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CHAPTER—4

SIMULATION OF SVC AND STATCOM

4.1 MODELING OF SVC

The focus of this chapter and research is simulation of single phase TCR/TSC

Static Var Compensator and STATCOM in MATLAB/Simulink to solve voltage

regulation and system dynamic performance deficiencies. Comparison of SVC

and STATCOM has also been done in this chapter. Transient Stability with SVC

and SIMULINK model of IG with SVC and STATCOM are also presented in this

chapter. SVC is thyristor based controller that provides rapid voltage control to

support electric power transmission voltages during immediately after major

disturbances. Since the advent of deregulation and the separation of generation

and transmission systems in electric industry, voltage stability and reactive power-

related system restrictions have become an increasing growing concern for

electric utilites. When voltage security or congestion problems are observed

during the planning study process, cost effective solution must be considered for

such problems. Voltage stability, voltage regulation and power system stability,

damping can be improved by using these devices and their proper control. Static

Var Compensators (SVC‘s), the most important FACTS devices, have been used

for a number of years to improve transmission line economics by resolving

dynamic voltage problems. The accuracy, availability and fast response enable

SVC‘s to provide high performance steady state and transient voltage control.

One approach to solving this problem is the application of ―Flexible AC

Transmission System‖(FACTs) technologies, such as the Static Var Compensator

(SVC). In an ideal ac power system, the voltage and frequency at every supply

point would be constant and free from harmonics, the power factor would be

unity. FACTS devices can regulate the active and reactive power control as well

as adaptive to voltage magnitude control simultaneously by their fast control

characteristics and their continuous compensating capability and so reduce flow

of heavily loaded lines and maintain voltages in desired level. Effect of FACTS

devices on power system security, reliability and loadability has been studied

according to proper control objectives. Some of papers have been tried to find

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suitable location for FACTS devices to improve power system security and

loadability. The SVC provides an excellent source of rapidly controllable reactive

shunt compensation for dynamic voltage control through its utilization of high-

speed thyristor switching/controlled reactive devices.

An SVC is typically made up of the following major components:

1. Coupling transformer

2. Thyristor valves

3. Reactors

4. Capacitors (often tuned for harmonic filtering)

Fig. 4.1 Single Phase of a TCR/TSC SVC

In general, the two thyristor valve controlled/switched concepts used with SVCs

are the thyristor-controlled reactor (TCR) and the thyristor-switched capacitor

(TSC). The TSC provides a ―stepped‖ response and the TCR provides a ―smooth‖

or continuously variable susceptance. A TCR consists of a fixed reactor in series

with a bi-directional thyristor valve. A TSC consists of a capacitor in series with a

bi-directional thyristor valve and a damping reactor. The thyristor switch acts to

connect or disconnect the capacitor for an integral number of half cycles of the

applied voltage. The capacitor is not phase controlled it is simply on or off.

Because of this TSC does not produce harmonic distortion. The reactor in the

TSC circuit serves to limit current under abnormal conditions, as well as to tune

the TSC circuit to a desired frequency.

Comprehensive power system study is required to develop appropriate

model to emphasized particular problems to be solved by SVC applications.

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Normally following studies are required for an SVC application from early

planning stage till operation.

1) load flow studies

2) small and large disturbance studies

3) Harmonics studies

4) Electromagnetic transient studies

5) Fault studies

The main objective of load flow analyses is to determine the node voltages

reactive and active power flow in the network branches, generations and loss. The

power flow studies related to SVC applications are

(1) To determine the location and preliminary rating of the SVC.

(2) To render information on the effects of the SVC on the system voltages

and power flows.

(3) To provide the initial condition for system transient analysis.

(4) And operating boundaries likewise inside or outside control range.

Principle of SVC operation:-

A TCR encompasses of a fixed reactor in series with bidirectional Thyristor

valve. Usually, the inductance in each phase is divided such that half of the

inductance is on each side of the Thyristor valve. This approach reduces the stress

on the Thyristor under fault conditions.Typically, air-core reactors are used in

TCRs.

A TSC comprises of a capacitor in series with bidirectional; Thyristor valve and

a damping reactor. The Thyristor switch acts to connect or disconnect the

capacitor for an integral number of half-cycles of the applied voltage. The

capacitor is not phase controlled; it is simply on or off. Because of this TSC does

not produce harmonics distortion. The reactor in the TSC circuit serves to limit

current under abnormal conditions as well as to tune the TSC circuit to a desired

frequency.

4.1.1 Simulation of TCR and TSC branch in SIMULINK

The simulation is done using Matlab Simulink version 7.8. As the snubber circuit

has already been specified with Th1, the snubber of Th2 must be eliminated.

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Thyristor blocks have an output identified by the letter m. This output returns a

Simulink vectorized signal containing the thyristor current and voltage. These

quantities are shown in scope Th1. At every cycle a pulse has to be sent to each

thyristor α degrees after the zero crossing of the thyristor commutation voltage.

For Pulse 1 phase delay parameter is set as 1/60 and for pulse 2 it is set as 1/60 +

1/120.

Fig. 4.2 Simulink Circuit Diagram of TCR

The simulation is also done using Matlab Simulink version 7.8. As the snubber

circuit has already been specified with Th1, the snubber of Th2 must be

eliminated. Thyristor blocks have an output identified by the letter m. This output

returns a Simulink vectorized signal containing the thyristor current and voltage.

These quantities are shown in scope_Th1. In this simulation a step block is used

to provide continuous firing signal to two thyristors. The value of the initial

voltage across the capacitor C1 should be -0.3141 V. This voltage is not exactly

zero because the snubber allows circulation of a small current when both

thyristors are blocked.

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Fig.4.3 Simulink Circuit Diagram of TSC

4.1.2 Simulation of TCR / TSC SVC in SIMULINK

Fig.4.4 Simulation of TCR/TSC SVC in SIMULINK

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4.2 SIMULATION OF SVC IN MATLAB /SIMULINK

Fig.4.5 Simulation of Static Var Compensator in Simulink

A static var compensator (SVC) is used to regulate voltage on a sytem .When

system voltage is low the SVC generates reactive power (SVC capacitive). When

system voltage is high it absorbs reactive power (SVC inductive).The SVC is

rated +100 Mvar capacitive and 50 Mvar inductive.The two 500 kV Three-Phase

Programmable Voltage Source is used to vary the system voltage to observe the

SVC performance.

The SVC is set in voltage regulation mode with a reference voltage Vref=1.0 pu.

The voltage droop is 0.03 pu/ 100MVA, so that the voltage varies from 0.97 pu to

1.015 pu when the SVC current goes from fully capacitive to fully inductive.

Initially the source is generating nominal voltage. Then, voltage is

successively decreased (0.97 pu at t = 0.1 s), increased (1.03 pu at t = 0.4 s) and

finally returned to nominal voltage (1 pu at t = 0.7 s). A positive of Q(pu) value

indicates inductive operation and a negative value of Q(pu) value indicates

capacitive operation. A positive value of SVC susceptance indicates that the SVC

is capacitive and a negative value indicates inductive operation.

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The SVC response speed depends on the voltage regulator integral gain Ki

(Proportional gain Kp is set to zero), system strength (reactance Xn) and droop

(reactance Xs). If the voltage measurement time constant and average time delays

Td due to valve firing are neglected, the system can be approximated by a first

order system having a closed loop time constant.

Tc= 1/(Ki (Xn+Xs)) (4.1)

With system parameters

Ki = 400

Xn = 0.0667 pu/100 MVA

Xs = 0.03 pu/100 MVA

4.2.1 SYSTEM MODEL WITHOUT SVC

Fig.4.6 System Model without SVC

In the present work system is simulated in simulink to observe the positive

sequence voltage profile in the absence of SVC. As SVC provides dynamic

response to voltage steps created through Three Phase Programmable Voltage

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Source, it will now be analysed from this model the changes that takes place in

positive sequence voltage profile of the system

4.3 MODELING OF STATCOM IN MATLAB/ SIMULINK

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

Transmission 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, STATCOM provides fast acting dynamic

reactive compensation for voltage support during contingency events which

would otherwise depress the voltage for a significant length of time. In emerging

electric power systems, increased transactions often lead to the situations where

the system no longer remains in secure operating region. The flexible AC

transmission system (FACTS) controllers can play an important role in the power

system security enhancement. However, due to high capital investment, it is

necessary to locate these controllers optimally in the power system. FACTS

devices can regulate the active and reactive power control as well as adaptive to

voltage-magnitude control simultaneously because of their flexibility and fast. In

this paper Control System Block diagram and V-I Characteristics of STATCOM

are described. STATCOM is modeled in MATLAB/ Simulink and simulation

results are shown and discussed.

The Static Synchronous Compensator (STATCOM) is a shunt device of the

Flexible AC Transmission Systems (FACTS) family using power electronics to

control power flow and improve transient stability on power grids.The

STATCOM regulates voltage at its terminal by controlling the amount of reactive

power injected into or absorbed from the power system. When system voltage is

low, the STATCOM generates reactive power (STATCOM capacitive). When

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

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Fig.4.7 STATCOM / SVC on a 500 kV Transmission Line

The power grid consists of two 500-kV equivalents (respectively 2000 MVA and

3500 MVA) connected by a 600-km transmission line. In this paper STATCOM

has a rating of +/- 100MVA."Droop" parameter should be set to 0.03 and the "Vac

Regulator Gains" to 5 (Proportional gain Kp) and 1000 (Integral gain Ki). "Step

Vref" block (the Red timer block connected to the "Vref" input of the

STATCOM) should be programmed to modify the reference voltage Vref as

follows: Initially Vref is set to 1 pu; at t=0.2 s, Vref is decreased to 0.97 pu; then

at t=0.4 s, Vref is increased to 1.03; and finally at 0.6 s, Vref is set back to 1 pu.

Here fault breaker at bus B1will not operate during the simulation.

4.4 COMPARISON OF STATCOM AND SVC

In an another experiment for comparison of STATCOM model with a SVC model

having the same rating (+/- 100 MVA) SVC is connected to a power grid similar

to the power grid on which our STATCOM is connected and disabling the "Step

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Vref" block by multiplying the time vector by 100 then program the fault breaker

by selecting the parameters "Switching of phase A, B and C" and verify that the

breaker is programmed.

Fig.4.8 SVC Power System

4.5 TRANSIENT STABILITY WITH SVC

A 1000 MW hydraulic generation plant (machine M1) is connected to a load

center through a long 500 kV, 700 km transmission line in

MATLAB/SIMULINK. The load center is modelled by a 5500 MW resistive

load. The load is fed by the remote 1000 MW plant and a local generation of 5000

MW (machine M2). In order to maintain system stability after faults, the

transmission line is shunt compensated at its center by a 200-Mvar Static Var

Compenstor (SVC). The two machines are equipped with a Hydraulic Turbine

and Governor (HTG), Excitation system and Power System Stabilizer (PSS).

Faults will be applied on the 500 KV system and impact of SVC on system

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stability will be observed. The machine M1 'Bus type' should be already

initialized as 'PV generator', indicating that the load flow will be performed with

the machine controlling its active power and its terminal voltage. Machine M2

will be used as a swing bus for balancing the power. Following parameters are

specified for M1 and M2:

M1: type = PV Terminal voltage (Vrms) = 13800 Active Power = 950e6

M2: type = Swing bus Terminal voltage (Vrms) = 13800 Active power = 4000e6.

First, SVC is set to operate in 'Var control (fixed susceptance)' mode with Bref

=0. Setting Bref to zero is equivalent to putting the SVC out of service.

Second, A 3-phase fault is applied and impact of the SVC is observed for

stabilizing the network during a severe contingency.

Fig.4.9 SVC Operating with T-G on 700 Km Transmission Line

A static var compensator (SVC) is used to regulate voltage on a sytem .When

system voltage is low the SVC generates reactive power (SVC capacitive). When

system voltage is high it absorbs reactive power (SVC inductive). SVC is rated as

200 Mvar.

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4.6 SIMULINK MODEL OF INDUCTION GENERATOR (IG) WITH

SVC AND STATCOM

The investigated power system network is modeled and simulated in MATLAB /

SIMULINK as shown in Fig. 4.10 and Fig. 4.11 to study the steady state behavior

with SVC and STATCOM. The fault is initiated between 10 and 10.1 sec from

starting of the simulation. The purpose of running simulation in this mode is to

verify the dynamic reactive power compensation capability of SVC and

STATCOM during the event of fault, while integrating wind power in a

distribution network. The network consists of a 132 kV, 50 Hz, grid supply point,

feeding a 33 kV distribution system through 132/33 kV, 62.5 MVA step down

transformer. There are two loads in the system; one load of 20 MW and another

load of 4 MW at 50 Km from the transformer. The 33 kV, 50 kM long line is

modeled as line. A 9 MW wind farm consisting of six 1.5 MW wind turbines is to

be connected to the 33 kV distribution network at 4 MW load point. Dynamic

compensation of reactive power is provided by a SVC or STATCOM located at

the point of wind farm connection. The 9 MW wind farm have conventional wind

turbine systems consisting of squirrel-cage induction generators and variable pitch

wind turbines. In order to limit the generator output power at its nominal value,

the pitch angle is controlled for winds exceeding the nominal speed of 9 m/s. Each

wind turbine has a protection system monitoring voltage, current and machine

speed. Test system is simulated in MATLAB/Simulink. Fig.4.10 and Fig. 4.11

shows the Simulink model of the test system. Phasor simulation is used to

simulate the test system; so as to make it valid for intended purpose. Variable-step

ode23tb solver is used for simulation. The simulation time is 20 sec. The problem

of voltage instability can be solved by using dynamic reactive compensation.

Shunt flexible ac transmission system (FACTS) devices, such as the SVC and the

STATCOM, have been widely used to provide high-performance steady state and

transient voltage control. The application of an SVC or a STATCOM to a wind

farm equipped with fixed-speed wind turbines (FSWTs) and squirrel-cage

induction generators (SCIGs) has been reported in open literatures for steady-state

voltage regulation for short-term transient voltage stability. FACTS Controller

inwind power station are used for improving voltage profile damping oscillations,

load ability, reduce active and reactive power losses, sub-state-of-the-art on

enhancement of different performance parameters of power systems such as

voltage profile, sub-synchronous resonance (SSR) problems, transient stability,

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and dynamic performance, by optimally placing of FACTS controllers such as

TCSC, SVC, STATCOM, SSSC, UPFC, IPFC, HPFC in wind power Systems.

Fig.4.10 Simulink Model of IG with SVC

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Fig.4.11 Simulink Model of IG with STATCOM

The network consists of a 132 kV, 50 Hz, grid supply point, feeding a 33 kV

distribution system through 132/33 kV, 62.5 MVA step down transformer. There

are two resistive loads in the system L1 AND L2.The 33 kV, 50 kM long line is

modeled as line. A 9 MW wind farm is to be connected to the 33 kV distribution

network. The total MVA loading on the system is 50 MVA; considering the T &

D losses in the system it is over loaded and representing weak distribution

network. Dynamic compensation of reactive power is provided by a STATCOM

located at the point of wind farm connection.

The simulation is run in three different modes, as follows –

i. Without Wind Farm and STATCOM,

ii. With Wind Farm and without STATCOM,

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iii. With Wind Farm and STATCOM

4.6.1 Without Wind Farm and STATCOM

In this mode the wind farm and STATCOM were skipped while running the

simulation. Only the distribution system and two loads were kept in the model.

The purpose of running the simulation in this mode is to ascertain that, the test

system is a weak system. Only voltages at 33 kV Bus – 1 and Bus – 2 are

measured.

Fig.4.12 Simulink Model without Wind Farm and STATCOM

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4.6.2 With Wind Farm and without STATCOM

Fig.4.13 Simulink Model with Wind Farm and without STATCOM

In this mode of simulation the wind farm is connected to the weak distribution

network in above mode. The purpose of running simulation in this mode is to try

integration of 9 MW wind power in weak distribution network, without dynamic

compensation of reactive power i.e. without using the STATCOM. Before

tripping the wind turbine generators have drawn reactive power from the network.

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4.6.3 With Wind Farm and with STATCOM

Fig.4.14 Simulink Model with Wind Farm and STATCOM

In this mode of simulation the wind farm with dynamic compensation by

STATCOM is connected to the weak distribution network in above mode. The

purpose of running simulation in this mode is to integrate 9 MW wind power in

weak distribution network, with dynamic compensation of reactive power using

the STATCOM.

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4.7 INDUCTION GENERATOR STABILITY CONDITIONS USING

STATCOM

The test network consists of a 33 kV, 50 Hz, grid supply point. A wind farm of 110

kW is integrated with a power grid of 33 kV by a three phase transformer of

400V/33 kV, 110 kVA. There are two loads in the system; one load of 50 KW

which is connected to 33 KV grids through another step down transformer of 33

kV/400 V, 250 kVA and another load of 2 kW at 6 kM away from each other. The

33 KV, 6 kM long line is modeled as line. Used generator in this model is squirrel

cage induction generators and stator windings are directly connected to the grid.

This grid is used to study and analyze machine and wind farm stability. Dynamic

compensation of reactive power is provided by a STATCOM located at the point

of wind farm connection. Here we study using MATLAB Simulation:

1. Induction generator stability conditions study without using STATCOM.

2. Induction generator stability conditions study with using STATCOM.

In this study, initially, Induction generators required reactive power is considered

by capacitor bank connected to 400V terminals at 400 KVAR rate. It's obvious that

in system different conditions more reactive power demand is provided by grid. A

3- phase short circuit is occurred at 1.0 seconds to 3.5 seconds.

Fig. 4.15 Single line diagram of test system

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4.7.1 SIMULATION MODEL

Fig.4.16 Simulation Model