Enhancing transient stability of power systems using a thyristor controlled series capacitor.

Abstract

The continuously growing demand for electric power requires transmitting larger amounts of power over long distances. An economically attractive solution to increase the power transfer through a long interconnection (up to a limit) without building new parallel circuits is to install series capacitor compensation on the transmission line. Large disturbances which constantly occur in power systems may disrupt the synchronous operation of the generators and lead to out-of-step conditions. Coordinated insertion and removal of the compensating capacitors in series with a transmission line is an approach that has been known for many years to be capable of enhancing the transient stability of power systems as well as providing additional damping to the power system oscillations. The relatively recent emergence of the thyristor controlled series capacitor (TCSC) has now made this method of transient stability enhancement practically feasible. This thesis compares a range of different strategies that have been proposed in the literature for control of series compensating reactance to enhance transient stability. Initially a simple swing-equation model of a single-generator power system, including an idealised controllable series compensator (CSC) is used to study the fundamental characteristics of the variable impedance control and its impact on transient stability. Subsequently, a detailed model of a small study system is developed, including a detailed representation of a TCSC, for more in-depth analysis. This detailed study system model is then used to compare three different transient stability control schemes for the TCSC, namely: generator speed-deviation based bang-bang control, discrete control based on an energy-function method, and nonlinear adaptive control. Time-domain results are presented to demonstrate the impact of the TCSC on first swing stability of the SMIB system with the above control schemes for various fault scenarios. The performance of each control scheme is also compared by evaluating the extent to which it extends the transient stability margin of the study system. For each of the three different TCSC control approaches considered, the results show that variable impedance control of the TCSC provides further improvement in the transient stability limits of the study system over and above the improvement that is obtained by having a fixed-impedance TCSC in the system. In the case of the bangbang and discrete control approaches, it is shown that a combination of a large steady state value of the TCSC compensation, together with a relative small range of variable TCSC reactance under transient conditions, offers. the best improvement in the transient stability limits for the studied system. The results also show that there is little difference in the extent to which the energy function method of TCSC control improves the transient stability limits over the improvement obtained using speed-deviation bang-bang control of the TCSC for the study system considered.