Security analysis black and white 2007
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POWER SYSTEM SECURITY
Viren B. Pandya
OUTLINE
System security and reliability
Illustrative example
Classification of power system states and
actions to be taken
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
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
Reliability has two componentsSecurity is the ability of the electric systems to withstand sudden disturbances such as electric short circuits or unanticipated loss of system elements.
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.
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)
An operator’s view of “security”
Security
Overload
Security
Voltage
Security
Angle/
Frequency security
Trans-former Overload
Line
Overload
Low
Voltage
Unstable
Voltage
Frequency instability
Rotor angle instability
Static security Dynamic security
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
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.
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
POWER SYSTEM SECURITY
Conti….
Secure post contingency: state of system when
contingency is applied to base operating
condition with correction
Example
500 MW
Unit 1
700 MW
Unit 21200 MW
250 MW
250 MW
Optimal Dispatch
Line max loadability is 400 MW
Example
500 MW
Unit 1
700 MW
Unit 21200 MW
500 MW
Post contingency
Line gets overloaded, correct it lowering generation on unit 1 to 400 MW to get secure dispatch
Example
Secure dispatch
To get secure dispatch, under no-contingency each line must take 200 MW, so reset generation
400 MW
Unit 1
800 MW
Unit 21200 MW
200 MW
200 MW
Example
Secure post contingency state
To get secure dispatch, under no-contingency each line must take 200 MW, so reset generation
400 MW
Unit 1
800 MW
Unit 21200 MW
400 MW
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 post-
contingency conditions are called “ Security
Constrained Optimal Power Flows or SCOPF”
Example
These programs can take account of many
contingencies and calculate adjustments to
generator MW, generator voltages,
transformer taps, interchange etc.
17
On-line Operator How to constrain the Operating rules,
assessment economic operation to on-line assessment,
(min-hours) maintain the normal state ? and Rs
Security-related decisions
Time-frame Decision maker Decision Basis for decision
Operational Analyst What should be the Minimum operating
planning operating rules ? criteria, reliability, (hrs-months) and Rs
Planning Analyst How to reinforce/maintain Reliability criteria
(months-years) the transmission system ? for system design,
and Rs
Power system “states” and actions
Normal (secure)
Emergency
Restorative
Extreme emergency.Separation, cascading
delivery pointinterruption,
load shedding
Alert,Not secure
Off-economic dispatch
Controlled loadcurtailment
Transmission loading reliefprocedures
Other actions(e.g. switching)
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.
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.
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.
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