Research On The Influence And Control Strategy Of Inrush Current On The Stable Operation Of Flexible DC System

Modular multilevel inverters have a series of advantages such as high efficiency,no commutation failure,and the ability to supply power to passive systems,which have been rapidly developed in recent years.Multiple high-voltage direct current transmission(MMC-HVDC)projects based on modular multi-level inverters have been built and put into operation worldwide.However,there have also been some urgent problems in the MMC-HVDC projects that have already been built,such as the problem of excitation inrush current caused by no-load closing of transformers near the converter station,which leads to the malfunction of broadband harmonic protection in the converter station.In the Fengning converter station of the Zhangbei Flexible DC Project,there have been multiple cases where the excitation inrush current generated during the no-load closing of the main transformer of the pumped storage power station connected to the converter station causes the harmonic distortion rate of the common connection point voltage to exceed the broadband harmonic protection setting,leading to the malfunction of the MMC converter broadband harmonic protection and the problem of MMC converter locking.Similar problems have also occurred in other converter stations of the Zhangbei Flexible DC Project.This problem seriously affects the safe and stable operation of the MMC-HVDC project,and effective solutions are urgently needed.This article conducts in-depth analysis and research on the issue of "excitation inrush current leading to misoperation of broadband harmonic protection in converter stations".Firstly,the reasons for the generation of excitation inrush current,the characteristics of excitation inrush current,and the characteristics of the second harmonic variation of excitation inrush current were analyzed.It was concluded that when the initial phase angle of closing is between 0 degrees and 90 degrees,as the initial phase angle of closing increases,the saturation of the transformer core becomes less severe,the discontinuity angle gradually increases,the amplitude of the second harmonic gradually decreases,and the proportion of the second harmonic gradually increases,and this conclusion was verified through simulation experiments in PSCAD.Afterwards,the effects of initial phase angle,residual magnetism,power supply voltage,and system impedance on excitation inrush current were analyzed,and the characteristics of excitation inrush current generated by no-load closing of the main transformer of the pumped storage power station connected to Fengning Station were studied.Secondly,the impact of excitation inrush current generated by the main transformer of the pumped storage power station connected to the Fengning converter station on the Fengning converter station was studied from four aspects,including the impact of excitation inrush current on the voltage at PCC,the impact of excitation inrush current on the DC bus voltage of the converter,the impact of excitation inrush current on the phase-locked loop,and the impact of excitation inrush current on the bridge arm.The mechanism of excitation inrush current causing broadband harmonic protection malfunction was analyzed.Finally,a modular multi-level converter grid side harmonic suppression strategy based on a quasi proportional resonant controller is proposed to solve the problem of broadband harmonic protection malfunction caused by excitation inrush current in the converter station.The harmonic suppression link is composed of a bandpass filter and a quasi proportional resonant controller.The harmonic suppression link is superimposed on the flexible DC inner loop current controller,achieving the effect of suppressing the grid side harmonic current and preventing the misoperation of broadband harmonic protection caused by excitation inrush current.The proposed grid side harmonic suppression strategy was applied in PSCAD to the equivalent simulation model of Zhangbei Flexible DC Project at Fengning Station for simulation experiments.The simulation results show that the proposed grid side harmonic suppression strategy has good harmonic suppression effect and can effectively prevent excitation inrush current from leading to broadband harmonic protection malfunction at Fengning Converter Station.