Low-capacitance cascaded H-bridge STATCOM for full power operation

A cascaded H-bridge STATCOM requires large DC filter capacitors to suppress the low-frequency voltage fluctuation in each of the H-bridge cells. Keeping the voltage stress of power device unchanged, increasing the fluctuation of the DC-link voltage is a possible method to significantly reduce the required capacitance for a STATCOM. However, excessive voltage fluctuation greatly degrades the operating range of the inductive conditions. This paper proposes a universal capacitance design method that can significantly reduce the required capacitance when compared with the traditional design method. First, the inductance is constrained in the form normalized by the system impedance. Then a new analysis method between the DC-link capacitance and the operating range is introduced. Experiment results obtained on a low-power 5-level cascaded H-bridge STATCOM prototype are presented to validate the correctness of the proposed design method.