Research On Control Strategy Of Reactive Circulating Current Suppression In Parallel Inverters Based On Virtual Capacitance

With the increasingly serious global energy crisis and the growing demand for electricity,distributed power generation has received widespread attention.Microgrid is proposed to reduce the negative effects between the distributed power supply and grid.As the photovoltaic,wind,fuel cells and other distributed power supply is connected to microgrid through the point of common coupling,the multiple parallel inverters are formed in microgrid.There are various control methods to realize the parallel operation of multiple inverters,where the droop control method is a kind of control technology without interconnection signal.Compared with other control methods,the droop control method has been widely used owing to simple system structure,high degree of redundancy and reliability.But when the system equivalent impedances of two inverters are mismatched,the circulating current between two parallel inverters will be produced.So the focus of this paper is how to suppress the circulating current effectively,improve the load power sharing precision and make parallel inverters more reliable.In this paper,the system of two single-phase full-bridge parallel inverters is used as the research object.Firstly,the control strategy of parallel inverters is analyzed from three aspects,which are the output power calculation,droop control principle and voltage and current double loop control strategy.From the start of the equivalent model of parallel inverter,the circulating current generation mechanism in low voltage micro grid is analyzed deeply.The conclusion is that the system equivalent impedance mismatch is the main reason to generate the circulating current in low voltage microgrid,and the coupling problem between the active power and reactive power is presented.Based on this,it is proposed to use the virtual capacitance method to make the system equivalent output impedance to be capacitive and decouple of active power and reactive power output of the system.Meanwhile,the reactive circulating current can be suppressed when the value of virtual capacitance ratio is matched with the capacity ratio.First of all,the suppression mechanism of virtual capacitance to system circulating current are deduced,then the value of virtual capacitance and its influence on the system performance are analyzed,and the reactive power compensation of virtual capacitance is analyzed simply.The parallel inverters based on LC and LCL filter are analyzed comparatively,and it is proposed to use LCL filter to suppress the high frequency circulating current of parallel inverters.Finally,the advantages and disadvantages of synchronous modulation and asynchronous modulation in parallel inverter are analyzed,and it is selected to combine the synchronous modulation and synchronous parallel operation control to suppress the circulating current further.In order to verify the control method,the simulation model is built by PSIM software,and the experimental platform with two single-phase full-bridge parallel inverters is also set up.Software programming and debugging are completed.The control strategy designed is verified.Simulation and experimental results verify the feasibility and effectiveness of the system control scheme.