The document discusses power system security, which includes maintaining reliability and preventing contingencies from causing violations. It is broken into three main functions: system monitoring, contingency analysis, and security constrained optimal power flow. Contingency analysis models different outages to operate the system defensively. Security constrained optimal power flow seeks optimal dispatches to prevent violations from any contingencies. The document defines different system states like normal, alert, emergency and explains control actions taken to transition between states or restore the system.

1. POWER SYSTEM SECURITY
Viren B. Pandya

2. OUTLINE
 System security and reliability
 Illustrative example
 Classification of power system states and
actions to be taken

3. POWER SYSTEM SECURITY
 Power system security is the ability of the
system to provide electricity with the appropriate
quality under normal and disturbance conditions
 In
security
applications,
we
refer
to
the
disturbances of interest as contingencies
 In
power
system
operations,
security
assessment analyzes the vulnerability of the
system to a set of postulated contingencies on a
real-time or near-real-time basis

4. POWER SYSTEM SECURITY
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

5. Reliability has two components
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.

6. An operator’s view of “security”
Security
Overload
Security
Angle/
Voltage
Security
Transformer
Overload
Line
Overload
Static security
Low
Unstable
Voltage
Voltage
Frequency
security
Frequency
instability
Rotor angle
instability
Dynamic security

7. POWER SYSTEM SECURITY
 Power system security is broken into three major
functions being done at control centre:
 System monitoring
 Contingency analysis
 Security constrained optimal power flow
 System monitoring is done by SCADA and state
estimator at central computer
 Contingency analysis gives results for different
known outages to operate system defensively

8. POWER SYSTEM SECURITY
 Several contingencies can be solved by power
flow programs and real time data and state
estimations
 Security constrained OPF: Here contingency
analysis is combined with OPF which seeks to
make changes optimal dispatches of generation
so that when security analysis is run, no
contingency results in violations.

9. POWER SYSTEM SECURITY
 Operating states of power system:
 Optimal dispatch: prior to contingency, OPF is
run but system may not be secure
 Post contingency: after contingency, security
violation i.e. line or Xmer beyond its flow limit
or bus voltage outside limit
 Secure dispatch: no contingency outages, but
corrections to operating parameters to
account for security violations

10. POWER SYSTEM SECURITY
Conti….
 Secure post contingency: state of system when
contingency is applied to base operating
condition with correction

11. Example
Line max loadability is 400 MW
250 MW
250 MW
700 MW
500 MW
Unit 1
1200 MW
Optimal Dispatch
Unit 2

12. Example
500 MW
700 MW
500 MW
Unit 1
1200 MW
Post contingency
Unit 2

13. Example
200 MW
200 MW
800 MW
400 MW
Unit 1
1200 MW
Secure dispatch
Unit 2

14. Example
400 MW
800 MW
400 MW
Unit 1
1200 MW
Unit 2
Secure post contingency state

15. Example
 Thus by adjusting generation on unit 1 and 2
we have prevented post contingency operating
state from getting overloaded. This is called
security correction.
 The
programs
which
can
make
control
adjustments to the base or pre-contingency
operation to prevent violations in the postcontingency conditions are called “ Security
Constrained Optimal Power Flows or SCOPF”

16. Example
 These programs can take account of many
contingencies and calculate adjustments to
generator
MW,
generator
transformer taps, interchange etc.
voltages,

17. Security-related decisions
Time-frame
Decision maker
Decision
Basis for decision
On-line
Operator
How to constrain the
Operating rules,
assessment
economic operation to
on-line assessment,
(min-hours)
maintain the normal state ?
and Rs
Operational
Analyst
Planning
(months-years)
Analyst
Minimum operating
operating rules ?
planning
(hrs-months)
What should be the
criteria, reliability,
and Rs
How to reinforce/maintain
Reliability criteria
the transmission system ?
for system design,
and Rs
17

18. Power system “states” and actions
Normal (secure)
Other actions
(e.g. switching)
Off-economic
dispatch
Restorative
Extreme emergency.
Separation, cascading
delivery point
interruption,
load shedding
Alert,
Not secure
Transmission
loading relief
procedures
Emergency
Controlled load
curtailment

20. Definition of states and control actions
(1) Real and Reactive power balance at each node (Equality
Constraints
(2) Limitations of physical equipment, such as currents and
voltages must not exceed maximum limits(Inequality
Constraints)
Normal (Secure) State: Here all equality (E) and inequality
constraints (I) are satisfied. In this state, generation is adequate
to supply the existing load demand and no equipment is
overloaded. Also in this state, reserve margins (for transmission
as well as generation) are sufficient to provide an adequate
level of security with respect to the stresses to which the system
may be subjected. The latter maybe treated as the satisfaction
of security constraints.

21. Definition of states and control actions
Alert (Insecure) State: The difference between this and the
previous state is that in this state, the security level is below
some threshold of adequacy. This implies that there is a
danger of violating some of the inequality (I) constraints when
subjected to disturbances (stresses). It can also be said that
security constraints are not met. Preventive control enables
the transition from an alert state to a secure state.
Emergency state: Due to a severe disturbance, the system
can enter emergency state. Here (I) constraints are violated.
The system, would still be intact, and emergency control
action (heroic measures) could be initiated to restore the
system to an alert state. If these measures are not taken in
time or are ineffective, and if the initiating disturbance or a
subsequent one is severe enough to overstress the system,
the system will breakdown and reach "In Extremis" state.

22. Definition of states and control actions
In Extremis State: Here, both (E) and (I) constraints are
violated. The violation of equality constraints implies that
parts of the system load are lost. Emergency control action
should be directed at avoiding total collapse.
Restorative State: This is a transitional state in which (I)
constraints are met from emergency control actions taken
but the (E) constraints are yet to be satisfied. From this
state, the system can transmit to either the alert or the
normal state depending on the circumstances.

23. STEADY-STATE SECURITY CONTROL
OBJECTIVE
 To prevent the system state from transitioning
from normal secure to emergency
 For an insecure normal state, two possible
responses are
 modification of the pre-contingency state to
eliminate the potential overload, in case the
contingency actually occurs
 a dispatch strategy to manage the emergency
once it occurs