A DSP-based digital controller for a thyristor controlled series capacitor

Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2007.The power transfer capability of long high voltage transmission lines is often limited by the inductive reactance of the transmission line. Series compensation is in some instances employed to lower the inductive reactance of the transmission line which increases the transmission line power transfer capability. Numerous methods have been employed to provide series compensation of a transmission line. One such method is to use a thyristor controlled series capacitor (TCSC). A thyristor controlled senes capacitor (TCSC) belongs to the flexible altemating CUlTent transmission systems (FACTS) family of devices. It is a variable capacitive and inductive reactance device that can be used to provide series compensation in high voltage transmission lines. One of the significant advantages that a TCSC has over other series compensation devices is that the TCSC's reactance is instantaneously and continuously variable. This means that the TCSC can be used not only to provide series compensation but can also be used to enhance the stability of the power system. However accurate control of the TCSC is challenging due to its highly non-linear variable reactance characteristic. The TCSC consists of back to back thyristors that control the reactance of the TCSC. By changing the trigger angle of these back to back thyristors it is possible to vary the reactance of the TCSC. The reactance characteristic becomes highly non linear at higher levels of compensation; at such operating points the trigger angle of the thyristors needs to be accurately controlled to avoid small variations in the thyristor trigger angle causing significant variation in the reactance of the TCSC. Literature has shown that there is an acceptable limit to the resolution of the thyristor trigger angle based on the parameters of the components used in the TCSC. If a controller is developed to meet this acceptable level of thyristor trigger angle resolution, then the operation of the TCSC will also be acceptable and its operation will not result in unwanted fluctuations in the transmission line variables. This thesis details the development of such a controller for use in a laboratory-scale TCSC. The thesis then goes on to present the practical results obtained from laboratory experiments on the laboratory-scale TCSC with the TCSC triggering controller being used to control the operation of the laboratory-scale TCSC. For purposes of comparison and benchmarking, a detailed simulation model of the laboratory-scale TCSC is developed to take into account the non-ideal properties of the components used in make-up of the laboratory-scale TCSC since the theoretical model is derived assuming ideal conditions. The detailed simulation model is also used to aid in the redesign the power circuit of the laboratory-scale TCSC in an attempt to improve the perfonnance of the laboratory-scale TCSC by obtaining better agreement between the theoretical and practical results. The redesigned laboratory-scale TCSC is used to obtain practical results to COnfill11 the findings of the simulation studies. Finally, the TCSC triggering controller is tested using a real time digital simulator (RTDS). The simulation model developed on the RTDS consisted of a two area, four generator power, with the TCSC connected between the two areas. The RTDS simulation model is used to study the ability of the TCSC to damp inter-area mode oscillations and hence the RTDS simulation model incorporated a power oscillation controller. The input of TCSC triggering controller was "connected" to the power oscillation damping controller and the output of the TCSC triggering controller was "connected" to the thyristors of the TCSC