Research On Multi Frequency Domain Virtual Impedance Control Of Islanded Microgrid

As a new type of distributed generation control system,microgrid is a vital part of smart distribution network.Multi-paralleled microgrid inverters have important functions of supporting voltage and frequency,distributing load and suppressing oscillation in islanded microgrid.However,micro sources and load are connected by line impedance,which are dispersed in microgrid.The resonance caused by impedance mismatch can lead to instability in the source-network-load impedance network.In addition,the impedance mismatch caused by the difference of lines leads to the inaccurate load power distribution between the microgrid inverters and threatens to the stable operation.This dissertation is organized as follows:(1)The influence factors of output impedance in multi frequency are analyzed based on the established impedance model of microgrid inverter.In order to provide a basis for subsequent multi-frequency impedance correction scheme,the characteristics and effects of virtual impedance are studied contrastively.(2)An impedance model of island microgrid containing two parallel microgrid inverters is established.The resonance mechanism between source and network in microgrid is analyzed by impedance analysis and closed loop pole analysis.Then,the significant impact of the time delay,line length and voltage controller parameters are determined.(3)For power distribution between inverters under nonlinear load,the effects of line impedance on fundamental and harmonic power distribution have been discussed by deducing the impedance matching conditions about reasonable distribution of fundamental and harmonic power.The restrictive relation between the harmonic power distribution and the power quality to the impedance requirement is analyzed.(4)A dual loop virtual impedance is proposed which contains an inner virtual impedance to suppress source-network resonance and an outer virtual impedance to distribute fundamental/harmonic power reasonably.In order to improve fundamental/harmonic power distribution and power quality,a double coordinate frequency division outer impedance scheme based on moving average filter is designed which take the fast response of the impedance loop and the accuracy of the steady state impedance into account.In order to eliminate the source-network resonance and improve stability of microgrid,an active damping scheme based on capacitance current PID control is applied as the inner virtual impedance which can eliminate the high frequency resonance and improve stability of microgrid.(5)A microgrid simulation model including two droop control inverters is built in Matlab/Simulink environment to verify the correctness of the theoretical analysis and the effectiveness of the proposed control method.The simulation results show that the proposed dual loop virtual impedance control scheme can effectively suppress the resonance generated by the source-network interaction,and achieve the fundamental and harmonic power sharing among the inverters.