Passivity-Based Zero-Sequence LC Resonance Suppression Method for Parallel Inverters System With Modified LCL Filter

Parallel inverters system with modified LCL (MLCL) filters may trigger zero-sequence LC (ZSLC) resonances. Especially in high-power applications, lower switching frequencies are usually adopted to reduce switching losses. In this situation, conventional inverter-side circulating current feedback (ICCF) control cannot behave as expected and introduces new stability issues due to high control delays. To deal with this situation, a passivity-based ZSLC resonance damping method is proposed. Firstly, the equivalent admittance model of ICCF control is established for the first time, which is decomposed into passive component and active component. The results show that the system stability is determined by the active component and the inverter output admittance ratio. Secondly, by analyzing the frequency-domain characteristics of active components, it is found that the control delay introduces a negative conductance that affects the stability of the ICCF control. Accordingly, a controllable admittance component is added to correspondingly reshape the inverter output admittance characteristics under high control delay conditions, which can mitigate the detrimental impact of control delay in the active component. Thus, the proposed control strategy can enhance the system stability and the circulating resonance current can be effectively attenuated in high-power applications. Finally, the feasibility and correctness of the proposed scheme are verified by the experiments.