Utility-Scale PV Inverter Impedances in D-Q Frame Under Different Q-Control Modes

Per the latest revision of the IEEE Std 1547, PV inverters will be required to provide reactive power regulation services in the near future. Accordingly, the stability impact of PV inverters in distribution grids attracts more and more attention. Compared to the positive sequence impedance method, the D-Q frame impedance matrix method is more accurate for grid-tied PV inverters that have non-symmetrical control in DQ frame, including DC voltage loop and phase locked loop (PLL). This paper uses the D-Q frame impedances of utility-scale PV farm and the generalized Nyquist Criterion (GNC) to assess the stability in medium voltage distribution grids. A comparison is done for PV farm under different Q control modes. The control mode of volt-var mode changes PV terminal impedance signs and magnitude significantly, and may cause stability problem, as a result of interaction of voltage control of multiple PV inverters in parallel. Hardware experiments show the accuracy of PV impedances from derivation based on small signal model.