Photovoltaic Synchronous Generator: Architecture and Control Strategy for a Grid-Forming PV Energy System

Transforming a conventional photovoltaic (PV) energy system from a grid-following to a grid-forming system is necessary when PV power generation is dominating the generation mix and for replacing traditional synchronous generators (SGs). The grid-forming PV energy system can provide frequency support functionality, which is vital for the stability of the power grid. This article presents a novel ac coupled solution that transforms an existing grid-following PV system to a grid-forming one without any hardware and software modification of the PV inverter. The resulting system, the PVSG, is achieved by an ac coupled supercapacitor-based energy storage system (ESS). The novel control of the PVSG is implemented in the ESS side. The novel control scheme includes fast and slow instantaneous power controls. The fast-instantaneous power flow control is fulfilled by the dc-link voltage control and ac voltage control. The cascaded voltage source controls enable fast-instantaneous power balance, while a slow instantaneous power control is used to implement inertia and grid synchronization. Two important fundamental functions are realized in the PVSG. The first one is the frequency inertia to resist the grid frequency variation where df/dt-based power control is achieved; the second one is the inertia against the PV intermittent power. The correctness and the effectiveness of the proposed PVSG are experimentally validated in a 480 V PVSG prototype with a TMS320F28379D DSP controller.