PV Controller Modification and its Impact on Assisting PV Penetration

Large-scale penetration of grid-following inverters into the electricity network presents various technical challenges to grid reliability. It is well-known that the ability of a grid to maintain a stable frequency is inhibited by adding traditional grid-tied photovoltaic (PV) generators. In this work, a detailed model of a simplified grid is presented, and it is shown that the proportion of PV generation and instability are positively correlated. The main instability phenomenon is captured by a Hopf Bifurcation in the field dynamics of the synchronous generator. Such a Hopf bifurcation severely constricts the feasible operating domain of the grid and may hinder normal operation. Modifying traditional grid-tied PV controllers and its impact on grid stability is assessed through small-signal, bifurcation and transient numerical analysis. Traditional PV controllers that are modified to virtual synchronous machine (VSM) type controllers show improvement in system damping. Unlike traditional grid-tied inverters, VSM inverters participate in critical modes of the synchronous generator (SG) and augment the operational domain of the SG+VSM system significantly, more importantly, almost eliminating the need for renewable energy curtailment. A case-study approach is used to present some key results on improvements in damping ratio, feasibility domain and transient stability. Finally, a feasibility domain curve is introduced and discussed in an aim to generalize the overall stability of any such system.