Page | 1 CHAPTER 1 INTRODUCTION The increasing number of power system blackouts in many countries in recent years, is a major source of concern. Power engineers are very interested in preventing blackouts and ensuring that a constant and reliable electricity supply is available to all customers. Incipient voltage instability, which may result from continues load growth or system contingencies, is essentially a local phenomenon. However, sequences of events accompanying voltage instability may have disastrous effects, including a resultant low-voltage profile in a significant area of the power network, known as the voltage collapse phenomenon. Severe instances of voltage collapse, including the August 2003 blackout in North - Eastern U.S.A and Canada, have highlighted the importance of constantly maintaining an acceptable level of voltage stability. The design and analysis of accurate methods to evaluate the voltage stability of a power system and predict incipient voltage instability, are therefore of special interest in the field of power system protection and planning. In planning and operating power systems, the analysis of voltage stability for a given system state involves the examination of two aspects: a) Proximity: how close is the system to voltage instability? Distance to instability may be measured in terms of physical quantities, such as load level, active power flow through a critical interface and reactive power reserve. b) Mechanism: how and when voltage instability occurs, what are the key contributing factors, what are the voltage-weak points, and what areas arc involved? What measures are most effective in improving voltage stability? Proximity gives a measure of voltage security whereas mechanism provides information useful in determining system modifications or operating strategies which could be used to prevent voltage instability.
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88481823 Improving Voltage Stability in Power Systems Using Modal Analysis
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Page | 1
CHAPTER 1
INTRODUCTION
The increasing number of power system blackouts in many countries in recent years, is a
major source of concern. Power engineers are very interested in preventing blackouts and
ensuring that a constant and reliable electricity supply is available to all customers. Incipient
voltage instability, which may result from continues load growth or system contingencies, is
essentially a local phenomenon. However, sequences of events accompanying voltage
instability may have disastrous effects, including a resultant low-voltage profile in a
significant area of the power network, known as the voltage collapse phenomenon. Severe
instances of voltage collapse, including the August 2003 blackout in North - Eastern U.S.A
and Canada, have highlighted the importance of constantly maintaining an acceptable level
of voltage stability. The design and analysis of accurate methods to evaluate the voltage
stability of a power system and predict incipient voltage instability, are therefore of special
interest in the field of power system protection and planning. In planning and operating
power systems, the analysis of voltage stability for a given system state involves the
examination of two aspects:
a) Proximity: how close is the system to voltage instability?
Distance to instability may be measured in terms of physical quantities, such as load level,
active power flow through a critical interface and reactive power reserve.
b) Mechanism: how and when voltage instability occurs, what are the key contributing
factors, what are the voltage-weak points, and what areas arc involved? What measures are
most effective in improving voltage stability?
Proximity gives a measure of voltage security whereas mechanism provides information
useful in determining system modifications or operating strategies which could be used to
prevent voltage instability.
[2]
CHAPTER 2
VOLTAGE STABILITY
The voltage stability of a power system refers to its ability to properly maintain steady,
acceptable voltage levels at all buses in the network at all times, even after being subjected to
a disturbance or contingency. A power system may enter a condition of voltage instability
when the system is subjected to a steady increase in load demand or a change in operating
conditions, or a disturbance (loss of generation in an area, loss of major transformer or major
transmission line). This causes an increased demand in reactive power. Voltage instability is
characterized by gradually decreasing voltage levels at one or more nodes in the power
system. Both static and dynamic approaches are used to analyze the problem of voltage
stability. Dynamic analysis provides the most accurate indication of the time responses of the
system.
Voltage stability is indeed a dynamic phenomenon and can be studied using extended
transient/midterm stability simulations. However, such simulations do not readily provide
sensitivity information or the degree of stability. They are time consuming in terms of CPU
and engineering required for analysis of results. Therefore, the application of dynamic
simulations is limited to investigation of specific voltage collapse situations, including fast or
transient voltage collapse and for coordination of protection and controls. Voltage stability
analysis often requires examination of a wide range of system conditions and a large number
of contingency scenarios. For such applications, the approach based on steady state analysis
is more attractive and if used properly, can provide much insight into the voltage/reactive
power problem.
2.1 Reasons of Voltage Collapse
Voltage collapse is a process in which, the appearance of sequential events together with the
voltage instability in a large area of system can lead to the case of unacceptable low voltage
[3]
condition in the network, if no preventive action is committed. Occurrence of a disturbance
or load increasing can leads to excessive demand of reactive power. Therefore, system will
show voltage instability. If additional resources provide sufficient reactive power support, the
system will be established in a stable voltage level. However, sometimes there are not
sufficient reactive power resources and the excessive demand of reactive power can leads to
voltage collapse.
Voltage collapse can be initiated due to small changes of system conditions (e.g. load
increasing) as well as large disturbances (e.g. line outage or generation unit outage). Under
these conditions, shunt FACTS devices such as SVC and STATCOM can improve the
system security with fast and controlled injection of reactive power to the system. However,
when the voltage collapse is due to excessive load increasing, FACTS devices cannot prevent
the voltage collapse and only postpone it until they reach to their maximum limits. Under
these situations, the only way to prevent the voltage collapse is load curtailment or load
shedding. So, reactive power control using FACTS devices is more effective in large
disturbances and contingencies should be considered in voltage stability analysis.
So the principle causes of voltage instability are:
The load on the transmission lines is too high.
The voltage source is too far from the load centre.
The source voltages are very low.
There is insufficient load reactive compensation.
2.2 Analysis and Methods of Prevention of Voltage Instability
A number of special algorithms have been proposed in the literature for voltage stability
analysis using the static approach. In general, these have not found widespread practical
application and utilities tend to depend largely on conventional power flow programs to
determine voltage collapse levels of various points in a network. However, this approach is
laborious and does not provide sensitivity information useful in making design decisions.
[4]
Some utilities use Q-V curves at a small number of load buses to determine the proximity to
voltage collapse and to establish system design criteria based on Q and V margins
determined from the curves. One problem with the Q-V curve method is that it is generally
not known apriori at which buses the curves should be generated. In producing a Q-V curve,
the system in the neighborhood of the bus is unduly stressed and results may be misleading.
In addition, by focusing on a small number of buses, system-wide problems may not be
readily recognized.
An approach using V-Q sensitivity and piecewise linear power flow analysis to find the
margin, measured in terms of total load growth, between a given operating condition and the
voltage collapse point is already described. There has been some indication that the linear
power flow solution may not be sufficiently accurate as the collapse point is approached.
Also, V-Q sensitivity information could be misleading when applied to a large system having
more than one area with voltage stability problems.
Most of the approaches proposed to date use conventional power flow models to represent
the system steady state. This may not always be appropriate, especially as the system
approaches critical condition. There is a need to consider more detailed steady state models
for key system components such as generators, SVCs, induction motors and voltage
dependent static loads. Load characteristics in particular could be critical and expanded sub-
transmission representation in the voltage collapse areas may be necessary.
There is a need for analytical tools capable of predicting voltage collapse in complex
networks, accurately quantifying stability margins and power transfer limits, identifying
voltage-weak points and areas susceptible to voltage instability, and identifying key
contributing factors and sensitivities that provide insight into system characteristics to assist
in developing remedial actions.
Modal analysis approach with the objective of meeting the above requirements is used
instead of the conventional methods. It involves the computation of a small number of
eigenvalues and the associated eigenvectors of a reduced Jacobian matrix which retains the
[5]
Q-V relationships in the network. However, by using the reduced Jacobian instead of the
system state matrix, the focus is on voltage and reactive power characteristics. The
eigenvalues of the Jacobian identify different modes through which the system could become
voltage unstable. The magnitude of the eigenvalues provides a relative measure of proximity
to instability. The eigenvectors, on the other hand, provide information related to the
mechanism of loss of voltage stability. Fast analytical algorithms for selective computation
of a specified number of the smallest eigenvalues make the approach suitable for the analysis
of large complex power systems.
2.3 Characteristics of Reactive Compensating Devices
There are different types of reactive compensating devices. How these devices influence
voltage stability are described below.
(a)Shunt capacitors
By far the most inexpensive means of providing reactive power and voltage support is the
use of shunt capacitors. They can be effectively used up to a certain point to exceed the
voltage stability limits by correcting the receiving end power factors. They can also be used
to free up “spinning reactive reserve” in generators and thereby help prevent voltage collapse
in many situations.
Shunt capacitors, however, have a number of inherent limitations from the viewpoint of
voltage stability and control:
In heavily shunt capacitor compensated systems, the voltage regulations tend to be
poor.
Beyond a certain level of compensation, stable operation is unattainable with shunt
capacitors.
The reactive power generated by shunt capacitors is proportional to the square of the
voltage; during system conditions of low voltage the var support drops, thus
compounding the problem.
[6]
(b) Regulated shunt compensation
A static var system (SVS) of finite size will regulate up to its maximum capacitive output.
There is no voltage control on instability problems within the regulating range. When pushed
to the limit, an SVS becomes a simple capacitor. The possibility of this leading to voltage
instability must be recognized.
A synchronous condenser, unlike an SVS, has an internal voltage source. It continues to
supply reactive power down to relatively low voltages and contributes to a more stable
voltage performance.
(c) Series capacitors
Series capacitors are self regulating. The reactive power supplied by series capacitors is
proportional to square of the line current and is independent of the bus voltages. This has a
favorable effect on voltage stability.
Series capacitors are ideally suited for effectively shortening long lines. Unlike shunt
capacitors, series capacitors reduce both the characteristics impendence (Zc) and the electrical
length of the line. As a result, both voltage regulation and stability are significantly
improved.
[7]
CHAPTER 3
DEFINING FACTS DEVICES
FACTS, an acronym which stands for Flexible AC Transmission System, is an evolving
technology-based solution envisioned to help the utility industry to deal with changes in the
power delivery business. The term FACTS refers to alternating current transmission systems
incorporating power electronic-based and other static controllers to enhance controllability
and increase power transfer capability. Technology concepts were conceived in the 1980’s
and projects sponsored by the Electric Power Research Institute (EPRI) demonstrated many
of these concepts with laboratory scale circuits.
The concept of Flexible AC Transmission Systems (FACTS) was first defined by Hingorani,
N.G. in 1988. Up to now, lots of advanced FACTS devices have been put forward due to the
rapid development of the modem power electronics technology. These FACTS devices have
a large potential ability to make power systems operate in a more flexible, secure and
economic way. Moreover, these FACTS devices can also make the power systems operate in
a more sophisticated way. A good coordination and adaptation is needed to fully exploit the
new characteristics of FACTS. Presently, the studies on FACTS are mainly focused on
FACTS devices developments and their impacts on the power system, such as power flow
modulation and control, transient stability enhancement, small-disturbance stability
improvement and oscillation damping. It is also significant to study the impact of the FACTS
devices on improving performance of power systems such as optimization related software
algorithms in modem Energy Management System (EMS).
3.1 Facts Controller Applications
The simplest way to identify the potential roles to be played by FACTS Controllers is to
examine their functions as they relate to conventional equipment. The definition of FACTS
systems incorporates both power electronic-based and other static controllers to enhance
[8]
controllability and increase power transfer capability. One of the system planners’ tasks is to
determine which combinations of controllers provide both the capacity to supply the reactive
power, dynamic reserve and continuous regulation needed for the application. Table 1 lists
the main functions that can be performed by FACTS Controllers and show both FACTS and
other conventional equipment that performs these functions.
Table 1- System Control Functions
Function Non FACTS Control Methods FACTS Controllers
Voltage
Control
Electric generators
Synchronous Condensers
Conventional Transformer tap-changer
Conventional Shunt Capacitor/Reactor
Conventional Series Capacitor/Reactor
Static Var Compensator (SVC)
Static Synchronous Compensator
(STATCOM)
Unified Power Flow Controller (UPFC)
Superconducting Energy Storage
(SMES)
Battery Energy Storage System (BESS)
Convertible Static Compensator (CSC)
Active
and
Reactive
Power
Flow
Control
Generator schedules
Transmission line switching
Phase Angle Regulator (PAR)
Series Capacitor (switched or fixed)
High Voltage Direct Current
Transmission (HVDC)
Interphase Power Controller (IPC)
Thyristor controlled Series Capacitor
(TCSC)
Thyristor Controlled Series Reactor
(TCSR)
Thyristor Controlled Phase Shifting
Transformer (TCPST)
UPFC
Static Synchronous Series Compensator
(SSSC)
Interline Power Flow Controller (IPFC)
[9]
Transient
Stability
Braking Resistor
Excitation Enhancement
Special Protection Systems
Independent Pole Tripping
Fast Relay Schemes
Fast Valving
Line Sectioning
HVDC
Thyristor Controlled Braking Resistor
(TCBR)
SVC, STATCOM, TCSC, TCPST,
UPFC
BESS, SMES, SSSC, CSC, IPFC
Dynamic
Stability
Power System Stabilizer
HVDC
TCSC, SVC, STATCOM, UPFC, SSSC,
TCPST, BESS, SMES, SSSC,CSC,
IPFC
Short
Circuit
Current
Limiting
Switched series reactors
Open circuit breaker arrangements
Thyristor switched series reactor, TCSC,
IPC, SSSC, UPFC; These are secondary
functions of these controllers and their
effectiveness may be limited.
3.2 Overview of Facts Controllers
The value of FACTS applications lies in the ability of the transmission system to reliably
transmit more power or to transmit power under more severe contingency conditions with the
control equipment in operation. If the value of the added power transfer over time is
compared to the purchase and operational costs of the control equipment, relatively complex
and expensive applications may be justified. Other economic considerations include the
market structure, transmission tariff and identification of winners and losers. Realization of
the value added by a proposed transmission project often requires a coordinated
implementation of conventional transmission equipment, possibly including transmission line
segments, FACTS Controllers, coordinated control algorithms and special operating
procedures.
Commonly used FACTS controllers are:
[10]
1. Static Var Compensator (SVC)
2. Static Synchronous Compensator (STATCOM)
3. Superconducting Magnetic Energy Storage (SMES)
4. Battery Energy Storage System (BESS)
5. Thyristor Controlled Series Capacitor (TCSC)
6. Static Synchronous Series Compensator (SSSC)
7. Unified Power Flow Controller (UPFC)
8. Interphase Power Controller (IPC)
3.3 Static Var Compensator (SVC)
The Static Var Compensator used for transmission system applications is a dynamic source
of leading or lagging reactive power. It is comprised of a combination of reactive branches
connected in shunt to the transmission network through a step up transformer. The SVC is
configured with the number of branches required to meet a utility specification as indicated
in Figure 3.1. This specification includes required inductive compensation and required
capacitive compensation. The sum of inductive and capacitive compensation is the dynamic
range of the SVC. One or more thyristor-controlled reactors may continuously vary reactive
absorption to regulate voltage at the high voltage bus. This variation is accomplished by
phase control of the thyristors, which results in the reactor current waveform containing
harmonic components that vary with control phase angle. A filter branch containing a power
capacitor and one or more tuning reactors or capacitors is included to absorb enough of the
harmonic currents to meet harmonic specifications and provide capacitive compensation. The
thyristor switched capacitor is switched on or off with precise timing to avoid transient inrush
currents.
[11]
Figure 3.1 Circuit diagram of a SVC containing a thyristor controlled reactor, a thyristor
switched capacitor and a double tuned filter
3.4 Static Synchronous Compensator (STATCOM)
The STATCOM shown in Figure 3.2 performs the same voltage regulation and dynamic
control functions as the SVC. However, its hardware configuration and principle of operation
are different. It uses voltage source converter technology that utilizes power electronic