An Active Damping Method based on Biquad Digital Filter for Parallel Grid-interfacing Inverters with LCL Filters

With the increasing penetration of renewable energy sources into modern power system, power electronic converters are commonly employed as the interface to the utility grid nowadays. During the operation of the interfacing converters, LCL-filters are usually used in order to reduce the harmonics around the switching frequency and its multiples. Although the LCL-filters have several advantages compared to single inductance filter, its resonance problem should be noticed. Conventionally, the resonance analysis is mainly focused on the single inverter system, whereas in a renewable energy system with parallel interfacing inverters, the resonance analysis should be expanded due to the parallel configuration of the LCL-filters. In this paper, the resonance phenomenon of the renewable energy system with multiple LCL-filters is studied. The frequency domain characteristics of the parallel LCL-filtered inverters are presented. It is shown that additional resonance peaks appear compared to the single inverter system, which further challenge the system stability. In order to facilitate efficient damping of the above multiple resonance peaks, biquad filter based active damping method is proposed. In contrast to the conventional active damping approaches, the biquad filter based active damping method does not require additional sensors and control loops. Meanwhile, the multiple instable closed-loop poles of the parallel inverter system can be moved to the stable region simultaneously. Real-time simulations based on dSPACE 1103 and experimental test are performed to validate the proposed damping approaches.