TRIBHUVAN UNIVERSITY

INSTITUTE OF ENGINEERING
PASHCHIMANCHAL CAMPUS

A PROJECT REPORT SUBMITTED IN PARTIAL FULFILLMENT OF THE

REQUIREMENTS FOR THE BACHELORS DEGREE IN
ELECTRICAL ENGINEERING
ON
CONTINGENCY ANALYSIS OF POWER SYSTEM
Subject Code: EE707

By:
Anish Sharma Poudel 073-BEL-204
Arjun Devkota 073-BEL-206
Dipendra Gaudel 073-BEL-220
Nisha Nepal 073-BEL-230

Under the supervision of:

Er. Ashwin Marahatta
Department of Electrical Engineering
Institute of Engineering, Paschimanchal Campus
Lamachaur, Pokhara

MARCH, 2020

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 ABSTRACT

Maintaining power system security is one of the challenging tasks for the power system
engineers. The security assessment is an essential task as it gives the knowledge about the system
state in the event of a contingency. Contingency analysis technique is being widely used to
predict the effect of outages like failures of equipment, transmission line etc, and to take
necessary actions to keep the power system secure and reliable. The off line analysis to predict
the effect of individual contingency is a tedious task as a power system contains large number of
components. Practically, only selected contingencies will lead to severe conditions in power
system. The process of identifying these severe contingencies is referred as contingency
selection and this can be done by calculating performance indices for each contingencies.

The main motivation of the work is to carry out the contingency analysis by calculating
the two kinds of performance indices; active performance index (PI P) and reactive power
performance index (PIV) and accordingly ranking the contingencies which is automatically done
by software DigSILENT . With the help of Fast Decoupled method the load flow analysis have
been calculated in DigSILENT environment. This provides an effective mean to know active
powers, reactive powers, voltages, phase angles of various buses, generators, lines, and loads.
The effectiveness of the method has been tested on 5-Bus in progress defense and now it has
been tested on 14 bus and also design of 33 bus is done.

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 TABLE OF CONTENTS

Contents Page No.

Title Page 1

Abstract 2

List of Figures 4

List of Tables 5

List of Abbreviations 6

1 Introduction 7

1.1 Background 7

1.2 Problem Statements 8

1.3 Objectives 10

1.4 Scope and Limitations 11

1.5 Report Organization 11

2 Literature Review 12

3 Methodology 13

3.1 Contingency Analysis Using Sentivities Factors 13

3.2 Contingency Analysis Using AC Power Flow 14

3.3 Contingency Analysis Using Fast Decoupled Load Flow Solution 15

3.4 Algorithm For The Fast Decoupled Load Flow 15

4 Results and Discussion 16

5 Further Works 21

6 References 22

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 List of Figures

Figure No. Title Page No.

Figure 1.2.a. Optimal Dispatch 9

Figure 1.2.b. Post-contingency state 9

Figure 1.2.c. Secure Dispatch 9

Figure 1.2.d. Secure Post Contingency State 9

Figure 4.1. 5- bus system 17

Figure 4.2. Ssingle line diagram of 14-bus system 18

Figure 4.3. Voltage magnitudes (load flow cases with individual grids) 19

Figure 4.4. Active power and reactive powers of generators 20

Figure 4.5. Single diagram of 33-bus system 26

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 List of Tables
Table No. Title Page No.
Table 4.1. Load demand 20

Table 4.2. Generator dispatch 21

Table 4.3. Generator controller setting 21

Table 4.4. Data of line in power factory model 21

Table 4.5. Data of transformers based on 100MVA 22

Table 4.6. Results of generators 22

Table 4.7. Results of buses 23

Table 4.8. Results of line data 23

Table 4.9. Ranking of the generators and lines according to PIp and PIV 25

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LIST OF ABBREVIATIONS Full Form

GSF Generation Shift Factor

LODF Line Outage Dstribution Factor

PIP Active power performance index

PIv Reactive power performance index

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 Chapter 1 INTRODUCTION

1.1. Background
Contingency analysis is the study of the outage of elements such as transmission lines,
transformers and generators, and investigation of the resulting effects on line power flows and
bus voltages of the remaining system. Contingencies referring to disturbances such as
transmission element outages or generator outages may cause sudden and large changes in both
the configuration and the state of the system. Contingencies may result in severe violations of
the operating constraints. Consequently, planning for contingencies forms an important aspect
of secure operation. System security can be said to comprise of three major functions that are
carried out in energy control center:
A. System monitoring
B. Contingency analysis
C. Corrective action analysis.
System monitoring supplies the power system operations or dispatches with pertinent
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 transit the data, voltages, currents,
current flows and the status of circuit breakers and switches in every substation in a
transmission network. The second major security function is contingency analysis. Modern
operation computers have contingency analysis programs stored in them. These foresee possible
system troubles (outages) before they occur. They study outage events and alert the operators to
any potential overloads or serious voltage violations. The third major security function,
corrective action analysis, permits the operators 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. Thus this provides preventive and post-contingency control. A simple example
of corrective action 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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1.2. Problem Statement
The major problems encountered in the power system are generation outages and line
outages. Contingencies are defined as potentially harmful disturbances for the steady state
operation of an electrical network. A power system under normal operating conditions may face
a contingency such as line outages or generator outages or loss of transformer, sudden change in
the load or faults.

Security analysis involves the power system to operate into four operating states :

 Optimal dispatch: In this state 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 the power system after a contingency has occurred, it is
being assumed that this condition has a security violation such as line or transformer are
beyond its flow limit, or a bus voltage is outside the limit.

 Secure dispatch: It is the state of the system with no contingency, but with corrections to
the operating parameters to account for security violations.

 Secure post-contingency: This is the state where the contingency is applied to the base
operating condition with corrections.

The concept of security analysis has been illustrated with a following example. Suppose a
power system consisting of two generators, a load, and a double circuit line, is to be operated
with both generators supplying the load as shown in Fig. 1.1(a) and ignoring the losses it is
assumed that the system as shown is in economic dispatch i.e. 500 MW is allotted for unit 1 and
the 700 MW for unit 2 as the optimum dispatch. Further, it is asserted that each circuit of the
double circuit line can carry a maximum of 400 MW, so that there is no loading problem in the
base-operating condition. This condition is being referred to as the optimal dispatch.

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 Fig.1.1. Various operating states of power system
Now, a failure in one of the two transmission lines has been postulated and it can be said
that a line contingency has occurred and this results in change in power flows in the other line
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causing the transmission line limit to get violated. The resulting flows have been shown Fig.
1.1(b), this sate of power system is being said to be post contingency state.
Now there is an overload on the remaining circuit. If the above condition is to be avoided,
the following security corrections have been done. The generation of unit 1 has been lowered
from 500 MW to 400 MW and the generation of unit 2 is raised from 700 MW to 800 MW. This
secure dispatch is illustrated in Fig. 1.1(c). Now, if the same contingency analysis is to done, the
post-contingency condition power flows is illustrated in Fig. 1.1(d).

Thus by adjusting the generation on unit 1 and unit 2, the overloading in other line is
prevented and thus the power system remains secure. These adjustments are called “security
corrections.” 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”. These programs can take account of many contingencies and
calculate adjustments to generator MW, generator voltages, transformer taps etc. Together with
the function of system monitoring, contingency analysis and the corrective actions the analysis
procedure forms a set of complex tools that can lead to the secure operation of a power system.

1.3. Objectives

The objective of the present work is selection of power system contingencies by

calculating the sensitivities factors; GSF and LODF. for transmission line outages using the Fast
decoupled load flow analysis or Newton-Raphson’s method.. The objective is also to compare
the performance of the method employing Newton-Raphson’s for various power system
networks. Since contingency analysis involves the simulation of each contingency on the base
case model of the power system, three major difficulties are involved in this analysis. First is the
difficulty to develop the appropriate power system model. Second is the choice of which
contingency case to consider and third is the difficulty in computing the power flow and bus
voltages which leads to enormous time consumption in the Energy Management System. So we
need to work considering this problems.

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1.4. Scopes and Limitations
In the large power systems, there is various buses, generators, lines, etc. The power
system components may go to outages or failures. To remove this effect the widely used
technology is contingency analysis. DC power flow method is used to when the active power is
to measured of various components and number of outages is less while Linear sensitivity factors
is used to when number of components is more and have to calculate active powers, voltages.
AC power flow method is most used in contingency selection and contingency analysis
for the large buses power system but it is difficult and slow process.

1.5. Report Organization

The work carried out in this report has been summarized in four chapters, Chapter 1
deliberates on the overview of the problem, objectives of work and organization of the report.
Chapter 2 discusses the brief of literature review. Chapter 3 contains various methods for
contingency analysis, the contingency selection by calculating the active and reactive power
performance indices using the Fast decoupled load flow and the results for various systems. In
The conclusions and the scope of further work are detailed in Chapter 4.

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 CHAPTER 2 LITERATURE REVIEW

The importance of power system security assessment for prediction of line flows and bus
voltages following a contingency has been presented. The paper also summarized the challenges
faced for the practical implementation of security analysis algorithms. The approximate changes
in the line flow due to an outage in generator or transmission line is predicted based on
distribution factors. The use of AC power flow solution in outage studies has been dealt.

Contingency screening or contingency selection is an essential task in contingency

analysis. This helps to reduce the numerous computations, the bounding method reduces the
number of branch flow computation by using a bounding criterion that helps in reducing the
number of buses for analysis and is based on incremental angle criterion. The 1P-1Q method for
contingency selection has been studied and would be presented in next session. In this method
the solution procedure is interrupted after an iteration of fast decoupled load flow.
Zaborzky et al. introduced the concentric relaxation method for contingency evaluation
utilizing the benefit of the fact that an outage occurring on the power system has a limited
geographical effect. The use of fast decoupled load flow proves to be very suitable for
contingency analysis. Contingency selection criterion based on the calculation of performance
indices has been first introduced by Ejebe and Wollenberg where the contingencies are sorted in
descending order of the values of performance index (PI) reflecting their severity.

The post contingency cases have been first studied using fast decoupled load flow. These
quantities have been assigned a degree of severity AC load flow would be applied to contingency
selection problem in for voltage ranking where the post contingent voltages are used to rank the
contingencies. The application of Genetic algorithm for contingency ranking would have been
studied in where the problem of contingency ranking problem is formulated as an optimization
problem with an objective of finding the critical cases, this approach reduces the computational
burden for contingency analysis. The authors attempted post voltage calculations for a
transmission line and transformer outages using Genetic algorithm.

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 CHAPTER 3 METHODOLOGY

Contingency analysis can be divided into three different stages namely contingency
definition, selection and evaluation. Contingency definition comprises of the set of possible
contingencies that might occur in a power system, it involves the process of creating the
contingency list. Contingency selection is a process of identifying the most severe contingencies
from the contingency list that leads to limit violations in the power flow and bus voltage
magnitude, thus this process eliminates the least severe contingencies and shortens the
contingency list. It uses some sort of index calculations which indicates the severity of
contingencies. On the basis of the results of these index calculations the contingency cases are
ranked. Contingency evaluation is then done which involves the necessary security actions or
necessary control to function in order to mitigate the effect of contingency.

3.1. Contingency Analysis using Sensitivity Factors

The problem of studying thousands of possible outages becomes very difficult to solve if
it is desired to present the results quickly. One of the easiest ways to provide a quick calculation
of possible overloads is to use sensitivity factors. These factors show the approximate change in
line flows for changes in generation on the network configuration and are derived from the DC
load flow. These factors can be derived in a variety of ways and basically come down to two
types:

 Generation Shift Factors

 Line Outage Distribution Factors

The generation shift factors are designated ali and have the following definition:

a li = ∆fl / ∆Pi ………………………………………….3.1

Where,

l=line index and i=bus index

∆fl = change in megawatt power flow on line l when a change in generation ∆Pi occurs at bus i

∆Pi = change in generation at bus i

The line outage distribution factors are used in a similar manner, only

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they apply to the testing for overloads when transmission circuits are lost. By
definition, the line outage distribution factor has the following meaning:
dlk = ∆fl / fk0 ………………………………………………….…3.2
where,

dl,k = line outage distribution factor when monitoring line l after an outage on line k
∆fl = change in MW flow on line l
fk0 = original flow on line k before it was outaged i.e., opened

By pre calculating the line outage distribution factors, a very fast procedure can be set up
to test all lines in the network for overload for the outage of a particular line. Furthermore, this
procedure can be repeated for the outage of each line in turn, with overloads reported to the
operations personnel in the form of alarm messages. The generator and line outage procedures
can be used to program a digital computer to execute a contingency analysis study of the power
system. It is to be noted that a line flow can be positive or negative so that we must check f l
against – flmax as well as flmax. It is assumed that the generator output for each of the generators in
the system is available and that the line flow for each transmission line in the network is also
available and the sensitivity factors have been calculated and stored.

3.2. Contingency Analysis using AC Power Flow

The calculations made with the help of network sensitivity factors for contingency
analysis are faster, but there are many power systems where voltage magnitudes are the critical
factor in assessing contingencies. The method gives rapid analysis of the MW flows in the
system, but cannot give information about MVAR flows and bus voltages. In systems where
VAR flows predominate, such as underground cables, an analysis of only the MW flows will not
be adequate to indicate overloads. Hence the method of contingency analysis using AC power
flow is preferred as it gives the information about MVAR flows and bus voltages in the system.
When AC power flow is to be used to study each contingency case, the speed of solution for
estimating the MW and MVAR flows for the contingency cases are important, if the solution of
post contingency state comes late, the purpose of contingency analysis fails. The method using
AC power flow will determine the overloads and voltage limit violations accurately. It does
suffer a drawback, that the time such a program takes to execute might be too long. If the list of

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outages has several thousand entries, then the total time to test for all of the outages can be too
long. However, the AC power flow program for contingency analysis by the Fast Decoupled
Power Flow (FDLF) provides a fast solution to the contingency analysis since it has the
advantage of matrix alteration formula that can be incorporated and can be used to simulate the
problem of contingencies involving transmission line outages without re inverting the system
Jacobian matrix for all iterations. Hence to model the contingency analysis problem the AC
power flow method, using FDLF method has been extensively chosen.

3.3. Fast Decoupled Load Flow Solution

The development of Fast Decoupled Load Flow (FLDF) method was developed by Stott
and this has been proposed based on the certain assumptions in the N-R method, hence to
understand the FLDF method it is required to consider the study of N-R method.

We have done load flow analaysis by using DigSILENT software.

Thus it can be concluded that fast decoupled power flow solution requires the least time per
iteration among all load flow techniques available, hence the power flow solution is obtained
very rapidly. Thus this technique is very useful in contingency analysis where numerous outages
are to be simulated in a very rapid manner.

3.4. Algorithm For Contingency Analysis Using Fast Decoupled Load Flow

The algorithm steps for contingency analysis using fast decoupled load flow solution are
given as follows:

Step 1: Read the given system line data and bus data.

Step 2: Set the counter to zero before simulating a line contingency.

Step 3: Simulate a line contingency.

Step 4: Calculate the active power flow for in the remaining lines and the maximum power flow
PMax

Step 5: Calculate the active power performance index PIP which give the indication of active

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 power limit violation

Step 6: Calculate the voltages at all the load buses following the line contingency.

Step 7: Calculate the reactive power performance index PIV which gives the voltage limit
violation at all the load buses due to a line contingency

Step 8: Check if this is the last line outage to be simulated; if not the step (3) to (7) is computed
till last line of the bus system is reached.

Step 9: The contingencies are ranked once the whole above process is computed as per the values
of the performance indices obtained.

Step 10: Do the power flow analysis of the most severe contingency case and print the results

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 CHAPTER 4 RESULT AND DISCUSSIONS

The main focus here is to perform the contingency analysis of 5-bus system, by
calculating generator shift factors (GSF) and line outage distribution factors (LODF). The
contingencies are then ranked where the most severe contingency is the one which is having the
highest sensitivity factor. The computation of these indices has been done based on load flow
analysis carried out using fast decoupled load flow (FDLF) under DigSILENT environment.

The study has been carried out for the 5-BUS system:

4.1. 5-Bus System:

The bus data and line data of 5-Bus System are detailed in table below.. The system as
shown in Fig. 4.1. consists a slack bus numbered 2 and 5 and load buses numbered 1, 3, and 4.
It has total five transmission lines and the active power flow in each transmission lines that has
been obtained using FDLF corresponding to the base case loading condition in DigSILENT
environment. This base case analysis is also referred a Pre- contingency state.

Fig.4.1. 5-Bus System

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4.2. 14-Bus System:

Figure 4.2. Single line diagram of 14-Bus system

The 14 Bus System consists of 14 buses (nodes), 5 generators, 11 loads, 16 lines, 5 transformers and one
shunt. 3 of these 5 transformers are used to represent one single 3-winding transformer. Figure 4.4. shows
the single line diagram.
• Bus 1 - Bus 5: 132 kV
• Bus 6, Bus 9 - Bus 14: 33 kV
• Bus 7: 1 kV
• Bus 8: 11 kV
The loads are not voltage-dependent, but have constant active and reactive power demand.
Generator “Gen 0001” is the slack generator, therefore voltage magnitude and voltage angle are
given (1.060 p.u., 0.0 degrees). The other generators are configured to control the active power
injection and voltage magnitudes at the connected buses. Therefore the active power dispatch
and controlled voltage magnitudes at their terminals are given.

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 Line data are given in per unit (p.u.) based on the base power Sb = 100MVA, without
information about the line length. The susceptance b in p.u. is twice the given line charging qc=2
in p.u., which is given as one-half of the total charging of lines. For the PowerFactory model
input data are required in /km and S/km (or F/km) respectively. Line data have been recalculated
for the network model with a nominal voltage. As there is no line length, the length of each line
in the PowerFactory model has been set to 1 km. The rated current of each line is not known and
therefore assumed to be 1 kA. The line between bus 1 and bus 2 is a double circuit, and has
therefore been modelled as two parallel lines with adapted parameters in the PowerFactory
model.

Figure 4.3. Voltage magnitudes (load flow cases with individual grids)

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Figure 4.4. Active and Reactive powers of generations

4.3. Tables with input data of 14-bus system

Table 4.1. Load Demand

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 Table 4.2.
Generator Dispatch

Table 4.3. Generator Controller Settings

Table 4.4. Data of llines based on 100MVA

21
 Table
4.4. Data of line in PowerFactory model

Table 4.5. Data of transformers based on 100MVA, with related voltages added in
PowerFactory Model

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Table 4.6. Results of generators

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 Table 4.7 Results of Buses

Table 4.8. Results of line data

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 Now the ranking of generators and lines have been done in above table 4.9. by calucating

performance indices PIP & PIv in the DigSilent PowerFactory environment.

Table 4.9. Ranking of the generators and lines according to PIp and PIV

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4.3. 33-bus system design

Figure 4.5 Single line diagram of 33-bus system

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 CHAPTER 5 FURTHER WORKS
We analysed the 14- bus system by using ac power flow method and hence calculating
performance indices ranking of the various generators and lines has been done using
DigSILENT. In the next session the work is to carry out in 33- bus and 66- bus system with the
help of Fast Decoupled Load Flow (FDLF), the PI P and PIV would be calculated in DigSILENT
environment and contingency ranking would be made. The effectiveness of the method would be
tested on 66-bus system.

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 CHAPTER 6 REFERENCES

 Wood A.J and Wollenberg B.F., “Power generation, operation and

control”, John Wiley & Sons Inc., 1996.

 Stott B, Alsac O and Monticelli A.J, “Security Analysis and

Optimization”, Proc. IEEE, vol. 75,No. 12, pp. 1623-1644,Dec 1987.

 Lee C.Y and Chen N, “Distribution factors and reactive power flow in
transmission line and transformer outage studies”, IEEE Transactions
on Power systems, Vol. 7,No. 1,pp. 194-200, February 1992.
 Pang C.K., Prabhakara F.S., El-Abiad A.H. and Koivo A.J., “Security evaluation in
power systems using pattern recognition”, IEEE Transaction on Power System
Apparatus and Systems,PAS-93,(2),pp.969-976 ,May/June 1974.

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