An improved grid-forming control for two-stage PV grid-connected systems

Abstract

Two-stage photovoltaic (PV) plants are increasingly expected to establish grid voltage and frequency rather than operate only as grid-following (GFL) sources. Existing PV grid-forming (GFM) schemes may nevertheless exhibit oscillations during GFM/GFL transitions, and the absence of direct PV-voltage regulation can leave the steady operating point ambiguous or even unstable. This paper proposes an improved GFM strategy for two-stage PV systems (IPVGFM). On the voltage-source-converter (VSC) side, a dynamic DC-link-voltage/frequency droop law uses the DC-link voltage to convey the AC-frequency deviation, and a phase-feedforward term is introduced to improve damping. On the DC/DC-converter side, the PV power commands is generated from the same DC-link-voltage signal and converted into a PV-voltage reference by an enhanced reserve-power-point-tracking (RPPT) algorithm. The algorithm combines perturb-and-observe logic with an adaptive perturbation step, so that the operating point is explicitly located and rapidly tracked. Root-locus analysis is used to examine the phase-compensation parameter, and hardware tests verify irradiance-disturbance response, frequency response, GFM/GFL transition, and RPPT dynamics.