Influence of phase-locked loop on input admittance of three-phase voltage-source converters

A regulated power converter can cause instability issues at its terminals with its input filter or the source. The Generalized Nyquist stability Criterion (GNC), when applied to the product of the source impedance and load admittance in the dq frame, can be used to analyze the stability of balanced three-phase ac systems. This paper presents the influence of the Phase-Locked Loop (PLL) on the input admittance of three-phase Voltage-Source Converters (VSC) for open loop, current-feedback and voltage feedback control conditions. Analytical results show that the PLL introduces a parallel admittance to the open loop admittance of the VSC, and that the current and voltage control loop have additional effects on it. Both simulation and experimental results are presented to verify this analysis. The possible instability due to different PLL design is also shown.