Research On Decentralized Reactive Power Circulating Current Suppression Of Paralleled Inverters

With the rapid development of human society,the traditional fossil energy is increasingly exhausted.The rational development and utilization of renewable energy is of great importance to today's society.The distributed generation technology has attracted more and more attention all over the world.As compared to the main grid,distributed generator is usually uncontrollable.Therefore,the direct connection of the distributed generation system with the main grid will deteriorate the power quality of the main grid.Consequently,the concept of micro-grid is proposed to coordinate the contradiction between the distributed generation system and the main grid.By flexibly controlling various distributed generators,micro-grid can improve the power supply reliability and power quality,and avoid the distributed generator supply alone,which will impact the safety,stability and power quality of power grid.The distributed generator is connected to the main grid through the inverter to ensure the stability of the main grid.The parallel operation of several inverters can expand capacity and realize redundancy in power system.Hence,it is considered as one of the important part of the development of inverter technology.The parallel operation of inverters can significantly improve the flexibility of the system,reduce the volume and weight of the power system greatly,and at the same time,and fundamentally enhance reliability,reduce costs and improve power density.This paper focuses on the power sharing issue of the paralleled inverters in micro-grid.The main efforts done in this paper include:(1)The research status of micro-grid and paralleled inverters technology is introduced in detail.Two control methods of paralleled inverters are discussed: interconnected lines and non-interconnected lines control strategy.The current research status of the power sharing problem between paralleled inverters is analyzed in detail.(2)The relationship between the power sharing of the parallel inverter system and the line impedance is elaborated,and the control strategy of the inverter is briefly introduced.The traditional droop control principle is expounded in detail,and the reasons of inaccurate power sharing of paralleled inverters with traditional droop controller is deduced.(3)Aiming at the existing control method of improving the accuracy of power sharing based on virtual impedance,an adjustable virtual impedance control method that can dynamically regulate its own impedance is proposed.Without any communication between the inverters and the measurement of line impedance,the proposed method can greatly improve the power sharing accuracy among all inverters.At the same time,the small signal stability model of the parallel inverters system with adjustable virtual impedance is established.Finally,a system of two inverters operate in parallel is established by MATLAB/SIMULINK to verify the correctness and superiority of the proposed control strategy.(4)Based on the conventional droop control strategy,an modified droop control strategy which can dynamically adjust the droop coefficient is proposed.An modified droop item is added to the Q-V droop,and the droop coefficient can be dynamically adjusted according to the reactive power measured by the inverters themselves.The effectiveness of the proposed modified droop control strategy is verified by simulation analysis.