Research On Three - Phase Photovoltaic Grid - Connected Control And Low Voltage Traverse Technology

Facing the deepening energy crisis and the increasingly deterioration of the environment, the development of grid-connected PV generation industry helps to solve this problem. The inverter is a key power electronic device in the operation process of the grid-connected PV system. What’s more, the chosen of the control strategy directly affect the performance of the whole system. In addition, aiming at the issue of high permeability of the photovoltaic effect on power system stability, a control strategy study is made based on the condition of normal operation state of the power grid and a low-voltage power grid voltage fault, to ensure the reliability of the system. As a key part of a large-scale grid-connected photovoltaic power systems,the research of LVRT technology has great practical significance.In view of the PV inverter, this paper mainly studied the control strategies of the grid-connected PV inverter under normal grid and grid fault conditions. Firstly, according to different coordinate system, two phase synchronous rotating coordinate system and two-phase static coordinate system of the mathematical model were established under the normal operation of the inverter. Also the corresponding of the PI controller under dq coordinate and the proportion of Quasi resonant(Q-PR) controller under αβ coordinate are designed. The simulation results show that two kinds of controller can achieve grid-connected task effectively, but the control strategy with Q-PR controller can inhibit the grid harmonic components in the current and improve grid power quality apparently.Secondly, the LVRT control strategy is adopted based on the active current control strategy, adding the DC unloading circuits to make a inhibition of the rise of the DC bus voltage under grid symmetric fault. All of the above ensures the stable operation of grid-connected PV generation system.Finally, the operation characteristics of the inverter are analyzed under unbalanced grid condition. In terms of the slow dynamic response of the traditional method under complex control system, the way of double current loop frame with the positive and negative sequence could not meet the needs, the LVRT control strategy based on Q-PR controller is introduced. The process of the positive and negative sequence separation of the current loop is saved and the design of control system is simplified. It helps improve the dynamic response of the system. The simulation results show that the designed control strategy of Q-PR controller can make the grid-connected PV generation system work well in cases of the asymmetric power grid failure and realize LVRT.