ECE 422/522
Power System Operations & Planning/
Power Systems Analysis II
1 – General Background
Spring 2014
Instructor: Kai Sun
1
Outline
• Structure of a power system
• US Electric Industry (utilities, deregulation, energy
resources)
• Overview of power system reliability and NERC guidelines
• Introduction of power system stability (basic concepts,
definitions and examples)
• Materials
– Part I (Chapters 1&2) of Kundur’s book
– Glossary of Terms Used in NERC Reliability Standards, Dec 21, 2012
– IEEE/CIGRE Joint Task Force on Stability Terms and Definitions, “Definition and
Classification of Power System Stability,” IEEE Transactions on Power Systems,
Vol. 19, No. 2., pp. 1387 – 1401, May 2004
2
1st 100 Years of Electric Industry
• 1882: Pearl Street Station, the 1st DC system by Edison, operated in NYC
• 1886: Commercially practical transformer and AC distribution system
developed by Stanley
• 1888: Development of poly-phase AC by Tesla started AC vs. DC battle
• 1889: 1st AC transmission line in the US (1-phase, 21km at 4kV in
Oregon)
• 1893: 1st 3-phase line (2.3kV, 12 km by SCE) in North America; AC vs.
DC battle ended when AC was chosen at Niagara Falls.
• 1912-1923: 1st 110kV and 220kV HVAC overhead lines
• 1950s: 345kV-400kV EHV AC lines by USA, Germany and Sweden
• 1954: 1st modern commercial HVDC transmission (96km submarine
cable) in Sweden.
• 1960s: 735-765kV EHV AC in Russia, USA and Canada
• 1972: 1st thyristor based HVDC Back-To-Back system between Quebec
and New Brunswick in Canada
3
Structure of an AC Power System
• Generation
– Low voltages <25kV due to
insulation requirements
• Transmission system
– Backbone system
interconnecting major power
plants (11~35kV) and load
center areas
– 161kV, 230kV, 345kV, 500kV,
765kV, etc.
• Sub-transmission system
– Transmitting power to
distribution systems
– Typically, 35/69kV-138kV
• Distribution system
– Typically, 4kV-34.5kV
4
Bulk Power System (Bulk Electric System)
• NERC definition
– The bulk electric system is a term commonly applied to the portion
of an electric utility system that integrates “the electrical generation
resources, transmission lines, interconnections with neighboring
systems, and associated equipment, generally operated at
voltages of 100 kV or higher.”
– Radial transmission facilities serving only load with one
transmission source are generally not included in this definition
• For short, a bulk electric system is the part of the transmission/subtransmission system connecting
– power plants,
– major substations, and
– HV transmission lines
• Most of power system reliability concerns are about bulk electric
systems
5
US Bulk Power Systems
6
Energy Resources for US Electricity Generation
From “Electricity sector of the United States” at Wikipedia.org
7
US Electric Industry Structure
• 3,195 utilities in the US in 1996.
Categories
Examples
Investor-owned utilities
240+, 66.1% of electricity
AEP, American Transmission Co., ConEd, Dominion
Power, Duke Energy, Entergy, Exelon, First Energy,
HECO, MidAmerican, National Grid, Northeast Utilities,
Oklahoma Gas & Electric, Oncor, Pacific Gas & Electric,
SCE, Tampa Electric Co., We Energies, Xcel,
Publicly owned utilities
2000+, 10.7%
Nonprofit state and local government agencies, including
Municipals, Public Power Districts, and Irrigation Districts,
e.g. NYPA, LIPA,
Federally owned utilities
~10, 8.2%
Tennessee Valley Authority (TVA), Bonneville Power
Administration (BPA), Western Area Power Administration
(WAPA), etc.
Cooperatively owned utilities
~1000, 3.1%
Owned by rural farmers and communities
Non-utilities, 11.9%
Generating power for own use and/or for sale in wholesale power markets, e.g. Independent Power Providers
(IPPs)
•
Fewer than 1000 are engaged in power generation
8
Deregulation: Competitive US Power Market
Structure
• The government sets down rules
and laws for market participants
to comply with
• On electricity prices
– Typically, determined by bidbased, security-constrained,
economic dispatch
– In a day-ahead market, the price
is determined by matching offers
from generators to bids from
consumers at each node to
develop a supply-demand
equilibrium price, usually on an
hourly interval.
– The price is calculated separately
for sub-regions in which the
system operator's power-flow
model indicates that constraints
will bind transmission imports.
Generation
Owner
Generation
Owner
…
Generation
Owner
Transmission
TransmissionOwner
Owner
Transmission Owner
Distribution
Owner
Distribution
Owner
Service
Provider
Service
Provider
9
…
Distribution
Owner
Service
Provider
• Compared to traditional economic dispatch, where the
actual fuel cost function Ci is known by the dispatcher.
dC3
=
5.8 + 0.018 P3 =
λ
dP3
dC2
=
5.5 + 0.012 P2 =
λ
dP2
dC1
=
5.3 + 0.008 P1 =
λ
dP1
Equal incremental
cost
10
California was the 1st
state to implement full
deregulation
11
Reliability Concerns with Deregulation
“California Electricity Crisis”
• Before passage of the deregulation law,
there was only one Stage-3 rolling
blackout (intentional load shedding by
utilities) declared.
• After passage, California had 38 Stage-3
rolling blackouts, mainly as a result of a
poorly designed market system that was
manipulated by traders and marketers.
• In order to sell electricity at a higher price,
some trader intentionally encouraged
suppliers to shut down plants (removing
power from the market) for unnecessary
maintenance.
Supply
Demand
(Source: Wikipedia.org and paper “Gaming and Price Spikes in Electrical Power Market,” by X. Guan,
et al, on IEEE Trans. Power Systems, Vol. 16, No. 3, Aug 2001.)
12
Power Blackouts in North America
Date
Area
Impacts
Nov 9, 1965
North America (NE)
20,000+MW, 30M people
13 hrs
Jul 13, 1977
North America (NY)
6,000MW,
26 hrs
Dec 22, 1982
North America (W)
12, 350 MW, 5M people
Jul 2-3, 1996
North America (W)
11,850 MW, 2M people
13 hrs
Aug 10, 1996
North America (W)
28,000+MW, 7.5M people
9 hrs
Jun 25, 1998
North America (N-C)
950 MW,
0.15MK people
19 hrs
Aug 14, 2003
North America (N-E)
61,800MW,
50M people
Sep 8, 2011
US & Mexico (S-W)
4,300MW,
5M people
9M people
13
Duration
2+ days
12hrs
NERC (North American Electric Reliability
Corporation)
• As a non-government organization, formed by the electric utility industry
in 1968 to promote the reliability of bulk power systems in North
America.
• Initially membership was voluntary and member systems followed the
reliability criteria for planning and operating bulk power systems to
prevent major system disturbances following severe contingencies
• As of June 2007, FERC (U.S. Federal Energy Regulatory Commission)
granted NERC the legal authority to enforce reliability criteria with all
users, owners, and operators of the bulk power systems in the U.S.
• NERC Membership is now mandatory. Member systems comply with
NERC’s Reliability Standards (approved by FERC) to promote reliable
operations and to avoid costly monetary penalties if caught noncompliant. Every system operator should read, understand and follow
NERC’s Reliability Standards. (Visit http://www.nerc.com for more
information on NERC.)
14
NERC Functional Model Diagram
15
Interconnections in North America
• Eight Regional Reliability Entities
(RREs) assisting NERC
– FRCC (Florida Reliability
Coordinating Council)
– MRO (Midwest Reliability
Organization)
– NPCC (Northeast Power
Coordinating Council)
– RFC (Reliability First Corporation)
– SERC (Southeastern Electric
Reliability Council)
– SPP (Southwest Power Pool)
– WECC (Western Electricity
Coordinating Council)
– TRE (Texas Reliability Entity)
35GW
180GW
650GW
70GW
From EPRI tutorial (Peak loads are based
on data in 2009)
16
NERC Reliability Coordinators
Code
Name
ERCOT
ERCOT ISO
FRCC
Florida Power & Light
TE
Hydro Quebec, TransEnergie
ISNE
ISO New England Inc.
MISO
Midwest ISO
NBSO
New Brunswick System Operator
NYIS
New York Independent System Operator
ONT
Ontario - Independent Electricity System Operator
PJM
PJM Interconnection
SPC
SaskPower
SOCO
Southern Company Services, Inc.
SPP
Southwest Power Pool
TVA
Tennessee Valley Authority
VACS
VACAR-South
WECC
WECC Reliability Coordinator
17
18
NERC Balancing Authority
• A Balancing Authority (BA) is a part of an interconnected power system
that is responsible for meeting its own load.
• Each BA operates an Automatic Generation Control (AGC) system to
balance its generation resources to load requirements.
– Generation resources: internal or purchased from other BAs and
transferred over tie-lines between BAs.
– Load requirements: internal customer load, losses, or scheduled
sales to other BAs.
19
NERC Balancing Authorities
• EI has about 90
BAs, which range in
load size up to
130GW peaks
• WI (WECC) has
about 30 BAs.
• ERCOT and Hydro
Quebec are each
operated as single
BAs.
20
System Control Centers
Duke Energy Control Center
(source: Patrick Schneider Photo.Com)
TVA Control Center
(Source: bayjournal.com)
(source: TVA.com
21
Reliable Electric Power Supply
• Requirements under both normal and
emergency conditions
– Voltage and frequency around normal values within
close tolerances
– Generators running synchronously with adequate
capacity to meet the load demand
– The “integrity” of the bulk power network
22
Reliability of Bulk Power Systems
• From both Planning and Operations perspectives:
– Power systems should be built and operated to achieve a reliable
electric power supply at the most economical cost
• Reliability is defined using two terms:
– Adequacy (planning): The ability of the electric systems to
supply the aggregate electrical demand and energy requirements of
their customers at all times, taking into account scheduled and
reasonably expected unscheduled outages of system elements.
– Security (operation): The ability of the electric systems to
withstand sudden disturbances, i.e. contingencies, such as electric
short circuits or unanticipated loss of system elements
23
Example of NERC’s Reliability Standards:
Performance under Normal and Emergency Conditions
24
Summary of NERC Contingencies
Category Description
Stability
Loss of load
A
No contingencies
Yes
No
B
N-1 (loss of 1 element)
Yes
No
C
Loss of ≥2 elements (local
events)
Yes
Planned or
controlled
D
Extreme events (loss of a
transmission path,
substation, power plant or
major load, cascading
outages, etc.)
25
Selecting contingencies for
evaluation
Contingencies to be studied
• Normal Design Contingencies (Categories A, B and C)
– Have a significant probability of occurrence
– Following any of these contingencies, the system is secure
(stability is maintained, and voltages and line and equipment
loadings are within applicable limits. )
• All facilities are in service, or
• A critical generator, transmission circuit, or transformer is out of
service, assuming that the area generation and power flows are
adjusted between outages by use of a reserve.
• Extreme Contingencies (Category D)
– After the analysis and assessment of selected extreme
contingencies, measures are developed to reduce the frequency of
occurrence of such contingencies or to mitigate the consequences
that are indicated by the simulations of such contingencies
26
NECR Contingencies
Unlikely but with
Extreme Impacts
Not Existing in Welldesigned Systems
• Most utilities manually select
NERC Category D
contingencies to simulate:
Consequences
o Loss of a key substation
o Loss of tie lines
o Outages close to a
generation/load pocket
D
C
Generator Outage
B
Needless to study
A
Credible and Acceptable
N-1 Line Outage
N-2 Line Outage
Frequency of Occurrence
Frequency may increase when system is stressed (e.g. Storm Approaching)
27
Extreme Events
How are reliability standards used?
• In Planning:
–Reliability standards should never be violated
in designing the system.
• In Operations:
–Reliability standards should never be
intentionally violated
–Sometimes, violations occur due to misoperations or delayed awareness of the realtime situation
28
Related Terms
• Operating quantities: Physical quantities (measured or
calculated) that can be used to describe the operating
conditions of a power system, e.g. real, reactive and
apparent powers, RMS values/phasors of alternating
voltages and currents.
• Steady-state operating condition of a power system: An
operating condition of a power system in which all the
operating quantities that characterize it can be considered
to be constant for the purpose of analysis.
29
• In designing and operating an interconnected power
system, its dynamic performance subjected to changes
(i.e. contingencies, small or large) is considered
• It is important that when the changes are completed, the
system settles to new operating conditions without
violation of constraints.
• In other words, not only should the new operating
conditions be acceptable (as revealed by steady-state
analysis) but also the system must survive the transition
to those new conditions. This requires dynamic analysis.
30
Related Terms (cont’d)
• Disturbance: a sudden change or a sequence of
changes in one or more parameters or
operating quantities of the power system.
P(δ)
• Small and large disturbances
– a small disturbance if the equations describing the
dynamics of the system may be linearized for the
purpose of accurate analysis, e.g. a load change
P
– a large disturbance if the equations that describe
the dynamics of the system cannot be linearized
for the purpose of accurate analysis, e.g. a short
circuit and loss of a generator or load.
δ
31
Related Terms (cont’d)
• Synchronous operation:
– A machine is in synchronous operation with another machine or a
network to which it is connected if its average electrical speed
(=ωr⋅P/2) is equal to the electric speed of the other machine or the
angular frequency of the ac network.
– A power system is in synchronous operation if all its connected
synchronous machines are in synchronous operation with the ac
network and with each other.
• Asynchronous operation: loss of synchronism or out of step
32
Stability of a Dynamical System
Consider a nonlinear dynamical system
Assume origin x=0 is an equilibrium, i.e.
The equilibrium point x=0 is stable in the sense of Lyapunov
such that
In other words, the system variable will stay in any given small region (ε)
around the equilibrium point once becoming close enough (δ) to that point.
x
δ
33
ε
Power System Stability
• Power system stability is the ability of a power system, for a given initial
operating condition, to regain an acceptable state of operating
equilibrium (i.e. the new condition) after being subjected to a
disturbance
• Considering an interconnected power system as a whole
– The stability problem with a multi-machine power system is mainly to
maintain synchronous operation of the machines (generators or
motors)
• Considering parts of the system
– A particular generator or group of generators may lose stability
(synchronism) without cascading instability of the main system.
– Motors in particular loads may lose stability (run down and stall)
without cascading instability of the main system.
34
Some Terms Related to System Dynamic
Performance
Secure (vs. Insecure)
Not violating given security criteria
Stable (vs. Unstable)
A system is able to regain an equilibrium following a disturbance.
(A stable power system may not be secure if the equilibrium or the
transition to the equilibrium violates security criteria)
Oscillatory
An operating quantity repetitively changes at some frequency around a central
value (equilibrium).
(When oscillation becomes uncontrollable to damage generators and other
equipment, the system will become insecure and even unstable)
35
Example: FIDVR (Fault-Induced Delayed Voltage Recovery)
NERC/WECC Planning standards require that following a Category B contingency,
•
voltage dip should not exceed 25% at load buses or 30% at non-load buses, and should not exceed
20% for more than 20 cycles at load buses
•
the post-transient voltage deviation not exceed 5% at any bus
36
Stability Classification
• Power system stability is essentially a single problem; however,
the various forms of instabilities that a power system may undergo
cannot be properly understood and effectively dealt with by
treating it as such.
• Because of high dimensionality and complexity of stability
problems, it helps to make simplifying assumptions to analyze
specific types of problems using an appropriate degree of detail of
system representation and appropriate analytical techniques.
• Analysis of stability, including identifying key factors that contribute
to instability and devising methods of improving stable operation,
is greatly facilitated by classification of stability into appropriate
categories
37
Stability Classification
• IEEE/CIGRE Joint Task Force on Stability Terms and Definitions, “Definition and
Classification of Power System Stability,” IEEE Trans. on Power Systems, Vol.19, No.2.,
pp. 1387-1401, May 2004.
• The classification of power system stability considers:
– The physical nature of the resulting mode of instability as indicated by
the main system variable (angle, frequency or voltage) in which
instability can be observed.
– The size of the disturbance (small or large disturbance) considered,
which influences the method of calculation and prediction of stability.
– The devices, processes and time span that must be taken into
consideration in order to assess stability. Typical ranges of time
periods
• Transient or short-term: 0-10s
• Mid-term: 10s to several minutes
• Long-term: several to tens of minutes
38
Stability Classification
Physical nature
Disturbance size
Time span
39
Homework #1
• Learn the IEEE paper “Definition and Classification of Power System Stability”
• Select 1 journal/conference paper published by IEEE since 2010 that introduces
or addresses some stability problems on bulk power systems
– Source: http://ieeexplore.ieee.org or http://scholar.google.com
– Keywords: e.g. “power system” + “stability”
• Write a 1-2 pages essay (not Q&A’s):
– Title, authors, source of the paper
– Background:
• What stability problem is concerned? (Which IEEE categories?)
• Why is the problem significant? (Any real-world stories?)
• In which aspect(s) was the problem not addressed well in earlier literature?
– Approach
• What new approach is proposed? (Outline of the procedure or steps)
• Any key techniques are applied by the approach?
• How does the new approach perform?
– Remark
• Any conclusions from the work, or any room for further work
• Give a 3-5 minutes talk on your chosen paper and hand in your essay in the class
of Jan 23 (Thursday). Please email me the paper title by Jan 22 (Wed.) 5pm
40
Rotor Angle Stability
• Rotor Angle Stability refers to the ability of synchronous machines of
an interconnected power system to remain in synchronism after
being subjected to a disturbance.
• Phenomenon: increasing angular swings of some generators leading
to their loss of synchronism with others.
41
Rotor Angle Stability (cont’d)
• Rotor angle stability depends on the ability to maintain/restore
equilibrium between electromagnetic torque (TE) and mechanical
torque (TM) of each synchronous machine in the system.
• A fundamental factor in this problem is the manner in which the power
outputs of synchronous machines vary as their rotor angles change
(Power vs. Rotor angle)
42
Rotor Angle Stability (cont’d)
V∠0
E∠δ
E V
P3φ = 3
sin δ
Xs
P3φ
E V
sin δ
Te = 3
=
ω
ωXs
Te (P3φ)
Ta=Tm-Te<0
(decelerates)
Steady-state limit:
Unstable
Tm
Pmax(3φ )
E V
=3
Xs
Te,max
E V
=3
ωXs
Small
disturbance
δ0
43
Ta=Tm-Te>0
(accelerates)
Large
disturbance
Rotor Angle Stability (cont’d)
For a simple power system consisting of a generator tied to a load
bus, only when both sides have rotating mass, rotor angle stability
can be a concern
44
Rotor Angle Stability (cont’d)
• Interconnected power system with multiple generators
45
Small signal stability
• Small-disturbance angle stability or small signal stability
is the ability of a power system to maintain synchronism
under small disturbances.
– The disturbances are considered to be sufficiently
small that linearization of system equations is
permissible for purposes of analysis
– Small signal stability depends on the initial operating
state of the system (eigenvalues of the linearized
system at the state).
– In today’s power systems, the small-signal stability
problem is usually associated with insufficient damping
of oscillations
46
Small signal stability (cont’d)
• Small signal stability problems may be either local or
global in nature.
– Local plant mode oscillations (at 0.7~2.0Hz): oscillations of a small
part of the power system (typically, a single power plant) against
the rest of the system
– Inter-area mode oscillations (at 0.1~0.7Hz): oscillations of a group
of generators against the rest of the system
• The time frame of interest is 10 to 20 seconds following a
disturbance. However, oscillations may last several
minutes
47
• 1.2 Hz local plant mode oscillations lasting 4 minutes
Small-signal unstable
Source: slides of
Gary Kobet (TVA)
48
Transient Stability
• Large-disturbance angle stability or transient stability is
concerned with the ability of the power system to maintain
synchronism when subjected to a severe disturbance, e.g. a
short circuit on a transmission line.
– The resulting system response involves large excursions
of generator rotor angles and is influenced by the
nonlinear power-angle relationship.
– Transient stability depends on both the initial operating
state of the system and the severity of the disturbance.
49
Transient Stability (cont’d)
• Transient instability is usually in the form of aperiodic
angular separation, which is often referred to as first
swing instability.
• However, in large power systems, transient instability may
occur after multiple swings as a result of, e.g.,
superposition of multiple oscillation modes.
• The time frame of interest in transient stability studies is
usually 3 to 5 seconds following the disturbance. It may
extend to 10-20 seconds (to observe a number of swings)
for very large systems with dominant inter-area
oscillations.
50
“Dynamic Stability”
• The term “dynamic stability” also appears in the literature
as a class of rotor angle stability.
– In the North American literature, it has been used
mostly to denote small signal stability.
– In the European literature, it has been used to denote
transient stability.
• Both CIGRE and IEEE have recommended that it not be
used.
51
Voltage Stability
• Voltage stability refers to the ability of a power system to maintain
steady voltages at all buses in the system after being subjected to a
disturbance from a given initial operating condition.
– It depends on the ability to maintain/restore equilibrium between
load demand and supply
– In order words, it depends on the ability to maintain bus voltages
so that when the system nominal load at a bus is increased, the
real power transferred to that load will increase.
52
Voltage Stability (cont’d)
• The term voltage collapse is also often used. It is the
process by which the sequence of events accompanying
voltage instability leads to a blackout or abnormally low
voltages in a significant part of the power system.
53
Voltage Stability (cont’d)
• Small-disturbance voltage stability
– ability to maintain steady voltages when subjected to small
perturbations such as incremental changes in system load.
– studies using linearized models for sensitivity analysis
• Large-disturbance voltage stability
– ability to maintain steady voltages following large disturbances
such as system faults, loss of generation, or circuit contingencies.
– studied using nonlinear models on involved devices, e.g. motors,
transformer tap changers, generator field-current limiters, etc.
54
Voltage Stability (cont’d)
• Short-term voltage stability
– involves dynamics of fast acting load components, e.g. induction
motors, electronically controlled loads and HVDC convertors.
– The study period of interest is in the order of several seconds
– requires solution of appropriate system differential equations
• Long-term voltage stability
– involves slower acting equipment, e.g. tap-changing transformers,
thermostatically controlled loads, and generator current limiters.
– the study period of interest may extend to several or many minutes
– requires long-term simulations
55
B. Gao, et al, “Towards the development of a systematic approach for voltage stability assessment
of large-scale power systems, IEEE Trans. Power Systems, Vol. 11 No. 3 Aug. 1996
56
Relationship between rotor angle instability
and voltage instability
• Typical systems vulnerable to two stability problems
– Rotor angle stability
– Voltage stability
• However, two problems often occur together
– For example, as rotor angles between two groups of generators approach
180o, the loss of synchronism causes rapid drop in voltages at
intermediate points in the network.
– Loss of synchronism of some generators may result from the outages
caused by voltage collapse or from operating conditions that violate
generator field current limits
57
System Operation
• Establish most economical operating conditions under “normal”
circumstances
• Operate the system such that if an unscheduled event occurs, it does
not result in uncontrolled (or cascading) outages
• Establish “Safe Operating Limits” for all situations
• Meet reliability criteria
– Voltage limits
– Line and component loading limits (thermal limits)
– Stability
– Dynamic performance
58
Transition due
to disturbance
Transition due
to control action
Normal
Secure with sufficient
margin; able to withstand
a contingency
Preventive
control
Alert
Secure with insufficient
margin; Contingency may
cause overloading
Corrective
control
Restorative
Emergency
control
Emergency
Insecure; system is
still intact
Restorative
control
Extreme
Power outages;
system separates
59
Cascading
events
Design and Operating Criteria for Stability
Design and operating criteria play an essential role in
preventing major system disturbances following severe
contingencies.
• The use of criteria ensures that, for all frequently
occurring contingencies (i.e. credible contingencies, e.g.
Categories B and C), the system will, at worst, transit from
the normal state to the alert state, rather than to a more
severe state such as the emergency state or the extreme
state.
• When the system enters the alert state following a
contingency, operators can take actions to return the
system to the normal state.
60
System Stability Studies
Types
Approach
Purposes
Small
signal
stability
• Using linear system analysis
tools to study the modal system
response to a small disturbance.
•
Obtain safe operating limits and guidelines
•
Identify poorly damped modes of
oscillation
• Details on the disturbance may
not be important
•
Setting of controls (e.g., exciters, power
system stabilizers)
•
New generation studies (to meet reliability
criteria at the least cost)
•
Transmission planning studies (to analyze
plans for future transmission expansion,
and to meet reliability criteria)
Transient • Using nonlinear system analysis
stability
tools to study the system
response to a large disturbance.
• Traditionally using time-domain
simulation to “track” the evolution
of system states and parameters •
during the transient period.
• Every study is for a completely
specified disturbance scenario
including the pre-disturbance
system condition and
disturbance sequence (any
change requires a new study)
Operations planning studies (to check if a
given system configuration or operations
schedule meets reliability criteria)
•
Special control to maintain stability (e.g.,
generation tripping, load shedding, etc.)
•
Severe disturbance (extreme contingency)
studies
•
Special purpose studies (e.g., system
blackstart or restoration plan, etc.)
61
Trends in North American Interconnections
• Fewer HV transmission lines built due to cost and environmental
concerns
• Heavier use of some power plants away from load centers due to
conservation of oil and natural gas
• Heavier loading of HV transmission due to growing electricity markets
under the “open transmission access” environment
• Generation trends have become more stability-conscious
–
–
–
–
–
Lower inertia
Higher short circuit ratio
More dependence on controls (e.g. excitation control)
Large concentration of generation
More power electronics based resources, e.g. renewables (intermittent)
may alter the basic inertial response
• Effects of HVDC systems and solid state electronic devices (e.g.
flexible ac transmission systems, or FACTS)
62
Structure of a Power System
and Associated Controls
63