Power System Security_ppt

Power System Security

Prepared by: Dhaval g.patel Assistant Professor CHARUSAT UNIVERSITY.

Introduction  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 the deficit without frequency drop. • Transmission line may be damaged and taken out by automatic relaying. • The specific time at which initiating event causes component failure is unpredictable. • Most equipment are protected by automatic devices ,so it may be switched out, if this limit is violated.

 If any event occurs on system that leaves it operating with limits violated, the event may be followed by series of further actions 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.  Ex:-A Single line is open due to insulation failure.(Cascading 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.

• 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.

Major Functions of Power System Security I. System Monitoring II. Contingency Analysis III. Corrective Action Analysis

System Monitoring • System monitoring supplies the power system operators or dispatchers with up to date information on the conditions of the 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, frequency, generator outputs and transformer tap positions in 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.

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. • Because of this aspect of systems operation, modern 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.

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 which seeks to make changes to the optimal dispatch of 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.

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 baseoperating condition-with corrections.

• 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.

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.

Continue………… • Normal state:- All equality and inequality constraints are satisfied. Generation is adequate to supply the existing load demand 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 the inequality constraints when subjected to disturbances. • Emergency state:- Due to severe disturbance, the system can enter emergency state. Here inequality constraints are 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, both equality 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.

Continue………… • Restorative state:- This is a transitional state in which inequality constraints are met from emergency control actions taken but the equality 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.

FACTORS affecting on Power System Security

• As a consequence of many wide spread blackout in interconnected 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. II. Within the constraints placed on the system operation by reliability considerations, the system will be operated most economically. • Engineering groups who have designed the power system’s transmission and generation systems have done so with reliability in mind. • This means that adequate generation has been installed to meet the load and that adequate transmission has been installed 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 would probably 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 as lightning strikes, object hitting transmission towers, or human errors in setting relays. • 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.

Continue………… • 1. 2. •

There are two major types of failure events, Transmission line outages Generation unit failures Transmission line failures cause changes in the line flows and voltages on 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 cause line flows and voltages to change in the transmission system, with the addition of dynamic problems involving system frequency and generator output.

Security analysis • 1. 2. • • • • 1. 2.

System security can be broken down into two major functions: Security assessment 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: System monitoring 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, selection and evaluation. • Contingency definition gives the list of contingencies to be processed whose probability of occurrence is high. This list is in terms of network changes, i.e. branch and/or injection outages. • These contingencies are ranked in rough order of severity employing contingency selection algorithm 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 post contingency violations are encountered.

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.

Contingency Analysis • The purpose of contingency analysis is to identify the list of contingencies that if occur would create violations in the system operating states. • They are ranked in order of severity.

Contingency analysis

Base case AC line flow

Continue………

Post Outage AC load flow

Continue………

Post outage AC load flow

• 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.

Contingency analysis

Sensitivity Factors • The problem of studying thousands of possible outages becomes very difficult to solve if it is desired to present the results 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 Factors 2. Line Outage Distribution Factors.

Generation Shift Factors αℓi = ∆fℓ / ∆Pi • Where , ℓ = line index, i= bus index • The αℓi factors then represents the sensitivity of the flow on line ℓ to a change in generation at bus i. • ∆fℓ = change in MW power flow on line ℓ when a change in generation, ∆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 the reference bus, and that all other generators remain fixed.

Continue………….. • If the generator in question was generating lost, then

MW and it was

New power flow on each line in the network can be calculated by precalculated set of “a” is given by, for l  1.....L 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 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.

• The generation shift sensitivity factors are linear estimates of the change in flow with a change in power at a bus. therefore the effects of simultaneous change on several generating buses can be calculated by superposition.

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. If we know the power flow on line ℓ and k, 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 fast procedure can be set up to test all lines in the network for overload for the outage of a particular line.

Contingency analysis using sensitivity factors

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 where voltage magnitudes are 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 only the MW flows will not be adequate to indicate overloads. • 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.

AC Power flow security analysis

Continue………. • This procedure will determine the overloads and voltage limit violations 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.  So we are confronted with dilemma. A methods which involving a and d factors can give rapid analysis but can’t 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.

AC power flow security analysis with contingency case selection

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.

Contingency selection(1P1Q) • To measure how much a particular outage might affect the power system, performance index (PI) is used.

• If n is a large no no., The PI will be small if all flows are within limit, and it will be large if one or more lines are overloaded. • If n=1, a table of PI value can be calculate quickly for each line in the network and order them from largest value to least. • The lines corresponding to the top of the list are then the candidates for the short list. • One procedure Simply ordered the PI table and then picked the top Nc entries from the list and placed them on the short list.

• 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 is just below its limit contributes to PI almost equal to one that is just over its limit limit. • Thus the PI’S ability to distinguish or detect bad cases is limited when n =1. Trying to develop an algorithm that can quickly calculate PI when n = 2 or larger has proven extremely difficult. • The solution procedure is interrupted after one iteration (one P - θ) calculation and one Q – V calculation; thus, the name 1P1Q.

Advantages:1. Give sufficient information in the solution at the end of the first iteration of the decoupled power flow to give a reasonable PI. 2. The voltages can also be included in the PI.

• where  Ei is the difference between the voltage magnitude as solved at the end of the 1P1Q procedure and the base-case voltage magnitude. •  E max is a value set by utility engineers indicating how much they wish to limit a bus voltage from changing on one outage case. • To complete the security analysis, the PI list is sorted so that the largest PI appears at the top.

Concentric Relaxation • An idea is considered to enter the field of security analysis in power systems is that an outage only has a limited geographical 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 line were a heavily loaded, high-voltage line, its loss will felt more than 20 miles away away.. • 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 as layer zero. The buses that are one transmission line or transformer from layer zero are then labeled layer one.

• 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 as a power flow with the outage in place. The buses in the highernumbered layers are kept as constant voltage and phase angle (i.e. as reference buses). • This procedure can be used in two ways ways:: either the solution of the layers included becomes the final solution of that 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.

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 N1 and the maximum phase angle appearing across any two buses in N3. • calculate the Pk and Pm the injections that will make the phase angles on buses k and m simulate the outage of line k-m.

0

• If we are given a transmission line in N2 with flow f 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 lower limit. Then, pq

------- (1)

we can translate this into a maximum change in phase angle difference as follows:

And finally ------ (2)

Thus, we can define the maximum change in the phase angle difference across pq.

The theorem states that ------ (3)

i,j = any pair of buses in N3, smallest in N3.

is the largest

in N3 and

is the

Eq.3    provides an upper limit to the maximum change in angular spread across any circuit in N2. Thus it provides us a limit as how far any of the N2 circuits can change their flow. i

j

By combining eq. (2) and (3),we obtain: ------ (4)

• Interpretation of bounding

• shows a graphical interpretation of the bounding process. There are two cases represented in above Figure: a circuit on the top of the figure that can’t go over limit, while that on the bottom could.. could • The horizontal line represents the change in flow on circuit pq times its reactance, f pq x pq . max  f • pq x pq represents the point where circuit pq will go into overload and is determined as explained previously.

• Any value of f pqmax x pq to the right of the dotted line represents an overload. • The solid line labeled i   j represents the upper limit on f pq x pq • 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 right of) the dotted line, the circuit may be shifted in flow due to the outage so as to violate a limit. • A completely safe N2 region would be one in which the maximum i   j 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 overload if the value of i   j m ax is less than the smallest value of  f p q x p q over all pairs pq, where pq corresponds to the buses. at the ends of circuits in N2 .

• If this condition fails, then we have to expand N1, calculate a new i   j in N3, and rerun the test over the newly defined N2 region circuits. When an N2 is found which passes the test, we are done and only region N1 need be studied in detail.

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

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.

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_____________. 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. 11. Contingency selection is usually performed using DC power flow model because__________ . 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.