Managing harmonic current distortion for grid connected converters with low per-unit filter impedances

The filter and controller design for a grid connected inverter is a challenging balance between size, cost and complexity with various standard harmonic requirements. Unfortunately, while LCL filters provide exceptional PWM frequency filtering they tend to generate significant baseband harmonics. This paper presents an analytical explanation as to why low per-unit filters, specifically LCL filters, generate more baseband harmonic distortion than a conventional L filter. Firstly it identifies that both grid distortion and baseband distortion created by deadtime and device volt drop contribute similar magnitudes of harmonic voltage, and hence potentially reinforce one another to drive harmonic currents into the grid. Next it is shown how the lower total inductance of an LCL filter significantly reduces its baseband impedance, generating more harmonic currents for a given level of voltage distortion. This outcome is further exacerbated by a reduced allowable controller bandwidth due to the LCL resonance. Consequently the paper proposes that feedback harmonic compensators are the best practical option for mitigation of both grid and converter baseband harmonic current distortion. The analysis is verified by simulation and experimental results.