Minimizing the effect of a distorted DC-bus voltage of a STATCOM

This paper proposes a control technique for minimizing the effect of a distorted DC-bus voltage on the operation of a PWM controlled STATCOM. A DC-bus voltage ripple occurs as a consequence of harmonic or unbalanced currents exchange between the compensator and the network and causes additional harmonic distortion on the AC side, which is not permissible for compensator operation. The proposed DC-ripple mitigation technique is based on the modulation of the switching function enabling the device operation with distorted DC-bus voltage and allowing the use of a relatively small capacitor on the DC side. The derived control algorithm is validated by means of digital simulation in the PSCAD/EMTDC program where a case of a STATCOM operating under unbalanced conditions (unbalanced network voltage) was evaluated.