PERFORMANCE EVALUATION OF CASCADED MULTILEVEL CONVERTER BASED STATCOM

Abstract

In this thesis the voltage control performance of the Cascaded Multilevel Converter (CMC) based STATCOM has been studied. Specifically, a 21 level CMC based STATCOM has been considered and its performance for controlling the bus voltage under fault conditions has been investigated in a practical power distribution system. For determining the switching angles of the CMC based STATCOM, an optimization methodology based on Sequential Quadratic Programming technique, has been used so as to minimize the THD of the STATCOM output voltage. Subsequently, a feedback linearization based on non-linear control law has been designed for the STATCOM voltage regulator. However it was found that under severe fault conditions, the performance of the non-linear voltage controller is quite unsatisfactory. To achieve satisfactory performance of the STATCOM voltage controller, Fuzzy Control systems have been proposed in this thesis. Two different fuzzy control systems namely indirect and direct have been developed in this thesis for this purpose. In the indirect control scheme, the .angle difference between the STATCOM bus voltage and the STATCOM output voltage (6) is used as the control signal, whereas the direct control system, both the modulation index and the angle S are used as control signals. The performances of these two developed control schemes have been studied with severe symmetrical and unsymmetrical faults. From detailed non-linear simulation studies it has been observed that the performance of the direct fuzzy control system is considerably superior to that of the indirect control scheme. IV