SRF-PLL with dynamic center frequency for improved phase detection

Phase detection is an integral component of any grid-connected, renewable-energy inverter and errors in the phase detector can degrade inverter control performance. To reduce these errors, an improved phase detector based on the synchronous-reference frame phase-locked loop is developed and demonstrated. This method is especially useful in weak three-phase variable-frequency systems, where fast and robust phase detection is important to inverter synchronization, but may be more difficult to achieve due to the varying frequency. The original SRF-PLL must trade off noise rejection with fast tracking capabilities, but by modifying the SRF-PLL to include a feed-forward dynamic center frequency, it is shown that both noise rejection and fast tracking can be achieved. The proposed modification to the SRF-PLL is demonstrated in hardware using dSPACE and a DC-sourced voltage source inverter paralleled to a variable-speed synchronous generator. The reductions in phase error are evaluated based on the reactive power injected by the inverter.