Research On Control Strategy Of Grid-connected Photovoltaic Inverter With LVRT Ability

Inverter is the core of photovoltaic grid-connected system, the design of its control strategies directly affects the performance of grid-connected system. As the installed capacity of photovoltaic power stations is increasing, the influence on stability of the power grid is bigger and bigger. Large-scale photovoltaic power stations must have low voltage ride through capability. This paper studies the low voltage ride through technology of photovoltaic power stations, the completed tasks are as follows:Photovoltaic grid-connected LVRT requirements at home and aboard, phase-locked loop technology and Crowbar low voltage protection circuit are introduced. Three methods of realizing low voltage ride through are analyzed. They are separately based on reactive compensation devices, energy storage devices and power restriction.Voltage-dip detection methods based on dq-PLL and SOGI are studied. Aiming at the problem that dq-PLL method cannot detect the voltage dip caused by injecting negative sequence component into the phase-locked loop when unsymmetrical voltage dip occurs, a detection method based on SOGI which can separate voltage phase sequences is studied. Only positive sequence component is allowed to phase-locked loop. The simulation results showed that the method based on dq-PLL could quickly detect symmetrical voltage dip and the method based on SOGI could rapidly detect unsymmetrical voltage dip.The topological structure of photovoltaic grid-connected inverter system is designed, its mathematical model is established. The inverter adopts SVPWM modulation mode. Double closed-loop control strategy of photovoltaic grid-connected inverter under normal grid operation and LVRT control strategy based on support of reactive power are studied. The key of LVRT is to ensure the inverter connected to grid under overcurrent due to voltage dip. Redistributing active current and reactive current reference value of photovoltaic grid-connected inverter output according to the grid voltage dip depth can avoid overcurrent and support reactive power to restore the grid voltage, thus realizing LVRT. The simulation results showed double closed-loop and LVRT control strategy are effective and feasible.