INTRODUCTION
1.
2.
The Bulk Power Supply
Reliability criteria
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3.
Reliability criteria
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4.
The 2 components
Security assessment from operators view
Requirements of a reliable electric service
NERC
Disturbance-performance table
How are they used
Security states
System operating limits
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INTRODUCTION
1.
The Bulk Power Supply System
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Elaborate, complex, interconnection of power components which
make up an interconnected power system.
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When we talk about reliability and security of power systems, we are
interested in what we call “THE BULK POWER SUPPLY SYSTEM”
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The part of the network which connects the power plants, the major
substations, and the main EHV/HV lines.
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Interruptions in the bulk power supply are very serious
- Many users are affected by these interruptions
- They can be costly
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They are to be avoided, and much effort is spent to do that
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2. RELIABILITY CRITERIA
Power Systems are built and operated with the following goal:
TO ACHIEVE A RELIABLE and ECONOMIC ELECTRIC POWER SUPPLY.
For the consumer to have a reliable and economic electric power
supply, a complex set of engineering analysis and design
solutions need to be implemented.
Reliability of a power system refers to the probability of its
satisfactory operation over the long run. It denotes the ability to
supply adequate electric service on a nearly continuous basis,
with few interruptions over an extended time period.
- IEEE Paper on Terms & Definitions, 2004
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Security and Adequacy
Security is the ability of the electric
systems to withstand sudden disturbances
such as electric short circuits or
unanticipated loss of system elements.
Security of a power system refers to the degree of risk in its ability to
survive imminent disturbances (contingencies) without interruption of
customer service. It relates to robustness of the system to imminent
disturbances and, hence, depends on the system operating condition as
well as the contingent probability of disturbances. (IEEE TermsDefs-’04)
Adequacy is 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 outage of system elements.
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Difference between reliability and security
• Reliability is the overall objective in power system design and operation.
To be reliable, the power system must be secure most of the time.
• Security is a time-varying attributes which can be judged by studying the
performance of the power system under a particular set of conditions.
Reliability, on the other hand, is a function of the time-average
performance of the power system; it can only be judged by
consideration of the system’s behavior over an appreciable period of
time.
- IEEE Paper on Terms and Definitions, 2004
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An operator’s view of “security”
Security
Overload
Security
Transformer
Overload
Line
Overload
Voltage
Security
Voltage
magnitude
out of limits
Static security (our interest)
Unstable
Voltage
“Any consequence of a
credible disturbance
that requires a limit”
Angle/
Frequency
security
Frequency
instability
Rotor angle
instability
Dynamic security
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Another View of “Security”
Security = dynamic security
Adequacy = static security
This view is strongly held by engineers that have been deeply
involved with “reliability”assessment tools for planning such as
TRELLS, Tplan, etc., which compute probabilistic indices based on
static security assessment. They will tell you that their tools are
concerned with adequacy, not security.
You must note the person’s background who uses the term “Security” in
order to understand the meaning being implied.
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REQUIREMENTS OF A RELIABLE ELECTRIC POWER SERVICE
• Steady-state and transient voltages and frequency must be held
within close tolerances
• Steady-state flows must be within circuit limits
• Synchronous generators must be kept running in parallel with
adequate capacity to meet the load demand
• Maintain “integrity” of bulk power network: avoid cascading outages
NERC, North American Electric Reliability Corporation:
Mission is to ensure reliability of the bulk power system in North
America. They develop/enforce reliability standards; assess reliability
annually via 10-year and seasonal forecasts; monitor the bulk power
system; evaluate users, owners, and operators for preparedness; and
educate, train, and certify industry personnel. NERC is a selfregulated organization, subject to oversight by the U.S. Federal
Energy Regulatory Commission & governmental authorities in
Canada. It is composed of 9 regional reliability councils &
encompasses virtually all power systems in US & Canada. NERC’s
activities play an essential role in preventing contingencies and
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mitigating their consequences.
Interconnections
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The Disturbance-Performance Table is the heart of reliability criteria
Disturbance
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HOW ARE RELIABILITY CRITERIA USED?
A) In System Planning or Design
– Make decisions on size, type and timing of new generation
and transmission facilities
– Design transmission network to withstand normal &
prescribed abnormal conditions
– The latter includes such things as short circuits (faults)
followed by loss of major components (to isolate the fault).
B) In System Operation
– Establish most economic operating conditions under “normal”
conditions
– Operate the system such that if an unscheduled event occurs,
it does not result in violation of reliability criteria.
– Establish “Safe Operating Limits” for all situations
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Power system operational “states” & actions
(given with respect to credible contingency list)
Normal (secure)
Other actions
(e.g. switching)
Off-economic
dispatch
Restorative
Extreme emergency.
Separation, cascading
delivery point
interruption,
load shedding
Alert,
Not secure
Emergency
Controlled load
curtailment
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Some comments about the previous slide:
• The use of criteria ensures (and the diagram illustrates that), for all
credible contingencies, 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 in extremis state.
• If a system is operated according to criteria, the system can
transition from normal state to emergency or in extremis state
only for a non-credible (extreme) contingency.
• When the alert state is entered following a contingency, operators
can take actions to return the system to the normal state, but such
actions should not include load shedding.
• Load shedding should only be performed under emergencies.
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System operating limits (SOLs)
System Operating Limits (SOL) are the values of operating
parameters (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. SOL are based upon ensuring operating
conditions are within pre- and post-contingency…
•Facility Ratings
•Transient Stability Ratings
•Voltage Stability Ratings
•System Voltage Limits
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System operating limits
300 MW
Bus 2
Question 1: Is it “secure”?
X23=1
X12=1
900 MW
X13=1
Bus 1
Bus 3
Continuous rating=1200MW
Emergency rating=1300 MW
1200
MW
Question 2: What is maximum
cct 1-3 flow such that reliability
criteria is satisfied?
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Treating power as if it is current….
V1
V2
P12
PG2
jx12
PG1
PD1
PD2
A very basic relation for power system engineers expresses the real
power flow across a transmission circuit as:
P12  V1 I12 cos 
(1)
Here, φ is the angle by which the voltage leads the current and is called
the power factor angle.
If we assume that electric loads are purely resistive, so that only real
power flows in the network, then φ≈0 (φ will not be exactly zero because
of line reactance). In this case, eq. (1) is:
P12  V1 I12
(2)
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Treating power as if it is current….
A basic fact of power system is that the voltages usually do not deviate
significantly from their nominal value. Under a system of
normalization (called per-unit), where all voltages are normalized with
respect to this nominal voltage, it will be the case that |Vk|≈1.0. As a
result, eq. (2) becomes:
P12  I12
(3)
In other words, the numerical value of the real power flowing on the circuit is
the same as the numerical value of the current magnitude flowing on that
circuit (under the system of normalization).
Useful conclusion: If we assume voltage magnitudes are all unity, and all
loads are purely resistive, then whatever rules we have of dealing with
currents also work with real pu power flows! (or Sbase×pu pwr flws)
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Two good approximations for parallel flows
1. Current division: For 2 parallel paths A and B,
power flows on path A according to
PTotal
XB
XA  XB
Bus 2
300
1
900
 300
2 1
X23=1
X12=1
900 MW
X13=1
Bus 1
2
900
 600
2 1
900 MW
Bus 3
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Two good approximations for parallel flows
1. Current division: For 2 parallel paths A and B,
power flows on path A according to
PTotal
300 MW
300
XB
XA  XB
Bus 2
1
 100
2 1
X23=1
X12=1
2
300

2 1
200
X13=1
Bus 1
100
Bus 3
300 MW
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Two good approximations for parallel flows
2. Superposition: Results of 2 independent calculations
will add
Bus 2
300 MW
300
100
300
Total=500
Total=200
200
900 MW
Total=700
Bus 1
600
100
Bus 3
1200 MW
Continuous rating=1200MW
Emergency rating=1300 MW
IS IT SATISFYING
RELIABILITY CRITERIA?
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System operating limits
300 MW
Bus 2
The answer to Question 1: Is
it “secure”?
Lose Cct 2-3!
900 MW
Total=1200
Bus 1
Bus 3
Continuous rating=1200MW
1200 MW
YES!!!
Emergency rating=1300 MW
IS IT SATISFYING
RELIABILITY CRITERIA?
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System operating limits
Bus 2
300 MW
Total=500
Total=200
900 MW
Total=700
Bus 1
Bus 3
Question 2: What is
maximum cct 1-3 flow
such that reliability
criteria is satisfied?
1200 MW
Depends on how flow is increased:
assume stress direction of Bus1/Bus3.
Desire precontingency limits to
reflect postcontingency effects
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System operating limits
Bus 2
300 MW
333
100
333
Total=533
Total=233
Question 2: What is
maximum cct 1-3 flow
such that reliability
criteria is satisfied?
200
1000MW
Total=767
Bus 1
667
100
Bus 3
1300 MW
Continuous rating=1200MW
Emergency rating=1300 MW
IS IT SATISFYING
RELIABILITY CRITERIA?
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System operating limits
300 MW
Bus 2
Lose Cct 2-3!
1000MW
Total=1300
Bus 1
Bus 3
1300 MW
Continuous limit=1200MW
Emergency limit=1300 MW
IS IT SATISFYING
RELIABILITY CRITERIA?
It is right at the limit!
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System operating limits
Bus 2
300 MW
Total=500
Total=200
900 MW
Total=700
Bus 1
SOL=767
Bus 3
Question 2: What is maximum cct 1-3
flow such that reliability criteria is
satisfied?
1200 MW
Answer
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System operating limits
It is common to develop x-y plots of operational parameters to communicate system
limits. Such plots are called nomograms. They may communicate limits for any kind of
security problem (overload, voltage, transient or oscillatory instability)
3000
2800
2600
2400
2200
2000
1800
1600
1400
1200
1000
800
INSECURE!
600
400
200
200
400
600
800
1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000
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