A variable inductor based harmonic filter design for multi-phase renewable energy systems with double closed-loop control

Abstract

Compared to L- or LC- type filters, LCL-type filters are superior in high-frequency harmonic attenuation ability. But the filtering inductors are designed with fixed values in traditional principles, which aim to limit the harmonic amplitude at the rated power. However, power converters may have to operate under load-shedding conditions due to the intermittent of both renewable energy and power consumers, which will worsen the harmonic issue on account of a skimpy filtering inductance. In this paper, in order to adapt the light-load conditions to a certain degree, an LCL-type filter with a variable converter-side inductor is designed for a multi-phase inverter system. The variable inductor can obtain about four times inductance at low currents compared to the inductance at the rated power. Meanwhile, a systematic double closed-loop control strategy is applied based on the provided extended coordinate transformation theory in multi-phase system, which can be generally applied in any n-phase systems. And in order to meet steady-state performance requirements of the system with the variable inductor, a system calibration method is proposed and introduced to design the controller parameters. Taking a five-phase inverter as an example, feasibility and validity of the proposed scheme have been verified based on the finite-element analysis (FEA), PLECS simulation platform and practical experiments.