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International Journal of Advanced Engineering Technology E-ISSN 0976-3945

IJAET/Vol.III/ Issue I/January-March, 2012/175-179

Research Paper

COMPENSATION BY TCSC IN OPEN LOOP CONTROL

SYSTEM1*

Sunita Tiwari,2S.P. Shukla

Address for Correspondence1*

Sr. Lecturer, Polytechnic,Durg2

Professor, Bhilai Institute of Technology, DurgABSTRACTThe FACTS controllers clearly enhance power system performance, improve quality of supply and also provide an optimalutilization of the existing resources. TCSC has been proposed to enhance the power transfer capability by changing the

reactive power distribution in the power system. This paper discusses the TCSCs power enhancement capability. It has also

discussed the effect of TCSC on steady state and transient stability. A transmission line model equipped with TCSC that issuitable for power transfer capability and transient stability analysis is proposed. This model is tested in a simpletransmission system for open loop control system on MATLAB 2007a software. Thyristor controlled series capacitors(TCSC) in closed loop system have been widely studied by many researchers but in this model the effect of TCSC in open

loop control system is discussed. The simulation result shows that TCSC is capable of increasing power level and improving

transient stability.

KEYWORDSTransient stability, Power enhancement, FACT, TCSC

I. INTRODUCTION

An increasingly competitive market where economicand environmental pressures limit their scope to

expand transmission facilities. The optimization of

transmission corridors for power transfer has become

a great importance. In this scenario, the FACTS

technology is an attractive option for increasing

system operation flexibility[1]

,New developments in

high-current, high-power electronics are making it

possible to control electronically the power flows on

the high voltage side of the network during both

steady state and transient operation.

One important FACTS component is the TCSC

which allows rapid and continuous changes of the

transmission line impedance[ 2 ]

. Active power flowsalong the compensated transmission line can be

maintained at a specified value under a range of

operating conditions. Fig. 1 is a schematic

representation of a TCSC module[2]

,which consists

of a series capacitor bank in parallel with a Thyristor

Controlled Reactor (TCR). The controlling element is

the thyristor controller, shown as a bidirectional

thyristor valve.

In this paper a short description of TCSC is given

along with the simulation of transmission line using

TCSC, a FACTS controller simulated in MATLAB-

R2007a. Analysis of the simulated transmission line

(compensated with TCSC) model shows that TCSC

can enhance power level of transmission line and has

the similar functions as a physical one.

The simulation of transmission line at different load

conditions is done and the results show that the

power transmitted through the line can be enhanced

with the application of TCSC. Change in value of

load affects the power level but, still, TCSC is

capable of increasing power level of the system in all

conditions.

Controlled series compensation can be applied

effectively to damp power oscillations. For damping

power oscillations, it is necessary to optimize theapplied compensation so as to counteract the

accelerating and decelerating swings of the disturbed

machine.

To examine the transient stability of the system with

and without TCSC, the same transmission line issubjected to transient disturbances i.e. a circuit

breaker, with specified switching time, is connected

in series with the transmission line and responses are

observed.

The main objective of this project is to demonstrate

how a TCSC influence the power of the load

connected to transmission line. The model developed

in this project was verified by simulation studies for a

series compensated system

II.THYRISTOR CONTROLLED SERIES

COMPENSATOR

It is obvious by series compensation technique that

power transfer between two station can be affectedby adjusting the net series impedance of line. One

such conventional and established method of

increasing transmission line capability is to install a

series capacitor, which reduces the net series

impedance, thus allowing additional power to be

transferred. Although this method is well known,

slow switching times is the limitation of its use.

Thyristor controllers, on the other hand, are able to

rapidly and continuously control the line

compensation over a continuous range with resulting

flexibility. Controller used for series compensation is

the Thyristor Controlled Series Compensator

(TCSC). TCSC controllers use thyristor-controlled

reactor (TCR) in parallel with capacitor segments of

series capacitor bank (Figure 1). The combination of

TCR and capacitor allow the capacitive reactance to

be smoothly controlled over a wide range and

switched upon command to a condition where the bi-

directional thyristor pairs conduct continuously and

insert an inductive reactance into the line.

A TCSC is a series controlled capacitive reactance

that can provide continuous control of power on the

ac line over a wide range. The functioning of TCSC

can be comprehended by analyzing the behavior of a

variable inductor connected in series with a fixedcapacitor, as shown in Figure 1.


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Fig.1. Thyristor Controlled Series Capacitor

(TCSC)

III.POWER SYSTEM STABILITY

Power system stability may be broadly defined as the

ability of a power system to remain in a state of

operating equillibrium under normal operating

conditions and to regain an acceptable state of

equilibrium after being subjected to a disturbance.[3]

Stability of power system has been a major concern

in system operation. The stability of a system

determines whether the system can settle down to the

original or close to the steady state after the transients

disappear. In general, power system stability is the

ability to respond to a disturbance from its normal

operation by returning to a condition where theoperation is again normal.[3]

A power system is said to be steady state stable for a

particular operating condition if, following any small

disturbance, it reaches a steady state operating

condition which is identical or close to the pre-

disturbance operating condition.[3]

Transient stability is defined as the ability of the

power system to maintain synchronism when

subjected to a severe transient disturbance. A system

is transiently stable if it can survive the initial

disturbance but it is transiently unstable if it cannot

survive. For the transiently stable system, a large

disturbance suddenly occurs, the system angle spreadstarts to increase but reaches a peak and then starts to

decline, making the system transiently stable. The

resulting system response involves large excursions

of generator rotor angles. Transient stability is

sometimes called first swing stability as the

instability often occurs during the first angle swing.[3]

IV.FUNDAMENTAL REACTANCE OF TCSC

The effective reactance of TCSC is given by

equations (1) and (2).[4]

Equation (1) assumes that

the capacitor voltage is free from harmonics and

considers the only the TCR current harmonics.

( )( )

=

CTCR

CTCRTCSC

XXj

XXX

1

11 ---------(1)

( )

+=

2

sin

22tan

2tan

1

2cos2

1

212

2

2

2

2

CTCSC XjX

---------(2)

Where

( )

=

sin1 LjX TCR

----------(3)

N

0= andCL

=1

0

On the other hand equation (2) gives a more accurate

representation of reactance of TCSC by considering

the harmonics of both the capacitor voltage and the

TCR current. Intuitively, in the case of equation (2)

the extra charge injected into the capacitor during the

capacitive vernier mode increases the fundamental

component of voltage, increasing the effective TCSC

capacitive reactance as seen by the power system. As

a result equation (2) results in higher value of TCSC

reactance for a given value of conduction angle when

compared to equation (1), in addition be presenting amore complete representation.

In the above equations, is the conduction angle, L is

the inductance of the TCR inductor, C is the

capacitance of the fixed capacitor, N is power

system frequency in radians per second and 0is the

resonant frequency of the TCSC circuit. Fig.2[5]

shows the effective reactance of TCSC.

Fig.-2 Reactance Characteristics of TCSC

Simulation results match more closely to

characteristics drawn using equation (4.2). The

negative and positive portions of the characteristics

represent capacitive and inductive vernier modes ofoperation.V. MODES OF OPERATION IN STEADY STATE

By controlling the firing angle of the thyristors the

effective reactance of the TCR can be varied. This

variable TCR reactance in parallel with a fixed

capacitor allows the TCSC to operate in four

different modes; blocking mode; bypass mode;

capacitive boost mode; and inductive boost mode.[4 ]

[5] [6]

Blocking Mode:

When the thyristor valve is not triggered and the

thyristors are kept in non-conducting state, the TCSC

is operating in blocking mode. In this mode, theTCSC performs like a fixed series capacitor.

Bypass Mode:

In bypass mode the thyristor valve is triggered

continuously and the valve stays conducting all the

time; so the TCSC behaves like a parallel connection

of the series capacitor with the inductor, Ls, in the

thyristor valve branch. In this mode, the resulting

voltage in the steady state across the TCSC is

inductive and the valve current is somewhat bigger

than the line current due to the current generation in

the capacitor bank. For practical TCSCs with XL/XC

ratio between 0.1 to 0.3 range, the capacitor voltage

at a given line current is much lower in bypass than

in blocking mode. Therefore, the bypass mode is

utilized as a means to reduce the capacitor stress

during faults.

Capacitive Boost Mode:


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In capacitive boost mode a trigger pulse is supplied to

the thyristor having forward voltage just before the

capacitor voltage crosses the zero line, so a capacitor

discharge current pulse will circulate through the

parallel inductive branch.

The discharge current pulse adds to the line current

through the capacitor and causes a capacitor voltage

that adds to the voltage caused by the line current.

The capacitor peak voltage thus will be increased inproportion to the charge that passes through the

thyristor branch. The fundamental voltage also

increases almost proportionally to the charge. From

the system point of view, this mode inserts capacitors

to the line up to nearly three times the fixed

capacitor. This is the normal operating mode of

TCSC.

Inductive Boost Mode

In inductive boost mode,the circulating current in the

TCSC thyristor branch is bigger than the line current.

In this mode, large thyristor currents result and

further the capacitor voltage waveform is very much

distorted from its sinusoidal shape. The peak voltage

appears close to the turn on.

The poor waveform and the high valve stress make

the inductive boost mode less attractive for steady

state operation. This mode increases the inductance

of the line, so it is in contrast to the advantages

associated with the application of TCSC for

increasing the line loadability by decreasing the line

impedance. Meanwhile, this mode is useful during

short circuits to decrease the fault current. This mode

is normally used as a current-limiting system, helping

to reduce the voltage sag during the faults.

V. TCSC MODELING USING SIMULINK

Figure 3. Model of SMIB system using

TCSC

The complete system has been represented in terms

of SIMULINK blocks in a single integral model.

SIMULINK is a software tool associated with

MATLAB, used for modeling, simulating and

analyzing dynamical systems. Single Machine

Infinite Bus (SMIB) system with all the required

components is modeled and is described. Simulink

model of SMIB system with TCSC has been shown

in Figure 3.

VII. SIMULATION RESULTSFor analyzing the effect of TCSC on transmission

system, three conditions of line is taken. In first

condition, at particular load, power transfer capability

of line is noted and the results are compared for (i)

uncompensated line (ii) line equipped with TCSC (at

three different firing angle).

In second condition, the load is changed, making it

more inductive and the results are identified in both

the condition i.e. when line is compensated (at one

particular firing angle) and when line is not

compensated.

In third condition, load is again changed, making it

more resistive, the results are identified in both thecondition, when line is compensated (at one

particular firing angle) and when line is not

compensated.

For analyzing the effect of TCSC on transient

stability of transmission system, transient disturbance

is applied on line in both the conditions, when it is

uncompensated, and when it is compensated with

TCSC (at three different firing angle) is observed and

results are compared.

Condition-I

(When load is P = 10 KW and QL= 1KVar )

CASE-1 Single phase transmission system

Fig. 4 Active, Reactive Power

(without compensation)

Table -1 Active/Reactive power output

CASE-2 Simulation of tr. Line with TCSC:

(i)When firing angle is 1500

Table-2 Active/Reactive power output

Fig. 5 Active, Reactive Power at =150o


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(ii). When firing angle, = 1620


(iii)When firing angle, = 1730

Table -4 Active/Reactive power output

Condition-II

(When load is P = 10 KW and QL=10 KVar)


Condition III

(When load is P = 10 KW and QL=100 Var)

Table- 6 Active/Reactive power output

It is clear from above simulations, for all the cases of

transmission line and all the conditions of load when

line is series compensated by TCSC, the transmission

capacity of line gets increased. It is also concluded

that transmission capacity can be controlled by

operating the model at different firing angle.

Moreover TCSC can be operated in capacitive mode

as well as inductive mode whenever it is required.

VIII. TRANSIENT STABILITY IMPROVEMENT

BY TCSC

After the application of transient disturbances, if

power oscillations persist for longer period and the

amplitude of oscillation is also high, then the system

is called unstable. To improve stability of the system

it is required that oscillations should damp fast.

Controlled series compensation can be applied

effectively to damp power oscillations. For damping

power oscillations it is necessary to optimize the

applied compensation so as to counteract the

accelerating and decelerating swings of the disturbed

machine.

To examine the transient stability of the system with

and without TCSC, the same transmission line is

subjected to transient disturbances i.e. a circuit

breaker, with specified switching time, is connected

in series with the transmission line and responses are

observed.

Case-1.When line is uncompensated:

Fig.-6 Power oscillation diagram

The amplitudes of oscillations are

1st positive Peak = above 740 MW

1st negative Peak = below 640 MW

2nd positive Peak = above 719 MW

2nd negative Peak = below 670 MW

3rd positive Peak = above 690 MW

3rd negative Peak = above 680 MW

Case-2.When line is compensated with TCSC:

(ii)When firing angle is 1500:

Fig.-7 Power oscillation diagram at 1500

The amplitudes of oscillations are-:

1st positive Peak = above 1580 MW1st negative Peak = below 1490 MW



(ii)When firing angle is 1620:


The amplitudes of oscillations:





(iii)When firing angle is 1730



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The amplitudes of oscillations are-





Comparison and discussion for stability:

Table-7 Oscillation Time

0.28 0.26 0.240.2

0

0.050.1

0.15

0.2

0.25

0.3

Seconds

Seconds

Seconds 0.28 0.26 0.24 0.2

Uncompe146 (with159 (with170 (with

1 2 3 4

Fig.10 Stability diagram for transmission lineBy comparing the four cases of power oscillations, it

is observed that oscillations damp faster when the

firing angle of thyristor is 170o, taking only 0.2

second and the amplitude of oscillations is

comparatively low. But, when the line is

uncompensated, oscillations damp after 0.28 seconds

and the amplitude of oscillations is high as comparedto compensated line. It is proven that TCSC improves

transient stability of the system.

X. CONCLUSIONS:

This paper analyzes the effect of TCSC on the

power flow through the buses with resistive and

inductive loads. The simulation results show one of

the salient features of TCSC, i.e., enhancement of

power by operating TCSC in capacitive region power

as it is the important issues of power transmission

system.

Table 8 Comparison of power (MW)

It is observed from the table 8 that without

compensation, the transmission line transfers 685

MW. When this transmission line is series

compensated with fixed capacitor, line transfers 1290

MW. With TCSC operated in capacitive region , the

power transfer capability of line increases and it

becomes 1550 MW which is 2.3 times more (126%)than the power when line is not compensated

(i.e..685MW). By comparing the four cases of power

oscillations, it is observed that oscillations damp

faster in compensated line as compared to

uncompensated line. Moreover the amplitude of

oscillations is lower in case of compensated line.

Now it is well proven that TCSC improves stability

of system.

REFERENCE1. N. H. Hingorani, "Flexible AC transmission systems,"IEEE

Spectrump. 4045, Apr. 1993.2. N. Cbristl, R. Hcdin, K. Sadck, P. Llitzelberger, p. E.

Krduse, S. M. McKcnna, A. H. Maiitaya, and D. Togerson,

"Advanced series compensiuion (ASC) with thyristor controllcdimpedance," Paper14/37/38-05.

3. MATLAB Based Simulation of TCSC FACTS Controller

Preeti Singh, Mrs.Lini Mathew, Prof. S. Chatterji ,N.I.T.T.T.R.Chandigarh. RIMT-IET, Mandi Gobindgarh. March 29, 2008.

4. The Impact of FACTS Devices on Digital Multi-functional

Protective Relays Mojtaba Khederzadeh, 2002.5. Identification of Thyristor Controlled Series Capacitor

(TCSC)Erivelton G. Nepomuceno1, Ricardo H. C. Takahashi1,

Luis A. Aguirre1, Oriane M. Neto2.

6. Power Quality Enhancement by TCSC Application to

Mitigate the Impact of Transformer Inrush Current

Mojtaba Khederzadeh, Senior Member, IEEE2008 IEEE.7 Selection of TCSC parameter:Inductor and CapacitorIEEE

2011, S. Meikandasivam, Rajesh Kumar Nema, and Shailendra

Kumar Jain.

Paper presented(i) Sunita Tiwari and S.P.Shukla Implementation of TCSC on

a Transmission Line model to analyze the variation in Power

Transfer capability, BITCON, National conference,Nov.2008.

(ii) Sunita Tiwari and S.P. Shukla Thyristor-Controlled Series

Capacitor and its application on Transmission System to

improve Transient stability AICON, National conference,

CSIT, Durg, Feb.2009.

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