Research On Low Voltage Ride Through Technology Of Photovoltaic System

As photovoltaic installed capacity increased year by year, the effects of PV systems offline for low voltage has becoming increasingly evident. The current national standard has detailed provisions about low voltage ride through capability of PV systems. It's a matter of great urgency to improve the LVRT capability of PV systems.We establish a model of two stage grid-connected PV systems in this paper, which has achieved the function of MPPT and grid-connected in unit power factor. To solve the problem of overcurrent in AC side and overvoltage in DC side when symmetrical voltage drop happened, this paper presents a method of reactive distribution based on capacity redundancy combined with national standards. The design principle of crowbar resistor suited for two stage grid-connected PV systems has also been presented. The effect of restraining overcurrent in AC side and overvoltage in DC side can be seen in Simulation results.PV system can offer reactive power with this control method at the same time. The control method is verified by the simulation. The limitation of reactive power output with this control method has been calculated through an actual grid model by simulation. It shows that this control method greatly promoted the LVRT capability of PV system.The reason of quadratic voltage fluctuation in DC side and negative sequence current in AC side has been analyzed in this paper when grid voltage asymmetrical drop happened. To improve this abnormal operating condition, this paper presents a positive and negative sequence independent decoupling method based on constant power. The essential distinction between this method and double loop control method based on voltage orientation has also been elaborated. The simulation results show that this control method can eliminate quadratic voltage fluctuation in DC side or negative sequence current in AC side. It also shows that overcurrent would not appear in AC side when voltage drop happened. In the end, split phase static var compensator has been adopted, in response to zero reactive power output when grid voltage asymmetrical drop happened. This compensator can export reactive power at PCC and improve 3-phase voltage imbalance. The LVRT capability of PV systems has been promoted in this way.