A Phase-locked Loop Method for Low Voltage Ride-Through Control in Three-phase Three-wire Photovoltaic Grid System

Low voltage ride through (LVRT) is regarded as one of the biggest challenges in photovoltaic grid equipment's designing, manufacturing and control technology. The fast and accurate automatic phase-locked control is the premise to achieve photovoltaic grid low voltage ride through. Considering that the traditional phase-locked loop method is difficult to adapt the performance requirements of low voltage ride through (LVRT) in three-phase three-wire photovoltaic grid system, this paper presents a new three-phase unbalanced phase-locked loop method. Improved second order generalized integral orthogonal signal generator(SOGI-OSG) is used for line voltage detection; Synchronous reference frame for phase-locked loop(SRF-PLL) is used for line voltage phase-locked and phase angle detection; Heron formula and Sine theorem is adopted to calculate the three-phase phase voltage amplitude and phase angle. Research results show that when grid voltage dips, this phase-locked loop method can detect the fault signal accurately. It provides a technical support for photovoltaic grid low voltage ride through system.