Design and implementation of a thyristor controlled series capacitor for research laboratory application.
Mazibuko, Ronnie Happy-Boy.
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The power transfer capability of a transmission line is determined by the magnitude of the voltage at each end of the line, angle difference of these voltages and the impedance of the line. This impedance is mainly inductive. Traditionally, fixed series capacitor banks have been used for series compensation. However, due to instability problems associated with loading transmission line close to their thermal limits, researchers have looked at other alternatives to line compensation by static devices such as fixed series capacitors. Flexible AC Transmission Systems (FACTS) has allowed power utilities to use existing transmission line networks close to their thermal limits without compromising stability of the power system. A FACTS series compensator is capable of influencing the transmission of power in a transmission line by dynamic control of the series compensating reactance inserted in the line. There are several different devices under the FACTS family, however, in this thesis only the Thyristor-Controlled Series Capacitor (TCSC) was considered. A TCSC comprises a fixed capacitor in parallel with a thyristor-controlled reactor (TCR). By varying the firing angle ex:. of the thyristors, the TCSC can be made to act in variable inductive or capacitive reactance mode. The thesis' overall objective was to design a practical TCSC for use in a research laboratory for further research initiatives. This thesis looks at different issues that need to be considered when designing and rating a TCSC compensator. In particular, the thesis examines the effects of different sizes of TCSC components on the rating of the device, the effects of harmonics on the TCSC ratings, sizing of TCSC's variable reactance, and the response time of TCSC to a step change in the firing angle. A mathematical model of a TCSC in a single-machine infinite bus (SMIB) system was developed and subsequently used in the initial design of the TCSC. Studies that were done using mathematical model of the TCSC module confirmed the ability of the TCSC controller to dynamically control the capacitive compensating reactance in the transmission line. The thesis then describes the development of a laboratory-scale TCSC for research investigations. Measured results from the laboratory demonstrate the ability of the TCSC series compensator to provide rapid control of series reactance of a transmission line. A detailed mathematical model of the SMIB equipped with TCSC module was developed, using parameter values of the laboratory scale prototype, to investigate power oscillation damping. Time-domain simulation results are presented in this thesis to demonstrate its ability to damp power swings in an electrical network.