In this paper, a holistic grid-forming (GFM) strategy is proposed and developed for PV systems without energy storage. The proposed strategy is based on the secondary level of the grid-forming control diagram, which makes the PV inverter operate smoothly between islanded, synchronization, GFM, and limited grid-forming (LGF) modes. The practical criteria for determining the operation modes are established and implemented. The proposed design is validated by case studies with a laboratory prototype using Siemens Software Defined Inverters.
