Responsibility Division for Harmonic Instability in the Multi-Paralleled Grid-Connected Inverters System Based on Impedance Analysis

In the multi-paralleled grid-connected inverters system, harmonic oscillations may be deteriorated or even lead to system instability by the coupling between the inverters and the grid impedance. This paper presents a quantify method to determine which inverters contribute more to the harmonic instability so that the corresponding inverters can be removed quickly or targeted oscillation suppression measures can be taken to restore the system to stable operation. In the method, a contribution factor (CFi) is defined which can quantitatively represent the responsibility of each inverter to system instability. The CFi are calculated by analyzing the variation of system stability margin after different inverters are removed based on the bode diagram, therefore, according to the corresponding CFi, which inverters have a significant impact on harmonic instability can be determined under different grid impedances. Besides used in the parallel system with different types of inverters (PSDI), the CFi can also be used for the parallel system with identical inverters (PSII) to determine the number of inverters that need to be removed to restore the system to stable operation. The reasonableness and effectiveness of the proposed method are verified by simulation results.