Harmonic Stability Assessment based on Global Admittance for Multi-Paralleled Grid-Connected VSIs using Modified Nyquist Criterion

The mutual interactions in the multi-paralleled grid-connected VSIs coupled through the grid impedance tend to result in the whole system oscillating-formed resonance, namely the harmonic stability issue. To address them, the traditional impedance-based stability criterion (IBSC) evaluates the stability by the impedance ratio, also known as minor loop gain (MLG). In this paper, the novel global admittance is proposed to assess the harmonic stability using the modified Nyquist criterion. Compared to the IBSC, the computation efforts would be reduced significantly and the resonance damping factor as the stability margin index can be obtained to assess the stability degree directly. The simulation results verify the effectiveness of the theoretical analysis.