Research On Key Technologies Of Three-phase Grid-connected Photovoltaic System

With the advantages of lo w carbon environmental and the convenience of management, maintenance, easy expansion, the large capacity of Grid-connected PV power station is becoming a focus direction of the distributed generation. But with the PV capacity has an increasing high proportion in the grid, its influence on the grid could not be more importance. To ensure the safe and stable operation of power grid, the state has proposed the strict technical specification on PV generation systems and formulated the relevant technical standards.This paper has deep researched on the two-stage three-phase grid-connected PV system and completed the design of the control system. Simultaneously, in view of the key technical problems such as the voltage-sag detection, LVRT operation an d control, islanding phenomenon detection and equipment of VSG and so on, the me ans of theoretical analysis, simulation and experimental research were taken. The m ain work and research results are as follows:Mathematical modeling of three-phase PV inverters is based on photovoltaic inverter grid characteristics in this paper. The voltage and current double closed- loop PI controller has been researched. Simulation and experimental results suggest that the inverter has reached the photovoltaic grid requirements to complete grid generation.Detection methods based on dq PLL voltage drop have been studied, and this paper proposed positive sequence component, positive and negative sequence component separation algorithm based on generalized second-order integration to improve the traditional methods. This method will achieve the goal of rapid and accurate detection of the voltage drop. Simulation results demonstrate that the two improved algorithms can effective ly suppress the influence of harmonics on the detection results. The improved algorithms have solved the asymmetric voltage drop detection failure problem, which lays the foundation for the research of the LVRT control strategy.In this paper, three power network fault simulation devices are summarized, and the transformer voltage sag generator is mainly studied, then the simulation of power network faults is realized. For the problem of voltage current double closed- loop control strategy during LVRT, the reactive power compensation control strategy has been proposed. The strategy can suppress the current increase by dynamic allocation of active and reactive current given value and simultaneously provide the reactive power to support the grid voltage recovery, and ult imately achieve the low voltage ride through.Given the significant harm of island phenomenon, the anti- islanding is an important research direction of photovoltaic power generation system. The reactive current- frequency positive feedback islanding detection method is deeply researched in this paper. The positive feedback amplification coeffic ient is changed from constant coefficient to variable coefficient for the purpose of speeding up the islanding detection rate. Experiments show that this method does not affect the quality of the output power while detects islanding quickly.Build the simulation model of photovoltaic inverter, and the validity of the theoretical analysis is verified by the way of the simulation. Finally, the partial functional tests of the 24.5 k W photovoltaic experiment platform are carried out, the function of the low voltage ride through and islanding protection are realized, so the effectiveness of the proposed control strategy is verified.