Multifrequency Impedance Model of Single-Phase Grid-Connected Parallel Inverters for Analysis on Circulating Resonant Current

The injected current quality of the multi-parallel grid-connected inverters is challenged by the circulating resonant currents which usually locates beyond the control bandwidth. Taking the digital control and the nonlinear PWM into consideration, this paper analyzes the resonant characteristics of the circulating currents. Firstly, assuming that the modulated wave is polluted with harmonics, based on the one-dimensional spectrum analysis method, the harmonic spectra of the bridge voltage is mathematically deduced. The multifrequency model of the parallel single-phase grid-connected inverters is then proposed. Applying the generalized Nyquist stability criterion to the proposed model, the resonant frequency of the circulating currents can be predicted, and the influence of the carrier phase on the parallel system stability is revealed. Finally, it is pointed out that the enlargement of the modulus of the output impedance contributes to suppress the circulating currents. The experimental results verified the correctness and effectiveness of the proposed model.