ABSTRACT
The study of the transient stability enhancement capability of Unified power flow
controller (UPFC) in a multi-machine power system is presented. The test system was
Nigerian 330kV power system and the focus was on the effect of disturbances on the
largest generating unit (Egbin) in the system. The original network was reduced to
retain the area of interest (i.e. South West buses including Egbin generator). The UPFC
was connected between Egbin and Ikeja-west buses and the analysis was conducted by
simulating a 3-phase fault at three locations; on the terminal of the largest generator
unit at Egbin bus, the bus with the largest load at Ikeja–west and the bus at Benin. The
response of the system for the three cases with and without the device in operation was
observed. The dynamic model for the series part of the device was developed in Python
programme using the application program interface (API) in Power System Simulation
for Engineering (PSS/E) software used for the analysis. Simulation results showed that,
with the UPFC in the network, the steady state voltage profiles of some buses were
improved by as much as 4.9% especially those close to the point of connection. In
addition, the device was able to damp power oscillation and the system transient
stability was enhanced. The enhancement is from the critical clearing time of the
system which was increased from 380ms to 590ms when the fault was at Egbin
generator terminal, from 470ms to 510ms following the fault at Ikeja-west, and from
770ms to 830ms when the fault was at Benin bus.
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TABLE OF CONTENTS
Title Page…………………………………………………………………………………i
Declaration………………………………………………………………………………ii
Certification……………………………………………………………………………..iii
Acknowledgements……………………………………………………………………..iv
Abstract…………………………………………………………………………………..v
Table of Contents……………………………………………………………………….vi
List of Figures……………………………………………………………………….…ix
List of Tables……………………………………………………………………..……xii
List of Appendices…………………………………………………………………………………….…..xiii
Abbreviations…………………………………………………………………………..xiv
1.0 INTRODUCTION…………………………………………………………………..1
1.1 Background……………………………………………………………………….1
1.2 Statement of Problem…………………………………………………………….3
1.3 Motivation…………………………………………………………………………4
1.4 Aim and Objectives………………………………………………………………5
1.5 Methodology………………………………………………………………………5
1.6 Significance of Study.……..………………………………………………………7
2.0 LITERATURE REVIEW……….……………………………………..……….…8
2.1 Introduction………………………………………………………………………….8
2.2 Review of fundamental Concepts………………………………………………….8
2.2.1 Power System Stability……………………………………………………………8
2.2.2 Rotor Angle Stability……………………………………………………………..9
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2.2.3 Transient Stability……………………………….……………………………….11
2.2.4 UPFC Principle of Operation……………………………………………………15
2.3 Review of Similar Works…………………………………………………………19
3.0 MATERIALS AND METHODS…………………………………………………25
3.1 Introduction……………………………………………………………………….25
3.2 Overview of Simulation Package (PSS/E)………………………………………25
3.3 Power Flow………………………………………………………………………..26
3.4 Power System Dynamic Equivalent..…………………………………………….27
3.4.1 Coherency-based Equivalent Method ………………..…………………………28
3.4.2 Dynamic Reduction Implementation…………………………………………….33
3.5 Multi-Machine Power System Model ………………………..………………….36
3.5.1 Mathematical Modelling…………………………………………………………37
3.6 Transient Stability Studies……………………………………………………….41
3.6.1 UPFC Model Implementation in PSS/E…………………………………………42
4.0 RESULTS AND ANALYSIS…………………………………………………….44
4.1 Introduction………………………………..………………………………………44
4.2 Network Reduction Results Evaluation….………………………………………44
4.3 Transient Stability Results and Analysis………………………………………..47
4.3.1 Steady State Performance of UPFC……………………………………………..47
4.3.2 Fault on Egbin Generator terminal………………………………………………48
4.3.3 Fault on Ikeja-West Bus…………………………………………………………52
4.3.4 Fault on Benin Bus………………………………………………………………55
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5.0 SUMMARY, CONCLUSIONS AND RECCOMMENDATIONS…………….59
5.1 Summary…………………………………………………………………………..59
5.2 Limitations of Study…………..………………………………………………….59
5.3 Conclusions………………………………………………………………………..59
5.4 Recommendations…………………………………………………………………61
REFERENCES.………………………………………………………………………62
APPENDICES……………………………………………………………….………..64
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CHAPTER ONE
INTRODUCTION
1.1 Background
Deregulation of power system around the world has brought to the forefront the issue
of power system stability. The stability issue stems from new regulatory requirements,
economic/environmental factors, and increase demand without a corresponding
increase in generation and transmission line reinforcement. All these results in power
systems being stressed beyond the capacity they were originally built to handle.
Considering that some generators are far from the load centres, the problem of transient
stability following a major disturbance will be a threat to the security of supply and
utility operators will find it critical to the daily operation of power systems. Power
system stability is the ability of synchronous machine to remain in synchronism with
one another after disturbances at various locations in the system. It can be broadly
divided into Steady-State or Small Signal Stability and Rotor Angle Transient Stability
(Padiyar, 2008). The former is the stability of the system under conditions of gradual or
relatively slow change in load while the latter refers to the maximum power transfer
possible through a point without losing stability with sudden and large changes in the
network conditions such as 3-phase bolted faults, or sudden loss of large
loads/generating unit(s). Stressed power systems are known to exhibit nonlinear
behaviour and the interactions among power systems components results in various
modes of oscillations. These oscillations if not properly damped, may be sustained for
several minutes affecting power flows and may even increase to cause loss of
synchronism between systems and ultimately lead to total or partial system outage.
The system dynamic response to disturbances and the risk of losing stability can be
reduced by additional element inserted into the system. Options involve connecting a
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Braking Resistor (BR) at the generator or substation terminals. This braking of the
accelerating rotor angle influences the rotor motion during disturbances. The switching
of the BR is made either trough a mechanical circuit breaker or with a power electronic
based devices. Also, Power system stabilizers (PSS) have been one of the fore-most
measures used to enhance the damping of power swings. PSS is a device which
provides additional control action via the automatic voltage regulator (AVR) system
loop. While conventional PSS can be considered an economical option to add damping
on critical electromechanical modes it might not be adequate to provide sufficient
damping of power swings where the transmission line loading over long distances is
quiet high (Kazemi and Mahamnia, 2008). An alternative to PSS are the Flexible AC
Transmission System (FACTS) controllers which can be designed to use a variety of
control signals and, in principle, can be placed at any location in the transmission
system to achieve the best possible damping. Advancement in power electronic devices
for high power application indicates that they will continue to find application in
electric power transmission and distribution systems. Power electronics provide
increased control, and as a result, operation of existing transmission and distribution
line close to their thermal limits. FACTS controllers can be series or shunt connected;
the Static Synchronous Series Compensator (SSSC) which can be defined as a static
synchronous generator is connected in series with a network and acts as a series
compensator whose output voltage is fully controllable, independent of line current,
with the aim of increasing or decreasing the voltage drop across the line, therefore
controlling the power flow.
A Static Var Compensator (SVC) is a shunt connected static var generator/load whose
output can be adjusted to exchange capacitive or inductive current with a network
where it is located. Primarily SVC is used to improve voltage stability. However,
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supplementary control can also provide oscillation damping. Another shunt connected
device is the STATic COMpensator (STATCOM) which in many respects resembles a
synchronous generator. It acts as a controllable shunt capacitive or inductive reactance,
thus allowing regulation of network voltage which is its main function. A
supplementary feedback controller can be added in order to enhance damping of
system’s oscillations. in addition, the Unified Power Flow Controller (UPFC) is another
FACTS device which is, technically, the most versatile and effective because, it
combines the functionality of a series (SSSC) and shunt (STATCOM) connected device
and has the capability of improving transient stability of a power system by controlling
the power flow through a transmission line via the series and shunt parameters, hence
its choice for this research works.
1.2 Statement of Problem
The Nigerian 330kV transmission network is a very large (but essentially radial)
network as shown in Figure 1.1. A section of the network involving the generator at
Egbin and the buses in the south west is used for this study. The choice of the study
area is based on the fact that Egbin thermal station is the single largest installed
electricity generation plant in Nigeria with an installed capacity of 1320 MW and the
majority of loads are concentrated in the southwest. Problems like sudden and large
changes in the network conditions such as 3-phase bolted faults, or sudden loss of
loads/generator unit on this section of the network, would weaken the system security.
Hence, the choice to study the response of the network to one of such problems while
incorporating a reported corrective device (i.e. UPFC) that is capable of appropriately
mitigating such problems.
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Figure 1.1: The Nigerian 330kV Transmission Network Showing the Area of Interest
1.3 Motivation
Lack of sufficient power to meet immediate and future demand is a problem in some
countries (Nigeria inclusive). Consequently there is the need to operate existing
infrastructure efficiently. As the power generation is stepped up, most of the
transmission corridors are pushed to their operating limits and with delay in building
new transmission lines and the associated cost, one problem that arise is the stability of
the network. Thus the present facilities on ground have to be optimised for best
performance. With the current state of the Nigerian power network where the
generation is inadequate to meet the load demand, security of the existing system is
very important as the network is increasingly been stressed and vulnerable to
instability. The reported stability enhancement capability of UPFC and its application
to a multi-machine power system coupled with the state of the study system is the
motivation for this study.
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1.4 Aim and Objectives
The aim of this project is to assess the transient stability enhancement capability of
UPFC in a multi-machine power system subjected to credible transient disturbance.
The research objectives are as follows:
i. Development of a typical multi-machine power system model
ii. Steady-state power flow study of the system leading to network reduction on
the study network by retaining the area of interest.
iii. Integration of steady-state and dynamic model of the UPFC in the power flow
and dynamic simulation studies.
iv. Transient stability analysis on the reduced multi-machine power system.
1.5 Methodology
The reduced Nigerian 330kV Grid is the test system for this study, and the data will be
obtained from the Power Holding Company of Nigeria (PHCN). To achieve the stated
objectives, the following research tasks were carried out:
I. Given that the transient stability analysis was conducted on the area including
the generator at Egbin and the buses in the south west, the impact of a
disturbance in this area on the rest of the network is not of immediate concern
thus a network reduction is performed. The network was divided into two
sections. An internal network where the impact of the disturbance is of interest,
this section of the network is the study system and an external network to be
represented by simplified models that still retain its effect on the study system
referred to as the dynamic equivalent. This is illustrated as shown in Figure 1.2.
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Figure 1.2: Dividing the entire Power System Network into two parts
Dynamic reduction of the network used centred on the coherency-based technique and
included the following steps:
i. Identification of coherent generator groups.
ii. Aggregation of generators in the group.
iii. Reduction of the proposed network.
II. A power flow study was performed on the reduced system in order to determine
the steady state pre-disturbance condition for the system. The simulation
software Power system Simulation for Engineering (PSS/E) will be used to
model the system and it components, and then the study is carried out
subsequently.
III. Transient stability analysis was conducted on the resulting multi-machine
system. The analysis is involved simulating a disturbance on the test system
without the UPFC in operation to observe the system behaviour during
transients. The UPFC model developed using Python programming is
subsequently activated in order to mitigate the resulting disturbances. The
system response to the device is examined.
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IV. Finally, simulation results produced by the simulation software are presented in
order to observe the enhancement capability of the UPFC during this transient
condition.
1.6 Significance of Study
The requirement for full control of transmitted power in a deregulated electrical power
market is imperative for system operators. This is necessitated because of the increased
and variable nature of the loads which usually exhibit non-linear behaviour. One of the
effects of this non-linear behaviour is the stability of the network following a severe
disturbance. Traditional means of ensuring maximum power transfer e.g. PSS BR etc
are increasingly not sufficient for modern power systems during contingencies and
FACTS devices have been reported to provide system operators with increased
flexibility of control of network parameters to ensure more power is delivered to their
consumers. This thesis has investigated the use of UPFC in improving transient
stability of a reduced Nigerian 330kV transmission network. The results obtained
could assist the system operator in future planning and daily operation of the network.
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