A Simplified D-Q Small-Signal Modelling method of Grid-Tied Inverters for Stability Analysis

Harmonic resonance caused by interactions between grid-tied inverters (GTIs) and weak grid are becoming increasingly prominent. Small-signal modeling of GTIs is the most fundamental work in to analyze the harmonic stability of GTI-weak grid coupling system. Different from generic small-signal modeling of GTIs in dq coordinate, a simplified impedance modeling method has been proposed in this paper after focusing interesting frequency band onto low-medium frequency range. In the simplified modeling method, a partial admittance is introduced, and coupling terms between d and q channels are identified as negligible parts and non-negligible parts, respectively. As a result, a simplified small-signal model of GTI has been obtained, and connections among terms of GTI’s impedance and its physical parts are highlighted to strengthen physical interpretations of dq impedances. In addition, concepts of active power admittance and reactive power admittance are introduced in this paper to demonstrate the influence mechanism of power-level’s variations on GTI’s impedance. Finally, the theoretical analysis in frequency domain and simulation results in time domain are provided to prove that the proposed simplified modelling method can be used to analyze the resonance mechanism and predict the stability of GTI-weak grid coupling system.