Fault Analysis And Control Of Droop-Controlled Inverters Operated In Parallel

With the increasingly prominent problems of energy and environment,the continuous development of distributed generation technology,the application of microgrid and its interface technology is more and more extensive.The droop control inverter is widely used in the microgrid because it can directly participate in the adjustment of the voltage and frequency of the microgrid and realize the smooth switching of the microgrid from off-grid to grid-connected.However,when the power grid fails,the droop control inverter is prone to overcurrent,causing the switching device to be blocked or even damaged.In addition,the difference between the fault response of the droop control inverter and the synchronous generator fault response results in a fault current calculation method for the conventional power system that is not applicable.This may result in severe degradation or even failure of the conventional protection device during fault transients.Aiming at the above problems,this paper studies the principle of microgrid fault analysis which is suitable for droop control inverter access,and analyzes the influence of control parameters,power parameters and nonlinear limiter on fault current characteristics.In addition,the coupling model of the parallel inverter is established,and the fault current expression of the parallel inverter is obtained.The main factors affecting the coupling characteristics of the inverter parallel system are discussed.Finally,this paper compares three typical fault ride-through methods,and proposes a fault ride-thourgh strategy based on smooth switching,which solves the problem of overvoltage and overcurrent generated by mode switching control when suppress fault current.The main research contents include:(1)For the droop control inverter fault operation mechanism is not clear,the fault analysis theory is not perfect,based on the analysis of the droop control inverter transfer function,the mathematical model of the inverter fault is established,and the symmetrical component method is used.On the basis of the fault boundary conditions and the Thevenin equivalent theorem,the fault mathematical model of the network side is established.On this basis,the first-order differential equations of each sorting network are obtained,and the fault current expressions of each phase are derived.Based on the fault current algorithm of iterative calculation,the practicability and correctness of the method are verified by simulation and experiment.Finally,the effects of droop coefficient,line parameters,fault time,asymmetry fault type and nonlinear limiter on fault current characteristics are revealed.(2)For the unclear problem of the fault characteritics of the parallel droopcontrolled inverter system,the multi-time scale coupling model of the parallel system is established.The interaction mechanism and coupling degree of the inverter are analyzed.The output fault current expression of the inverter is analyzed in detail to discuss the influence of the change of the inverter i parameter on the output fault current of the inverter j.The output voltage,current,frequency and other electrical properties of the inverter parallel system are studied.The fault characteristics of the quantity and the main influence factors that affect the coupling characteristics between the inverters are obtained.(3)In view of the grid fault ride-through strategy of the droop control inverter,this paper compares three typical control strategies(virtual impedance current limiting strategy,reduced internal potential limiting strategy and direct switching to current control strategy),and summarizes the advantages and disadvantages of the three types control methods.The problem of overcurrent and overvoltage generated when adoping the internal potential limiting strategy and the direct switching current control strategy are analyzed.Then,by re-activating the offline controller to make the closed-loop tracking of the corresponding reference value during the fault,the smooth reverse switching performance is achieved,and the fault ride-through of the bumpless switching current limiting control is realized.The simulation and experimental analysis verify the effectiveness of the proposed strategy.