Research On Power Control Strategy Of Parallel Inverters In Isolated Microgrid

At present,in order to develop in the direction of high power,the parallel operation of multi-module inverters is an important method.Meanwhile,parallel operation technology is a key technology for the transformation of inverter from traditional centralized power distribution to distributed power distribution.However,parallel operation of multi-module inverter is more difficult than that of multi-module DC power supply.Under ideal conditions,the output voltage amplitude,frequency,phase and other parameters of the multi-module inverter in parallel are the same,that is,there is no voltage difference of the inverter output.However,in the actual parallel inverters system,the output voltage of the parallel inverters are different due to the difference of various parameters,which results in the deviation of the output power of the parallel inverters.In order to eliminate the power deviation caused by the voltage difference and achieve the effect of power sharing,this thesis takes two parallel inverters as the research object,improves the traditional droop control and the traditional voltage-current double closed-loop control,and applies this improved method to the power control strategy of islanded microgrid.The simulation results show that this method is applied to eliminating the deviation of power allocation and achieving the effect of the power sharing.The main research contents are as follows:(1)The droop control of a single inverter and the equivalent topology and mathematical model of two parallel inverters are established,and the real-time active and reactive power of the inverter was calculated by using the instantaneous power theory.The traditional droop control and antidroop control are obtained according to the different impedance characteristic of microgrid lines.(2)Based on the traditional droop control method,the double-closed loop control topology of inverter voltage and current are established,and the traditional double loop control strategy is analyzed.A fractional-order proportional-integral controller with BP algorithm for parameter tuning is improved,aiming at the difficulty of achieving ideal results with constant parameter proportional-integral control.The simulation results show that the new improved control strategy can not only achieve the following effect of voltage and current,but also further improve the stability of voltage and frequency.of microgrid system(3)The traditional droop control method is analyzed,and the equivalent circuit topology of parallel inverters are established.Because of the difference of line impedance,the output voltage and current of the two parallel inverters are not equal.Through power calculation and final value theorem,it is proved that under the traditional droop control method,active power can be distributed proportionally,while reactive power can not be distributed proportionally,and there is a deviation in reactive power distribution.In response to this problem,two PI control links are added to the traditional droop control.The PI control links ensure that the output voltage of the two inverters is the same,thus achieving the effect of reactive power proportional distribution.The simulation results show that the control stategy can eliminate the deviation of reactive power distribution between two parallel inverters.(4)By analyzing the traditional antidroop control,it is concluded that the reactive power can be accurately distributed according to the inverse ratio of the sag coefficient,and active power has deviation due to the difference of line impedance.In order to solve this problem,an anti-droop control method combining increasing droop coefficient,changing no-load voltage and virtual resistance is adopted.The mathematical formulas of the droop control equation for increasing droop coefficient and reducing no-load voltage are deduced and proved,and the anti-droop curves for increasing droop coefficient and reducing no-load voltage are drawn according to the deduced mathematical formulas.The simulation results show that the improved anti-droop control method can effectively eliminate the deviation of active power distribution between two parallel inverters.