Important Short Questions and Answers : Thyristor Controlled Series Capacitor (TCSC) and Applications

THYRISTOR CONTROLLED SERIES CAPACITOR (TCSC) AND APPLICATIONS

1. What is meant by TCSC?

TCSC is a thyristor controlled series capacitor. It has one parallel connected thyristor controlled inductor and a series capacitor connected with the transmission line. It provides continuous variable capacitive reactance and variable inductive reactance to control the transmission line parameters.

2.Write down the expression for equivalent impedance, capacitive and inductive reactance of a TCSC.

Equivalent impedance of a TCSC, Zeq = -j ((1) / (ωc-1/ ωL))

If (ωc - (1/ ωL)) < 0 - the TCSC provides variable capacitive reactance mode. If (ωc - (1/ ωL)) > 0 - the TCSC provides variable inductive reactance mode.

3. What are the different modes of operation of TCSC?

Bypassed- thyristor mode

Blocked - thyristor mode

Partially conducting thyristor or Vernier mode.

4. Give short notes on Bypassed- thyristor mode.

In this mode the TCSC module behaves like a parallel capacitor - inductor combination. The susceptance (increase in power flow) value of inductor is higher than the capacitor. This mode is employed for control purposes and initiating certain protective functions.

5. Give short notes on Blocked - thyristor mode.

In this mode, also known as the waiting mode, here the firing pulses of thyristor switches are blocked. The TCSC behaves like a fixed series capacitor and the net TCSC reactance is capacitive. In this mode the dc-offset voltages of the capacitors are monitored and quickly discharged using a dc-offset control without causing any harm to the transmission system transformers.

6. Give short notes on capacitive Vernier mode.

In this mode the TCSC provides continuously controllable capacitive reactance. It is achieved by varying the thyristor pair firing angle in an appropriate range. A variant of this mode is the capacitive vernier mode, in which the thyristors are fired when the capacitor voltage and current have opposite polarity. This condition causes a thyristor controlled reactor (TCR) current that has a direction opposite that of the capacitor current thereby resulting in a loop-current flow in the TCSC controller. This loop current increases the voltage across the fixed capacitor (FC), effectively enhancing the equivalent capacitive reactance and the series compensation level.

7. Give short notes on inductive vernier mode.

In this mode the TCSC provides continuously controllable inductive reactance. Here the TCSC can be operated by having a high level of thyristor conduction. In this mode the direction of the circulating current is reversed and the controller presents a net inductive impedance.

8. What are the conclusions made from the TCSC modes of operation?

Thyristor switched series capacitor (TSSC), which permits a discrete control of the capacitive reactance.

Thyristor controlled series capacitor (TCSC), which offers a continuous control of capacitive or inductive reactance. Practically TSSC is more commonly used.

9. What are the modeling techniques involved in TCSC?

Variable reactance model (1. Transient stability model 2. Long term stability model)

An advanced transient stability studies model

10. What is the need for modeling of a TCSC?

A TCSC involves continuous - time dynamics, relating to voltages and currents in The capacitor and reactor, and nonlinear discrete switching behavior of thyristors. So it is Very important to derive a model for a TCSC controller to maintain the stability of a power system.