Study Of The Low Voltage Ride Through Control Scheme For Grid-connected Photovoltaic Inverter

At present, the grid-connected photovoltaic (PV) system has become the focus of theresearching of new energy power generation field as an important technology in the usageof solar energy. As the proportion of grid-connected photovoltaic system in energystructure becoming heavier, if they take off from the grid when grid voltage sags, the gridwould be impacted, the grid fault would be expanded, the power supply of the load wouldbe influenced and the recovery time of the grid would be delayed. For these reasons, thelatest standards of grid-connected photovoltaic system provisions that grid-connectedinverter of PV system should have low voltage ride through (LVRT) capability, at homeand abroad. Therefore, on the view of security and stability, the study of low voltage ridethrough capability of the grid-connected inverter has great engineering value.In this paper, the mathematical models of the grid-connected inverter under thecondition of grid voltage symmetric and asymmetric cases are established respectively，besides, three sequence separation methods are analyzed and the second order generalizedintegrator (SOGI) is chose to achieve the separation of the positive and negative sequenceof grid voltage. The balanced positive sequence control of low voltage ride throughmethod is introduced briefly on the basis of the analyzing above, with which, the currentwill stand balanced and would not over flow in the case of grid voltage drops. In addition,according to the national standard of photovoltaic power station about low voltage ridethrough capability request, a control algorithm of low voltage ride through ofgrid–connected inverter with reactive power compensation ability is designed to controlthe positive sequence and negative sequence of grid current, respectively. This algorithmcan make sure that the grid current do not distorted or over the threshold, besides, it makesreactive power injected reasonable to supporting the voltage of the point ofinterconnection of PV power station.On the basis of theoretical research, simulations of both control methods are carriedon, the simulation results show that each method can realize its own goals of control,meeting the requirements of low voltage ride through. Finally, photovoltaic grid-connected inverter experiment platform is set up based on TMS320F2812and thesoftware program is written in C language that realizes digital control, from which, thebalanced positive sequence control of low voltage ride through method in this paper isverified.