Headroom-Ordered Hybrid Grid-Forming/Grid-Following Photovoltaic Units: Equilibrium, Stability, and Current-Limit Analysis

A photovoltaic plant can act as a grid-forming source only while it holds power in reserve. This paper analyzes a two-stage photovoltaic unit whose inverter follows a modified REGFM_C1, the generic hybrid grid-forming/grid-following inverter model of the Western Electricity Coordinating Council, made fully dynamic and extended with a direct-current port and a coordination law. Three actuators regulate the direct-current link at staggered set-points. The boost converter curtails the array while reserve remains, the grid-following branch yields next, and the grid-forming branch gives way last. Without limiting or saturation, every equilibrium engages at most one of the curtailment and the yield and delivers the droop response clipped by the available power; with the limiters active, the link voltage still sits at its upper set-point whenever reserve remains. In a reduced model, the direct-current mode is stable at every positive reserve, with a margin independent of it and a damping floor set by the array-side capacitor, and link-voltage excursions obey a tight bound. Under current limiting, damping that acts through the current reference loses its authority along the current direction, where a separate feedforward filter supplies damping. On the 9-bus system, the unit rides through balanced and unbalanced faults, and, unlike a mode-switched scheme, it keeps its grid-forming frequency response in a deficit without switching.