Control strategy study of STATCOM based on cascaded PWM H-bridge converter with delta configuration

This paper describes a transformerless static synchronous compensator (STATCOM) system based on cascaded pulse-width-modulation (PWM) H-Bridge converter with delta configuration. It is intend for dynamically compensating reactive power and negative-sequence load current, as well as for improving power quality and reliability of power distribution system. In this paper, attention is paid to reactive-power control, negative-sequence current compensation, along with voltage-balancing control consisting of clustered balancing control and individual balancing control. Clustered balancing control that keeps the dc voltage of each cluster equal to the dc mean value of all the three clusters is achieved by introducing a zero-sequence current flowing through the loop of the three delta-connected clusters in three phases. The aim of this control scheme is to fully extend power-quality- improving capability of STATCOM system. A 100-V 3-kVA downscaled laboratory system is designed, constructed and tested, using large-capacity electrolytic capacitors as dc-voltage sources. Experimental results obtained from the laboratory system verify the viability of this cascaded multilevel PWM STATCOM system.