Security analysis black and white 2007

23
POWER SYSTEM SECURITY Viren B. Pandya

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Transcript of Security analysis black and white 2007

Page 1: Security analysis black and white 2007

POWER SYSTEM SECURITY

Viren B. Pandya

Page 2: Security analysis black and white 2007

OUTLINE

System security and reliability

Illustrative example

Classification of power system states and

actions to be taken

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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

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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

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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)

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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

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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

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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.

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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

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POWER SYSTEM SECURITY

Conti….

Secure post contingency: state of system when

contingency is applied to base operating

condition with correction

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Example

500 MW

Unit 1

700 MW

Unit 21200 MW

250 MW

250 MW

Optimal Dispatch

Line max loadability is 400 MW

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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

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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

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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

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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”

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Example

These programs can take account of many

contingencies and calculate adjustments to

generator MW, generator voltages,

transformer taps, interchange etc.

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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

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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)

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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.

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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.

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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.

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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