Power System Security

Power System Security•Introduction•Major functions of power system security•Operating states of power system•FACTORS affecting on power system security•Security analysis•Contingency analysis•Sensitivity factor•AC Power flow method•Contingency relaxation

Introduction• System security involves practices suitably designed to keep

the system operating when components fail.• If the process of cascading failures continues, the system as a

whole or its major parts may completely collapse. This isnormally referred to as a System blackout.

• A particular system state is said to be secure only withreference to one or more specific contingency cases, and agiven set of quantities monitored for violation.

• Most power systems are operated in such a way that any singlecontingency will not leave other components heavilyoverloaded, so that cascading failures are avoided.

Major Functions of Power System Security

I. System MonitoringII. Contingency AnalysisIII. Corrective Action Analysis

System Monitoring• System monitoring supplies the power system operators or

dispatchers with up to date information on the conditions ofthe power system on real time basis as load and generationchange.

• Telemetry systems measure, monitor and transmit the data,voltages, currents, line flows, status of circuit breakers,frequency, generator outputs and transformer tap positions inevery substation in a transmission network.

• Digital computers in a control center then process thetelemeter data and place them in a data base form and informthe operators in case of an overload or out of limit voltage.

• Alarm or warnings must be given if required.

Contingency Analysis• Many of the problems that occur on a power system can cause

serious trouble within such a quick time period that theoperator could not take action fast enough. This is often thecase with cascading failures.

• Because of this aspect of systems operation, modernoperations computers are equipped with contingency analysisprograms that model possible system troubles before theyarise.

• These programs are based on a model of the power system andare used to study outage events and alarm the operators to anypotential overloads or out of limit voltages.

Corrective Action Analysis

• Corrective action analysis permits the operator to change theoperation of the power system if a contingency analysisprogram predicts a serious problem in the event of theoccurrence of a certain outage.

• Thus this provides preventive and post contingency control.• A simple example of corrective action is the shifting of

generation from one station to another.• This may result in change in power flows and causing a

change in loading on overloaded lines.

Operating state of power system• Equality constraints:- Real and Reactive power balance at each

node.• Inequality constraints:- Limitations of physical equipment

such as currents and voltages must not exceed maximumlimits.

• Five operating states of power systemI. Normal stateII. Alert stateIII. Emergency stateIV. Extremis stateV. Restorative state.

Continue…………• Normal state:- All equality and inequality constraints are

satisfied. Generation is adequate to supply the existing loaddemand and no equipment is overloaded.

• Alert state:- The security level is below some threshold ofadequacy. This implies that there is a danger of violating someof the inequality constraints when subjected to disturbances.

• Emergency state:- Due to severe disturbance, the system canenter emergency state. Here inequality constraints are violated.The system would still be intact, and emergency control actioncould be initiated to restore the system to an alert state.

• Extremis state:- Here, both equality and inequality constraintsare violated. The violation of equality constraints implies thatparts of the system load are lost. Emergency control actionshould be directed at avoiding total collapse.

Continue…………• Restorative state:- This is a transitional state in which

inequality constraints are met from emergency control actionstaken but the equality constraints are yet to be satisfied.

From this state the system can transmit to either the alert or thenormal state depending on the circumstances.

FACTORS affecting on power system security

• As a consequence of many wide spread blackout ininterconnected power systems, the priorities for operation ofmodern power system have evolved to the following:

I. Operate the system in such a way that power is deliveredreliably.

II. Within the constraints placed on the system operation byreliability considerations, the system will be operated mosteconomically.

• Engineering groups who have designed the power system’stransmission and generation systems have done so withreliability in mind.

• This means that adequate generation has been installed tomeet the load and that adequate transmission has beeninstalled to deliver the generated power to the load.

Continue…………• If the operation of the system went on without sudden failures or

without experiencing unanticipated operating states, we wouldprobably have no reliability problems.

• However, any piece of equipment in the system can fail, eitherdue to internal causes or due to external causes such as lightningstrikes, object hitting transmission towers, or human errors insetting relays.

• It is highly uneconomical to build a power system with so muchredundancy (extra transmission line, reserve generation, etc..) thatfailures never cause load to be dropped on a system.

• Rather, system are designed so that the probability of droppingload is small.

• Thus, the power systems are designed to have sufficientredundancy to withstand all major failures events.

Continue…………• There are two major types of failure events,1. Transmission line outages2. Generation unit failures• Transmission line failures cause changes in the line flows and

voltages on the transmission equipment remaining connectedto the system.

• Therefore, the analysis of transmission failures requiresmethod to predict these flows and voltages so as to be surethey are within their respective limits.

• Generation failures can also cause flows and voltages tochange in the transmission system, with the addition ofdynamic problems involving system frequency and generatoroutput.

Security analysis• System security can be broken down into two major functions:1. Security assessment2. Security control• The former gives the security level of the operating state.• The later determines the appropriate security constrained

scheduling required to optimally attain the target security level.• System security assessment is the process by which any

violations are detected.• System assessment involves two functions:1. System monitoring2. Contingency analysis

Continue…………• System monitoring provides the operator of the power system

with up to date information on the current conditions of the P.S.• Contingency analysis is much more demanding and normally

performed in three states, i.e. Contingency definition, selectionand evaluation.

• Contingency definition gives the list of contingencies to beprocessed whose probability of occurrence is high. This list is interms of network changes, i.e. branch and/or injection outages.

• These contingencies are ranked in rough order of severityemploying contingency selection algorithm to shorten the list.

• Contingency evaluation is then performed (using AC powerflow) on the successive individual cases in decreasing order ofseverity.

• The evaluation process is continue up to the point where no postcontingency violations are encountered.

Continue…………• The second major function, security control, allows operating

personnel to change the power system operation in the eventthat a contingency analysis program predicts a seriousproblem, should a certain outage occur.

• Normally it is achieved through Security ConstrainedOptimization(SCO) program.

Contingency Analysis

• The purpose of contingency analysis is to identifythe list of contingencies that if occur would createviolations in the system operating states. They areranked in order of severity.

Contingency analysis

Base case AC line flow

Continue………

Post Outage AC load flow

Continue………

Post outage AC load flow

Contingency analysis

Sensitivity Factors• The problem of studying thousands of possible outages

becomes very difficult to solve if it is desired to present theresults quickly.

• It is easy to solve it with linear sensitivity factors.• These factors show the approximate change in line flows for

changes in generation on the network configuration.• There are two types;1. Generation Shift Factors2. Line Outage Distribution Factors.

Generation Shift Factors

• Where , l= line index,i= bus index

• = change in megawatt power flow on line l when a change ingeneration, ∆Pi occurs at bus i.

• ∆Pi= change in generation at bus i.• It is assumed that the change in generation , ∆Pi , is exactly

compensated by an opposite change in generation at thereference bus, and that all other generation remain fixed.

• The factors then represents the sensitivity of the flow online l to a change in generation at bus i.

Continue…………..• If the generator was generating MW and it was lost, then

new power flow on each line in the network is given by,

Where, = flow on line l after the generator on bus i fails.= flow before the failure.

• The outage flow on each line can be compared to its limit andthose exceeding their limit flagged for alarming.

• This would tell the operations personnel that the loss of thegenerator on bus i would result in an overload on line.

Line outage distribution factor• It is apply to the testing for overloads when transmission

circuits are lost.

• Where ,= line outage distribution factor when monitoring line

l after an outage on line k.= Change in MW flow on line l.= Original flow on line k before it was outage.

The flow on line l with k out can be given by,

Continue……..• Where,

, = Preoutage flows on lines l and k, respectively.= Flow on line l with line k out.

• By precalculating the line outage distribution factor a very fastprocedure can be set up to test all lines in the network foroverload for the outage of a particular line.

AC Power flow method

Continue……….

• This procedure will determine the overloads and voltage limitviolations accurately.

• Drawback:- Its take more time to execute.• If the list of outages has several thousands entries then total

time to test for all of the outages can be too long.• Solution:-Select contingencies in such a way that only those

that are likely to result in an overload or voltage limit violationwill study. The other cases will go unanalyzed.

Continue…..

Continue…..

• There are two sources of error can arise during selecting the bad cases from full outage case list.

1. Placing too many cases on the short list.2. Skipping cases.:-A case that would have shown a problem is

not placed on the short list and results in possibly having that outage take place and cause trouble without the operators being warned.

Contingency selection(1P1Q)• To measure how much a particular outage might affect the

power system, performance index (PI) is used.

• The PI will be small value if all flows are within limit, and itwill be large if one or more lines are overloaded.

1. Calculate PI value for each line and order them fro largestvalue to least. The lines corresponding to the top of the listare then the candidates for the short list.

2. Simply ordered the PI table and then picked the top Nc entriesfrom the list and placed them on the short list.

Continue…..• 1P1Q method is used to perform an outage case selection.• Decoupled power flow is used.• The solution procedure is interrupted after one iteration.• Advantage:-1. Give sufficient information in the solution at the end of the

first iteration of the decoupled power flow to give areasonable PI.

2. The voltages can also be included in the PI.