Power System Security_ppt

8/10/2019 Power System Security_ppt

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Power System Security

Prepared by:

Dhaval g.patel

Assistant Professor

CHARUSAT UNIVERSITY.


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Minimization cost

Maintain system Security

1. Generating unit may have to taken off-line because of

auxiliary equipment failure.

2. By maintaining spinning reserve, remainning unit can make up

Introduction

Transmission line may be damaged and taken out by automatic

relaying.

The specific time at which initiating event causes componentfailure is unpredictable.

Most equipment are protected by automatic devices ,so it may be

switched out, if this limit is violated.


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If any event occurs on system that leaves it operating with

limits violated, the event may be followed by series of furtheractions that switched other equipment out of service.

If the process of cascading failures continues, the system as a

whole or its major parts may completely collapse. This is

normally referred to as a System blackout.- .

failure)

System Security can be divided in three major functions that

are carried out in operation control center.1.System monitoring

2.Contingency analysis

3.Security-constrained optimal power flow.


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Most power systems are operated in such a way that any single

contingency will not leave other components heavily

overloaded, so that cascading failures are avoided.

System security involves practices suitably designed to keep

the system operating when components fail.


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Major Functions of Power System

Security

I. System Monitoring

II. Contingency Analysis

III. Corrective Action Analysis


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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 generation

change.

Telemetry systems measure, monitor and transmit the data,

voltages, currents, line flows, status of circuit breakers,

,

every substation in a transmission network.

Digital computers are installed in a control center, to gather the

telemetered data then process and place them in a data base

from which operators can display information on large display

monitors.

Computer check incoming against prestored limits and alarm

the operators in case of an overload or out of limit voltage.


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Contingency Analysis

It allows the system to be operated defensively.

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

serious trouble within such a quick time period that the

operator could not take action fast enough. This is often the

case with cascading failures.,

operations computers are equipped with contingency analysis

programs that model possible system troubles before they

arise.

These programs are based on a model of the power system and

are used to study outage events and alarm the operators to any

potential overloads or out of limit voltages.


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Corrective action analysis

Corrective action analysis permits the operator to change the

operation of the power system if a contingency analysis

program predicts a serious problem in the event of the

occurrence of a certain outage.

Contingency analysis is combined with optimal power flow

generation, and also other adjustments so that when security

analysis is run, no contingency result in violations.

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.


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Operating state of Power System

Optimal dispatch:-The power system is in prior to any

contingency. It is optimal with respect to economic operation,

but it may not be secure.

Post Contingency:-it is the state of power system after a

contingency has occurred. This condition has a security

violation.

Secure dispatch:- it is the state of power system with no

contingency outages. but with corrections to the operating

parameters to account for security violations.

Secure post contingency:-contingency:-It is the state of the

power system when the contingency is applied to the base-

operating condition-with corrections.


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By adjusting the generation on unit 1 and 2 ,we have prevented

the post-contingency operating state from an overload. This is

the essence of security corrections.

Programs which make control adjustment to the base operation

to prevent violations in post-contingency conditions are called

SCOPF.


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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 maximum

limits.

I. Normal state

II. Alert state

III. Emergency state

IV. Extremis state

V. Restorative state.


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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 of

adequacy. This implies that there is a danger of violating some

of theinequality constraintswhen subjected to disturbances.- ,

enter emergency state. Hereinequality constraintsare violated.

The system would still be intact, and emergency control action

could be initiated to restore the system to an alert state.

Extremis state:- Here, bothequality and inequality 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.


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Continue

Restorative state:- This is a transitional state in whichinequality constraintsare met from emergency control actions

taken but theequality constraintsare 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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FACTORS affecting on Power System

Security

As a consequence of many wide spread blackout ininterconnected power systems, the priorities for operation of

modern power system have evolved to the following:

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

reliably..

reliability considerations, the system will be operated most

economically.

Engineering groups who have designed the power systemstransmission and generation systems have done so with

reliability in mind.

This means thatadequate generation has been installed to

meet the load and that adequate transmission has been

installed to deliver the generated power to the load.


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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, either

due to internal causes or due to external causes such aslightning

strikes, object hitting transmission towers, or human errors in

It is highly uneconomical to build a power system with so much

redundancy (extra transmission line, reserve generation, etc..) that

failures never cause load to be dropped on a system.

Rather, A system are designed so that the probability of dropping

load is small.

Thus, the power systems are designed to have sufficient

redundancy to withstand all major failures events, but this does

not guarantee that system will be 100 % reliable.


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Continue There are two major types of failure events,

1. Transmission line outages2. Generation unit failures

Transmission line failures cause changes in theline flowsand

voltageson the transmission equipment remaining connected

to the system. Therefore, the analysis of transmission failures requires

method to predict these flows and voltages so as to be sure

they are within their respective limits.

Generation failures can also causeline flowsandvoltagestochange in the transmission system, with the addition of

dynamic problems involvingsystem frequencyandgenerator

output.


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Security analysis

System security can be broken down into two major functions:1. Security assessment

2. 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 monitoring

2. Contingency analysis


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Continue System monitoringprovides 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, selection

and evaluation.

Contingency definition gives the list of contingencies to be.

terms of network changes, i.e. branch and/or injection outages.

These contingencies are ranked in rough order of severity

employingcontingency selectionalgorithm to shorten the list. Contingency evaluation is then performed (using AC power

flow) on the successive individual cases in decreasing order of

severity.

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


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Continue

The second major function, security control, allows operating

personnel to change the power system operation in the event

that a contingency analysis program predicts a serious

problem, should a certain outage occur.

Normally it is achieved through Security Constrained

Optimization(SCO) program.


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Contingency analysis

Base case AC line flow


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Continue

Post Outage AC load flow


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Continue

Post outage AC load flow


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Operators must know which line or generator outages will

cause flow or voltage outside the limit..to predict the effect

of outages, contingency analysis is used.

The methodological problem to cope with in C.A. is the speed

of solution of the model used.

Problem of selection of credible outages.

Use of DC load flow model to gain the speed of solution.


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Contingency analysis


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Sensitivity Factors

The problem of studying thousands of possible outagesbecomes very difficult to solve if it is desired to present the

results quickly.

It is easy to solve it withLinear sensitivity factors.

These factors show the approximate change in line flows for

c anges n genera on on e ne wor con gura on.

There are two types :-

1. Generation Shift Factors

2. Line Outage Distribution Factors.


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Generation Shift Factors

i =

f/

Pi

Where ,= line index,

i= bus index

The i

factors then represents the sensitivity of the flow on

lineto a change in generation at bus i.

f= change in MW power flow on line when a change in

generation,Pioccurs 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 the

reference bus, and that all other generators remain fixed.


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Continue..

If the generator in question was generating MW and it was

lost, then

New power flow on each line in the network can be calculated by

precalculated set of a is given by,

for 1.....l L

Where, = flow on lineafter the generator on bus i fails.

= flow before the failure.

The outage flow on each line can be compared to its limit

and those exceeding their limit flagged for alarming.

This would tell the operations personnel that the loss of the

generator on bus i would result in an overload on line.


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The generation shift sensitivity factors are linear estimates of

thechange in flowwith achange in powerat a bus. therefore

the effects of simultaneous change on several generating busescan be calculated by superposition.


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Line outage distribution factor

It is apply to the testing for overloads when transmission

circuits are lost.

Where ,

= line outage distribution factor when monitoring linea er an ou age on ne .

= Change in MW flow on line .

= Original flow on line k before it was outage.

If we know the power flow on lineand k,

The flow on linewith k out can be given by,


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Continue.. Where,

, = Preoutage flows on linesand k, respectively.

= Flow on linewith line k out.

By precalculating the line outage distribution factor a very fast

overload for the outage of a particular line.


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Contingency analysis using sensitivity factors


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AC Power flow method The calculations made by network sensitivity methods are

faster than those made by AC power flow methods.

There are many power systems wherevoltage magnitudesare

the critical factor in assessing contingencies. In addition, there

are some systems where VAR flows predominate on some

circuits, such as underground cables, and an analysis of onlye ows w no e a equa e o n ca e over oa s.

When AC power flow is to be used to study each contingency

case, speed of solution and no. of cases to be studied are

critical. So, AC power flow program used NR method or decoupled

power flow method.


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AC Power flow security analysis


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Continue.

This procedure will determine theoverloadsandvoltage limit

violationsaccurately.

Drawback:- Its take moretimeto execute.

If the list of outages has several thousands entries then total

time to test for all of the outages can be too long.

.

involvingaanddfactors can give rapid analysis but cant give

MVAR flows and voltages. Where as AC power flow gives

full accuracy but take long time.

Solution:-Select contingencies in such a way that only those

that are likely to result in an overload or voltage limit violation

will study. The other cases will go unanalyzed.


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AC power flow security analysis with contingency case selection


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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 :-This is conservative

approach which leads to longer run times for the security

analysis procedure to execute.

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.


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However, when n=1, PI dose not snap from near zero to near

infinity as branch limit exceeds. Instead its rises as a quadratic

function.

A line that isjustjust belowbelow itsits limitlimitcontributes to PI almost equal to

one that isjustjust overover itsits limitlimit.

Thus the PIS ability to distinguish or detect bad cases is limited

when n =1. Trying to develop an algorithm that can quicklycalculate PI when n = 2 or larger has proven extremely difficult.

The solution procedure is interrupted after one iteration (one P -)

calculation and one Q V calculation; thus, the name 1P1Q.


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Concentric Relaxation An idea is considered to enter the field of security analysis in

power systems is that an outage only has a limitedgeographical effect.

The loss of a transmission line does not cause much effect a

thousand miles away; in fact, we might it doesn't cause much

trouble beyond 20 miles from the outage, although if the linewere a eav y oa e , g - o tage ne, ts oss w e tmoremore

thanthan 2020 milesmiles awayaway..

To realize the benefit from the limited geographical effect of

an outage, power system is divided in 2 parts.(1) affected part (2) unaffected part.

To make this division, the buses at the end of the outaged line

are marked aslayer zero. The buses that are one transmission

line or transformer from layer zero are then labeledlayer one.


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This same process can be carried out, layer by layer, until all

the buses in the entire network are included.

Some arbitrary number of layers is chosen and all buses

included in that layer and lower-numbered layers are solved asa power flow with the outage in place.The buses in the higher-

numbered layers are kept as constant voltage and phase angle

(i.e. as reference buses).

ThisThis procedureprocedure cancan bebe usedused inin twotwo waysways::either the solutiono t e ayers nc u e ecomes t e na so ut on o t at case

and all overloads and voltage violations are determined from

this power flow, or the solution simply is used to form a

performance index for that outage.

Disadvantages:-It requires more layers for circuits.

B di


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Bounding N1 = the subsystem immediately surrounding the outaged line

N2 = the external subsystem that we shall not solve in detail

N3 = the set of boundary buses that separate N1 and N2

This method is based on the assumptions about the phase angle

spread across the lines in N2, given the injections in N1and the

maximum phase angle appearing across any two buses in N3.

angles on buses k and m simulate the outage of line k-m.k m


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If we are given a transmission line in N2 with flow then

there is a maximum amount that the flow on pq can shift. i.e. it

can increase to its upper limit or it can decrease to its lowerlimit. Then,

0

pqf

we can translate this into a maximum change in phase angle

difference as follows:

------- (1)

And finally

Thus, we can define the maximum change in the phase angle

difference across pq.

------ (2)


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The theorem states that

i,j = any pair of buses in N3, is the largest in N3 and is the

smallest in N3.

Eq.3 provides an upper limit to the maximum change in

angular spread across any circuit in N2. Thus it provides us a

------ (3)

i j

By combining eq. (2) and (3),we obtain:

m as ow ar any o e 2c rcu s can c ange e r ow.

------ (4)


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Interpretation of bounding

shows a graphical interpretation of the bounding process. There

are two cases represented in above Figure: a circuiton the top

of the figure thatcant go over limit, while that on the bottombottomcouldcould..

The horizontal line represents the change in flow on circuit pq

times its reactance, .

represents the point where circuit pq will go intooverload and is determined as explained previously.

q pqf xmax

pq pq

f x


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Any value of to the right of the dotted line representsAny value of to the right of the dotted line represents

an overload.an overload.

The solid line labeled represents the upper limit on

Thus, if the solid line is below (to the left) of the dotted line,

then the circuit theory upper limit predicts that the circuit

cannot go into overload; if on the other hand, the solid line is

above (to the rightof) the dotted line, the circuit may be shifted

in flow due to the outage so as to violate a limit.

max

pq pqf x

j

pq pqx

A completely safe N2 region would be one in which the

maximum will become smaller and smaller. Therefore,

the test to determine whether the N1 region encompasses all

possible overloaded circuits should be as follows: All circuits in N2 are safe from overloadif the value of

is less than the smallest value of over all pairs pq,

where pq corresponds to the buses. at the ends of circuits in N2 .

j

j

m ax

q p qf x


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If this condition fails, then we have to expand N1, calculate a

new in N3, and rerun the test over the newly defined

N2region circuits. When an N2is found which passes the test,

we are done and only region N1need be studied in detail.

j


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REFERENCES

1. Modern Power System Analysis by D.P.Kothari &

I.J.Nagrath .

2. Power Generation, Operation and Control by

A.J.Wood & B.F.Wollenberg


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1. An operationally secure power system is one with____________.

A. Medium probability of system blackout.

B. Low probability of system blackout.

C. High probability of system blackout.

D. Zero probability of system blackout.

2. Contingency analysis provides operating constrains to be employed in______________.

A. Unit commitment.

B. Emission dispatch.

C. Economic dispatch.

D. (A) and (B).

3. A Security analysis program normally uses_____________.

A. DC load flow.

B. AC load flow.

C. AC-DC flow.

D. Any of the above.

4. Energy management system ensures_____________.

A. Minimum cost.

B. Minimum environment cost.

C. High security.

D. Any of the above.


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5. Energy control center is supposed to perform the following security functions____________.

A. System monitoring.

B. Contingency analysis.

C. Corrective action analysis.

D. All of the above.

6. A Power system is operationally secure means______________.

A. All line flows and load bus voltages are within limit.

B. The operating cost is minimum.

C. It is safe from lightning strokes .

D. It is safe against switching surges.

7. Line outage distribution factors are primarily useful for________________.

A. System monitoring.

B. Contingency definition.

C. Contingency selection.

D. Security control.

8. Pre-contingency corrective rescheduling is required for system operation to be___________.

A. Most Economical.

B. In corrective by secure.

C. In security level-I.

D. None of the above.

9 Contingency definition gives the list of component outages


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9. Contingency definition gives the list of component outages_____________.

A. Which provides the contingency in decreasing order of severity.

B. Which includes the contingency with high probability of occurence.

C. For outage simulation.

D. Any one of the above.

10. A power system has secure and economic operation implies that__________.

A. Operating cost is minimum subject to emission constraints.

B. Operating cost is minimum subject to line flow constraints.

C. Magnitude of the line flows are minimum.

D. Transmission real power losses are minimum.

. __________ .

A. Accurate results are required.

B. Masking is to be removed.

C. Limited accuracy results are required.

D. All of the above.

12. Pre-plus post-contingency corrective rescheduling is required at times for___________.

A. The network operation is required in alert state .

B. The network operation is non-corrective emergency.

C. The network corrective capability is limited.

D. None of the above.


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