Zero Voltage Ride-through Technology Research On Single-stage Grid-connected Photovoltaic System

With the increasing of human demand for energy and the increasingly decline of the coal,oil,natural gas and so on,people have realized the importance of the development of renewable energy.The key technology of using of renewable energy is how to integrate renewable energy into the power grid,and the photovoltaic power generation technology can just solve this difficult problem.However,Large-scale photovoltaic power plants connected to the grid,will impact on the security and stability of the power grid.Therefore,In the GB/T19964-2012 "provisions of PV power plants power system access technology" required that photovoltaic(PV)power generation system keep up the operation of the merged power networks,and should have a zero voltage ride-through(ZVRT)capability.considering the characteristics of grid voltage phase and frequency are difficult to obtain during grid voltage drops to zero,design a method of using historical period value as PLL output.Grid-connected PV systems realize the ZVRT,by detecting the voltage amplitude of the PCC point to switch control strategy real-timely,in addition inject a certain amount of reactive current into the broken-down grid to support grid voltage recovery.Building a real-time simulation platform of grid-connected PV system based on RT-LAB and DSP-F2812,the results demonstrate the effectiveness of the control strategy.Taking into account that PV systems will send a certain amount of reactive current to the grid,and cause the change of system power factor during the Low Voltage Ride-Through(LVRT).To rely on the current size of the threshold value,and rely on the phase relationship between the detected voltage and current to determine the fault protection with a non-negligible impact.Therefore,this article made an in-depth study of differential protection sensitivity of output lines of photovoltaic power station.when the output line of grid-connected photovoltaic systems with the Low Voltage Ride-Through capability occurs three-phase short-circuit failures.It is found that the use of LVRT results in the decline of sensitivity of the ratio-restraining characteristic and product-restraining characteristic differential protection.The model of grid-connected photovoltaic(PV)system is set up,and verified by simulink simulation.And further conclusion shows that the impact of differential protection of product-restraining characteristic on sensitivity is smaller,when the reactive power compensation is injected,and is better for the protection of output lines of photovoltaic power station,when the short-circuit fault occurs.For fault characteristic of grid-connected photovoltaic(PV)systems,the sensitivity of ratio-restraining zero-sequence differential protection based on different control strategies is studied,when the output line of grid-connected PV systems occur Single-phase short-circuit ground failures.The results show that sensitivity of differential protection based on constant power control strategy is higher,compared with differential protection under control strategy oriented by grid voltage positive sequence component.After further comparative analysis,in the state of reactive power compensation,sensitivity of differential protection based on constant power control strategy with the increasing of reactive power compensation is reduced.However,it still possesses a higher sensitivity than control strategy oriented by grid voltage positive sequence component,so it is better for the protection of output lines of photovoltaic power station.This conclusion is verified by Simulink model of grid-connected photovoltaic system.