Research On Active Impedance Reconstruction Device For Multi-paralleled Single Phase Grid-tied Inverter System

Nowadays,the new energy has been attracting more and more attention,the key element for new energy is grid-tied inverter.All those inverters are paralleled to scale up the capacity.However,due to the existence of equivalent grid impendance,the inverters will be couplled with each other,which will bring the challenge for the stability of inverters.On other hand,compared with single L filter,the LCL filter is usually employed in the inverter system,because of the excellent harmonic rejection ability and the reduction of inductor volume.However,the third-order LCL filter would cause the instability problem due to the resonance peak.In order to avoid such potential issue,many damping methods have been proposed,which is mainly classified as active damping and passive damping.The active damping adds additional variable feedback or feedforward into the control loop,such as capacitor voltage feedbacking and capacitor voltage feedforward.The passive damping inserts the resistor in paralleled with or in series with the inductor or capacitor of LCL filter.The passive damping has excellent preformance of the resonance suppression,Neverthelsss,it brings in additional power losses.The main contents in this thesis are listed as follows:1)The inverter control strategy including the filter resonance suppression methods,harmonic current rejection methods,stability criterion for muti-paralleled inverters are introduced.2)The traditional proportional resonance plus harmonic compensation control trategy is introduced.Then,the principle of capacitive current feedback-based active damping is interpreted,which is followed by the inverter modeling.Finally,the corresponding control parameters are designed,and the effectiveness of the abovementioned control methods are verified experimentally.3)A series-type active impedance reconstruction device is proposed.The control principle and equivalent model of the impedance reconstruction device are analyzed.Finally,the effectiveness of the inverter with the proposed series-type active impedance reconstruction device under different grid conditions is verified by simulation and experiments.4)A hybrid active impedance reconstruction device is derived based on the impedance modeling.The control strategy of the proposed method is also analyzed.Finally,the effectiveness of the proposed hybrid active impedance reconstruction device on the paralleled inverter system under different grid conditions is verified by simulation and experiment.5)Finally,the work of this paper is summarized,and future work direction is given.