Investigation and resonances damping of multiple PV inverters

This paper addresses the resonance problem in multiple PV inverters based microgrid. Unlike the performance of single grid-connected PV inverter where the resonance frequency is mainly fixed by the output LCL filter parameters, the multiple-PV inverter based microgrid system presents a more challenging picture where inverter interactions will excite complex resonances at various frequencies. As a result, line currents of inverters can be severely distorted even the control schemes and filter circuits are properly designed based on the single inverter model. This paper first develops a microgrid model based on discrete-time domain closed-loop Norton´s equivalent circuits. Multiple resonances can then be evaluated using the developed model. To mitigate the resonances, this paper also designs a virtual harmonic resistance based active damping method. The proposed damping scheme is seamlessly incorporated into the conventional deadbeat current tracking schemes through the direct control reference modification. Simulation and experimental results are provided to validate the correctness of the developed resonance modeling and active damping methods.