Research On Parallel Technology Of Distributed Inverter System For Flexible Power Sharing Control

The control technology of the distributed inverter system has become an important research field nowadays,in the background of the rapid development of the renewable energy distributed generation industry.In AC microgrids,the parallel operation of distributed generators is actually the parallel operation of distributed inverters.It is a fundamental requirement for optimal system operation that the load power should be properly shared among the distributed generators.This paper intensively studies the flexible power sharing technology and correlated issues for distributed inverters in parallel operation,based on the world's current level of parallel technology for the distributed inverter system.To accomplish the goal of flexible power sharing among the inverters,this study employs the communication technology in the distributed inverter system and takes the advantage of hierarchical control and multi-agent theory in the area of the distributed system.Several improved control strategies are proposed for parallel operation and power sharing for distributed inverters,which enhance the system performance effectively.Firstly,in the existing methods of secondary voltage control for reactive power sharing,the information of reactive power is required to transmit in the communication network,which increases communication burden and system complexity.An improved control strategy of secondary voltage control for reactive power sharing,named as'Voltage compensation signal integration method',is proposed in this paper to overcomes this shortcoming.In an inverter's local controller,an integral term is introduced into the voltage droop function,where the voltage compensation signal is utilized as the common reactive power reference for each inverter.Thus double function for both reactive power sharing among the distributed inverters and common bus voltage restoration can be realized.Experimental results from a distributed inverter system prototype including two three-phase inverters indicate that the reactive power sharing performance of the proposed strategy is comparable to that of the existing methods which demand power information exchange.Moreover,the effect of communication delay is studied.One analysis method for system dynamic performance in the presence of communication delay is presented.Experimental results in the presence of delay validate the proposed strategy's robustness to communication delay.Then,in the existing centralized adaptive virtual impedance methods,the central controller leads to a single point of failure,which decreases the system reliability.An adaptive virtual impedance method based on distributed control of multi-agent systems is proposed in this paper to overcome this shortcoming.By introducing the consensus cooperative control theory,the problem of reactive power sharing among distributed inverters is equivalent to the consensus problem of the weighted reactive power.Each inverter only utilizes its local and neighbouring power information to adjust virtual impedance magnitude adaptively,thus the demand for global power information can be avoided and system reliability is enhanced.Furthermore,impedance phase is adjusted according to the power factor to enhance system robustness.Thus,the problem in existing method that adaptive virtual impedance magnitude with fixed impedance angle cannot generate the required voltage compensation at some work points is solved.Using the optimization criterion of minimal communication link number,a communication network topology is given.It satisfies the demand for containing a spanning tree and has redundancy.A distributed inverter system prototype with three three-phase inverters is built,from which experimental results indicates the proposed strategy's robustness to communication link failure.Finally,after the intensive study for Q-V droop control,an improved V restoration mechanism based on the changing rate of the weighted average inverter voltage is proposed in this paper.It solves the problem that the existing V restoration mechanism causes improper reactive power sharing.The voltage changing rate of all the inverters is restored using the changing rate of the weighted average voltage,thus the reactive power sharing performance is not affected by the V restoration process.By utilizing the multi-agent theory,a distributed calculation method is given to obtain the changing rate of the weighted average inverter voltage.It is theoretically proved that the communication topology should contain a spanning tree for the calculation result in each inverter's local controller converging to the actual value.The performance of the proposed strategy is verified by experimental results from a distributed inverter system prototype with three three-phase inverters.