Supporting The Grid-connected Point Voltage Strategyof Photovoltaic Generation System Under Unbalanced Grid Voltage

Along with photovoltaic grid-connected system capacity keep growing, make the interaction between photovoltaic power system and power grid has become increasingly prominent. Grid converters control problem is particularly prominent under unbalanced power grid because asymmetry faults of power grid or asymmetry nonlinear load, etc can cause three-phase voltage unbalance. So research strategy about supporting effect of power injection from photovoltaic power system on voltage at grid-connected point during asymmetrical faults of power system has important significance.The tracking control of grid-connected current is the central technology that is studied in Photovoltaic Generation System. Grid required injection current reference may be symmetrical or asymmetrical even distortion under asymmetric fault. So in order to effectively trace the reference currents, it is necessary to start from the following two aspects: One is appropriate modulation technique is the key to accurate injection current to the power grid. The another one is grid synchronization technology, it must be noted that any grid converters current controller heavily rely on accurate grid synchronization technology.On the one hand, for the sake of accuracy and fast track the reference currents, on the analysis of principle of fixed hysteresis control method, the paper design a combination of fuzzy control hysteresis current tracking technology. It not only retain the advantages of high precision, fast dynamic response of fixed hysteresis control, but also effectively reduce the switching frequency range, effectively limiting the switching frequency peak frequency. On the other hand, double frequency lock loop of 2 order generalized integrators is given which has the function of frequency adaptive and phase sequence extraction for unbalanced voltage of power grid. Analysis of the performance of filter, choosing appropriate discretization algorithm for filter. Then testing the basic performance of frequency locked loop while power grid voltage amplitude, frequency, phase mutation, verify its correctness.This paper research focuses on grid-connected reference current settings about supporting effect of power injection from photovoltaic power system on voltage at grid-connected point during asymmetrical faults of power system. By analyzing the positive and negative sequence equivalent circuit of grid converters for photovoltaic power system, the principle of supporting power grid voltage is pointed out. And determine power value injection from photovoltaic power system. Put forward reference current set flow chart about supporting effect of power injection from photovoltaic power system on voltage at grid-connected point by synthesizing the factors of photovoltaic power system's actual capacity, negative unbalance factor of grid voltage, line impedance and maximum allowable current of system.Simulating the equivalent model of grid converter for 1MVA photovoltaic power system through 10 kv transformer access to power system under asymmetrical faults of power system. The simulation results proved that reference current setting method is deduced in this paper have varying degrees of support effect to voltage at grid-connected point for different unbalance factor.