An Optimal Combination Modulation Strategy for a Seven-level Cascade Multilevel Converter Based STATCOM

This paper proposes an optimal modulation strategy for a static synchronous compensation (STATCOM) using a seven-level cascade multilevel converter without any filter. Cascade multilevel converters have been used in STATCOM application due to its ability to reduce harmonics and increase output power by increasing the level of output voltages for a given semiconductor device without device series connection. Challenges in high power application include relatively low switching frequency (few hundred hertz to 1 kHz) limited by the thermal handling capability of semiconductor switch, and the low harmonic requirement specified by IEEE 519 standard. With carrier based PWM, the STATCOM can generate lower current harmonic distortions at the expense of working at higher switching frequency. If step modulation strategy is used, the inverter works with lower switching frequency, but the STATCOM injects higher current harmonic distortions to power system, which generally leads to usage of extra filters. The proposed optimal combination modulation strategy works with low switching frequency and has low current harmonic distortions. The simulation results show that it can satisfy the device thermal requirements and IEEE 519 harmonic requirements simultaneously