Computation of Frequency Characteristics of Grid Inverters by General Small-Signal Analysis Method

This paper proposes an automatic formulation and analysis method, which utilizes a general-purpose simulation algorithm for frequency analysis of an averaged power converter system, from connection and model information of circuit and control elements. This method is generally applicable even to a converter with nonlinear elements as far as the objective system can be linearized at the operating point. As application examples, a single-phase grid-connected inverter with digital control and a three-phase grid-forming inverter with power control are analyzed for the frequency characteristics of the output admittances to apply the Nyquist criterion. The proposed method is accurate enough up to the Nyquist frequency of the switching cycles. The impedance characteristics are utilized for stability analysis.