Transient Characteristic Analysis And Fault Ride-through Method Of Voltage Source Inverter Parallel System

With the prominence of energy crisis and environmental problems,new energy power generation methods such as photovoltaics and wind power are gradually replacing traditional fossil energy power generation.At the same time,power electronic devices represented by voltage source inverters are used as new energy grid-connected interfaces,which are gradually changing the operating characteristics of power systems due to their fast transient response,easy oscillation,and lack of rotating parts.The voltage source inverter has a flexible control method and can provide voltage and frequency support for the grid.However,due to the weak overcurrent capability of power electronic devices,when a fault occurs on the grid side,the protection device operates,and the off-grid operation cannot continue to work during the fault.Therefore,the research on the fault ride-through strategy of the inverter has always been a research hotspot.This dissertation takes the voltage source inverter as the research object,analyzes its operating characteristics during faults around two typical control techniques(droop control and virtual synchronous control),and considers the system coupling characteristics and interaction effects under the condition of parallel operation.The main work of the fault full-cycle current flexible control strategy is as follows:(1)First,the basic principles of droop control and virtual synchronous control are introduced,and the dynamic regulation of P-f and Q-U is briefly described.Secondly,the control characteristics of two voltage source inverter control strategies are introduced,and the droop control inverter with low-pass filtering can be equivalent to virtual synchronous machine in mathematical model.Finally,the power angle and current transient characteristics of the symmetrical fault droop control inverter and virtual synchronous machine are deduced in detail.The theoretical analysis shows that when a symmetrical fault occurs on the grid side,the inverter has a fault overcurrent risk.(2)A fault ride-through method is proposed in which two virtual synchronous machines(VSG-VSG)form a parallel system.First,the inverter fault ride-through method is adopted to switch the mode switching by directly switching the current command value,and through the simulation analysis,it is found that there is an inrush current during the two mode switching processes at the beginning and the end of the fault.Afterwards,the theoretical analysis shows that the causes of the inrush current can be divided into the off-line switching of the inverter’s own control loop and the asynchronous switching between the parallel inverters,and it is emphasized that when the two inverters have a fault mode switching time difference,the two interaction mechanism.Finally,based on the theoretical analysis,a flexible control strategy for the full-cycle fault current of the VSG-VSG parallel system is proposed.(3)A fault ride-through method is proposed in which a virtual synchronous machine and a droop control inverter(VSG-Drop)constitute a parallel system.First,the theoretical analysis shows that the transient responses of the droop control inverter and the virtual synchronous machine are not synchronized,and the voltage and current of the droop control inverter change faster.Subsequently,the effect of the circulating current caused by the mismatch of the output impedance under stable conditions is introduced,and the circulating current is suppressed by the virtual impedance method.Finally,in order to solve the asynchronous switching problem of the VSG-Droop parallel system,it is proposed to add synchronization modules in the power loop and the voltage outer loop to further realize the synchronous switching,and the feasibility of this strategy is verified by simulation.