Short Circuit Current Suppression Of PV Generation System Under Asymmetrical Fault In Power Grid

With the large-scale photovoltaic power supply connected to the power grid,the interaction between the power grid and the photovoltaic system becomes increasingly significant.The access of the photovoltaic power supply will lead to an increase in the short-circuit current at the short-circuit point under fault in power grid.When asymmetrical faults in the power grid cause the grid voltage to drop,it is necessary that the photovoltaic power supply can continue to deliver power to the grid without exiting the operation.PV grid-connected inverters may inject non-sinusoidal current with the DC-link voltage rises,the active power and reactive power will oscillate at a double frequency under unbalanced voltage sags.The over-current problem of the PV grid-connected inverter seriously affects the reliability of the system operation.In order to solve the above problems,the details are as follows:In this paper,the short-circuit current contribution of photovoltaic power generation system is simulated and analyzed in condition of power grid fault.This paper studies the three-phase inverter control strategy,a current suppression strategy based on constant active power control is proposed,according to the reference current during asymmetric faults.The maximum possible amplitude determines the active power reference value,and the active power reference value is determined according to the demand for injecting reactive power into the photovoltaic system during the voltage dip of the grid.This method can effectively suppress the magnitude of the fault current of the photovoltaic power generation system during asymmetric faults.Aiming at the problems of the double-frequency oscillation of active power and the rise of DC-link voltage in two-stage three-phase grid-connected PV system during asymmetric faults,a new current reference algorithm is proposed and the expression of the current reference is given.In the control method,the PV inverter injects a sinusoidal current under the asymmetrical grid fault and can reduce the current amplitude,and can adjust the rise of the DC-link voltage and the oscillation of the active power under the unbalanced fault,thereby improving the operational reliability of the system.On this basis,an asymmetric fault current limiting control scheme is proposed for the overcurrent injected by the PV system during asymmetrical faults which is non-maximum power tracking fault operation mode when the grid voltage is asymmetrically dips.The MPPT mode and the non-MPPT fault mode switching standard for DC-DC converters of PV power generation systems have been established.The current limiting method is highly efficient and easy to implement.While eliminating the active power oscillation,the injection current is effectively suppressed within the rated value.This paper mainly studies the control strategy of three-phase inverter and DC-DC converter,current suppression method based on constant active power control and fault current suppression method in non-MPPT mode are proposed,which can reduce the contribution of the PV generation system to the short-circuit current during asymmetrical faults,reduce the DC-link voltage ripple and effectively suppresses the active power oscillation,and can effectively prevent the inverter from being burned due to over-current.The proposed method was simulated and the results verified the effectiveness of the proposed method.