A study of renewable energy system harmonic resonance based on a DG test-bed

Harmonic resonance involving grid-parallel inverters is becoming a critical issue in power systems with increasing amount of renewable energy and other distributed generation (DG) resources. Such harmonic resonance can occur at the grid interconnect point of individual or multiple grid-parallel solar or wind inverters, and is usually the result of impedance mismatch between the grid and the inverters. The problem typically appears as a sustained resonance dominated by certain frequency component(s). This paper presents an impedance-based approach to detecting and characterizing renewable energy system harmonic resonance, and use it to evaluate various harmonic resonance behaviors observed on a distributed generation system test-bed with controllable grid impedance. Design and capabilities of the DG test-bed are presented along with measurement results involving single-phase solar inverters and a three-phase wind inverter. The impedance-based system stability analysis technique is applied to quantify the observed harmonic resonance problems and to explain their origins.