Comparative analysis of stability limitations in weak grid-connected synchronous generator, VSC, and DFIG systems considering the power flow control dynamics

In the conventional power systems, the synchronous generators (SGs) are known to suffer stability issues under weak grid conditions. Recently, similar problems are observed in the inverter interfaced systems, including the full voltage source converter (VSC)-based high-voltage DC, and partial VSC interfaced doubly fed induction generator (DFIG) systems, but the root causes of these instabilities in reality are not completely understood yet. This study aims to provide a possible unified explanation for the origin of these instability issues. With the experiences of SG's instability mechanism, for the VSC and DFIG systems, the dynamics of outer power flow controls are mainly focused, and the reduced-order models are parallelly presented first. Then, centralised around the active power control loops of these systems for analysis, it is found that an open-loop zero will move to the right half plane with grid strength decrease. The borders for the occurrence of right-half-plane zeros (RHPZs) are analytically derived. Further, a general interpretation regarding these identified RHPZs' limitations, including the adverse phase lag effect on stability and the attraction effect on the stability boundary, is illustrated. In addition, a uniform explanation for the origin of the RHPZs in these systems is also provided. Time-domain simulations are conducted to validate the analysis.