Analysis Of Circulating Current In Parallel With Multiple Inverters And Research On Suppression Strategy

A microgrid is a small power distribution system that is effectively assembled by generators,loads,energy conversion devices,and control conversion devices.When there are multiple distributed power sources in the microgrid as the load capacity increases,the small power distribution system,that is,the microgrid system,will have the phenomenon of multiple inverters in parallel.The use of non-interconnected wire technology is the core of the droop control strategy used in the multi-inverter parallel system based on grid-connected mode.The principle is to adjust the output voltage and amplitude of each inverter through the droop control equation,so as to realize the effective equalization of the output power of the system.This paper proposes an improved droop control strategy based on the traditional droop control strategy mode,in order to solve the problems existing in the traditional droop control strategy in the multi-inverter parallel system based on the grid-connected mode.First of all,when the system based on the parallel connection of multiple inverters in the grid-connected mode has different impedance characteristics,the output voltage difference between the parallel systems,and the resistance-inductance of the output equivalent line impedance,there is no corresponding The droop control type will have a significant impact on the average distribution accuracy,including: the output reactive power of inverter with inductance equivalent line impedance and the output active power of inverter with resistance equivalent line impedance,and the system will generate circulating current.In this regard,this paper proposes a method to introduce virtual complex impedance.When the output equivalent line obtained by this method is resistive and inductive,there is a corresponding droop control equation,and the output power of the system can be effectively shared and the circulating current of the system can be effectively suppressed.Secondly,due to the basic characteristics of three-phase inverter system,it is impossible to analyze it effectively.Therefore,this paper obtains the overall mathematical model under the coordinate system through coordinate transformation processing,and then performs linearization processing by introducing small disturbances,and finally obtains the overall small-signal model of the inverter system under the coordinate system.The voltage and current double closed-loop decoupling method and robust design method are proposed,and then the parameters are designed,including LC filter,current inner loop and voltage outer loop.Finally,through the simulation test platform,the proposed robust design is verified,which verifies the correctness and effectiveness of the proposed robust design method.Finally,due to the traditional droop control strategy adopted by the system,due to the mismatch of the equivalent line impedance of the system and the easy fluctuation of the grid voltage,the output power cannot be effectively shared,and even the system is seriously unstable.Therefore,this paper proposes a method to introduce a proportional term and a virtual complex impedance into the droop control formula.Finally,two parallel inverter systems based on grid connected mode are built on the simulation experimental platform.The results of this paper are verified and analyzed in detail,including reliability,accuracy,effectiveness,flexibility,practicability and stability.