Operation of Thyristor Controlled Reactor and Thyristor Switched Capacitor of Static var Compensator for Voltage Variations

The deviations that occur in electrical power supplied by utilities to consumers are termed as power quality disturbances. Due to power quality disturbances, a change is evident for a short duration in voltage, current or frequency. In order to maintain constant voltage to the connected load, compensation devices are used based on flexible AC transmission systems (FACTS) technology. Based on an increase or decrease in voltage, suitable correction action can be taken by power electronic based devices. The voltage and current variations of static VAR compensator, shunt connected flexible AC transmission system device, is analyzed. The variations in system voltage are due to sag and swell. The operation of thyristor-controlled reactor (TCR) and thyristor switched capacitor (TSC) are analyzed which together comprise static VAR compensator.


Introduction
Electric power supplied by utilities must be free of disturbances and supply voltage must be within the range specified.Any violation of these conditions results in erroneous operation of power consuming equipment.In order to minimize the impact of changes in supply voltage, the quality of power supplied must be monitored.In flexible AC transmission systems (FACTS), control is increased and power transfer capability are increased [1,11].shunt and series connected devices, based on FACTS technology.The main objective of an electrical power system is to meet variations of load requirement with power supplied from generating stations.The capacitive and inductive nature of loads connected results in variations of reactive power.The ratio of active power to the reactive power is termed as

Voltage variations
Sag is a decrease in voltage to between 0.1 per unit (pu) and 0.9 pu for durations from 0.5 cycles to 1 minute [2,10].Swell is an increase in voltage above 1.1 pu for durations from 0.5 cycle to 1 minute [2,9].Due to momentary or persistent disturbances in supply voltage, the connected loads in the system can be severely affected.Some of the reasons for the disturbances to occur are load changes, faults, lightning, and switching of loads with reactive components [3,8]

Static var Compensator (SVC)
Static var compensator (SVC) comes under the category of variable impedance type FACTS devices.SVC injects or absorbs reactive power to regulate voltage at a given bus.Fig. 2 shows simulation model to obtain different waveforms necessary to analyze operation of SVC.Without and with compensation provided by SVC is explained in [4,12].In [5,13], optimization techniques are used for SVC operation.The effects of total harmonic distortion (THD) on load profile is considered in [6].
Modeling and simulation of IEEE 14 bus system were carried out in [7] SVC consists of parallel connection of: One number of thyristor controlled reactor (TCR).

Fig.2 Simulink model to depict the operation of SVC
A transmission line forming a part of power system is considered for analysis with SVC connected across the line.In case of TSC, switch has only ON and OFF possibilities and no control is possible.

Different waveforms obtained during SVC operation in MATLAB Simulink environment
Input signals are measured voltage and current for SVC controller.Firing angle-controlled pulses for TCR and on/off pulses for TSCs are obtained as output.Fig. 3 shows SVC connection of one TCR and one TSC out of the three TSCs.
fig.1,In fig.1 sag is initiated from 0.1 to 0.2 seconds and swell is initiated from 0.4 to 0.5 seconds.

Fig. 3 .
Fig. 3. (a) A portion of TCR bank (b) A portion of one of the three TSC bank The TSC components or banks which are made on/ off are decided and the delay angle provided by reactor is calculated.TSCs must be switched to boost up voltage during sag and reactive power generation takes place.TCR must be fired into the power system during swell for reactive power absorption.In case of TSC, switch has only ON and OFF possibilities and no control is possible.In case of TCR, impedance can be controlled by varying firing angle of the pulse generators Figures 4, 5 and 6 respectively represent voltage and current of TSC 1, TSC 2, TSC 3, voltage, current of Thyristor 1 and 2 of TSC.

Fig. 4
Fig.4 Voltage and current of TCR

Fig. 9
Fig.9 Voltage and current of TSC3