FACTS: Thyristor-Controlled Series Compensation

Thyristor-Controlled Series Compensation (TCSC) is used in power systems to dynamically control the reactance of a transmission line in order to provide sufficient load compensation.  The benefits of TCSC are seen in its ability to control the amount of compensation of a transmission line, and in its ability to operate in different modes. These traits are very desirable  since loads are constantly changing and cannot always be predicted.

TCSC designs operate in the same way as Fixed Series Compensation, but provide variable control of the reactance absorbed by the capacitor device. The basic structure of a TCSC can be seen below:

A thyristor-controlled series compensator is composed of a series capacitance which has a parallel branch including a thyristor-controlled reactor.

TCSC operates in different modes depending on when the thyristors for the inductive branch are triggered.  The modes of operation are as listed:

• Blocking mode: Thyristor valve is always  off, opening inductive branch, and effectively causing the TCSC to operate as FSC
• Bypass mode:  Thyristor valve is always on, causing TCSC to operate as capacitor and inductor in parallel, reducing current through TCSC
• Capacitive boost mode: Forward voltage thyristor valve is triggered slightly before capacitor voltage crosses zero to allow current to flow through inductive branch, adding to capacitive current. This effectively increases the observed capacitance of the TCSC without requiring a larger capacitor within the TCSC.

Because of TCSC allowing different operating modes depending on system requirements, TCSC is desired for several reasons. In addition to all of the benefits of FSC, TCSC allows for increased compensation simply by using a different mode of operation, as well as limitation of line current in the event of a fault. A benefit of using TCSC is the damping of sub synchronous resonance
caused by torsional oscillations and inter-area oscillations.  The ability to dampen these oscillations is due to the control system controlling the compensator. This results in the ability to transfer more power, and the possibility of connecting the power systems of several areas over
long distances.

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This article was taken from the introduction of a report which was written by a partner and I, submitted to ECE3333: Power Systems I, taught by Professor Rajiv Varma at the University of Western Ontario.

FACTS: Series Compensation

The main purpose of series compensation in power systems is to decrease the reactive impedance of the transmission line to reduce voltage drop over long distances and to reduce the Ferranti effect.  By adding series capacitors to the line, engineers can compensate for the physical inductance inherent in the transmission line.  The voltage drop across the line is reduced with more compensation, allowing more power to be received by the load for any given sending power.  Two main types of series compensation are fixed series compensation, and thyristor controlled series compensation, each with their own advantages.

Fixed Series Compensation

Fixed series compensation (FSC) of a line is desirable for power transmission due to the effects of line reactance modification.  By adding series capacitance, the reactive impedance of the line decreases, thus lowering the voltage drop across the transmission line.  This effect can be seen through the simplified power flow equation (see the post about Power Transfer) obtained by neglecting line resistance and line charging capacitance.

Line reactance is counteracted by a series capacitance, resulting in overall lower line impedance and a lower voltage drop across the line.

By adding the series capacitance, it can be seen that the receiving line end voltage will be closer to the sending line end voltage.  This decrease in voltage drop across the line allows more power to be transferred over the line for any given sending line end voltage.

The advantage to using FSC compared to thyristor controlled series compensation is price.  Usually FSC allows for a majority of compensation for a lower cost when compared to thyristor controlled series compensation. The following phasor diagram demonstrates the effect of series compensation:

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This article was taken from the introduction of a report which was written by a partner and I, submitted to ECE3333: Power Systems I, taught by Professor Rajiv Varma at the University of Western Ontario.

Power Transfer Simulation using PowerWorld

Recently, I completed my first “lab” for ECE4464: Power Systems II. Like ECE3333 (Power Systems I), this course is being taught by one of the most inspiring professors I have ever had, Prof. Rajiv Varma, Ph.D.

Using PowerWorld’s Simulator software, we repeated one of our basic labs from ECE3333 as an introduction to computerized modelling of power systems. We connected a single synchronous machine to an infinite bus across a 600km, 1000MW-SIL power line.

It is simplest for me to just lift the objectives from my lab report:

In this lab, our objective is to simulate a simple single machine infinite-bus configuration using the PowerWorld Simulator software.  We design a local generator system (a synchronous generator) having a nominal generation capacity of 500MW and with no predefined peak generation (that is, the generator is modelled as having infinite generation capability).

In this manner, we can explore various phenomena like power transfer, power system stability and the effect of shunt compensation on the midline.  We model a 600km span of transmission line with a shunt compensation device installed at the midline (300km from both ends) and determine the stability limit with and without this compensation device enabled.

Please see the following images, which show the simulation being run in PowerWorld:

500 MegaWatt generation, no compensation

500 MegaWatt generation with Synchronous Condenser Compensation

For my full report, see: Power Systems 4464 Lab 1 (PDF). Note that the synchronous condenser installed at the midline is a Switched Shunt Compensation unit. I thought the standard inductor/capacitor schematic symbol looked a little boring, so I overlaid a synchronous condenser on top of it.