Operating range extension of cascaded H-bridge star using hybrid converter topology for STATCOM under unbalanced conditions

Abstract

A controller for a hybrid converter for STATCOM applications, based on a cascaded H-bridge star in series with a generic converter connected at the star point, is proposed. The controller utilizes the generic converter in an optimized way to maximize the operating range of the converter, with focus on unbalanced operating conditions. So for each operating condition, the generic converter reference voltage minimizes the voltage rating of the star part. Theoretical investigations of the hybrid converter using the proposed control approach show that the hybrid converter allows to effectively extend the operating range of the converter under unbalanced conditions of the system, given that the generic converter is large enough to circulate the needed active power between the converter phase legs. This indicates that, depending on the requirements from the operator in terms of negative-sequence injection capability, the hybrid converter can be a valuable solution to overcome the limitations of the CHB star.