Research On Power Proportion Strategy For Paralleled Inverters Based On Hierarchical Control

The conventional power system uses fossil fuel to generate electrical power which increasingly affects the environment.As a result,there is trend in integrating renewable energy sources(RESs)in them.Microgrid is an effective way to integrate RES s,improve system voltage support and increase its efficiency and reliability.Droop control method is a basic way for active and reactive power sharing in Microgrid,but it's not suitable for low voltage and medium voltage networks where line impedance is mainly resistive.Thus,This paper focuses on the power sharing strategies for microgrid with low rated voltage or rated medium voltage.Main contents are as follows.(1)The paper studies the control strategies for microgrid inverters from unit level and system level,analysises the pros and cons of them.Comprehensive introduction of paralleled invert control strategies is given.Meanwhile,it summarizes the key technologies and areas of recent research.(2)Three phase inverter model is established in rotating coordinates.Droop controller model,voltage and current loop controller model are established based on the power transmission characteristics.Frequency domain method is used to analysis the closed-loop control characteristics and to choose controller parameters as well as filter parameters.Based on the model of paralleled inverts,cause and essence of circulation is concluded for the follow-up work.(3)The basic principles and limitations of droop control based traditional hierarchical control are described.The concept of virtual impedance is described.An adaptive virtual impedance hierarchical control strategy is proposed:Based on traditional primary control,a power equivalent impedance concept is introduced and the mismatch of local load is solved for power sharing.Power regulation is added based on traditional secondary control which only regulates voltage and frequency.The significance of feeding line parameter identification is introduced,and the recursive least squares(RLS)method with forgetting factor is used to identify the line parameters,thus distributed control strategy without communication reliance is realized.Simulation verifies that improved hierarchical control strategy is an effective way to solve the power sharing problem.(4)The hardware in the loop platform is built.Model based design method is introduced in detail.The proposed control strategy is verified by the hardware in the loop experiment.