Suppression control of module capacitor voltage fluctuation for cascade STATCOM

This paper presents suppression control of the module capacitor voltage fluctuation for the Cascade Static Synchronous Compensator (STATCOM). In the control of the reactive current of the cascade STATCOM, the instantaneous power of the each module fluctuates by twice the source frequency. Therefore, the module capacitor voltage of the cascade STATCOM fluctuates by twice the source frequency. If the capacitor voltage fluctuation is suppressed, the average capacitor voltage can be reduced. Then, the large margin to the breakdown voltage of the switching device is obtained and the switching losses can be reduced. The authors propose the suppression control of the module capacitor voltage fluctuation for the cascade STATCOM. The suppression of the capacitor voltage fluctuation can be achieved by superimposing the third-order harmonic voltage to the output ac module voltage. The relation between the third-order harmonic voltage and the suppression of the voltage fluctuation are derived. The effectiveness of the proposed control of the cascade STATCOM has been verified by experiments.