Comparison and analysis on common modulation strategies for the cascaded multilevel STATCOM

With the application of multilevel inverter in the high voltage and large power occasions in recent years, its modulation strategy has become a research hot point in the field of power electronics. As STATCOM is a typical application of multilevel converter, its common modulation strategies such as Step Wave modulation, Specific Harmonic Elimination modulation, Carrier PWM modulation, Space Vector modulation are summarized in this paper. It also points out the particularity of the most popular cascaded multilevel structure to other main circuit topologies of STATCOM and pays heavy attentions on two typical modulation strategies: SHE-PWM and CPS-SPWM. Simulations on these two algorithms in a cascaded 5-level STATCOM are done. By results discussion and comparison, it is known that these two methods are both suitable for the cascaded multilevel topology. The harmonic elimination ability of SHE-PWM is better than that of CPS-SPWM, while its calculation work becomes tremendous and difficult with the level number increasing. CPS-SPWM is easy to realize and its calculation speed is fast even under a very high level number. The proper modulation strategy of a cascaded multilevel STATCOM will be chosen based on the switch frequency limitation, STATCOM volume and its main circuit topology requirements.