Research On Impedance-based Passivization Methods For The Grid-connected Three-phase Voltage Source Converters

With the proposal of the dual-carbon goal by the government of China,developing renewable energy distributed generation(DG)is the trend and consensus in the future power system.The grid-connected converters are the core elements in the DG,and the stable operation of the grid-connected converters has become a research hotspot with the increasing penetration of the renewable energy DG in the whole power system.The conventional s-domain stability analysis and design methods for a single grid-connected converter are no longer suitable for multiple grid-connected converters in a complex DG,so that the impedance-based stability analysis method based on the equivalent admittance of the converter that is suitable for the multiple converters in a complex grid scenario is proposed by researchers.The passivization of the equivalent admittance of the converter is a sufficient condition for the stability of the system.Therefore,we take the passivization of the equivalent admittance of the voltage source converter(VSC)as the research goal and researched a series of impedance-based passivation methods,such as passive power filter design,novel Smith predictor,Luenberger observer-based active damping,phasecompensated voltage feedforward.The main content and contributions of this dissertation are as follows:1.The impedance model of the grid-current controlled LCL-type VSC is built to research the passivity of the VSC’s equivalent admittance.As the total delay of the VSC and the anti-resonant frequency of the LCL filter determine the non-passive region of the grid-current controlled VSC,an optimal parameter design method for the LCL filter that can achieve all frequencies’ passive equivalent admittance of the VSC is proposed.The method is applicable for the grid-side current feedback and grid-side voltage feedforward VSC with a proportional resonance controller in a stationary reference frame.The influence of various popular passive damping methods on the passivity of the VSC’s equivalent admittance is studied.In addition,it is also found that all frequencies’ passive equivalent admittance cannot be guaranteed for the trap filter through the proposed passivity-based optimal parameter design method.The effectiveness of the proposed method is validated by the simulation.2.The passivity of the equivalent admittance for the converter-side-current controlled VSC is researched.Since the non-passive region is merely caused by the total time delay of the VSC for the converter-side-current controlled VSC.A novel Smith predictorbased delay compensation method that can compensate for the calculation delay is proposed to improve the internal and external stability of the VSC.It is found that the proposed method can not only increase the phase-margin-based internal stability of the VSC,but also improve the passivity-based external stability of the VSC.Finally,the experimental results proved that the achieved VSC can broadly adapt to the grid impedance variations so that the robustness of the VSC is improved.3.In terms of the grid-side-current and converter-side-current controlled VSC,the Luenberger observer-based capacitor-current feedback active damping is researched to reduce the additional capacitor current sensors used in the conventional capacitor current feedback active damping method.In terms of the passivity,it is found that observer without one-step prediction is better than that with the one-step prediction for the converterside-current controlled VSC,otherwise,for the grid-side-current controlled VSC,with one-step prediction is superior to that without one-step prediction.With the anti-resonant frequency of the LCL limited in a region,all frequencies passivity of the VSC can be achieved so that the two additional current sensors can be saved.Finally,the experimental results validated that the proposed method can significantly improve the robustness of the VSC to grid impedance variations.4.A unified impedance model of VSC with various currents and voltage sampling positions,such as grid-or converter-side-current,capacitor-or grid-voltage,is proposed.The algebraic expression of the real part of the VSC’s equivalent admittance is derived to research the passivity.The effection of the traditional capacitor-current feedback active damping and the voltage feedforward filter on the passivity of the VSC’s equivalent admittance is researched.The unit voltage feedforward is compared to find out the mechanism that the feedforward part affects the passivity of the VSC’s equivalent admittance.A phase-compensated band-pass filter is proposed to enhance the passivity of the VSC’s equivalent admittance at the critical frequency.Furthermore,the passivity-based parameter design guideline is also presented.The experimental results proved that the proposed phase-compensation band-pass filter can effectively improve the robustness of the VSC.