Control of square-wave inverters in high power hybrid active filter systems

This paper presents a new control scheme for a hybrid parallel active filter system intended for high power applications (up to 50 MW nonlinear loads) to meet IEEE 519 recommended harmonic standards. The active filter is realized with small rated-1%-2%-of the load rating, square-wave inverters at the dominant harmonic frequencies. The controller achieves harmonic isolation even in the presence of supply voltage harmonic distortions for desired dominant harmonic frequencies such as 5^th and 7^th harmonics for six pulse rectifier loads or 11^th and 13^th for twelve pulse loads. A novel method of active filter inverter DC bus control, as proposed here, achieves power balancing by exchanging energy at the fundamental frequency and at the dominant harmonic frequency. The proposed controller directly generates inverter voltage commands and enables low switching frequency inverter implementations including square-wave inverters. This increases the practical viability and cost-effectiveness of this hybrid parallel active filter system especially for high power applications. Simulation results are presented to validate the proposed scheme