complete seminar report (11eel19016_dimpal soni)pdf

8/9/2019 Complete Seminar Report (11EEL19016_DIMPAL SONI)PDF

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Seminar

On

STATIC SYNCHRONOUS COMPENSATOR (STATCOM)

SUBMITTED IN PARTIAL FULFILLMENT OFREQUIREMENT OF THE DEGREE OF

B.TechBACHELOR OF TECHNOLOGY

INELECTRICAL ENGINEERING

Supervised by Submitted byDr. G.K. Joshi Dimpal Soni

Head Enroll. No. : - 12/22526

Roll No. :-

B.Tech (Electrical Eng.)

DEPARTMENT OF ELECTRICAL ENGINEERING

M.B.M. ENGINEERING COLLAGE

JAI NARAYAN VYAS UNIVERSITYJODHPUR

2014


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ACKNOWLEDGEMENT

By the blessings of Lord Shiva, the academic ou tcome in the form of a

seminar work on STATIC SYNCHRONOUS COMPENSATOR(STATCOM)could take the shape of reality.

It is privilege for me to express my sincere gratitude towards my esteemed

guide without the support of whom, this would have been very difficult for me to

bring out this seminar in this form. I m grateful to Dr. G.K. Joshi (Head of

Department) for all the valuable guidance, constant moral encouragement

extended at his end.

I like to remember the motivation initiated by My Father Shree Brijesh

Soni and My Mother Smt. Meena Soni whose love and latent blessings are the

basis for me to bring out this seminar.

I am thankful to my friend Deepak Soni for his constant encouragement

and all those who helped me directly or indirectly in my endeavor. This

acknowledgement is intended to be a thanks giving gesture to all those people

involved directly or indirectly with my work.

Wednesday, November 13, 2013

Dimpal Soni

Enroll. No. : - 12/22526

Roll No.: -

B.Tech (Electrical Eng.)

Department of Electrical

Engineering

J.N.V. University Jodhpur


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CERTIFICATE

This is to certify that Miss Dimpal Soni, B.tech. Scholar in Electricalengineering bearing Roll no. 111103365 &Enroll. No. 12/22526 has carried outher seminar onSTATIC SYNCHRONOUS COMPENSATOR (STATCOM)

under my supervision.The work presented in this seminar has not been submitted elsewhere for

award of any other degree or diploma.

Dr. G. K. Joshi

Supervisor

HeadDeptt. Of Electrical Eng.

J.N.V. University, Jodhpur

Counter Signed by

Prof. Manoj Kumar Bhaskar

Deptt. Of Electrical Engineering

J.N.V. University, Jodhpur


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LIST OF TABLES

Table No. Particulars Page No.

1 Partial derivatives of STATCOM model 33


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LIST OF FIGURES

Fig. No. Particulars Page No.

1.1 Operational Limits of Transmission lines 2for different voltage levels

1.2 Overview of Major Facts Devices 3

1.3 SVC building blocks and voltage / current

Characteristic 6

1.4 SVC Outlook 6

1.5 SVC using a TCR and FC 7

1.6 Comparison of the loss characteristics of 7

TSCTCR, TCRFC compensators and

Synchronous condenser

1.7 SVC of combined TSC and TCR type 8

2.1 Reactive power generation by a STATCOM 10

2.2 STATCOM operating in inductive or

capacitive modes 11

2.3 Current controlled block diagram of STATCOM 11

2.4 Voltage controlled block diagram of STATCOM 12

2.5 Static Synchronous Compensator 14

2.6 Waveform for Operation of Statcom 15

2.7 Two machine system with STATCOM 15

2.8 Transmitted power versus transmission 17

angle characteristic of a STATCOM

2.9 V-I characteristic of a STATCOM 18

2.10 STATCOM structure and voltage / 19current characteristic


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2.11 6 Pulses STATCOM 20

2.12 STATCOM Equivalent Circuit 21

2.13 Substation with a STATCOM 21

3.1 TCSC Circuit and Characteristics 23

3.2 Principal configuration of DFC 24

3.3 Operational diagram of a DFC 25

3.4 Principle configuration of an UPFC 26

3.5 UPFC functional scheme 27

4.1 Thevenin Equivalent Circuit Diagram of 30STATCOM: (a) STATCOM Schematic Diagram;

(b) STATCOM Equivalent Circuit


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Chapter 1 INTRODUCTION 1-8

1.1 INTRODUCTION 1

1.2 FACTS DEVICES 11.2.1 Facts for Transmission System 2

1.3 MAJOR FACTS DEVICES 3

1.4 CONFIGURATION OF FACTS DEVICES 4

1.4.1 Shunt Devices 4

1.4.2 SVC 4

1.5 SVC USING A TCR AND AN FC 6

1.5.1 SVC of the FC/TCR type 7

1.6 SVC USING A TCR AND TSC 8

Chapter 2 STATIC SYNCHRONOUS COMPENSATOR 9-21

(STATCOM)

2.1 INTRODUCTION 9

2.2 STRUCTURE OF STATCOM 9

2.3 CONTROL OF STATCOM 10

2.3.1 Two Modes of Operation 102.3.2 Current Controlled STATCOM 11

2.3.3 Voltage Controlled STATCOM 12

2.4 BASIC CONFIGURATION AND PRINCIPLE OF OPERATION 13

2.5 CHARACTERISTICS OF STATCOM 15

2.6 STATCOM V-I CHARACTERISTIC 18

2.7 FUNCTIONAL REQUIREMENTS OF STATCOM 18

Chapter 3 OTHER SERIES AND SHUNT DEVICES 22-28


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3.1 SERIES DEVICES 22

3.2 TCSC 223.2.1 Advantages 23

3.3 DYNAMIC POWER FLOW CONTROLLER 233.3.1 (TSC / TSR) 24

3.4 UNIFIED POWER FLOW CONTROLLER 263.4.1 OPERATING PRINCIPLE OF UPFC 26

Chapter 4 STATIC SYNCHRONOUS COMPENSATOR POWER 29-33

FLOW MODEL

4.1 STATCOM POWER FLOW MODEL 294.2 LINEARISED POWER EQUATION 31

4.3 NEWTON-RAPHSON-ALGORITHM 31

Chapter 5 APPLICATIONS, CONCLUSION AND FUTURE WORK 34-35

5.1 APPLICATIONS OF STATCOM 34

5.2 SCOPE FOR FUTURE RESEARCH 34

5.3 CONCLUSION 35

References36


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CHAPTER: 1

INTRODUCTION

1.1 INTRODUCTION:-

Flexible AC transmission system (FACTS) controllers are power electronics based controllers.

With the applications of FACTS technology, bus voltage magnitude and power flow along the

transmission lines can be more flexibly controlled. Among the FACTS controllers, the most

advanced type is the controller that employs Voltage Sourced Converter (VSC) as synchronoussources. Representative of the VSC type FACTS controllers are the Static Synchronous

Compensator (STATCOM), which is a shunt type controller, the Static Series Compensator

(SSSC), which is a series type controller and the Unified Power Flow Controller (UPFC), a

combined series-shunt type controller. Of all the VSC the most widely used is the STATCOM. Itcan provide bus voltage magnitude control. Computation and control of power flow for power

systems embedded with STATCOM appear to be fundamental for power system analysis andplanning purposes. Power flow studies incorporating STATCOM requires accurate model in

solution algorithms.

There are mainly two models of STATCOM which have well tested in power systems. There are

the Current Injection Model (CIM) and the Power Injection Model (PIM). The CIM STATCOMhas a current source connected in shunt the bus for voltage magnitude control. The PIM models

the STATCOM as shunt voltage source behind an equivalent reactance or impedance, which is

also referred to as voltage source model (VSM). This steady state power injection model ofSTATCOM has proved reliable when incorporated in power systems and is well documented.

The use of this STATCOM in power system simulators has therefore increased over the last onedecade and is therefore adopted implementation in this work with the voltage expressed in

rectangular coordinate.

1.2 FACTS DEVICES:-

Flexible AC Transmission Systems, called FACTS, got in the recent years a well known termfor higher controllability in power systems by means of power electronic devices. SeveralFACTS-devices have been introduced for various applications worldwide. A number of new

types of devices are in the stage of being introduced in practice.

In most of the applications the controllability is used to avoid cost intensive or landscaperequiring extensions of power systems, for instance like upgrades or additions of substations

and power lines. FACTS-devices provide a better adaptation to varying operational conditions

and improve the usage of existing installations. The basic applications of FACTS-devices are:

Power flow control,

Increase of transmission capability,


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The left column in Figure 1.2 contains the conventional devices build out of fixed or

mechanically switch able components like resistance, inductance or capacitance together withtransformers. The FACTS-devices contain these elements as well but use additional power

electronic valves or converters to switch the elements in smaller steps or with switching

patterns within a cycle of the alternating current. The left column of FACTS-devices uses

Thyristor valves or converters. These valves or converters are well known since several years.

They have low losses because of their low switching frequency of once a cycle in theconverters or the usage of the Thyristors to simply bridge impedances in the valves.

The right column of FACTS-devices contains more advanced technology of voltage source

converters based today mainly on Insulated Gate Bipolar Transistors (IGBT) or Insulated Gate

Commutated Thyristors (IGCT). Voltage Source Converters provide a free controllable voltagein magnitude and phase due to a pulse width modulation of the IGBTs or IGCTs. High

modulation frequencies allow to get low harmonics in the output signal and even to compensate

disturbances coming from the network. The disadvantage is that with an increasing switchingfrequency, the losses are increasing as well. Therefore special designs of the converters are

required to compensate this.

1.4 CONFIGURATION OF FACTS DEVICES:

1.4.1 Shunt Devices:

The most used FACTS-device is the SVC or the version with Voltage Source Converter calledSTATCOM. These shunt devices are operating as reactive power compensators. The main

applications in transmission, distribution and industrial networks are:


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Reduction of unwanted reactive power flows and therefore reduced network losses.

Keeping of contractual power exchangeswith balanced reactive power.

Compensation of consumers and improvement of power quality especially with huge demand

fluctuations like industrial machines, metal melting plants, railway or underground trainsystems.

Compensation of Thyristor converters e.g. in conventional HVDC lines.

Improvement of static or transient stability.

Almost half of the SVC and more than half of the STATCOMs are used for industrial

applications. Industry as well as commercial and domestic groups of users require powerquality. Flickering lamps are no longer accepted, nor are interruptions of industrial processes

due to insufficient power quality. Railway or underground systems with huge load variations

require SVCs or STATCOMs.

1.4.2 SVC:

Electrical loads both generate and absorb reactive power. Since the transmitted load varies

considerably from one hour to another, the reactive power balance in a grid varies as well. The

result can be unacceptable voltage amplitude variations or even a voltage depression, at theextreme a voltage collapse.

A rapidly operating Static Var Compensator (SVC) can continuously provide the reactivepower required to control dynamic voltage oscillations under various system conditions and

thereby improve the power system transmission and distribution stability.

Applications of the SVC systems in transmission systems:

a. To increase active power transfer capacity and transient stability margin

b. To damp power oscillations

c. To achieve effective voltage control

In addition, SVCs are also used

1. in transmission systems

a. To reduce temporary over voltages

b. To damp sub synchronous resonances

c. To damp power oscillations in interconnected power systems


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2. in traction systems

a. To balance loads

b. To improve power factor

c. To improve voltage regulation

3. In HVDC systems

a. To provide reactive power to acdc converters

4. In arc furnaces

a. To reduce voltage variations and associated light flicker

Installing an SVC at one or more suitable points in the network can increase transfer capabilityand reduce losses while maintaining a smooth voltage profile under different network

conditions. In addition an SVC can mitigate active power oscillations through voltageamplitude modulation.

SVC installations consist of a number of building blocks. The most important is the Thyristor

valve, i.e. stack assemblies of series connected anti-parallel Thyristors to provide

controllability. Air core reactors and high voltage AC capacitors are the reactive power

elements used together with the Thyristor valves. The step up connection of this equipment tothe transmission voltage is achieved through a power transformer.

F ig 1.3 SVC bui lding blocks and voltage / cur rent characteri stic

In principle the SVC consists of Thyristor Switched Capacitors (TSC) and Thyristor Switchedor Controlled Reactors (TSR / TCR). The coordinated control of a combination of thesebranches varies the reactive power as shown in Figure. The first commercial SVC was installed

in 1972 for an electric arc furnace. On transmission level the first SVC was used in 1979. Since

then it is widely used and the most accepted FACTS-device.


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The main disadvantage of this configuration is the significant harmonics that will be generated

because of the partial conduction of the large reactor under normal sinusoidal steady-state

operating condition when the SVC is absorbing zero MVAr. These harmonics are filtered in thefollowing manner. Triplex harmonics are canceled by arranging the TCR and the secondary

windings of the step-down transformer in delta connection. The capacitor banks with the help

of series reactors are tuned to filter fifth, seventh, and other higher-order harmonics as a high-pass filter. Further losses are high due to the circulating current between the reactor andcapacitor banks.

Fig.1.6 Comparison of the loss character istics of TSCTCR, TCRFC compensators and

synchr onous condenser

These SVCs do not have a short-time overload capability because the reactors are usually of

theair-core type. In applications requiring overload capability, TCR must be designed for short-time overloading, or separate thyristor-switched overload reactors must be employed.

1.6 SVC USING A TCR AND TSC:-

This compensator overcomes two major shortcomings of the earlier compensators by

reducing losses under operating conditions and better performance under large systemdisturbances. In view of the smaller rating of each capacitor bank, the rating of the reactor bank

will be 1/n times the maximum output of the SVC, thus reducing the harmonics generated by

the reactor. In those situations where harmonics have to be reduced further, a small amount of

FCs tuned as filters may be connected in parallel with the TCR.


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F ig. 1.7 SVC of combined TSC and TCR type

When large disturbances occur in a power system due to load rejection, there is a

possibility for large voltage transients because of oscillatory interaction between system and the

SVC capacitor bank or the parallel. The LC circuit of the SVC in the FC compensator. In theTSCTCR scheme, due to the flexibility of rapid switching of capacitor banks without

appreciable disturbance to the power system, oscillations can be avoided, and hence the

transients in the system can also be avoided. The capital cost of this SVC is higher than that ofthe earlier one due to the increased number of capacitor switches and increased control

complexity.

CHAPTER: 2

STATIC SYNCHRONOUS COMPENSATOR (STATCOM)

2.1 INTRODUCTION:-

The STATCOM is a solid-state-based power converter version of the SVC. Operating as ashunt-connected SVC, its capacitive or inductive output currents can be controlled

independently from its terminal AC bus voltage. Because of the fast-switching characteristic of

power converters, STATCOM provides much faster response as compared to the SVC. In


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addition, in the event of a rapid change in system voltage, the capacitor voltage does not change

instantaneously; therefore, STATCOM effectively reacts for the desired responses. For

example, if the system voltage drops for any reason, there is a tendency for STATCOM toinject capacitive power to support the dipped voltages.

STATCOM is capable of high dynamic performance and its compensation does not depend onthe common coupling voltage. Therefore, STATCOM is very effective during the power

system disturbances.

Moreover, much research confirms several advantages of STATCOM. These advantages

compared to other shunt compensators include:

Size, weight, and cost reduction

Equality of lagging and leading output

Precise and continuous reactive power control with fast response

Possible active harmonic filter capability

This chapter describes the structure, basic operating principle and characteristics of

STATCOM. In addition, the concept of voltage source converters and the corresponding

control techniques are illustrated.

2.2 STRUCTURE OF STATCOM:-

Basically, STATCOM is comprised of three main parts (as seen from Figure below): a voltage

source converter (VSC), a step-up coupling transformer, and a controller. In a very-high-voltage system, the leakage inductances of the step-up power transformers can function as

coupling reactors. The main purpose of the coupling inductors is to filter out the current

harmonic components that are generated mainly by the pulsating output voltage of the powerconverters.


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F ig. 2.1 Reactive power generation by a STATCOM

2.3 CONTROL OF STATCOM

The controller of a STATCOM operates the converter in a particular way that the

phase angle between the converter voltage and the transmission line voltage is dynamically

adjusted and synchronized so that the STATCOM generates or absorbs desired VAR at thepoint of coupling connection. Figure 3.4 shows a simplified diagram of the STATCOM

with a converter voltage source __1E and a tie reactance, connected to a system with avoltage source, and a Thevenin reactance, XTIEX_THVTH.

2.3.1 Two Modes of Operation

There are two modes of operation for a STATCOM, inductive mode and the

capacitive mode. The STATCOM regards an inductive reactance connected at its terminal

when the converter voltage is higher than the transmission line voltage. Hence, from thesystems point of view, it regards the STATCOM as a capacitive reactance and the

STATCOM is considered to be operating in a capacitive mode. Similarly, when the systemvoltage is higher than the converter voltage, the system regards an inductive reactanceconnected at its terminal. Hence, the STATCOM regards the system as a capacitive

reactance and the STATCOM is considered to be operating in an inductive mode


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.

F ig. 2.2 STATCOM operati ng in inductive or capaciti ve modes

In other words, looking at the phasor diagrams on the right of Figure 3.4, when1I, the reactive

current component of the STATCOM, leads (THVE1) by 90, it is in inductive mode and

when it lags by 90, it is in capacitive mode.

This dual mode capability enables the STATCOM to provide inductive compensation as wellas capacitive compensation to a system. Inductive compensation of the STATCOM makes it

unique. This inductive compensation is to provide inductive reactance when overcompensation

due to capacitors banks occurs. This happens during the night, when a typical inductive load isabout 20% of the full load, and the capacitor banks along the transmission line provide with

excessive capacitive reactance due to the lower load. Basically the control system for a

STATCOM consists of a current control and a voltage control.

2.3.2 Current Controlled STATCOM

F ig. 2.3 Curr ent controlled block diagram of STATCOM


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Figure above shows the reactive current control block diagram of the STATCOM. An

instantaneous three-phase set of line voltages, vl, at BUS 1 is used to calculate the reference

angle, , which is phase-locked to the phase a of the line voltage, vla . An instantaneous three-phase set of measured converter currents, il, is decomposed into its real or direct component,

I1d, and reactive or quadrature component, I1q, respectively. The quadrature component is

compared with the desired reference value, I1q*

and the error is passed through an erroramplifier which produces a relative angle, , of the converter voltage with respect to thetransmission line voltage. The phase angle, 1, of the converter voltage is calculated by adding

the relative angle, , of the converter voltage and the phaselock-loop angle, . The reference

quadrature component, I1q*, of the converter current is defined to be either positive if the

STATCOM is emulating an inductive reactance or negative if it is emulating a capacitive

reactance. The DC capacitor voltage, vDC, is dynamically adjusted in relation with the converter

voltage. The control scheme described above shows the implementation of the inner current

control loop which regulates the reactive current flow through the STATCOM regardless of theline voltage.

2.3.3 Voltage Controlled STATCOM

In regulating the line voltage, an outer voltage control loop must be implemented. The outervoltage control loop would automatically determine the reference reactive current for the inner

current control loop which, in turn, will regulate the line voltage.

F ig. 2.4 Voltage control led block diagram of STATCOM

Figure shows a voltage control block diagram of the STATCOM. An instantaneous three-phase

set of measured line voltages, v1, at BUS 1 is decomposed into its real or direct component,

V1d, and reactive or quadrature component, V1q, is compared with the desired reference value,

V1*, (adjusted by the droop factor, Kdroop) and the error is passed through an error amplifier

which produces the reference current, I1q*, for the inner current control loop. The droop factor,

Kdroop, is defined as the allowable voltage error at the rated reactive current flow through the

STATCOM.


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2.4 BASIC CONFIGURATION AND PRINCIPLE OF OPERATION

Basically, shunt connected FACTS device can be realized by either a VSC or a CSC. But theVSC topology is preferred because CSC topology is more complex than VSC in both power and

control circuits. In CSC such as GTO (Gate Turn Off Thyristor) is used, a diode has to be placed

in series with each of the switches. This almost doubles the conduction losses compared with thecase of VSC. The DC link energy storage element in CSC topology is inductor where as that in

VSC topology is a capacitor. Thus, the efficiency of a CSC is expected to be lower than that of a

VSC. The modeled STATCOM using VSC topology is being used in the test system to supplyreactive power to increase the transmittable power and to make it more compatible with the

prevailing load demand.

Thus, the shunt connected FACTS device should be able to minimize the line over voltage underlight load condition and maintain voltage levels under heavy load condition. Two VSC

technologies can be used for the VSC. One of them, VSC is constructed with IGBT/GTO-based

SPWM inverters. This type of inverter uses sinusoidal Pulse-Width Modulation (SPWM)

technique to synthesize a sinusoidal waveform from a DC voltage source with a typical choppingfrequency of a few kilohertz. Harmonic voltages are cancelled by connecting filters at the AC

side of the VSC.

This type of VSC uses a DC link voltage Vdc. Output voltage is varied by changing the

modulation index of the SPWM modulator. Thus modulation index has to be varied for

controlling the reactive power injection to the transmission line. In another type VSC isconstructed with GTO-based square-wave inverters and special interconnection transformers.

Typically four three-level inverters are used to build a 48-step voltage waveform. Special

interconnection transformers are used to neutralize harmonics contained in the square waves

generated by individual inverters. In this type of VSC, the fundamental component of output

voltage is proportional to the voltage Vdc. Therefore Vdc has to be varied for controlling thereactive power.

The shunt controller is like a current source, which draws from or injects current into the system

at the point of connection. The shunt controller may be variable impedance, variable source or a

combination of these. Variable shunt impedance connected to the line voltage causes a variable

current flow and hence represents injection of current into the line. As long as the injectedcurrent is in phase quadrature with the line voltage, the shunt controller only supplies or

consumes reactive power. When system voltage is low, the STATCOM generates reactive power

(STATCOM capacitive). When system voltage is high, it absorbs reactive power (STATCOM

inductive).

The variation of reactive power is performed by means of a VSC connected on the secondary

side of a coupling transformer. The VSC uses forced-commutated power electronic devices(GTOs, IGBTs or IGCTs) to synthesize a voltage V2 from a DC voltage source. Any other phase

relationship will involve handling of real power as well. So, the shunt controller is therefore a

good way to control the voltage at and around the point of connection through injection ofreactive current (leading or lagging) alone or a combination of active and reactive current for a

more effective voltage control and damping of voltage dynamics.


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The real power (P) and reactive power (Q) are given by:

F ig. 2.5 Static Synchronous Compensator

E is the line voltage of transmission line. V is the generated voltage of VSC. X is the equivalent

reactance of interconnection transformer and filters and is the phase angle of E with respect to

V.

In steady state operation, the voltage V generated by the VSC is in phase with E (=0), so that

only reactive power is flowing (P=0). If V is lower than E, Q is flowing from E to V

(STATCOM is absorbing reactive power). On the reverse, if V is higher than E, Q is flowingfrom V to E (STATCOM is generating reactive power).

Since we are using here a VSC based on SPWM inverters hence modulation index is varied for

controlling the reactive power injection to the transmission line. A capacitor is connected on theDC side of the VSC acts as a DC voltage source. In steady state the voltage V has to be phase

shifted slightly behind E in order to compensate for transformer and VSC losses and to keep the

capacitor charged.


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F ig. 2.6 Waveform for Operation of Statcom

2.5 CHARACTERISTICS OF STATCOM

The derivation of the formula for the transmitted active power employs considerable

calculations. Using the variables defined in Figure below and applying Kirchoffs laws thefollowing equations can be written;

F ig. 2.7 Two machine system with STATCOM


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By equaling right-hand terms of the above formulas, a formula for the current I1 is obtained as

Where UR is the STATCOM terminal voltage if the STATCOM is out of operation, i.e. when Iq= 0. The fact that Iq is shifted by 90 with regard to UR can be used to express Iq as

Applying the sine law to the diagram in Figure below the following two equations result

From which the formula for sin is derived as


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The formula for the transmitted active power can be given as

To dispose of the term UR the cosine law is applied to the diagram in Figure above Therefore,

F ig. 2.8 Transmitted power versus transmission angle character istic of a STATCOM

With these concepts of STATCOM, it is thus important to utilize these

principles in accommodating shunt compensation to any system. Since this thesis only reflectson the voltage control and power increase, the requirements of the STATCOM would be

further elaborated.


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2.6 STATCOM V-I CHARACTERISTIC:-

A V-I characteristic of a STATCOM is depicted in Fig.2.9 . As can be seen, the STATCOM cansupply both the capacitive and the inductive compensation and is able to independently control

its output current over the rated maximum capacitive or inductive range irrespective of the

amount of ac-system voltage. That is, the STATCOMcan provide full capacitive-reactive power at any system voltage even as low as 0.15 pu. The

characteristic of a STATCOM reveals strength of this technology: that it is capable of yielding

the full output of capacitive generation almost independently of the system voltage (constant-current output at lower voltages). This capability is particularly useful for situations in which the

STATCOM is needed to support the system voltage during and after faults where voltage

collapse would otherwise be a limiting factor.

F ig. 2.9 V-I characteri stic of a STATCOM

2.7 FUNCTIONAL REQUIREMENTS OF STATCOM:-

The main functional requirements of the STATCOM in this thesis are to provide shuntcompensation, operating in capacitive mode only, in terms of the following;

Voltage stability control in a power system, as to compensate the loss voltage along

transmission. This compensation of voltage has to be in synchronism with the AC system

regardless of disturbances or change of load.

Transient stability during disturbances in a system or a change of load.


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Direct voltage support to maintain sufficient line voltage for facilitating increased reactive

power flow under heavy loads and for preventing voltage instability

Reactive power injection by STATCOM into the system

The design phase and implementation phase (as presented in the next chapter) would refer tothe theoretical background of STATCOM in providing the requirements

In 1999 the first SVC with Voltage Source Converter called STATCOM (STATic

COMpensator) went into operation. The STATCOM has a characteristic similar to the

synchronous condenser, but as an electronic device it has no inertia and is superior to thesynchronous condenser in several ways, such as better dynamics, a lower investment cost and

lower operating and maintenance costs. A STATCOM is build with Thyristors with turn-off

capability like GTO or today IGCT or with more and more IGBTs. The static line between the

current limitations has a certain steepness determining the control characteristic for the voltage.

The advantage of a STATCOM is that the reactive power provision is independent from theactual voltage on the connection point. This can be seen in the diagram for the maximumcurrents being independent of the voltage in comparison to the SVC. This means, that even

during most severe contingencies, the STATCOM keeps its full capability.

In the distributed energy sector the usage of Voltage Source Converters for grid interconnection

is common practice today. The next step in STATCOM development is the combination with

energy storages on the DC-side. The performance for power quality and balanced networkoperation can be improved much more with the combination of active and reactive power.

F ig. 2.10 STATCOM structure and voltage / curr ent character istic


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STATCOMs are based on Voltage Sourced Converter (VSC) topology and utilize either Gate-

Turn-off Thyristors (GTO) or Isolated Gate Bipolar Transistors (IGBT) devices.

The STATCOM is a very fast acting, electronic equivalent of a synchronous condenser. If the

STATCOM voltage, Vs, (which is proportional to the dc bus voltage Vc) is larger than bus

voltage, Es, then leading or capacitive VARS are produced. If Vs is smaller then Es thenlagging or inductive VARS are produced.

F ig 2.11 6 Pulses STATCOM

The three phases STATCOM makes use of the fact that on a three phase, fundamentalfrequency, steady state basis, and the instantaneous power entering a purely reactive devicemust be zero.

The reactive power in each phase is supplied by circulating the instantaneous real powerbetween the phases. This is achieved by firing the GTO/diode switches in a manner that

maintains the phase difference between the ac bus voltage ES and the STATCOM generated

voltage VS. Ideally it is possible to construct a device based on circulating instantaneous powerwhich has no energy storage device (ie no dc capacitor).

A practical STATCOM requires some amount of energy storage to accommodate harmonic

power and ac system unbalances, when the instantaneous real power is non-zero. Themaximum energy storage required for the STATCOM is much less than for a TCR/TSC type of

SVC compensator of comparable rating.


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F ig. 2.12 STATCOM Equivalent Cir cuit

Several different control techniques can be used for the firing control of the STATCOM.

Fundamental switching of the GTO/diode once per cycle can be used. This approach willminimize switching losses, but will generally utilize more complex transformer topologies. As

an alternative, Pulse Width Modulated (PWM) techniques, which turn on and off the GTO or

IGBT switch more than once per cycle, can be used. This approach allows for simpler

transformer topologies at the expense of higher switching losses.

The 6 Pulse STATCOM using fundamental switching will of course produce the 6 N thharmonics. There are a variety of methods to decrease the harmonics. These methods include

the basic 12 pulse configuration with parallel star / delta transformer connections, a complete

elimination of 5th and 7th harmonic current using series connection of star/star and star/delta

transformers and a quasi 12 pulse method with a single star-star transformer, and twosecondary windings, using control of firing angle to produce a 30 degree phase shift between

the two 6 pulse bridges. This method can be extended to produce a 24 pulse and a 48 pulse

STATCOM, thus eliminating harmonics even further. Another possible approach for harmoniccancellation is a multi-level configuration which allows for more than one switching element

per level and therefore more than one switching in each bridge arm. The ac voltage derived has

a staircase effect, dependent on the number of levels. This staircase voltage can be controlled toeliminate harmonics.

F ig 2.13 Substation wi th a STATCOM


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CHAPTER: 3

OTHER SERIES AND SHUNT DEVICES

3.1 SERIES DEVICES:-

Series devices have been further developed from fixed or mechanically switched

compensations to the Thyristor Controlled Series Compensation (TCSC) or even VoltageSource Converter based devices.

The main applications are:

Reduction of series voltage decline in magnitude and angle over a power line,

Reduction of voltage fluctuations within defined limits during changing power transmissions,

Improvement of system damping resp. damping of oscillations,

Limitation of short circuit currents in networks or substations,

Avoidance of loop flows resp. power flow adjustments.

3.2 TCSC:-

Thyristor Controlled Series Capacitors (TCSC) address specific dynamical problems intransmission systems. Firstly it increases damping when large electrical systems are

interconnected. Secondly it can overcome the problem of Sub Synchronous Resonance (SSR), aphenomenon that involves an interaction between large thermal generating units and series

compensated transmission systems.

The TCSC's high speed switching capability provides a mechanism for controlling line power

flow, which permits increased loading of existing transmission lines, and allows for rapid

readjustment of line power flow in response to various contingencies. The TCSC also can

regulate steady-state power flow within its rating limits.

From a principal technology point of view, the TCSC resembles the conventional seriescapacitor. All the power equipment is located on an isolated steel platform, including the

Thyristor valve that is used to control the behavior of the main capacitor bank. Likewise the

control and protection is located on ground potential together with other auxiliary systems.Figure shows the principle setup of a TCSC and its operational diagram. The firing angle and

the thermal limits of the Thyristors determine the boundaries of the operational diagram.


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F ig. 3.1 TCSC Cir cuit and Character istics

3.2.1 Advantages

Continuous control of desired compensation level

Direct smooth control of power flow within the network

Improved capacitor bank protection

Local mitigation of sub synchronous resonance (SSR). This permits higher levels ofcompensation in networks where interactions with turbine-generator torsional vibrations or

with other control or measuring systems are of concern.

Damping of electromechanical (0.5-2 Hz) power oscillations which often arise between

areas in a large interconnected power network. These oscillations are due to the dynamics ofinter area power transfer and often exhibit poor damping when the aggregate power tranfer over

a corridor is high relative to the transmission strength.

3.3 DYNAMIC POWER FLOW CONTROLLER:-

A new device in the area of power flow control is the Dynamic Power Flow Controller (DFC).The DFC is a hybrid device between a Phase Shifting Transformer (PST) and switched series

compensation.

A functional single line diagram of the Dynamic Flow Controller is shown in Figure 3.2. TheDynamic Flow Controller consists of the following components:

Astandard phase shifting transformer with tap-changer (PST)

Series-connected Thyristor Switched Capacitors and Reactors


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quite different characteristics. The steady state control range for loadings up to rated current is

illustrated in Figure 3.3 , where the x-axis corresponds to the throughput current and the y-axis

corresponds to the injected series voltage.

F ig3.3. Operational diagram of a DFC

Operation in the first and third quadrants corresponds to reduction of power through the DFC,whereas operation in the second and fourth quadrants corresponds to increasing the power flow

through the DFC. The slope of the line passing through the origin (at which the tap is at zero

and TSC / TSR are bypassed) depends on the short circuit reactance of the PST.

Starting at rated current (2 kA) the short circuit reactance by itself provides an injected voltage(approximately 20 kV in this case). If more inductance is switched in and/or the tap is

increased, the series voltage increases and the current through the DFC decreases (and the flowon parallel branches increases). The operating point moves along lines parallel to the arrows in

the figure. The slope of these arrows depends on the size of the parallel reactance. The

maximum series voltage in the first quadrant is obtained when all inductive steps are switchedin and the tap is at its maximum.

Now, assuming maximum tap and inductance, if the throughput current decreases (due e.g. tochanging loading of the system) the series voltage will decrease. At zero current, it will not

matter whether the TSC / TSR steps are in or out, they will not contribute to the series voltage.

Consequently, the series voltage at zero current corresponds to rated PST series voltage.

Next, moving into the second quadrant, the operating range will be limited by the line

corresponding to maximum tap and the capacitive step being switched in (and the inductivesteps by-passed). In this case, the capacitive step is approximately as large as the short circuit

reactance of the PST, giving an almost constant maximum voltage in the second quadrant.


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3.4 UNIFIED POWER FLOW CONTROLLER:-

The UPFC is a combination of a static compensator and static series compensation. It acts as a

shunt compensating and a phase shifting device simultaneously.

F ig3.4. Pri nciple conf iguration of an UPFC

The UPFC consists of a shunt and a series transformer, which are connected via two voltagesource converters with a common DC-capacitor. The DC-circuit allows the active power

exchange between shunt and series transformer to control the phase shift of the series voltage.

This setup, as shown in Figure 1.21, provides the full controllability for voltage and powerflow. The series converter needs to be protected with a Thyristor bridge. Due to the high efforts

for the Voltage Source Converters and the protection, an UPFC is getting quite expensive,

which limits the practical applications where the voltage and power flow control is required

simultaneously.

3.4.1 OPERATING PRINCIPLE OF UPFC

The basic components of the UPFC are two voltage source inverters (VSIs) sharing a common

dc storage capacitor, and connected to the power system through coupling transformers. OneVSI is connected to in shunt to the transmission system via a shunt transformer, while the other

one is connected in series through a series transformer.

A basic UPFC functional scheme is shown in fig.3.5


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F ig.3.5. UPFC functional scheme

The series inverter is controlled to inject a symmetrical three phase voltage system (Vse), of

controllable magnitude and phase angle in series with the line to control active and reactivepower flows on the transmission line. So, this inverter will exchange active and reactive power

with the line. The reactive power is electronically provided by the series inverter, and the active

power is transmitted to the dc terminals. The shunt inverter is operated in such a way as to

demand this dc terminal power (positive or negative) from the line keeping the voltage acrossthe storage capacitor Vdc constant. So, the net real power absorbed from the line by the UPFC

is equal only to the losses of the inverters and their transformers. The remaining capacity of the

shunt inverter can be used to exchange reactive power with the line so to provide a voltage

regulation at the connection point.

The two VSIs can work independently of each other by separating the dc side. So in that case,the shunt inverter is operating as a STATCOM that generates or absorbs reactive power to

regulate the voltage magnitude at the connection point. Instead, the series inverter is operating

as SSSC that generates or absorbs reactive power to regulate the current flow, and hence the

power low on the transmission line.

The UPFC has many possible operating modes. In particular, the shunt inverter is operating in

such a way to inject a controllable current, ish into the transmission line. The shunt inverter canbe controlled in two different modes:

VAR Control Mode: The reference input is an inductive or capacitive VAR request. The shuntinverter control translates the var reference into a corresponding shunt current request and

adjusts gating of the inverter to establish the desired current. For this mode of control a

feedback signal representing the dc bus voltage, Vdc, is also required.


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Automatic Voltage Control Mode: The shunt inverter reactive current is automatically

regulated to maintain the transmission line voltage at the point of connection to a reference

value. For this mode of control, voltage feedback signals are obtained from the sending end busfeeding the shunt coupling transformer.

The series inverter controls the magnitude and angle of the voltage injected in series with theline to influence the power flow on the line. The actual value of the injected voltage can be

obtained in several ways.

Direct Voltage Injection Mode: The reference inputs are directly the magnitude and phase

angle of the series voltage. Phase Angle Shifter Emulation mode: The reference input is phase

displacement between the sending end voltage and the receiving end voltage. Line ImpedanceEmulation mode: The reference input is an impedance value to insert in series with the line

impedance. Automatic Power Flow Control Mode: The reference inputs are values of P and Q

to maintain on the transmission line despite system changes.


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F ig. 4.1 Thevenin Equivalent Cir cuit Diagram of STATCOM: (a) STATCOM Schematic

Diagram; (b) STATCOM Equi valent Circuit

Using the rectangul ar coordinate representati on,

Where V STCand STC are the STATCOM voltage magnitude and angle respectively. e kand f k

are the real and imaginary parts of the bus voltage respectively.e STC and f STC are the real and imaginary parts of the STATCOM voltage respectively. The

active and reactive powers for the STATCOM and node k respectively are:

(6)

(7)


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And

(8)

(9)

4.2 LINEARISED POWER EQUATIONA single-phase power network with n-buses is described by 2(n-1) non-linear equations. The

inclusion of one STATCOM model augments the number of equations by two. The solution ofthe combined system of non-linear equations is carried out by iteration using the full Newton-

Raphson method.

The Jacobian used in conventional power flow is suitably extended to take account of the new

elements contributed by the STATCOM. The set of linearised power flow equations for thecomplete system is

The Jacobian elements in equation (10) are given in table 4.1 ahead.

4.3 NEWTON-RAPHSON-ALGORITHM

1. We assume a suitable solution for all the buses except the slack bus. We assume a flat voltage

profile i.e. Vp=1.0+j0.0 for p=1,2,,n, ps, Vs=a+j0.0.

2. We then set a convergence criterion = i.e. if the largest of absolute of the residues exceeds ,

the process is repeated, or else its terminated.

3. Set the iteration count K=0.

4. Set the bus count p=1.

5. Check if a bus is a slack bus. If that is the case, skip to step 10.


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6. Calculate the real and reactive powers Pp and Qp respectively, using the equations derived for

the same earlier.

7. Evaluate

8. Check if the bus p is a generator bus. If that is the case, compare Qkpwith the limits. If it

exceeds the limits, fix the reactive power generation to the corresponding limit and treat the busas a load bus for that iteration and go to the next step. If lower limit is violated, set Q sp=Qp min. If

the limit is not violated evaluate the voltage residue.

9. Evaluate

10. Increment the bus count by 1, i.e. p = p+1 and finally check if all the buses have been taken

into consideration. Or else, go back to step 5.

11. Determine the largest value among the absolute value of residue.

12. If the largest of the absolute value of the residue is less than , go to step 17.

13. Evaluate the Jacobian matrix elements.

14. Calculate the voltage increments

15. Calculate the new bus voltage Evaluate cos and sin

of all voltages.

16. Advance iteration count K=K+1 and go back to step 4.

17. Evaluate bus and line powers and output the results.


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TABLE 1


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CHAPTER: 5

APPLICATIONS, CONCLUSION AND FUTURE WORK

5.1 APPLICATIONS OF STATCOM

Usually a STATCOM is installed to support electricity networks that have a poorpower

factor and often poorvoltage regulation.There are however, other uses, the most common use is

for voltage stability. A STATCOM is a voltage source converter (VSC)-based device, with the

voltage source behind a reactor. The voltage source is created from a DCcapacitor and thereforea STATCOM has very little active power capability. However, its active power capability can be

increased if a suitable energy storage device is connected across the DC capacitor. The reactive

power at the terminals of the STATCOM depends on the amplitude of the voltage source. For

example, if the terminal voltage of the VSC is higher than the AC voltage at the point ofconnection, the STATCOM generates reactive current; on the other hand, when the amplitude of

the voltage source is lower than the AC voltage, it absorbs reactive power. The response time of

a STATCOM is shorter than that of an SVC, mainly due to the fast switching times provided bytheIGBTs of the voltage source converter. The STATCOM also provides better reactive power

support at low AC voltages than an SVC, since the reactive power from a STATCOM decreases

linearly with the AC voltage (as the current can be maintained at the rated value even down tolow AC voltage).

STATCOM has following applications in contr oll ing power system dynamics. Damping of power system oscillations. Damping of subsynchronous oscillations. Balanced loading of individual phases. Reactive compensations of AC-DC converters and HVDC links.

Improvement of transient stability margin. Improvement of steady-state power transfer capacity. Reduction of temporary over-voltages. Effective voltage regulation and control. Reduction of rapid voltage fluctuations (flicker control).

5.2 SCOPE FOR FUTURE RESEARCH

Although this research has covered most of the interesting issues and challenges of the advanced

STATCOM and several aspects of the integration of ESS into STATCOM, there are certain

aspects that might be interesting for future investigations which are given below:

Due to the excessive number of semiconductor devices andpassive components, a fault

protection scheme to enhance the ride-though capability in various faults scenarios remains as an

important challenge

In the investigation of the interface topology, the ES was assumed to be charged to a

voltage level that is not higher than the DC-side voltage of the VSC. It might be valuable to
http://en.wikipedia.org/wiki/Power_factorhttp://en.wikipedia.org/wiki/Power_factorhttp://en.wikipedia.org/wiki/Voltage_regulationhttp://en.wikipedia.org/wiki/Capacitorhttp://en.wikipedia.org/wiki/Insulated_gate_bipolar_transistorhttp://en.wikipedia.org/wiki/Insulated_gate_bipolar_transistorhttp://en.wikipedia.org/wiki/Capacitorhttp://en.wikipedia.org/wiki/Voltage_regulationhttp://en.wikipedia.org/wiki/Power_factorhttp://en.wikipedia.org/wiki/Power_factorhttp://en.wikipedia.org/wiki/Power_factor


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investigate the possibility of charging ES to a higher extent and the related issues such as

protection issues.

Research on the CMC based topology with ESS can be implemented for real and reactive

power compensation in wind farms with FSIGs or Double Fed Induction Generator

5.3 CONCLUSION

Among FACTS controllers, the shunt controller STATCOM have shown feasibility in terms ofcost effectiveness in a wide range of problem-solving abilities from transmission to distribution

levels. A comparison between the STATCOM and the SVC is made and based on several aspects

it is concluded that a STATCOM is more preferred when compared to SVC and other

compensation devices. Instead of directly deriving reactive power from the energy storagecomponents, the STATCOM basically circulates power with the connected network. Therefore,

the reactive components used in the STATCOM are much smaller than those in the SVC.

The location of the shunt FACTS device depends on the application for which it is installed.

Shunt compensation FACTS devices are installed at the end points of transmission lines (buses)

when used for applications, such as bus voltage regulation and improving HVDC link

performance, etc. However, from simulation results it is observed that for increasing the powertransfer capability of long transmission lines (tie lines connecting two major grids), midpoint of

the lines is the best location for shunt connected multi pulse STATCOM device. When

connected at the midpoint the real power is improved and the load ability margin. The midpointsitting of STATCOM also facilitates the independent control of reactive power at both the ends

of the transmission line. For a given voltage limit, the midpoint sitting controls a larger reactive

power because each side of the STATCOM device addresses only half the line impedance and

not the full line impedance as in the case of the transmission line receiving end sitting andsending end sitting. The simulation study shows that a STATCOM with real power capability

can improve the real power and enhance load stability margin, damp the power system

oscillations ore effectively and stabilize the system faster if the STATCOM-SMES controller islocated at the midpoint. Various concepts regarding the FACTS technology and the important

features of some of the FACTS devices have been presented. The Newton raphson method has

been presented to solve the power flow problem in the power system with static synchronouscompensator (STATCOM).

The study of the basic principles of the STATCOM is carried out as well as the basics of reactive

power compensation using a STATCOM. A power flow model of the STATCOM is attemptedand it is seen that the modified load flow equations help the system in better performance. The

bus system shows improved plots and the thus we can conclude that the addition of a STATCOM

controls the output of a bus in a robust manner.

Hence our objective to maintain voltage stability has been successfully achieved with theincorporation of Static Synchronous Compensator (STATCOM).


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REFERENCES:-

[1]. How FACTS controllers benefits AC transmission systems: John J. Paserba, Fellow IEEE.

[2]. How FACTS improve the performance of electrical grid: Rolf Grunbaum, Ake Petersson,

Bjom Thorvaldsson (ABB Review 3/2002)

[3] Gyugyi L, Schauer C.D., Williams S.L., Rietman T.R., Torgerson D.R., Edris A., (1995),

The Unified Power Flow Controller: A New Approach to Power Transmission Control, IEEE

Transactions on Power Delivery, vol. 2, pp. 1085-1097.

[4] Edris A, Mehraban A.S., Rahman M, Gyugyi L, Arabi S, Reitman T., (1998),ControllingThe Flow of real and reactive power, IEEE Computer Applications in power, 20-5.

[5] Haque M.H., Yam C.M., (2003), A simple method of solving the controlled load flow

problem of a power system in the presence of UPFC, Electric Power Systems Research 65(1),pp. 55-62.

[6] Hingorani N.G. and Gyugyi, L. (2000), Understanding FACTS, The Institute of Electrical

and Electronics Engineers, New York.

[7] Sen, K.K., (1999), STATCOM-static synchronous compensator theory, modelling and

applications, IEEE PES Winter Meeting 2, pp.1177-1183.

[8] Chun, L., Qirong, J., Xiaorong, X. and Zhonghong, W. (1998), Rule-based control for

STATCOM to increase power system stability, Power System Technology, Proceedings 1998

International Conference on POWERCON, pp. 372376.

[9] Rahim, A. H. M. A., Al-Baiyat, S. A. and Al-Maghrabi, H. M.: 2002, Robust dampingcontroller design for a static compensator, IEE Proceedings on Generation, Transmission and

Distribution 149, 491496.

[10] Haque, M. H.: (2004), Improvement of first swing stability limit by utilizing full benefit of

shunt FACTS devices, IEEE Transactions on Power Systems 19(4), 18941902.

[11] Acha, E., Fuerte-Esquivel, C.R., Ambriz-Perez, H. and Angeles-Camacho, C. (2004),

FACTS Modelling and Simulation in Power Networks, John Wiley & Sons, Chichester.

[12] Wei, X., Chow, J.H., Fardanesh, B. and Edris, A.A. (2004), A Common modelling

framework of voltage sourced converters for power flow, sensitivity, and dispatch analysis,

IEEE Transactions Power on System, vol.19, pp. 934-941.

[13] Zhang, X. P., Rehtanz, C. and Pal, B. (2006) Flexible AC Transmission Systems:Modelling and Control, Springer-Verlag Berlin Heidelberg, Germany.

[14] Milano, F. (2005), An Open Source Power System Analysis Toolbox, IEEE Transactions

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