Grid-Connected Photovoltaic LVRT Strategy Based on Improved DDSRF-PLL and Reactive Power Control

When the grid voltage contains high harmonics, decouple double synchronous reference frame phase lock loop (DDSRF-PLL) will not extract the synchronizing information accurately. And conventional grid-connected control strategy makes insufficient use of photovoltaic (PV) inverter's reactive power output capacity when grid is faulty. For above problem, this paper proposes a low voltage riding through control strategy based on improved DDSRF-PLL and reactive current control. After analyzing the structure of DDSRF-PLL module, high-order harmonic filters are introduced before decoupling calculation, so that the module can extract grid synchronizing information and positive/negative sequence value more accurately. Furthermore, by establishing relationship between voltage drop and output reactive current, PV inverter can provide reactive power support required from grid during LVRT, and provide active power as much as possible. When bus capacitor voltage rises due to power imbalance between front and rear stage of system, the crowbar circuit is used to ensure DC voltage stability. Finally, A simulation model is build in MATLAB/simulink to verify the effectiveness of this LVRT strategy.