Research On Low Voltage Ride Through Technology Of Large-scale Photovoltaic Power Station

With the traditional energy crisis becoming more and more serious, countries in the world have been committed to the development and utilization of new energy. Solar energy receives more and more people’s favor for its unique advantage, such as abundant, clean, cheap and so on. Photovoltaic(PV) generation has became a major form of using solar energy. With the installed capacity of grid-connected PV plants are continuously increasing, A series of technical indicators must be required to PV grid-connected system for reduce the shock and influence of grid, the low voltage ride through(LVRT) capability is a very important indicator of them. The LVRT technologies of large-scale grid-connected PV plant were chosen as study research contents in this paper, the main work are as follows:The LVRT requirements at home and abroad and three typical LVRT realization methods based on energy storage equipment, reactive compensation device, voltage support through reactive current were presented.The test devices for LVRT implementation based on impedance form, transformer form and power electronic converter form were illustrated.The voltage-sag detection method based on dq-PLL was studied. For the method can not detect voltage unsymmetrical sag faults because of the negative sequence components of voltage, a sequence separation method based on second order generalized integrator (SOGI) was adopted to separate the sequences of voltage, only entering the positive sequence components into enter the PLL. The simulation results verified the above method has higher detection accuracy for symmetrical or unsymmetrical voltage sag faults, the feasibility of the method was proved.The topological structure and operating principle of the three-phase voltage-sourced PV grid-connected inverters were analyzed. The mathematical model in dq-reference frame of it was built. The double-closed-loop control strategy of inverter when is working normally was studied. Through analysis show that the direct cause for PV plants be off the grid is that the inverter output current increase abruptly, then the active current control strategy for LVRT was proposed, which can inhibit inverter output current increasing and provide dynamic reactive power to support grid voltage by inverter. The simulation results by Simulink verified the active current control strategy can effectively inhibit inverter output current increasing caused by voltage-sag and support voltage of grid. In this way, the operation reliability of PV grid-connected systems is improved.The simulation models of test systems based on passive reactor devices for LVRT were established in MATLAB/Simulink, the grid-connected inverter with500kW rated power in a PV plant was simulation tested under various voltage sag depth conditions. The topological structure and operating principle of the test devices were presented, the steps to calculate the minimal sum of passive reactors were deduced. The simulation verified the strong abilities of the inverter under test and the feasibility of the device for LVRT test.