Research On Maximum Active Power Output Strategy For Two-stage Grid- Connected Photovoltaic System Under Unbalanced Grid Faults

In recent years,with the increasingly prominent environmental problems and increasing demand for electricity,renewable energies such as photovoltaic power generation are gradually replacing traditional fossil fuels and their penetration into power systems are increasing.Therefore,when an unbalanced grid fault occurs,grid codes require the grid-connected PV system to keep connected to the grid——to reduce the active deficit of the system and to prevent the large-scale load rejection from affecting the stable operation of the power grid.This requires that the PV grid-connected system needs to inject active power as much as possible to support the grid during the fault,but the prerequisite is to ensure the safety of the grid-connected inverter.Therefore,it is important to analyze the maximum active power output capability of grid-connected inverter during grid fault.This paper first introduced the components of a two-stage grid-connected photovoltaic system,including photovoltaic panels,Boost converters and three phase grid-connected inverters,built a simulation model and designed parameters for related controllers.Simulations were performed under normal and fault conditions to verify that the design parameters were reasonable.Then taking the voltage source converter(VSC)as the research object,this paper considered overcurrent,active power oscillation that the inverter may face under unbalanced grid faults,the safe operation region(SOR)of inverter was analyzed.According to the SOR,we obtained the analytical expressions of maximum active power output capacity under different fault types with different fault strength.Then,the working mode of the pre-stage Boost converter could be determined after comparing the result with the maximum output power capability of the PV panel.When the maximum output power capability of the PV panel is small or the fault is not serious,the system may operate in low power mode,and then we can optimize the proposed control strategy in two ways: minimize the maximum phase current or minimize active the power oscillation.In addition,the active power oscillation was no longer simply limited to 0,but limited within a safe range.Although this causes the third harmonic current to rise to a certain degree,it has not exceeded the requirement of the grid codes.The key is that this greatly increases the maximum active power output capability,for example,the relative percentage is up to 30% under single-phase ground fault.Finally,the simulation results verified that the analysis of the active power SOR and the maximum active power output capability are correct.The experimental results verified that the proposed strategy can improve the maximum active power output capability of the inverter under unbalanced grid fault,and that the optimal control of the maximum phase current minimization and the active power oscillation minimization are effective.The two-stage photovoltaic experimental platform achieves safe operation online under unbalanced fault conditions.