VOLTAGE STABILITY IMPROVEMENT USING STATIC VAR COMPENSATOR IN POWER SYSTEM

BY

EMEZUE EMMANUEL C.

12H/0018/EE

A PROJECT RESEARCH SUBMITTED TO THE DEPARTMENT OF ELECTRICAL/ ELECTRONIC SCHOOL OF ENGINEERING TECHNOLOGY, FEDERAL POLYTECHNIC

OWERRI IMO STATE.

IN PARTIAL FULFILLMENT FOR THE REQUIREMENT FOR THE AWARD OF HIGHER NATIONAL DIPLOMA (HND) IN ELECTRICAL AND ELECTRONIC

ENGINEERING.

OCTOBER 2014

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CERTIFICATION PAGE

This is to certify that is work Voltage Stability Improvement using Static Var Compensator in Power System was carried out by EMEZUE EMMANUEL .C. With the Reg. No: 12H/0018/EE in the Department of Electrical-Electronic Engineering, under the supervision.

……………………………………….. ……………………Engr. U.S Osondu Date(Supervisor)

……………………………………….. ..……………..Engr. M.U Anyaehie Date(Co-coordinator)

………………………………………….. ………………Engr. (Lady) M.I Aririguzo Date(H.O.D)…………………………………………….. ……………..External Examiner Date

TABLE OF CONTENTTITLE PAGE

CERTIFICATION PAGE i

ABSTRACT ii

TABLE OF CONTENT iii

CHAPTER ONE

1.0 INTRODUCTION ………………………………….……………………11.1 STATIC VAR COMPENSATOR…………………………………………..3

CHAPTER TWO

2.0 LITERATURE REVIEW………………………………………………………42.1 SYSTEM MODEL……………………………………………………………….4

CHAPTER THREE

3.0 METHODOLOGY……………………………………………………………….73.1 MODEL OF SVC WITH PSCAD PRESENTATION………………73.2 SIMULATION AND RESULT……………………………………………..10

CHAPTER FOUR

4.0 CONCLUSION………………………………………………………………..12

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REFERENCE …………………………………………………………………………..13

ABSTRACT

This paper investigates the effects of static VAR compensator (SVC) on voltage stability of a power system. The functional structure for svc built with a Thruster controlled reactor (TCR), and its model are described. The model is based on representing the controller as variable impedance that changes with the firing angle of the TCR. A power system computer aided Design/Electromagnetic Transients including DC (PSCAD/EMTDC) is used to carry out simulations of the system under study and detailed results are shown to access the performance of svc on the voltage stability of the system.

CHAPTER ONE

1.0 INTRODUCTION

Dynamic voltage support and reactive power compensation have been long recognized as a very significant measure to improve the performance of electric power systems. The rapid advances in power electronics area have made it both practical and economic to design powerful thyristor controlled reactive power compensation devices, Static Var Compensators (SVC) [ 13. Both theoretical analysis and field tests have proved the excellent SVC performances.

Power systems suffer greatly from voltage instability especially due to excessive consumption or injection of reactive power by the system elements and the consumer’s load.

The voltage instability caused by the variation in the reactive power requirement of the system’s elements and the consumer’s load either result in excessive high or low voltage which may cause damage to the system and the consumer’s load since the system elements and the consumer’s load are designed to operate within a specific voltage goes high if there is excessive injection of reactive power by the system elements or the consumer’s load, but goes low if there is excessive consumption of reactive power by the system elements or the consumer’s load. As a result the system the system’s reactive power needs to be congruously adjusted through effective reactive power compensation of the variation in the system must be kept within the allowance range. To achieve this several methods have been used, like the switching in/out the shunt reactive, series compensation capacitor, synchronous generator etc.

With these methods the desired objectives were at effectively achieved with wear and tear in the mechanical components and slow response being the major problems. Research work were carried out leading to the discovery of FACTS devices which have been mainly used in solving power system steady state control these FACTS devices

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include, static VAR comparator (SVC), Thyristor controlled series capacitor (TCSD) static synchronous compensator (STATCOM) etc.

Today’s changing electric power systems create a growing need for flexibility, reliability, fast response and accuracy in the fields of electric power generation, transmission, distribution and consumption. Flexible Alternating Current Transmissions Systems (FACTS) are new devices emanating from recent innovative technologies that are capable of altering voltage, phase angle and/or impedance at particular points in power systems. Their fast response offers a high potential for power system stability enhancement apart from steady-state flow control.

Among the FACTS controllers, Static Var Compensator (SVC) provides fast acting dynamic reactive compensation for voltage support during contingency events which would otherwise depress the voltage for a significant length of time.

The primary purpose of SVC applications is to maintain bus voltage at or near a constant level. In addition SVC may improve transient stability by dynamically supporting the voltage at key points and steady state stability by helping to increase swing oscillation damping. The theory of transient stability improvement by SVC is now well understood in light of the equal area criterion. As for the damping improvement, although computer simulations and field test results have shown the improvement [2,3,4], the author feels that some fun damental issues still need to be discussed in order to explain why power system damping can be improved by SVC applications and how. This paper attempts to address these issues by using the well-known equal area criterion.

It is found that a bus voltage controlled SVC does not contribute significantly to system damping. A significant contribution to system damping can be achieved when an SVC is controlled by some auxiliary signals superimposed over its voltage control loop[2,3]. Usually SVC control systems are designed to have a voltage control loop with continuous auxiliary damping control signal superimposed. It was found in [2] that while the voltage regulation with continuous damping control signal design has good performances when system oscillations are small in magnitude, it fails in providing damping in some critical large oscillation cases. It has been suggested that discontinuous "bang-bang" type of controls should be used for damping large oscillations. In the application of SVC to a power system its contribution to system damping depends on SVC location in the system. In the best SVC location to achieve the maximum damping improvement in a two-area power system was studied by numerical methods. It is found that the mid-point of the transmission circuit is the best SVC location

SVC also dampens power swings and reduces system losses by optimized reactive power control. Power System Computer Aided Design/Electromagnetic Transient’s Direct Current (PSCAD/EMTDC) has been used in this paper to conduct simulations on voltage regulation at the point of connection of SVC to the system.

However, the aim of this paper is to enhance voltage stability using static Var compensator at the event of occurrence of fault in the system.

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1.1 SIGNIFICANCE OF THE PROJECT

The main advantage of SVCs over simple mechanically switched compensation schemes is their near-instantaneous response to changes in the system voltage. For this reason they are often operated at close to their zero-point in order to maximise the reactive power correction they can rapidly provide when required.

They are, in general, cheaper, higher-capacity, faster and more reliable than dynamic compensation schemes such as synchronous condensers. However, static VAR compensators are more expensive than mechanically switched capacitors, so many system operators use a combination of the two technologies (sometimes in the same installation), using the static VAR compensator to provide support for fast changes

1.2 CAUSES OF UNDERVOLTAGE AND OVERVOLTAGE

OVERVOLTAGE: The cause of over-voltage on a power system may be broadly divided into two major categories viz.

Internal causes External causesINTERNAL CAUSES: internal cause of over-voltage on the power system is primarily due to oscillations set up by the sudden changes in the circuit conditions. These circuit changes may be a normal switching operation such as opening of circuit breaker or it may be fault condition such as grounding of a line conductor.

Internal causes do not produce surges of large magnitude. Experience shows that sages due to internal causes are generally taken care of by providing proper insulation to the equipment in the power system.

Internal causes of over-voltage on a power system include the following:

Switching surges Insulation failure Arcing ground Resonance

Switching surges: The over-voltage produced on the power system due to switching operation is known as switching surges. The following cause can set up over-voltages:i) case of an open lineii) case of a loaded line iii) current chopping

Insulation failure: The most common of insulation failure in the system is the grounding of conductor i.e. insulation failure between line and earth. Below illustrates this:-

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Fig.1:

Suppose a line at potential E is earthed at point x. the earthing of the line causes two equal voltages of –E to travel along XO and XP containing currents –E/Zn and +E/Zn respectively. Both these currents pass through X to earth so that current to earth is 2E/Zn (where Zn is the natural impendence given by Zn = √L/C).

Arcing ground: this is the phenomenon of intermittent arc taking place in line –to-ground fault of a 3-ø system with consequent production of transients. These transients are cumulative and may cause serious damage to the equipment in the power system by causing breakdown of insulation.

Resonance: under resonance, the impedance of the circuit is equal to the inductance of the circuit and the power factor is unity. Resonance causes high voltages in the electrical system.EXTERNAL CAUSE: External causes are mainly surges due to lightening which are always very severe and may increase the system voltage to several the normal values. Over-voltage due to lightering is known as voltage spike.

UNDERVOLTAGE: This is generally chronic problem caused by a number of factors beyond the end user’s control. These factors which can lead to under voltage may include. Weather, high demand and others, distribution characteristics of the system can also contribute to the chronically low voltage situations. For example customers at the end of a long transmission line may be subject to a permanent voltage drop due to line loses on top of the utility voltage variations. Even under ideal conditions, most customers will see a drop in utility voltage levels to over the course of the day as demand begins to increase around 8.00am and peaks around 3.00pm to 4.00pm.

1.3 THE STATIC VAR COMPENSATOR

A static VAR compensator is a set of electrical devices for providing fast-acting reactive power on high-voltage electricity transmission networks.[1][2] SVCs are part of the Flexible AC transmission system [3][4] device family, regulating voltage, power factor, harmonics and stabilizing the system. Unlike a synchronous condenser which is

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(-E,E/Zn) X (-E,-E/Zn)

P Q

a rotating electrical machine, a static VAR compensator has no significant moving parts (other than internal switchgear). Prior to the invention of the SVC, power factor compensation was the preserve of large rotating machines such as synchronous condensers or switched capacitor banks.[5]

The SVC is an automated impedance matching device, designed to bring the system closer to unity power factor. SVCs are used in two main situations:

Connected to the power system, to regulate the transmission voltage ("Transmission SVC")

Connected near large industrial loads, to improve power quality ("Industrial SVC")

In transmission applications, the SVC is used to regulate the grid voltage. If the power system's reactive load is capacitive (leading), the SVC will use thyristor controlled reactors to consume VARs from the system, lowering the system voltage. Under inductive (lagging) conditions, the capacitor banks are automatically switched in, thus providing a higher system voltage. By connecting the thyristor-controlled reactor, which is continuously variable, along with a capacitor bank step, the net result is continuously variable leading or lagging power.

In industrial applications, SVCs are typically placed near high and rapidly varying loads, such as arc furnaces, where they can smooth flicker voltage.[1][6]

Static Var Compensators are shunt connected static generators/absorbers whose outputs are varied so as to control the voltage of the electric power system.

They are applied by utilities in transmission application for the several purposes. The primary purpose is usually for rapid control of voltage at weak points in a network. Installations may be at the midpoint of transmission interconnections or at the line ends.

In its simple form, SVC is connected as Thyristor Controlled Reactor Fixed Capacitor (TCR-FC) configuration values, and capacitors (for harmonic filtering through tuning).

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CHAPTER TWO

2.0 LITERATURE REVIEW

2.1 SYSTEM MODEL

Studies have being performed on a single machine connected to a constant voltage bus through two transformers Z1 and Z4 and a transmission line divided equally into two sections Z2 and Z3 as shown in Fig below. An SVC device is connected at the middle bus. An SVC device is connected at the middle bus. The SVC is a combination of the reactor and capacitors. It can be controlled quickly by thyristors switching. The SVC acts as a variable susceptance. The main inputs to the SVC controller are the reference susceptance and a fuzzy logic controller. The fuzzy logic controller is adapted using the identified model.

Fig. 2 Test Power System To Analyzing Svc For Transient Stability.

Static Var Compensator (Svc) Description And Modelling: the SVC uses conventional thyristors to archive fast control of shunt – connected capacitors and reactors which basically consist of Constance capacitors (C) and a thyristor control reactor (L). The delay angle control of the thyristor bank determines the equivalent shunt admit ant presented to the power system (Zhang, 2003a, b),

New version of SVC is basically a shunt connected static var generator /load whose output is adjusted to exchange capacitive or inductive current so as to maintain or control special power system variables; typically, the controlled variables is the SVC bus voltage (Stagg and El- Abiad, 2002; saadat, 2002). One of the major reasons for installing a SVC is to improve Dynamic voltage control and thus increase system load ability. An additional stabilizing signal, and supplementary control, super impose on the voltage control loop of a SVC can provide damping of system oscillation as discussed. In this paper, the SVC is basically represented by a variable reactance with

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maximum inductive and capacitive limits to control block and signals to damp oscillation.

The model considers SVC as shunt- connected variable susceptance, Basic which is adapted automatically to achieve the voltage control. The equivalent susceptance, Beq is determined by the firing angle’’ of the thyristors that is instant the fundamental frequency equivalent neglecting harmonics of the current results (Gyugyi, 1988; Carsten, 2002):

As the reactive power demand at the bus varies, the susceptance is varied subject to the limits. However, the reactive power is a function of the square of the bus voltage. Hence the reactive power generated decreases as the voltage decreases

CHAPTER THREE

3.0 METHODOLOGY

3.1 MODEL OF SVC WITH PSCAD REPRESENTATION OF ITS CONTROL

In order to investigate the impact of SVC on power systems, appropriate SVC model is very important. In this section, SVC and its mathematical model will be introduced. SVC is built up with reactors and capacitors, controlled by thruster valves which are in parallel with a fixed capacitor bank. It is connected in shunt with the transmission line through a shunt transformer and thus, represented in figure 1 (1). Figure 2 shows the equivalent circuit at which svc is modeled.

3.1.1 FUNCTIONAL AND EQUIVALENT CIRCUIT DIAGRAM OF A TYPICAL SVC

Fig.3: Functional diagram of a TCR-FC SVC

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Fig.4: Equivalent circuit of the SVC

The model considers SVC as shunt-connected variable susceptance, Bsvc which is adapted automatically to achieve the voltage control. The equivalent susceptance, Beq

is determined by the firing angle α of the thyristors that is defined as the delay angle measured from the peak of the capacitor voltage to the firing instant. The fundamental frequency equivalent neglecting harmonics of the current results in (4);

Beq = BL (α) +BC

Where

BL (α) = - 1 -2α – sin (α), BC = ώC and 0O < α <

ώL Ω Ω

If the real power consumed by the SVC is assumed to be zero, them;

PSVC = O

QSVC = - V2BSVC

Where V is the bus voltage magnitude;

As the reactive power demand at the bus varies, the susceptance is varied subject to the limits. However, the reactive power is a function of the square of the bus voltage. Hence the reactive power generated decreases as the voltage decreases.

The SVC can both absorb as well as supply reactive power at the bus it is connected to by control of the firing angle of the thyristor elements. By controlling the firing angle α of the thyristors (i.e. the angle with respect to the zero crossing of the phase voltage), the device is able to control the voltage magnitude. Changes in α results in changes on the current and hence, the amount of reactive power consumed

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by the inductor. When α = 90o, the inductor is fully activated but is deactivated when α = 180o. Actually, the basic control strategy is typically to keep the transmission bus voltage within certain narrow limits defined by a controller droop and the firing angle α limits (90o < α> 180o).

Studied system

Fig 4. A 230kv power system with svc connected at bus 3

Figure 4 shows a 230KV power network to which SVC is connected as bus 3. The power system comprises a generator generating 16KV which is stepped up by a transformer to 230KV. This voltage is transmitted via the transmission line to bus 3. The models of the generator and the SVC as well as the transformer parameters are the IEEE standard models of PSCAD/EMTDC (5)

3.2 SIMULATION AND RESULTS

The system under study is carried out in PSCAD/EMTDC when the generator is operating at its rated power level. The voltage at bus 3 having a load of 4MW and 3MVar is found approximately to be 230KV. But, when the load is increased to 150MW and 30MVar, the voltage dropped to 229.151KV. So, further increase in load resulted in the corresponding voltage drop at bus 3 as is shown in Table 1.

However, with SVC connected to bus 3, the voltage required (230KV) is maintained even at the increase of load. It is observed that the voltage magnitude at bus 3 is maintained averagely to 230.65KV during load variation. When SVC is unconnected, the firing angle is 180o showing that the inductor is deactivated. However, the inductor is activated when SVC is connected and the firing angle is 116.9o. Hence optimizing the reactive power and improving the voltage profile (See figure 5-6).

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It should be noted here that changes in firing angle result on changes in the current, and hence the amount of reactive power consumed by the inductor. As the load increases, the firing angle decreases and hence more amount of reactive power is consumed by the inductor and vice versa.

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CHAPTER FOUR

4.0 PRINCIPLE OF OPERATION

Typically, an SVC comprises one or more banks of fixed or switched shunt capacitors or reactors, of which at least one bank is switched by thyristors. Elements which may be used to make an SVC typically include:

Thyristor controlled reactor (TCR), where the reactor may be air- or iron-cored

Thyristor switched capacitor (TSC)

Harmonic filter(s)

Mechanically switched capacitors or reactors (switched by a circuit breaker)

4.1 ONE-LINE DIAGRAM OF A TYPICAL STATIC VAR COMPENSATOR

Fig 5. One-line diagram of a typical SVC configuration; here employing a thyristor controlled reactor, a thyristor switched capacitor, a harmonic filter, a mechanically switched capacitor and a mechanically switched reactor.

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By means of phase angle modulation switched by the thyristors, the reactor may be variably switched into the circuit and so provide a continuously variable MVAR injection (or absorption) to the electrical network.[2] In this configuration, coarse voltage control is provided by the capacitors; the thyristor-controlled reactor is to provide smooth control. Smoother control and more flexibility can be provided with thyristor-controlled capacitor switching.

Fig.6 Thyristor Controlled Reactor(TCR), shown with Delta connection

Fig.7 Thyristor Switched Capacitor (TSC), shown with Delta connection

The thyristors are electronically controlled. Thyristors, like all semiconductors, generate heat and deionized water is commonly used to cool them.[5] Chopping reactive load into the circuit in this manner injects undesirable odd-order harmonics and so banks of high-power filters are usually provided to smooth the waveform. Since the filters themselves are capacitive, they also export MVARs to the power system.

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More complex arrangements are practical where precise voltage regulation is required. Voltage regulation is provided by means of a closed-loop controller.[7] Remote supervisory control and manual adjustment of the voltage set-point are also common

4.2 ADVANTAGES OF SVCs OVER COMPENSATION SCHEMES

The main advantage of SVCs over simple mechanically switched compensation schemes is their near-instantaneous response to changes in the system voltage. For this reason they are often operated at close to their zero-point in order to maximise the reactive power correction they can rapidly provide when required.

They are, in general, cheaper, higher-capacity, faster and more reliable than dynamic compensation schemes such as synchronous condensers.[7] However, static VAR compensators are more expensive than mechanically switched capacitors, so many system operators use a combination of the two technologies (sometimes in the same installation), using the static VAR compensator to provide support for fast changes and the mechanically switched capacitors to provide steady-state VARs.

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CHAPTER FIVE

5.0 CONCLUSION

In this paper, the basic structure of an SVC operating under typical bus voltage control and its model are described. The model; is based on representing the controller as a variable impedance that changes with the firing angle of the Thyristor Controlled Reactor (TCR), which is used to control voltage in the

System. Simulations carried out confirmed that static Var compensator could provide the fast acting voltage support necessary to prevent the possibility of voltage reduction and voltage collapse at the bus to which it is connected.

In this study, the effectiveness of static Var compensator (SVC) has been studied in improving the transient stability of a sample two-area power system with various and different studied such as investigation the response of SVC to transient phenomena due to various faults such as single line to ground-line to line fault, line to line to ground fault and finally the three phase to ground faults are investigated.

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