Load Loss
Non-Interruptible Load loss that does not include: (1) Consequential Load Loss, (2) the response of
voltage sensitive Load, or (3) Load that is disconnected from the System by end-user equipment. (NERC
Glossary of Terms)
Non-Firm Energy
Electrical energy that may be interrupted by either the provider or the receiver of the energy by giving
advance notice to the other party to the transaction. This advance notice period is equal to or greater
than the minimum period agreed to in the contract. Non-firm energy may also be interrupted to maintain
system reliability of third-party TSPs. Non-firm energy must be backed up by reserves.
Non-Firm Point-To-Point Transmission Service
Point-to-point Transmission service under the Tariff that is reserved & scheduled on an as-available basis
& is subject to curtailment or interruption.
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Non-Firm Transmission Service
Transmission service that is reserved on an as-available basis and is subject to curtailment or
interruption. (NERC Glossary of Terms)
Non-Motor Load
A simplified grouping of all non-spinning type load. Non-motor load includes resistive heaters, lighting,
etc. Total load is composed of motor load plus non-motor type load.
Non-Spinning Reserve
1. That generating reserve not connected to the system but capable of serving demand within a specified
time. 2. Interruptible load that can be removed from the system in a specified time. (NERC Glossary of
Terms)
Normal Clearing
A protection system operates as designed and the fault is cleared in the time normally expected with
proper functioning of the installed protection systems. (NERC Glossary of Terms)
Normal Excitation
A condition in which the generator’s excitation system is supplying exactly the excitation current needed
to maintain the magnetic field of the generator. A normally excited generator is neither supplying nor
absorbing reactive power from the system.
Normal (Pre-Contingency) Operating Procedures
Operating procedures that are normally invoked by the system operator to alleviate potential facility
overloads or other potential system problems in anticipation of a contingency.
Normal Rating
The rating as defined by the equipment owner that specifies the level of electrical loading, usually
expressed in megawatts (MW) or other appropriate units that a system, facility, or element can support or
withstand through the daily demand cycles without loss of equipment life. (NERC Glossary of Terms)
Normal Voltage Limits
The operating voltage range on the interconnected systems that is acceptable on a sustained basis.
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North American Energy Standards Board (NAESB)
The acronym for the North American Energy Standards Board. NAESB is responsible for business
practice standards in the wholesale electric energy industry.
Notice of Proposed Rulemaking (NOPR)
In the case of a Federal agency, a written announcement published in the Federal Register which
includes information regarding the time, place, & nature of a public rulemaking proceeding, the legislative
authority under which the agency is proposing the rule, either the terms or substance of the proposed rule
or a description of the subjects & issues involved & an invitation to interested persons to submit
comments to the agency regarding the proposed rule. The publication of a Notice of proposed
rulemaking in the Federal Register is usually the first step in a rulemaking proceeding eventually leading
up to the agency adopting of a Final Rule.
NPCC
Acronym for the Northeast Power Coordinating Council. NPCC is 1 of the eight NERC REs.
Nuclear Plant Interface Requirements (NPIR)
The requirements based on NPLRs and Bulk Electric System requirements that have been mutually
agreed to by the Nuclear Plant Generator Operator and the applicable Transmission Entities. (NERC
Glossary of Terms)
Off-Peak
Those hours or other periods defined by NAESB business practices, contract, agreements, or guides as
periods of lower electrical demand. (NERC Glossary of Terms)
On-Peak
Those hours or other periods defined by NAESB business practices, contract, agreements, or guides as
periods of higher electrical demand. (NERC Glossary of Terms)
Ohm’s Law
A basic electrical law that relates the voltage (V), current (I), & impedance (Z). Ohm’s law is commonly
stated as:
V=IxZ
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Open Access
FERC Order No. 888 requires public utilities to provide non-discriminatory transmission service over their
transmission facilities to third parties to move bulk power from 1 point to another on a nondiscriminatory
basis for a cost-based fee. Order 890 expanded Open Access to cover the methodology for calculating
ATC; improvements that opened a coordinated transmission planning processes; standardization of
energy & generation imbalance charges; & other reforms regarding the designation & un-designation of
transmission network resources.
Open Access Same Time Information Service (OASIS)
An electronic posting system that the Transmission Service Provider maintains for transmission access
data and that allows all transmission customers to view the data simultaneously. (NERC Glossary of
Terms)
Open Access Transmission Tariff (OATT)
Electronic transmission tariff accepted by the U.S. Federal Energy Regulatory Commission requiring the
Transmission Service Provider to furnish to all shippers with nondiscriminating service comparable to that
provided by Transmission Owners to themselves. (NERC Glossary of Terms)
Operating Criteria
The fundamental principles of reliable interconnected systems operation.
Operating Instruction
A command by operating personnel responsible for the Real-time operation of the interconnected Bulk
Electric System to change or preserve the state, status, output, or input of an Element of the Bulk Electric
System or Facility of the Bulk Electric System. (A discussion of general information and of potential
options or alternatives to resolve Bulk Electric System operating concerns is not a command and is not
considered an Operating Instruction.) (NERC Glossary of Terms)
Operating Plan
A document that identifies a group of activities that may be used to achieve some goal. An Operating
Plan may contain Operating Procedures and Operating Processes. A company-specific system
restoration plan that includes an Operating Procedure for black-starting units, Operating Processes for
communicating restoration progress with other entities, etc., is an example of an Operating Plan. (NERC
Glossary of Terms)
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Operating Procedure
A document that identifies specific steps or tasks that should be taken by one or more specific operating
positions to achieve specific operating goal(s). The steps in an Operating Procedure should be followed in
the order in which they are presented, and should be performed by the position(s) identified. A document
that lists the specific steps for a system operator to take in removing a specific transmission line from
service is an example of an Operating Procedure. (NERC Glossary of Terms)
Operating Reserve
That capability above firm system demand required to provide for regulation, load forecasting error,
equipment forced and scheduled outages and local area protection. It consists of spinning and nonspinning reserve. (NERC Glossary of Terms)
Operating Reserve - Spinning
The portion of Operating Reserve consisting of:


Generation synchronized to the system and fully available to serve load within the Disturbance
Recovery Period following the contingency event; or
Load fully removable from the system within the Disturbance Recovery Period following the
contingency event.
Operating Reserve - Supplemental
The portion of Operating Reserve consisting of:


Generation (synchronized or capable of being synchronized to the system) that is fully available
to serve load within the Disturbance Recovery Period following the contingency event; or
Load fully removable from the system within the Disturbance Recovery Period following the
contingency event.
Operating Security
The ability of a power system to withstand or limit the adverse effects of any credible contingency to the
system including overloads beyond emergency ratings, excessive or inadequate voltage, loss of stability
or abnormal frequency deviations.
Operating Security Limit
The value of a system operating parameter (for example, total power transfer across an interface) that
satisfies the most limiting of prescribed pre- & post- contingency operating criteria as determined by
equipment loading capability & acceptable stability & voltage conditions.
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Operating Transmission Limit
The maximum value of the most critical system operating parameters which meets: (a) pre-contingency
criteria as determined by equipment loading capability & acceptable voltage conditions, (b) transient
performance criteria, or, (c) post-contingency loading & voltage criteria.
Operating Voltage
The voltage level by which an electrical system is designated and to which certain operating
characteristics of the system are related; also, the effective (root-meansquare) potential difference
between any two conductors or between a conductor and the ground. The actual voltage of the circuit
may vary somewhat above or below this value. (NERC Glossary of Terms)
Operational Planning Analysis
An analysis of the expected system conditions for the next day’s operation. (That analysis may be
performed either a day ahead or as much as 12 months ahead.) Expected system conditions include
things such as load forecast(s), generation output levels, Interchange, and known system constraints
(transmission facility outages, generator outages, equipment limitations, etc.). (NERC Glossary of Terms)
Operations Support Personnel
Individuals who perform current day or next day outage coordination or assessments, or who determine
SOLs, IROLs, or operating nomograms, in direct support of Real-time operations of the Bulk Electric
System. (NERC Glossary of Terms)
Oscillations
Cyclic variations in voltage, current, frequency, or power flows. The voltage & current of the power
system naturally oscillates at 60 HZ. The term oscillations is typically used to refer to low frequency (a
few HZ) oscillations that may occur.
Oscillatory Stability / Instability
An angle stability classification. The oscillatory stability limit of a power system is the maximum amount
of MW that can be transmitted across the system without excessive power oscillations leading to a loss of
synchronism. No large disturbance need occur. The response of generator control systems (governor &
exciter) are very important to oscillatory stability / instability.
Outage
The period during which a generating unit, transmission line, or other facility is out of service. Outages
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are typically categorized as forced, due to unanticipated problems that render a facility unable to perform
its function &/or pose a risk to personnel or to the system, or scheduled / planned for the sake of
maintenance, repairs, or upgrades.
Out-of-Step (OOS)
To lose synchronism. Out-of-step is best viewed in terms of rotating magnetic fields. When a generator
is out-of-step with the power system it connects to, the generator’s rotating magnetic field is no longer instep or in synchronism with the rotating magnetic field of the system.
Out-of-Step Protective Relay (78)
A protective relay that is designed to detect out-of-step conditions & initiate a user determined response.
Out-of-step protective relays are often based on the same principles as impedance relays. An out-of-step
relay differentiates between an out-of-step condition & a fault condition by measuring the speed at which
the measured impedance changes.
Outage
The period during which a generating unit, transmission line, or other facility is out of service.
Outage Transfer Distribution Factor (OTDF)
In the post-contingency configuration of a system under study, the electric Power Transfer Distribution
Factor (PTDF) with one or more system Facilities removed from service (outaged). (NERC Glossary of
Terms)
Overcurrent Relay
A protective relay that activates in response to a high current magnitude. Overcurrent relays can be
either timed or instantaneous & directional or non-directional.
Overexcite
A generator is overexcited when the applied excitation is greater than that needed to support the
generator’s magnetic field. The excess reactive power produced by the over-excitation condition flows to
the system. An overexcited generator supplies reactive power to the system. The terms “lagging” &
“boosting” are also used to refer to an overexcited generator. A transformer may also overexcite. Overexcitation of a transformer results from either applying to high a voltage magnitude or operating at to low
of a frequency.
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Overlap Angle ()
When commutating between 2 valves in an HVDC converter, a natural overlap period occurs in which
both valves are simultaneously conducting. The length of the overlap period is measured in degrees & is
called the overlap angle. The symbol for the overlap angle is the Greek lower case letter mu ().
Overlap Regulation Service
A method of providing regulation service in which the Balancing Authority providing the regulation service
incorporates another Balancing Authority’s actual interchange, frequency response, and schedules into
providing Balancing Authority’s AGC/ACE equation. (NERC Glossary of Terms)
Parallel Circuit
An electrical circuit in which all the positive terminals are connected to a common point. All the negative
terminals are connected to a second point. The voltage drop is the same across each full element of the
parallel circuit.
Parallel Path Flows
The difference between the scheduled & actual power flow, assuming zero inadvertent interchange, on a
given transmission path.
Parallel Resonance
A resonance condition in a circuit with a parallel combination of inductance & a capacitance. At
resonance a parallel circuit reaches its maximum impedance equal to a multiple of the circuit’s resistance
value.
Parking
Parking is a commercial term that defines energy or transmission that is not complete. The transaction
sink, source, & path are identified when tagged – prior to transaction implementation. The priority of the
transaction is also identified when tagged. The merchant buys a “link” of the final transmission path &
“parks” the generation without identifying source or sink (but must show POR & POD). The merchant can
also buy a “link” of energy & park it. The ATC for a parked transaction should be decremented – there is
an impact on a flowgate, but it is not calculated until the source & sink are known. Parking applies to both
firm & non-firm energy & transmission.
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Participation Factors
A set of dispatch rules such that given a specific amount of load to serve, an approximate generation
dispatch can be determined. To accomplish this, generators are assigned a percentage that they will
contribute to serve load. (NERC Glossary of Terms)
Parties
The TSP & transmission customer receiving service under the Tariff.
Peak Demand
1. The highest hourly integrated Net Energy For Load within a Balancing Authority Area occurring within a
given period (e.g., day, month, season, or year). 2. The highest instantaneous demand within the
Balancing Authority Area. (NERC Glossary of Terms)
Peak Load
As defined by NERC, the highest hourly integrated net energy for load (generation plus imports minus
exports) within a BA area occurring within a given period (e.g., day, month, season, or year), or the
highest instantaneous demand within the BA area.
Peak Period
A period consisting of 2 or more calendar months but less than 7 calendar months, which includes the
period during which the responsible entity’s annual peak demand is expected to occur.
Peaking Capacity
Capacity used to serve peak demand. Peaking generating units operate a limited number of hours per
year, & their capacity factor is normally less than 20%.
Peaking Generating Units
Peaking plants operate primarily during times when load or demand increases rapidly to a maximum level
& remains there for only a short time, e.g., on hot summer afternoons when air conditioning causes
electricity usage to reach its highest level in the daily cycle. Peaking plants are often powered by natural
gas, but they can also be powered by water at hydroelectric dams or by fuel oil. These plants can be
brought online & taken offline quickly, in response to changing demand.
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Penstock
A water pipe or conduit that carries water from the upper water reservoir to the turbine at a hydroelectric
unit.
Performance-Reset Period
The time period that the entity being assessed must operate without any violations to reset the level of
non-compliance to zero. (NERC Glossary of Terms)
Period
The time for 1 complete cycle of a repeating wave. For example, a 60 HZ current steps through 60 cycles
per second. The period of 1 complete cycle is therefore 1/60th of a second.
Permanent Magnet Generator (PMG)
A simple generator that uses a permanent magnet (a material that retains its magnetism) to provide the
required magnetic field. PMGs are often part of governor control systems. The PMG is used to produce
an output voltage whose magnitude is representative of a generator’s shaft speed.
Per-Unit (P.U.)
A system for reporting quantity magnitudes. A per-unit number is stated in terms of a base quantity. For
example if the base voltage is 345 kV then a voltage of 359 kV is 1.04 per-unit (359/345=1.04).
Petersen Coil
A grounding reactor, used in otherwise ungrounded AC transmission systems, designed to limit the
current flowing to ground at the location of a fault almost to zero by setting up a reactive current to ground
that balances the capacitive current to ground flowing from the lines. Also known as an arc-suppression
coil.
Phase
AC power systems use 3 conductors to efficiently generate & transmit large amounts of power. Each of
the conductors is called a phase. The phases are each assigned a letter designation; “A”, “B”, & “C”.
Customer load can be connected as single-phase (1), 2-phase (2), or 3-phase (3).
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Phase Angle
The angle by which 1 waveform leads or lags another waveform. A phase angle can exist between 2
voltages, 2 currents, or between a current & a voltage.
Phase Shifting Transformer (PST)
A transformer designed to shift the phase of the incoming voltages. PSTs use a tap changing winding in
the similar manner to a conventional tap changer. The voltage of a PSTs tap changer is added in
quadrature to the incoming voltage to create a phase shift. PSTs are also referred to as phase angle
regulators or PARs.
Phasors
Similar to a vector but also includes information about the frequency of the quantity. A phasor diagram is
a collection of lines that, like vectors, illustrate a quantities magnitude & direction. However, phasor
diagrams must also specify the frequency of the quantity. AC voltages & currents can be represented by
phasors.
Photosphere
The lowest visible layer of the Sun’s atmosphere. Sunspots are observed in the photosphere.
Physical Access Control Systems (PACS)
Cyber Assets that control, alert, or log access to the Physical Security Perimeter(s), exclusive of locally
mounted hardware or devices at the Physical Security Perimeter such as motion sensors, electronic lock
control mechanisms, and badge readers. (NERC Glossary of Terms)
Physical Security Perimeter
The physical border surrounding locations in which BES Cyber Assets, BES Cyber Systems, or Electronic
Access Control or Monitoring Systems reside, and for which access is controlled. (NERC Glossary of
Terms)
Physically Adjacent Balancing Authorities
2 BAs that are directly interconnected with each other.
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Pick-Up Point
An operating setting for a protective relay that determines at what point the relay will activate.
Pilot Relay
A protective relay system typically used to protect high voltage transmission lines. Pilot relays use
telecommunication systems to communicate between the terminals of the transmission line.
Planned Derating
The removal of a component for repairs that is scheduled well in advance & has a predetermined
duration.
Planned Outage
An outage that is planned well in advance.
Planning
The process by which the performance of the electric system is evaluated & future changes & additions to
the bulk electric systems are determined.
Planning Assessment
Documented evaluation of future Transmission System performance and Corrective Action Plans to
remedy identified deficiencies. (NERC Glossary of Terms)
Planning Authority (PA)
The responsible entity that coordinates and integrates transmission facility and service plans, resource
plans, and protection systems. (NERC Glossary of Terms)
Planning Coordinator (PC)
See Planning Authority. (NERC Glossary of Terms)
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Planning Reserve
The difference between an entity’s expected annual peak capability & its expected annual peak demand
expressed as a percentage of the annual peak demand.
Point of Delivery (POD)
A location that the Transmission Service Provider specifies on its transmission system where an
Interchange Transaction leaves or a Load-Serving Entity receives its energy. (NERC Glossary of Terms)
Point of Receipt (POR)
A location that the Transmission Service Provider specifies on its transmission system where an
Interchange Transaction enters or a Generator delivers its output. (NERC Glossary of Terms)
Point-To-Point (PTP) Transmission Service
The reservation and transmission of capacity and energy on either a firm or non-firm basis from the
Point(s) of Receipt to the Point(s) of Delivery. (NERC Glossary of Terms)
Poles
(1) The electrical circuits formed by the field winding on the rotor of a synchronous generator. When DC
current is passed through the field winding the poles become magnetic north or south poles. (2) The
opposite ends of a magnet where the field is most concentrated, designated as the north & south poles.
In a synchronous generator, the magnetic poles are established by DC current passing through the field
winding on the rotor which is essentially the coil of an electromagnet. (3) In AC electrical equipment,
particularly in switches & circuit breakers, poles refer to the contact assemblies associated with a
particular phase. For example, it is common to refer to pole A, B, or C of a 3 phase disconnect switch.
Potential Difference
A difference in voltage magnitudes between 2 locations. Current can flow along a closed path if a
potential difference exists across the path.
Potential Transformers (PT)
A low power transformer (an instrument transformer) used to reduce normally high power system voltages
to low values ( 110 Volt). A PT has a high number of turns on the primary winding & a low number of
turns on the secondary winding. The low magnitude secondary voltages of a PT are typically input to
meters, relays, etc.
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Power (P)
The rate at which energy is expended to do work. Power is measured in watts, kW, MW, or GW.
Power Angle (δ)
The phase angle between 2 voltage waveforms. A power angle is the same as a voltage phase angle
difference. The power angle is a major factor in determining the amount of MW flow between 2 locations.
The Greek letter delta (δ) is the symbol for power angle.
Power Converter
A mechanical or solid-state device for converting AC power to DC power or vice versa. Modern power
converters are thyrister based devices that are typically strong sources of harmonics.
Power Factor (PF)
The ratio of the MW to MVA. The cosine of the phase angle between a load’s voltage & current is the
power factor of the load. A unity power factor load draws no MVar, just MW.
Power Flow Program
A computerized algorithm that simulates the steady-state behavior of the power system under a given set
of conditions.
Power Pool
2 or more interconnected electric systems planned & operated to supply power for their combined
demand requirements.
Power System
The collective name given to the elements of the electrical system. The power system includes the
generation, transmission, distribution, substations, etc. The term power system may refer to 1 section of
a large interconnected system or to the entire interconnected system.
Power System Stabilizer (PSS)
A feature added to an excitation system that is designed to assist with the damping of low frequency (1
HZ) power system oscillations. A typical PSS provides positive damping to power oscillations by ensuring
that voltage corrections made by the excitation system are in-phase with detected frequency oscillations.
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Power Transfer Distribution Factor (PTDF)
In the pre-contingency configuration of a system under study, a measure of the responsiveness or
change in electrical loadings on transmission system Facilities due to a change in electric power transfer
from one area to another, expressed in percent (up to 100%) of the change in power transfer. (NERC
Glossary of Terms)
Power-Angle Curve
A graphical representation of the MW transfer equation. The power-angle curve is a plot of the active
power transfer as the power angle is varied between 0 & 180. The power-angle curve is a good tool for
analyzing the angle stability of a simple (2 bus) power system.
Power-Circle Diagram
A graphical method of illustrating how MW & MVAr flows change as the power angle changes. Powercircle diagrams are composed of circular characteristics of the power flow out of the sending end & into
the receiving end of a 2 bus system.
Primary Frequency Control
The power delivered to the interconnection in response to a frequency deviation through generator
governor response, load response (typically from motors), demand response (designed to arrest
frequency excursions), & other devices that provide an immediate response to frequency based on local
(device‐ level) control systems, without human or remote intervention.
Primary Inadvertent Interchange
The component of area (n) inadvertent interchange caused by the regulating deficiencies of the area (n).
Primary Winding
The winding of a transformer that is connected to the power input or source end of the transformer.
Prime Mover
A mechanism that converts thermal or hydraulic energy into mechanical power. For example, a coal fired
boiler with a steam turbine is a prime mover as it converts the thermal energy of coal into the mechanical
power to turn the turbine.
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Pro Forma Tariff
Usually refers to the standard OATT and/or associated transmission rights mandated by the U.S. Federal
Energy Regulatory Commission Order No. 888. (NERC Glossary of Terms)
Production Cost
The costs associated with starting, operating, & stopping generating units.
Protected Cyber Assets (PCA)
One or more Cyber Assets connected using a routable protocol within or on an Electronic Security
Perimeter that is not part of the highest impact BES Cyber System within the same Electronic Security
Perimeter. The impact rating of Protected Cyber Assets is equal to the highest rated BES Cyber System
in the same ESP. A Cyber Asset is not a Protected Cyber Asset if, for 30 consecutive calendar days or
less, it is connected either to a Cyber Asset within the ESP or to the network within the ESP, and it is
used for data transfer, vulnerability assessment, maintenance, or troubleshooting purposes. (NERC
Glossary of Terms)
Protection System





Protective relays which respond to electrical quantities,
Communications systems necessary for correct operation of protective functions
Voltage and current sensing devices providing inputs to protective relays,
Station dc supply associated with protective functions (including batteries, battery chargers, and
non-battery based dc supply), and
Control circuitry associated with protective functions through the trip coil(s) of the circuit breakers
or other interrupting devices.
(NERC Glossary of Terms)
Protection System Maintenance Program (PSMP)
An ongoing program by which Protection System and automatic reclosing components are kept in
working order and proper operation of malfunctioning components is restored. A maintenance program
for a specific component includes one or more of the following activities:





Verify — Determine that the component is functioning correctly.
Monitor — Observe the routine in-service operation of the component.
Test — Apply signals to a component to observe functional performance or output behavior, or to
diagnose problems.
Inspect — Examine for signs of component failure, reduced performance or degradation.
Calibrate — Adjust the operating threshold or measurement accuracy of a measuring element to
meet the intended performance requirement.
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(NERC Glossary of Terms)
Protective Relay
A mechanical or electronic device used to sense abnormal power system conditions & respond to isolate
the abnormal equipment typically by opening CBs.
Provider of Last Resort
A legal obligation (traditionally given to utilities) to provide service to a customer where competitors have
decided they do not want that customer’s business.
Pseudo-Tie
A time-varying energy transfer that is updated in Real-time and included in the Actual Net Interchange
term (NIA) in the same manner as a Tie Line in the affected Balancing Authorities’ control ACE equations
(or alternate control processes).
Pumped Storage Hydro Electric
A plant that usually generates electric energy during peak load periods by using water previously pumped
into an elevated storage reservoir during off-peak periods when excess generating capacity is available to
do so. When additional generating capacity is needed, the water can be released from the reservoir
through a conduit to turbine generators located in a power plant at a lower level.
Public Utilities Regulatory Policies Act (PURPA)
The Public Utility Regulatory Policies Act of 1978, passed by the U.S. Congress. This statute requires
States to implement utility conservation programs & create special markets for co-generators & small
producers who meet certain standards, including the requirement that States set the prices & quantities of
power the utilities must buy from such facilities.
Purchasing-Selling Entity (PSE)
The entity that purchases or sells, and takes title to, energy, capacity, and Interconnected Operations
Services. Purchasing-Selling Entities may be affiliated or unaffiliated merchants and may or may not own
generating facilities. (NERC Glossary of Terms)
P-V Curve
A power versus voltage curve. A plot of the power transferred to a bus versus the voltage at that bus. PV curves are a graphical tool used to analyze a power system’s voltage stability.
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Pythagorean Theorem
A mathematical relationship which states that in a right triangle the square of the hypotenuse length is
equal to the sum of the squares of the lengths of the remaining 2 sides.
Quadrature (Q)
At an angle of 90. When 2 vectors are in quadrature they are perpendicular to 1 another. The symbol
“Q” for reactive power is derived from the word Quadrature.
Qualified Controllable Device (WECC)
A controllable device installed in the Interconnection for controlling energy flow; the WECC Operating
Committee has approved using the device for controlling the USF on the qualified transfer paths.
Qualifying Facility (QF)
A cogeneration or small power production facility that meets certain ownership, operating, & efficiency
criteria established by the FERC pursuant to the PURPA.
Quality (Q)
A factor for measuring the frequency response of an electrical circuit. A circuit’s quality is dependent
upon the relative magnitudes of its reactive & resistive elements.
Radiation Storm
Space radiation storm happens when an explosion on the Sun accelerates solar protons toward Earth.
These protons stream past our planet where they are (mostly) deflected by Earth's magnetic field.
NOAA’s SWPC has defined 5 types of radiation storms (S-1 to S-5), ranging from mild (S-1) to extreme
(S-5). Radiation storms have minor impacts on power systems but can impact telecommunication
systems.
Ramp Period
The time between generation ramp start & end times usually expressed in minutes.
Ramp Rate or Ramp
(Schedule) The rate, expressed in megawatts per minute, at which the interchange schedule is attained
during the ramp period.
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(Generator) The rate, expressed in megawatts per minute, that a generator changes its output.
Ramp Window
The time period that occurs each hour for adjusting a BA’s generation. A typical ramp window is from 10
minutes to the hour till 10 minutes after the hour. If all BAs use the same ramp windows, frequency
deviations will be reduced.
Rate Base
The value of property upon which a utility is permitted to earn a specified rate of return as established by
a regulatory authority. The rate base generally represents the value of property used by the utility in
providing service & may be calculated by any 1 or a combination of the following accounting methods:
fair value, prudent investment, reproduction cost, or original cost. Depending on which method is used,
the rate base includes cash, working capital, materials & supplies, & deductions for accumulated
provisions for depreciation, contributions in aid of construction, customer advances for construction,
accumulated deferred income taxes, & accumulated deferred investment tax credits.
Rate-of-Change (Protective Relay)
A type of protective relay that monitors the rate at which a quantity changes. For example, a rate-ofchange relay may monitor the rate at which the MW flow along a transmission line varies. The relay could
be set to trip the line if the rate of MW flow change exceeds a specified value.
Rated System Path Methodology
The Rated System Path Methodology is characterized by an initial Total Transfer Capability (TTC),
determined via simulation. Capacity Benefit Margin, Transmission Reliability Margin, and Existing
Transmission Commitments are subtracted from TTC, and Postbacks and counterflows are added as
applicable, to derive Available Transfer Capability. Under the Rated System Path Methodology, TTC
results are generally reported as specific transmission path capabilities. (NERC Glossary of Terms)
Rating
The operational limits of a transmission system element under a set of specified conditions. (NERC
Glossary of Terms)
Reach
The extent of protection that an impedance relay provides to a transmission line. The reach is typically
defined in terms of the impedance of the line. For example, a zone #1 impedance relay may reach 85%
into the protected line.
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Reaction Turbine
A water turbine in which the pressure difference across the turbine blades causes the blades to turn. A
Francis Turbine is an example of a reaction turbine.
Reactive Capability Curve
A graphical method of illustrating the complex power output limits of a synchronous generators. The
reactive capability curve is sometimes called a “D-Curve” as it is typically shaped like the letter “D”.
Reactive power (Q)
The portion of electricity that establishes and sustains the electric and magnetic fields of alternatingcurrent equipment. Reactive power must be supplied to most types of magnetic equipment, such as
motors and transformers. It also must supply the reactive losses on transmission facilities. Reactive
power is provided by generators, synchronous condensers, or electrostatic equipment such as capacitors
and directly influences electric system voltage. It is usually expressed in kilovars (kvar) or megavars
(Mvar). (NERC Glossary of Terms)
Reactive Supply & Voltage Control from Generating Sources Services
An ancillary service or an IOS. Provides reactive supply through changes to generator reactive output to
maintain transmission line voltage & facilitate electricity transfers.
Reactor
A device intentionally designed to act as an inductor & store energy in its magnetic field. A shunt reactor
acts as a sink (absorber) of reactive power from the system. Series reactors are devices that are used to
increase the inductive reactance (XL) of a transmission path.
Real Power (P)
The portion of electricity that supplies energy to the load. (NERC Glossary of Terms)
Reallocation
The total or partial curtailment of Transactions during TLR Level 3a or 5a to allow Transactions using
higher priority to be implemented. (NERC Glossary of Terms)
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Recallable Available Transmission Capability (RATC)
TTC less the TRM, less recallable transmission service, less non-recallable transmission service
(including the CBM). RATC must be considered differently in the planning & operating horizons. In the
planning horizon, the only data available are recallable & non-recallable transmission service
reservations, whereas in the operating horizon transmission schedules are known.
Recallability
The right of a TSP to interrupt all or part of a transmission service for any reason, including economic,
that is consistent with FERC policy & the TSP’s transmission service tariffs or contract provisions.
Receiving Balancing Authority
The Balancing Authority importing the Interchange. (NERC Glossary of Terms)
Reclosing Relay
A relay that automatically (after a few cycles or a few seconds) recloses a transmission line following a
fault.
Rectifier
An HVDC converter operated to convert AC power to DC power.
Region
1 of the 8 NERC REs.
Regional Entity (RE)
1. An entity that ensures that a defined area of the BES is reliable, adequate & secure. 2. A member of
the North American Electric Reliability Council. The RE organization can serve as the compliance
monitor
Regional Transmission Group (RTG)
Voluntary organization of transmission owners, transmission users, & other entities interested in
coordinating transmission planning & expansion & use on a regional & interregional basis.
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Regional Transmission Organization (RTO)
An organization that is independent from all generation & power marketing interests & has exclusive
responsibility for electric transmission grid operations, short-term electric reliability, & transmission
services within a multi-State region. To achieve those objectives, the RTO manages transmission
facilities owned by different companies & encompassing 1, large, contiguous geographic area.
Regulating Reserve
An amount of reserve responsive to Automatic Generation Control, which is sufficient to provide normal
regulating margin. (NERC Glossary of Terms)
Regulating Transformer
A transformer used to regulate voltage or phase angle. Conventional tap changing (ULTC) transformers
& phase shifting transformers (PSTs) are regulating transformers.
Regulating Unit
A generator used for the regulation of system frequency. To serve as a regulating unit the generator
must have available spinning reserve.
Regulation
The ability to maintain a quantity within acceptable limits. For example, frequency regulation is the
control or regulation of the system frequency to within a tight bandwidth of 60 HZ. Voltage regulation is
the control of a voltage level within a set bandwidth.
Regulation & Frequency Response Service
Provides for following the moment-to-moment variations in the demand or supply in a BA & maintaining
scheduled Interconnection frequency.
Regulation Reserve Sharing Group (RRSG)
A group whose members consist of two or more Balancing Authorities that collectively maintain, allocate,
and supply the Regulating Reserve required for all member Balancing Authorities to use in meeting
applicable regulating standards. (NERC Glossary of Terms)
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Regulation Service
The process whereby one Balancing Authority contracts to provide corrective response to all or a portion
of the ACE of another Balancing Authority. The Balancing Authority providing the response assumes the
obligation of meeting all applicable control criteria as specified by NERC for itself and the Balancing
Authority for which it is providing the Regulation Service. (NERC Glossary of Terms)
Relative Acceleration
For torque & power angles to change, a relative acceleration must exist for a period of time. 1 part of the
system must accelerate with respect to another part. Once relative acceleration occurs, any speed
difference that has developed will continue the increase or decrease in torque or power angles. Torque &
power angles will not stop changing until all sections of the system are running at the same frequency.
Relay
An electrical or mechanical device that responds to a measured input with a user determined output.
Types of relays include auxiliary relays, monitoring relays, regulating relays, & protective relays.
Reliability
The degree of performance of the elements of the BES that results in electricity being delivered to
customers within accepted standards & in the amount desired. Reliability may be measured by the
frequency, duration, & magnitude of adverse effects on the electric supply.
Reliability Adjustment Arranged Interchange
The process whereby one Balancing Authority contracts to provide corrective response to all or a portion
of the ACE of another Balancing Authority. The Balancing Authority providing the response assumes the
obligation of meeting all applicable control criteria as specified by NERC for itself and the Balancing
Authority for which it is providing the Regulation Service. (NERC Glossary of Terms)
Reliability Assessment
The evaluation & verification of the reliability details of arranged interchange (path contiguity, ramping
ability, & transmission system availability) during initial request for interchange & the evaluation &
verification of the reliability details of a reliability adjustment arranged Interchange to confirmed &
implemented interchange.
Reliability Coordinator (RC)
The entity that is the highest level of authority who is responsible for the reliable operation of the Bulk
Electric System, has the Wide Area view of the Bulk Electric System, and has the operating tools,
processes and procedures, including the authority to prevent or mitigate emergency operating situations
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in both next-day analysis and real-time operations. The Reliability Coordinator has the purview that is
broad enough to enable the calculation of Interconnection Reliability Operating Limits, which may be
based on the operating parameters of transmission systems beyond any Transmission Operator’s vision.
(NERC Glossary of Terms)
Reliability Coordinator Area
The collection of generation, transmission, and loads within the boundaries of the Reliability Coordinator.
Its boundary coincides with one or more Balancing Authority Areas. (NERC Glossary of Terms)
Reliability Coordinator Information System (RCIS)
The system that Reliability Coordinators use to post messages and share operating information in real
time. (NERC Glossary of Terms)
Reliability Criteria
Principles used to design, plan, operate, & assess the actual or projected reliability of an electric system.
Reliability Directive
A communication initiated by a Reliability Coordinator, Transmission Operator, or Balancing Authority
where action by the recipient is necessary to address an Emergency or Adverse Reliability Impact.
(NERC Glossary of Terms)
Reliability Must Run (RMR) Unit
A unit that must run for operational or reliability reasons, regardless of economic considerations. An RMR
unit would not otherwise be operated unless it is necessary to provide voltage support, stability or
management of localized transmission constraints.
Reliability Standard
A requirement, approved by the United States Federal Energy Regulatory Commission under this Section
215 of the Federal Power Act, or approved or recognized by an applicable governmental authority in other
jurisdictions, to provide for reliable operation [Reliable Operation] of the bulk-power system [Bulk-Power
System]. The term includes requirements for the operation of existing bulk power system [Bulk-Power
System] facilities, including cyber security protection, and the design of planned additions or modifications
to such facilities to the extent necessary to provide for reliable operation [Reliable Operation] of the bulkpower system [Bulk-Power System], but the term does not include any requirement to enlarge such
facilities or to construct new transmission capacity or generation capacity. (NERC Glossary of Terms)
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Reliable Operation
Operating the elements of the bulk-power system [Bulk-Power System] within equipment and electric
system thermal, voltage, and stability limits so that instability, uncontrolled separation, or cascading
failures of such system will not occur as a result of a sudden disturbance, including a cyber security
incident, or unanticipated failure of system elements. (NERC Glossary of Terms)
Remedial Action Scheme (RAS)
Scheme designed to detect predetermined system conditions & automatically take corrective
actions that may include, but are not limited to, adjusting or tripping generation (MW & MVar),
tripping load, or reconfiguring a system. RAS accomplish objectives such as:





Meet requirements identified in the NERC Reliability Standards
Maintain BES stability
Maintain acceptable BES voltages
Maintain acceptable BES power flows
Limit the impact of cascading or extreme vents
The following do not individually constitute a RAS:
a) Protection systems installed for purpose of detecting faults on BES Elements & isolating the
faulted elements
b) Schemes for automatic UFLS & UVLS comprised of only distributed relays
c) OOS tripping & power swing blocking
d) Automatic reclosing schemes
e) Schemes applied on an element for non-fault conditions, such as, but not limited to, generator
loss-of-field, transformer top-oil temperature, overvoltage, or overload to protect element
against damage by removing it from service
f) Controllers that switch or regulate 1 or more of the following: series or shunt reactive
devices, FACTS devices, PSTs, VFTs, or tap-changing transformers; &, that are located at
& monitor quantities solely at the same station as the element being switched or regulated
g) FACTS controllers that remotely switch static shunt reactive devices located at other
stations to regulate the output of a single FACTS device
h) Schemes or controllers that remotely switch shunt reactors & shunt capacitors for
voltage regulation that would otherwise be manually switched
i) Schemes that automatically deenergize a line for a non-fault operation when 1 end of the line
is open
j) Schemes that provide anti-islanding protection (e.g., protect load from effects of being
isolated with generation that may not be capable of maintaining acceptable frequency and
voltage)
k) Automatic sequences that proceed when manually initiated solely by a System Operator
l) Modulation of HVDC or FACTS via supplementary controls, such as angle damping or
frequency damping applied to damp local or inter-area oscillations
m) SSR protection schemes that directly detect subsynchronous quantities (e.g., currents or
torsional oscillations)
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n) Generator controls such as, but not limited to, AGC, generation excitation [e.g. AVR &
PSS], fast valving, & speed governing
Remote Terminal Unit (RTU)
An electronic device, installed in a substation or generator, which acts as an interface between a
company’s EMS system & the generator or substation.
Reportable Cyber Security Incident
A Cyber Security Incident that has compromised or disrupted one or more reliability tasks of a functional
entity. (NERC Glossary of Terms)
Reportable Disturbance
Any event that causes an ACE change greater than or equal to 80% of a Balancing Authority’s or reserve
sharing group’s most severe contingency. The definition of a reportable disturbance is specified by each
Regional Reliability Organization. This definition may not be retroactively adjusted in response to
observed performance. (NERC Glossary of Terms)
Request for Interchange
A collection of data as defined in the NAESB Business Practice Standards submitted for the purpose of
implementing bilateral Interchange between Balancing Authorities or an energy transfer within a single
Balancing Authority. (NERC Glossary of Terms)
Request Status
The overall status of a transaction/tag, generated by the Tag Authority Service. (For example; pending,
implemented, or dead.)
Requesting PSE
The PSE submitting the interchange transaction tag.
Rerating
A change in the capability of a generator due to a change in conditions such as age, upgrades, auxiliary
equipment, cooling, etc.
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Reserve Sharing Group (RSG)
A group whose members consist of two or more Balancing Authorities that collectively maintain, allocate,
and supply operating reserves required for each Balancing Authority’s use in recovering from
contingencies within the group. Scheduling energy from an Adjacent Balancing Authority to aid recovery
need not constitute reserve sharing provided the transaction is ramped in over a period the supplying
party could reasonably be expected to load generation in (e.g., ten minutes). If the transaction is ramped
in quicker (e.g., between zero and ten minutes) then, for the purposes of Disturbance Control
Performance, the Areas become a Reserve Sharing Group. (NERC Glossary of Terms)
Resistance (R)
The property of a material that opposes or resists current flow by converting electric energy to heat. The
symbol for resistance is the letter “R”.
Resistivity
A measure of the ability of a material to conduct electricity. The greater the resistivity, the greater the
opposition to current flow. An insulator has a high resistivity.
Resonance
In an electrical circuit, resonance is achieved when the magnitudes of the inductive & capacitive elements
match. Resonance may be achieved by inputting energy at the circuit’s resonant frequency or by varying
the size of the inductive or capacitive elements. A radio receiver is tuned to resonance at the channel the
operator wants to receive.
Resonant
To achieve resonance.
Resonant Frequency (FR)
The frequency at which resonance is achieved in an electrical circuit. The formula for the resonant
frequency is:
FR 
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Resource Planner
The entity that develops a long-term (generally one year and beyond) plan for the resource adequacy of
specific loads (customer demand and energy requirements) within a Planning Authority Area. (NERC
Glossary of Terms)
Resources
Resources is typically used to refer to available generation. For example, load must match resources to
maintain frequency.
Response Rate
The Ramp Rate that a generating unit can achieve under normal operating conditions expressed in
megawatts per minute (MW/Min). (NERC Glossary of Terms)
Responsive Reserves
Reserve capacity that is available to respond to system frequency disturbances.
Responsive Spinning Reserve
That portion of spinning reserve available to respond to frequency disturbances as a result of the
generator’s governor response.
Restoration
The process of returning generators & transmission system elements & restoring load following an outage
on the electric system.
Restrike
The re-ignition of an electric arc.
RFC
Acronym for the Reliability First Corporation. RFC is 1 of the 8 NERC REs.
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Right-of-Way (ROW)
The corridor of land under a transmission line(s) needed to operate the line(s). The width of the corridor is
established by engineering or construction standards as documented in either construction documents,
pre-2007 vegetation maintenance records, or by the blowout standard in effect when the line was built.
The ROW width in no case exceeds the applicable Transmission Owner’s or applicable Generator
Owner’s legal rights but may be less based on the aforementioned criteria. (NERC Glossary of Terms)
Right Triangle
A triangle in which 1 of the 3 internal angles is equal to 90.
Rolling Blackout
Also known as rotating outages, these are controlled, temporary interruptions of service to customers,
most commonly initiated by switching off selected distribution circuits intended to reduce load during
times of capacity shortfalls due to significant forced outages of generation &/or transmission facilities.
The service interruptions are transferred from 1 group (or block) of customers to another over time so that
no 1 group bears the entire burden of the necessary reduction in load.
Root Mean Square (RMS)
The effective value of an AC voltage or current. The RMS value of an AC quantity would produce the
same amount of heat in a DC resistive circuit. For example if an AC circuit has an RMS current of 10
amps, then 10 amps of DC current would have the same heating effect. Most AC meters read RMS
values.
Rotational Energy
Energy stored within a rotating mass. For example, a spinning generator contains rotational energy. The
terms inertial energy, stored energy, & rotational energy are often used interchangeably to refer to the
energy stored in the rotating elements (motors & generators) of the power system.
Rotor
The rotating component of a motor or generator.
Runner
The rotating element of a hydro turbine.
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Saturation
Condition reached in a transformer where the magnetic flux has reached its maximum value.
Transformers can saturate from exposure to excessive voltage, excessive current, or exposure to DC
currents. Damaging effects of saturation include a failure to follow the turns ratio, excessive heat
production & harmonic creation.
Scenario
Possible event. (NERC Glossary of Terms)
Schedule (Noun)
An interchange schedule. (NERC Glossary of Terms)
Schedule (Verb)
To set up a plan or arrangement for an interchange transaction. (NERC Glossary of Terms)
Schedule Confirmation
The process of verifying the accuracy of an interchange schedules between all the entities to the
transaction.
Schedule Implementation
The process of entering the details of a negotiated schedule into the control systems of BAs involved in a
transaction of power & energy.
Schedule Period
The length of time between the nominal starting & ending time of each schedule.
Scheduled
The desired or target value. For example, for a 345 kV bus, the scheduled value of voltage may be 358
kV. System operators would try to hold this bus voltage at 358 kV.
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Scheduled Frequency
60.0 Hertz, except during a time correction.
Scheduled Interchange
Electric power scheduled to flow between entities, usually the net of all sales, purchases, & wheeling
transactions between those areas at a given time.
Scheduled Losses
The scheduled power transfer to a TSP for compensation of losses incurred on that TSP’s transmission
system due to a transfer of power between PSEs.
Scheduled Net Interchange (NIS)
Algebraic sum of all scheduled MW transfers, including dynamic schedules, with adjacent BAs, & taking
into account the effects of schedule ramps. BAs directly connected via asynchronous ties to another
Interconnection may include or exclude MW transfers on those tie lines in their scheduled interchange,
provided they are implemented in the same manner for actual net interchange.
Scheduled Total Interchange
The net of all interchange schedules with all Adjacent BAs. It is, in essence, the scheduled interchange
with the Interconnection.
Scheduling Path
The Transmission Service arrangements reserved by the Purchasing-Selling Entity for a Transaction.
(NERC Glossary of Terms)
Scroll Case
A spiral-shaped steel intake guiding the flow of water into the wicket gates located just prior to the turbine
of a hydro-electric facility.
Scrubber
A pollution control system used in fossil fuel units for removing sulfur from the exhaust gases.
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Seams
The boundaries between adjacent electricity-related organizations. Differences in regulatory
requirements or operating practices may create “seams problems”.
Secondary Frequency Control
Actions provided by an individual BA or its RSG intended to restore primary control response & restore
frequency from the arrested frequency back to scheduled frequency, or to maintain scheduled frequency
deployed in the minutes time frame. Secondary control comes from either manual or automated dispatch
from a centralized control system. Secondary control also includes initial reserve deployment for
disturbances & maintains the minute‐ to‐ minute balance throughout the day & is used to restore
frequency to normal following a disturbance & is provided by both spinning & non‐ spinning reserves.
Secondary Inadvertent Interchange
The component of Balancing Area “X” inadvertent interchange caused by the regulating deficiencies of
the Balancing Area “X”.
Secondary Winding
The winding of a transformer that is connected to the power output or load end of the transformer.
Sectionalizing
The process of reducing the load on a distribution feeder using switching actions.
Self-Excitation
A possible operating condition for a generator in which the charging current from a high voltage
transmission system takes over as the excitation current source of the generator. Self-excitation is a
dangerous condition as high voltages can develop in the generator & in the generator auxiliaries.
Self-Provision
An alternate means (other than purchase from the TSP) by which a transmission customer may fulfill its
IOS obligations. Alternatives for Self-Provision may include: 1) use of the transmission customer’s own
IOS Resource, or 2) purchase of an IOS Resource from a third party IOS supplier.
Sending Balancing Authority
The Balancing Authority exporting the Interchange. (NERC Glossary of Terms)
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SERC
Acronym for the Southeastern Electric Regional Reliability Council. SERC is 1 of the eight NERC REs.
Series Circuit
Electrical circuit in which elements are connected end to end. The same current flows through each
element of a series circuit.
Series Resonance
A resonance condition in a circuit with a series combination of inductance & capacitance. At resonance a
series circuit reaches its minimum impedance equal to the circuit’s resistance value.
Servomotor
A device used to create a force based on a control signal input. For example, in a governor control
system a control signal is first developed using a control valve. The control signal is input to a
servomotor. The servomotor then drives steam valves, wicket gates, etc. The servomotor may be an
electric motor, an hydraulic piston, or any other means of developing a force.
Settling Frequency
Refers to the point when the frequency stabilizes following a generation trip. Point B represents the
interconnected system frequency at the point immediately after the frequency stabilizes due to governor
action but before the contingent BA takes corrective AGC action.
Shield Wires
Conductors strung across the top of transmission lines that are designed to protect the transmission line
from lightning strikes.
Short Circuit
The introduction of a low impedance path between conductors energized at different voltages. A short
circuit is the same as a fault.
Short Circuit Ratio (SCR)
The ratio between the short circuit MVA of the local system & the MVA rating of a piece of equipment.
For example, the SCR of a generator is equal to the MVA that would flow to a 3 fault applied at the
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generator’s high side bus divided by the MVA rating of the generator. SCRs can also be calculated for
HVDC systems & other electrical devices.
Shunt Capacitor Bank
Shunt capacitors are capacitors connected from the power system to an electrical ground. They are used
to supply KVAr to the system at the point where they are connected. A shunt capacitor bank is a group of
shunt capacitors.
Sine
The sine of either of the unknown angles of a right triangle is the ratio of the side directly opposite the
unknown angle to the hypotenuse.
Sine Wave
The graphical representation of a mathematical function that describes the smooth, symmetrical, &
periodic variation of a quantity that oscillates in magnitude or amplitude. In AC electric power systems,
the voltage & current are characterized by sine waves having a frequency of 60 Hz. These waveforms,
starting from a zero baseline, traverse a path that increases to a crest (positive maximum), then falls back
to zero, continues downward to a trough (equal but opposite to the crest, i.e., in the negative direction), &
back to zero in 1/60th of a second.
Single Contingency
The sudden, unexpected failure or outage of system facilities or elements (generating unit, transmission
line, transformer, etc.). Elements removed from service as part of the operation of a RAS are considered
part of a single contingency.
Sink
Final POD for the transaction: the actual load.
Sink Balancing Authority
The Balancing Authority in which the load (sink) is located for an Interchange Transaction and any
resulting Interchange Schedule. (NERC Glossary of Terms)
Six-Pulse Converter
An HVDC converter that has six pulses to the DC output voltage for each cycle of the AC supply voltage.
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Sliding Pressure
A method of operation available in some thermal units. Normally a unit’s steam valves are adjusted to
control the amount of steam sent to the turbine stages. In a sliding pressure mode of operation, the
steam valves are operated wide open to minimize valve throttling losses. The boiler pressure is then
varied to adjust the MW output of the unit.
Slip
The difference between the synchronous speed of an induction motor & the speed at which its rotor
actually rotates.
Slip-Rings
Metal rings attached to the shaft of a synchronous machine. The rotor’s field winding terminates on the
inner surface of the slip-rings while the brushes ride on the smooth outer surface of the slip-rings.
Small Power Producer
Under the PURPA, a small power production facility (or small power producer) generates electricity using
waste, renewable (water, wind, & solar), or geothermal energy as a primary energy source. Fossil fuels
can be used, but renewable resource must provide at least 75 percent of the total energy input.
Smoothing Reactor
A series reactor attached to an HVDC transmission line that smoothes the ripple of an HVDC converter’s
DC output voltage & assists with the power conversion process.
Software
A series of instructions written to enable computer hardware to perform useful tasks.
Solar Flare
Intense, short-lived releases of energy from the surface of the Sun. Solar flares are seen as bright areas
on the Sun in optical wavelengths & as bursts of noise in radio wavelengths. Solar flares can last from
minutes to hours. Flares are our solar system’s largest explosive events. The primary energy source for
flares appears to be the tearing & reconnection of strong magnetic fields. CMEs are often associated
with large flares.
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Solar Magnetic Disturbance (SMD)
See Geomagnetic Disturbance (GMD)
Solar Maximum
Months during a sunspot cycle when the sunspot number reaches a maximum.
Solar Minimum
Months during a sunspot cycle when the sunspot number reaches a minimum.
Solar Storm
Energy disturbance on the surface of the Sun.
Solar Wind
Outward flow of solar particles & magnetic fields from the Sun. Solar wind speed varies between 300 &
1,000 kM/sec.
Solenoid
An electro-magnet that, when energized, is used to perform a mechanical function such as opening a
switch.
Solid State Relay
A relay that uses semiconductor components.
Source
The initial point of receipt for the transaction: the actual generation facility.
Source Balancing Authority
The Balancing Authority in which the generation (source) is located for an Interchange Transaction and
for any resulting Interchange Schedule. (NERC Glossary of Terms)
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Special Protection System (SPS)
An automatic protection system designed to detect abnormal or predetermined system conditions, and
take corrective actions other than and/or in addition to the isolation of faulted components to maintain
system reliability. Such action may include changes in demand, generation (MW and Mvar), or system
configuration to maintain system stability, acceptable voltage, or power flows. An SPS does not include
(a) underfrequency or undervoltage load shedding or (b) fault conditions that must be isolated or (c) outof-step relaying (not designed as an integral part of an SPS). Also called Remedial Action Scheme.
(NERC Glossary of Terms)
Spinning Reserve
Unloaded generation that is synchronized and ready to serve additional demand. (NERC Glossary of
Terms)
SPP
Acronym for the Southwest Power Pool. SPP is 1 of the 8 NERC REs.
Stability
The ability of an electric system to maintain a state of equilibrium during normal and abnormal conditions
or disturbances. (NERC Glossary of Terms)
Stability Limit
The maximum power flow possible through some particular point in the system while maintaining stability
in the entire system or the part of the system to which the stability limit refers. (NERC Glossary of Terms)
State Estimator (SE)
Computer software that takes measurements of quantities related to a system state as input & provides
an estimate of the complete system state (as its output). The SE is used to confirm that the monitored
power system is operating in a secure state by simulating the system both at the present time & 1 step
ahead, for a particular network topology & loading condition. With the use of an SE & its associated
contingency analysis software, system operators can review each critical contingency to determine
whether each possible future state is within reliability limits.
Static Var Compensator (SVC)
A combination of shunt reactors & shunt capacitors that use thyrister based switches & thyrister control to
regulate the var output.
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Static Var System (SVS)
A combination of an SVC & other reactive power equipment. A common control system controls both the
SVC & the other reactive equipment.
Station
A location in an electrical network where 1 or more elements are connected. Examples include
generating stations & substations.
Station Service
The electric supply for the ancillary equipment used to operate a generating station or substation.
Station Service Generator
A generator (usually found in hydro plants) used to normally supply electric energy for station service
equipment.
Stator
The stationary component of a motor or generator.
Steady State Time Period
Time period before a disturbance occurs. It is a stable pre-event condition for the existing system
configuration, which includes all existing BES elements, including elements on maintenance, planned or
unplanned outage.
Steady State Stability / Instability
An angle stability classification. The steady state stability limit of a power system is the maximum amount
of active power that can be transmitted across the system without a loss of synchronism occurring. No
large disturbance need occur.
Storage
Energy transferred from 1 entity to another entity that has the ability to conserve the energy (that is,
stored as water in a reservoir, coal in a pile, etc.) with the intent that the energy will be returned at a time
when such energy is more useable to the original supplying entity.
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Stored Energy
Energy stored within a rotating mass. For example, a spinning generator contains stored energy. The
terms inertial energy, stored energy, & rotational energy are often used interchangeably to refer to the
energy stored in the rotating elements (motors & generators) of the power system.
Subregion
A subdivision of a RE.
Substation
An element of the power system that contains CBs, disconnect switches, transformers, reactors,
capacitors, & other equipment. A central control house is often provided to house control & protective
equipment.
Subsynchronous
A frequency below synchronous speed.
Subsynchronous Resonance (SSR)
An electric power system condition where the electric network exchanges energy with a turbine/generator
at 1 or more of the natural frequencies of the combined system. The frequency of the energy exchange is
below the synchronous frequency (subsynchronous) of the system.
Subtransmission
A functional or voltage classification relating to lines at voltage levels between the distribution &
transmission levels.
Sunspot
Dark areas on the Sun’s surface. Sunspots contain strong magnetic fields that are constantly shifting. A
moderate sized sunspot is about as large as the Earth. Sunspots form & dissipate over periods of days or
weeks. Sunspots occur when strong magnetic fields emerge through the Sun’s surface & allow the area
to cool slightly, from a background value of 6000 ° C down to about 4200 ° C. This area appears as a
dark spot in contrast with the Sun.
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Sunspot Cycle
Over the last 300 years, the average number of sunspots has regularly waxed & waned in an 11-year
sunspot cycle. During the Sun’s 11 year cycle the polarities of its north & south poles reverse. The
sunspot cycle is a useful way to mark changes in the Sun. Solar Minimum refers to the several Earth
years when the number of sunspots is lowest. Solar Maximum occurs in the years when sunspots are
most numerous. During Solar Maximum, activity on the Sun & its effects on our terrestrial environment
are high. Sunspot cycle is also called solar cycle.
Sunspot Number
A daily index of sunspot activity.
Supercritical Boiler
A boiler system that is operated at a much higher temperature & pressure than a conventional drum type
boiler. Supercritical boilers are also called “once through” type boilers. A supercritical boiler does not
have any significant steam storage, which affects its response to governor commands.
Supersynchronous
A frequency above synchronous speed.
Supervisory Control
A form of remote control comprising an arrangement for the selective control of remotely located facilities
by an electrical means over 1 or more communication media.
Supervisory Control & Data Acquisition (SCADA)
A system of remote control and telemetry used to monitor and control the transmission system. (NERC
Glossary of Terms)
Supplemental Regulation Service
A method of providing regulation service in which the Balancing Authority providing the regulation service
receives a signal representing all or a portion of the other Balancing Authority’s ACE. (NERC Glossary of
Terms)
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Surge
A transient variation of current, voltage, or power flow in an electric circuit or across an electric system.
(NERC Glossary of Terms)
Surge Impedance Loading (SIL)
The MW loading on a transmission line at which the line’s natural reactive power production equals its
reactive power usage.
Sustained Outage
The deenergized condition of a transmission line resulting from a fault or disturbance following an
unsuccessful automatic reclosing sequence and/or unsuccessful manual reclosing procedure. (NERC
Glossary of Terms)
Switching Order
A sequence of steps to accomplish a desired switching action.
Switching Process
The process by which the status (open, closed, etc.) of power system equipment (switches, CBs, etc.) is
adjusted to perform maintenance or enhance operations.
Switching Station
Power system equipment used to tie together 2 or more electric circuits through switches. The switches
are selectively arranged to permit a circuit to be disconnected, or to change the electric connection
between the circuits.
Switching Surge
The sudden changes to voltage & current waveforms that accompany transmission system switching
events.
Synch-Check Relay (25)
A protective relay that will not allow a CB to be closed unless the frequency difference, voltage magnitude
difference, & voltage angle across the open CB are within acceptable limits.
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Synchronize
The process of bringing 2 electrical systems together by closing a circuit breaker at an interface point
when the voltages & frequencies are properly aligned. Also, when generators are brought on-line, they
are said to be synchronized to the system.
Synchronous
To be in-step with a reference. A synchronous generator rotates in synchronism with the power system.
Synchronous Condenser
A synchronous machine that operates as a synchronous motor. The MW to turn the machine’s shaft is
drawn from the power system. The full capabilities of the machines excitation system (to absorb & supply
MVAr) are then available for voltage control purposes. Hydroelectric generators can often be operated in
synchronous condenser mode. The unit’s water turbine is typically de-watered & the unit’s rotor turned as
if it were a motor.
Synchronous Machine
An AC machine whose rotor rotates in synchronism with the power system to which it is attached.
Synchronous machines can be either generators or motors. A synchronous machine also includes a
source of DC excitation current (the excitation system).
Synchronous Speed
The speed at which the rotor of a synchronous generator must rotate in order to stay in synchronism with
the rotating magnetic field of the system. The synchronous speed is determined by the frequency of the
power system & the number of magnetic poles in the rotor. For example, the synchronous speed of a 2
pole steam-turbine generator in a 60 Hz system is 3600 RPM, while the synchronous speed of a 24 pole
hydro generator is 1/12th of that, or 300 RPM.
Synchroscope
A device for comparing the frequency difference & voltage angle across an open CB.
System
A combination of generation, transmission, and distribution components. (NERC Glossary of Terms)
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System Operating Limit (SOL)
The value (such as MW, MVar, Amperes, Frequency or Volts) that satisfies the most limiting of the
prescribed operating criteria for a specified system configuration to ensure operation within acceptable
reliability criteria. System Operating Limits are based upon certain operating criteria. These include, but
are not limited to:
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Facility Ratings (Applicable pre- and post-Contingency equipment or facility ratings)
Transient Stability Ratings (Applicable pre- and post-Contingency Stability Limits)
Voltage Stability Ratings (Applicable pre- and post-Contingency Voltage Stability)
System Voltage Limits (Applicable pre- and post-Contingency Voltage Limits)
(NERC Glossary of Terms)
System Operator
An individual at a Control Center of a Balancing Authority, Transmission Operator, or Reliability
Coordinator, who operates or directs the operation of the Bulk Electric System (BES) in Real-time.
(NERC Glossary of Terms)
System Personnel
Those people who have the capability to affect system operations & who must abide by the authority
vested in the system operator. May include power plant operators, system maintenance personnel,
power schedulers, power marketers, etc.
Tag
Refers to the collection of information in the electronic request for an energy schedule & subsequent
responses utilized in the electronic Transaction Information System (TIS) implemented by NERC.
Tag ID
A unique identifying number assigned to each tag by the Tag Agent, currently represented by combining
the codes for the Source BA, PSE, & Sink BA together with a unique number.
Taps
Fixed electrical contacts at different positions on a transformer’s winding. Taps are adjusted to change
the voltage ratio of a transformer.
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Target
An indicator on a relay that is displayed when the relay operates. The term “flag” is often used to refer to
a target.
Task
1 of the elements that make up a function in the Functional Model.
Telemetering
The process by which measurable electrical quantities from substations and generating stations are
instantaneously transmitted to the control center, and by which operating commands from the control
center are transmitted to the substations and generating stations. (NERC Glossary of Terms)
Telemetry
Equipment for measuring a quantity (amps, volts, MW, etc.) & transmitting the result via a
telecommunication system (radio, microwave, etc.) to a remote location for indication or recording.
Tertiary Frequency Control
Encompasses actions taken to get resources in place to handle current & future changes in load or
contingencies. Reserve deployment & reserve restoration following a disturbance is a common type of
tertiary frequency control.
Tertiary Winding
An additional, often third, winding added to a power transformer. The tertiary winding may be used to
connect a reactor, capacitor, or to provide station service.
Thermal Limit
A power flow limit based on the possibility of damage by heat. Heating is caused by the electrical losses
that are equal to I2R.
Thermal Rating
The maximum amount of electrical current that a transmission line or electrical facility can conduct over a
specified time period before it sustains permanent damage by overheating or before it sags to the point
that it violates public safety requirements. (NERC Glossary of Terms)
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Theta ()
A Greek symbol used to indicate an angular measurement.
3-Part Communication
A communications protocol where information is verbally stated by a party initiating a communication, the
information is repeated back correctly to the party that initiated the communication by the second party
that received the communication, & the same information is verbally confirmed to be correct by the party
who initiated the communication.
Thrust Bearing
The bearing that opposes axial forces in the generator shaft.
Thyrister
A solid-state electronic component whose ability to conduct current is controlled via its voltage polarity or
a gate signal. Thyristers are also called silicon controlled rectifiers (SCR). Thyristers are combined in
series/parallel arrangements to perform rapid switching actions. Thyristers are used in modern static var
compensators (SVC), adjustable speed drives (ASD), & HVDC converters.
Tie Facility
The transmission facility interconnecting BA Areas.
Tie-Line
A circuit connecting two Balancing Authority Areas. (NERC Glossary of Terms)
Tie-Line Bias (TLB)
A mode of Automatic Generation Control that allows the Balancing Authority to 1.) maintain its
Interchange Schedule and 2.) respond to Interconnection frequency error. (NERC Glossary of Terms)
Tie-Line Telemetry
Telemetry equipment used to measure power flow data on a tie-line connecting BAs. The power flow
data is then transmitted to both BAs.
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Time Error
The difference between the Interconnection time measured at the Balancing Authority(ies) and the time
specified by the National Institute of Standards and Technology. Time error is caused by the
accumulation of Frequency Error over a given period. (NERC Glossary of Terms)
Time Error Correction (TEC)
An offset to the Interconnection’s scheduled frequency to return the Interconne ction’s Time Error to a
predetermined value. (NERC Glossary of Terms)
Title Transfer
An exchange of energy ownership; may or may not be associated with a physical delivery of energy.
TLR Log
Report required to be filed after every TLR Level 2 or higher in a specified format. The NERC IDC
prepares the report for review by the issuing Reliability Coordinator. After approval by the issuing
Reliability Coordinator, the report is electronically filed in a public area of the NERC Web site. (NERC
Glossary of Terms)
Torque
The torque is a force that produces a rotating or twisting action.
Torque Angle ()
The angle by which the rotating magnetic field of synchronous machine leads or lags the rotating
magnetic field of the system to which it connects. A generator has a positive torque angle while a motor
has a negative torque angle. The symbol for the torque angle is the letter “” (Greek lower case letter
delta).
Torque-Speed Curves
A graphical means of illustrating the relationship between the torque developed by a motor & the speed of
rotation of the motor shaft.
Torsional
A twisting force.
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Total Actual Interchange
The algebraic sum of all interchange metered with all Physically Adjacent BAs. It is, in essence, the
actual interchange with the Interconnection.
Total Flowgate Capability (TFC)
The maximum flow capability on a Flowgate, is not to exceed its thermal rating, or in the case of a
flowgate used to represent a specific operating constraint (such as a voltage or stability limit), is not to
exceed the associated System Operating Limit. (NERC Glossary of Terms)
Total Harmonic Distortion (THD)
A computed value used to quantify the harmonic content of a waveform. The THD is a measure of the
percent of the harmonic components content as compared to the magnitude of the fundamental
component.
Total Transfer Capability (TTC)
The amount of electric power that can be moved or transferred reliably from one area to another area of
the interconnected transmission systems by way of all transmission lines (or paths) between those areas
under specified system conditions. (NERC Glossary of Terms)
Transfer Capability
The measure of the ability of interconnected electric systems to move or transfer power in a reliable
manner from 1 area to another over all transmission lines (or paths) between those areas under specified
system conditions. The units of transfer capability are in terms of electric power, generally expressed in
MW. The transfer capability from “Area A” to “Area B” is not generally equal to the transfer capability from
“Area B” to “Area A”
Transfer Capability
The measure of the ability of interconnected electric systems to move or transfer power in a reliable
manner from one area to another over all transmission lines (or paths) between those areas under
specified system conditions. The units of transfer capability are in terms of electric power, generally
expressed in megawatts (MW). The transfer capability from “Area A” to “Area B” is not generally equal to
the transfer capability from “Area B” to “Area A.” (NERC Glossary of Terms)
Transfer Distribution Factor (TDF)
See Distribution Factor. (NERC Glossary of Terms)
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Transfer Limit
The maximum amount of power that can be transferred in a reliable manner from 1 area to another over
all transmission lines (or paths) between those areas under specified system conditions.
Transformer
A device that operates on magnetic principles to increase (step up) or decrease (step down) voltage.
Transient Time Period
Transitional time period beginning after a disturbance in which high-speed automatic actions occur in
response to the disturbance. This time period starts at the time of the disturbance & can continue for
seconds or until a new steady state is achieved.
Transient Stability
The ability of an electric system to maintain synchronism between its parts when subjected to a
disturbance & to regain a state of equilibrium following that disturbance.
Transmission
An interconnected group of lines and associated equipment for the movement or transfer of electric
energy between points of supply and points at which it is transformed for delivery to customers or is
delivered to other electric systems. (NERC Glossary of Terms)
Transmission Constraint
A limitation on one or more transmission elements that may be reached during normal or contingency
system operations. (NERC Glossary of Terms)
Transmission Customer
1. Any eligible customer (or its designated agent) that can or does execute a transmission service
agreement or can or does receive transmission service. 2. Any of the following responsible entities:
Generator Owner, Load-Serving Entity, or Purchasing-Selling Entity. (NERC Glossary of Terms)
Transmission Line
A system of structures, wires, insulators and associated hardware that carry electric energy from one
point to another in an electric power system. Lines are operated at relatively high voltages varying from
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69 kV up to 765 kV, and are capable of transmitting large quantities of electricity over long distances.
(NERC Glossary of Terms)
Transmission Loading Relief (TLR)
A procedure used to manage congestion on the electric transmission system.
Transmission Operator (TOP)
The entity responsible for the reliability of its “local” transmission system, and that operates or directs the
operations of the transmission facilities. (NERC Glossary of Terms)
Transmission Owner (TO)
The entity that owns and maintains transmission facilities. (NERC Glossary of Terms)
Transmission Overload
A state where a transmission line has exceeded either a normal or emergency rating of the electric
conductor.
Transmission Planner (TP)
The entity that develops a long-term (generally one year and beyond) plan for the reliability (adequacy) of
the interconnected bulk electric transmission systems within its portion of the Planning Authority Area.
(NERC Glossary of Terms)
Transmission Reliability Margin (TRM)
The amount of transmission transfer capability necessary to provide reasonable assurance that the
interconnected transmission network will be secure. TRM accounts for the inherent uncertainty in system
conditions and the need for operating flexibility to ensure reliable system operation as system conditions
change. (NERC Glossary of Terms)
Transmission Service
Services provided to the Transmission Customer by the Transmission Service Provider to move energy
from a Point of Receipt to a Point of Delivery. (NERC Glossary of Terms)
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Transmission Service Provider (TSP)
The entity that administers the transmission tariff and provides Transmission Service to Transmission
Customers under applicable transmission service agreements. (NERC Glossary of Terms)
Traveling Wave
Energy can be viewed as a wave traveling through space. When a disturbance occurs in the power
system the disturbance can be viewed as a wavefront of current & voltage propagating out from the
disturbance point. A frequency disturbance can also be viewed in terms of a frequency deviation
propagating out from the disturbance point in the shape of a wave.
TRE
Acronym for the Texas Reliability Entity. TRE is 1 of the 8 REs.
Trip
The opening of CBs on an electric system, normally to electrically isolate a particular element of the
system to prevent it from being damaged by fault current or other potentially damaging conditions.
Trigonometry
A branch of mathematics that deals with the relationships between the lengths of sides & the angles
between the sides of triangles.
Triplen Harmonics
The odd numbered harmonic orders that are evenly divisible by the number 3. For example, the 3rd, 9th,
15th, etc. are Triplen Harmonics.
Turbine
A rotating mechanical device. A turbine is rotated by the force of a working fluid. The working fluid is
typically steam, water, or combustion gas.
Turbine Follow
A mode of operation of a fossil unit’s turbine/boiler control system in which the unit’s response to a load
(MW) change request is delayed if the boiler’s temperature &/or pressure moves outside set boundaries.
The MW response of the unit’s turbine “follows” the response of the boiler.
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Turning Gear
A method of rotating the shaft of a horizontally mounted turbine/generator. The turning gear may rotate
the shaft at 2 RPM to prevent shaft warp.
Turns Ratio
The ratio of the number of turns in the primary winding of a transformer to the number of turns in the
secondary winding.
Twelve Pulse Converter
An HVDC converter that has 12 pulses to the DC output voltage for each cycle of the AC supply voltage.
Unbundling
Separating electric utility service into its basic components & offering each component separately for sale
with separate rates for each component.
Under Load Tap Changer (ULTC)
A tap changer that is designed to change the turns ratio when the transformer has current flow across its
windings. The term load tap changer (LTC) is also used to refer to this type tap changer.
Underexcite
A generator is underexcited when the applied excitation is less than that needed to support the
generator’s magnetic field. The deficiency in the reactive power needs of the generator is supplied by the
system. An underexcited generator absorbs reactive power from the system. The terms “leading” &
“bucking” are also used to refer to an underexcited generator.
Underfrequency Load Shedding (UFLS)
The tripping of customer load based on magnitudes of system frequency. For example, a utility may
dump 5% of their connected load if frequency falls below 59.3 HZ, dump an additional 10% if frequency
falls below 58.9 HZ, & dump a final 10% if frequency falls below 58.5 HZ. These 3 steps of load shedding
would form this utilities UFLS plan. The purpose of UFLS is a final effort to arrest a frequency decline.
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Undervoltage Load Shedding (UVLS)
The tripping of customer load based on magnitudes of system voltage. For example, a utility may dump
5% of their connected load if voltage falls below 92% of nominal & an additional 10% of their load if
voltage falls below 90% of nominal. These 2 steps of load shedding would form this utilities UVLS plan.
The purpose of UVLS is typically to avoid a voltage collapse.
Unilateral Payback
Inadvertent interchange accumulations paid back unilaterally controlling to a target of non-zero ACE.
Controlling to a non-zero ACE ensures that the unilateral payback is accounted for in the CPS
calculations. The unilateral payback control offset is limited to BA’s L10 limit & shall not burden the
Interconnection.
Unit Commitment
The process of selecting which generating units will be placed on line to serve the load & reserve
requirements.
Universal Coordinated Time (UTC)
Time standard used by the Electronic Tagging System for communication purposes; also referred to as
Greenwich Mean Time (GMT).
Unscheduled Power Flow
Power does not flow where it is scheduled but rather according to the relative impedance of the available
paths. When power is scheduled to flow from system “A” to system “B” some of the power may flow
through an adjoining system “C”. The power that flows through system “C” is called unscheduled power
flow. Also called parallel flow or loop flow.
Valve
General name given to a mercury-arc or thyrister based device that is used to conduct current when a
control signal is applied. A valve is turned on by a gate or grid pulse. When turned on a valve conducts
current. When turned off a valve blocks the flow of current. A valve is turned off by removing the forward
biased voltage & stopping the current flow. Power converters are composed of several valves connected
in different arrangements depending on the converter design.
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Vectors
Line segments that are used to represent the magnitude & direction of physical quantities. DC voltages &
currents can be represented by vectors.
Vegetation Inspection
The systematic examination of vegetation conditions on a Right-of-Way and those vegetation conditions
under the applicable Transmission Owner’s or applicable Generator Owner’s control that are likely to
pose a hazard to the line(s) prior to the next planned maintenance or inspection. This may be combined
with a general line inspection. (NERC Glossary of Terms)
Viscosity
A property of a fluid that tends to prevent the fluid from flowing when subjected to an applied force. Highviscosity fluids resist flow; low-viscosity fluids flow easily.
Voltage (V)
The electrical force (a separation of charge) that causes current to flow. Symbol is “V” & units are volts.
Voltage Phase Angle
The phase angle between 2 voltages. The voltage phase angle is the same as the power angle. (In a
generator the voltage phase angle between the internal generator voltage & the stator terminal voltage is
the equivalent of the torque angle.)
Voltage Collapse
A process in which a voltage unstable system experiences an uncontrollable reduction in system voltage.
Voltage Control
The control of transmission voltage through adjustments in generator reactive output & transformer taps,
& by switching capacitors & inductors on the transmission & distribution systems.
Voltage Limits
A hard limit above or below which is an undesirable operating condition. Normal limits are between 95 &
105 percent of the nominal voltage at the bus under discussion.
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Voltage Ratio
The ratio between the primary & secondary voltages of a transformer. There is a strong (but not identical)
relationship between a transformer’s voltage ratio & its turns ratio.
Voltage Reduction
A procedure designed to deliberately lower the voltage at a bus. It is often used as a means to reduce
demand by lowering the customer’s voltage.
Voltage Regulator
A component of an excitation system. The voltage regulator monitors the generator’s output voltage &
causes an adjustment in excitation current when required. Voltage regulators can be operated in either a
manual or automatic mode.
Voltage Relay (27 & 59)
A protective relay that activates in response to either a high (59) or low (27) voltage.
Voltage Source
A device capable of producing a voltage. Generators & batteries are voltage sources.
Voltage Stability
The ability of a power system to maintain voltage so that when the system nominal load is increased the
actual power transferred to that load will increase. In a voltage stable power system the power transfer &
the system voltages are controllable by the system operators.
V-Q Curve
A voltage versus reactive power curve. A plot of the voltage at a bus versus the reactive power injected
into that bus. V-Q curves are a graphical tool used to analyze a power system’s voltage stability.
Watthour (Wh)
A unit of measure of electrical energy equal to 1 watt of power supplied to, or taken from, an electric
circuit steadily for 1 hour.
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WebSAS
Acronym for a web based security analysis system. This software tool is used in the WECC to assist with
congestion management. WebSAS is the WECC’s wide area reliability tool or WART.
WECC
Acronym for the Western Electricity Coordinating Council. WECC is 1 of the 8 NERC REs.
Wheeling
The movement of electricity from 1 system to another over transmission facilities of interconnecting
systems. Wheeling service contracts can be established between 2 or more systems.
Wheeling Charges
Payments required for providing wheeling services.
Wicket Gate
Adjustable elements that control the flow of water to the turbine of a hydro-electric facility.
Wide Area
The entire Reliability Coordinator Area as well as the critical flow and status information from adjacent
Reliability Coordinator Areas as determined by detailed system studies to allow the calculation of
Interconnected Reliability Operating Limits. (NERC Glossary of Terms)
Wide Area Reliability Tool (WART)
This generic term is intended to reflect in a tool neutral manner those wide-area reliability assessment
tools (such as the IDC in the Eastern Interconnection or WebSAS in the Western Interconnection)
acknowledged by NERC as a decision making tool among various reliability entities.
Wind Generating Station
A collection of wind turbines electrically connected together & injecting energy into the grid at 1 point,
sometimes known as a wind farm.
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Wye Connection
A 3 electrical connection in which 1 end of each of the 3 phases is connected to a common point. The
common point is often grounded. Transformer windings are often connected in a Wye configuration.
Year 1
The first twelve month period that a Planning Coordinator or a Transmission Planner is responsible for
assessing. For an assessment started in a given calendar year, Year One includes the forecasted peak
Load period for one of the following two calendar years. For example, if a Planning Assessment was
started in 2011, then Year One includes the forecasted peak Load period for either 2012 or 2013. (NERC
Glossary of Terms)
Zonal Market
A market for electric energy divided into regional pricing zones. Generators within a zone receive the
same price for the power they provide, & transmission lines crossing zonal boundaries are assessed
additional costs due to market congestion when the power flowing through them reaches operational
constraints.
Zones of Protection
The zone or area within which a protective relay can sense abnormal conditions.
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