Evaluation of low-voltage ride-through capability of a two-stage grid-connected three-level photovoltaic inverter

Low-voltage ride-through (LVRT) requirements demand inverter-interfaced renewable energy power generation systems to remain connected in the presence of grid faults, by injecting required reactive current for voltage support. In this paper, a two-stage grid-connected photovoltaic inverter consists of a boost converter and a three-level T-type inverter is investigated. A stable decoupled double synchronous reference frame phase-locked loop (DDSRF-PLL) is adopted appropriately for three-phase grid synchronization under faulty grids or abnormal conditions. Moreover, the DDSRF could extract and separate both positive- (PS) and negative-sequence (NS) components of grid voltage and current. During normal operation, the cascaded control structure of T-type inverter is composed of an outer proportional-integral-derivative (PID) dc-link voltage control loop and four inner multivariable-PI current control loops. Once grid voltage sags are detected by the separated d-axis PS voltage component, the current references are calculated according to the sag depth so as to stay in maximum power point tracking (MPPT) mode or switch to Non-MPPT mode, meeting the latest LVRT requirements of China. The performance of the LVRT capability is evaluated by simulation and experimental results.