The Impact Of Converter Transformer Inrush Current On 50Hz Protection And Its Preventive Strategy

The converter transformer no-load closing is used as the basic operation during the converter station debugging and production operation of the HVDC transmission system.In recent years,the events that 50 Hz protection malfunction caused by the converter transformer no-load closing have emerged in China Southern Power Grid,seriously affecting the safety and stability of HVDC transmission system and power supply.There initiatives to circumvent on the operation management,takes the corresponding inrush current suppression strategy,and finally modifies the protection settings to leapfrog the inrush current to prevent the protection malfunction in actual projects,but the control effect of protection maloperation isn’t well.Based on the theory and simulation,it studies the impact of converter transformer inrush current on 50 Hz protection and its preventive strategy in paper,so the transformer new simulation model considering hysteresis characteristic is established,the influence mechanism of inrush current on 50 Hz protection malfunction is clarified,the influence boundary of inrush current on 50 Hz protection action is further analyzed.Finally,the countermeasures of the effect of inrush current on 50 Hz protection are put forward.Aiming at modeling for transformer considering hysteresis characteristic and credibility evaluation,In this paper,PSCAD/EMTDC transformer transient new model considering the hysteresis characteristic is proposed for the reliability problem of the classical transformer model for the inrush current in the PSCAD/EMTDC HVDC control and protection model.Comparative analysis shows that the peak and harmonic characteristics of inrush current in the transient process is consistent using hysteresis center line instead of hysteresis loop to simulate the transformer core excitation characteristics.Taking hysteresis loop data of model core is not easy to obtain into account,PSCAD/EMTDC classical transformer model can also achieve the purpose of this study.In view of transformer inrush current event in HVDC project of China Southern Power Grid and playing a preventive role to the operation and management of the HVDC operation system,influence mechanism of inrush current on 50 Hz protection malfunction is analyized in detail from the perspective of harmonic transformation,which found that the positive order second harmonic of sympathetic inrush current changed and enlarged by the HVDC system is the fundamental reason for 50 Hz protection malfunction.At the same time,the influence factors and boundary of 50 Hz protection malfunction are studied,the converter transformer closing angle,converter transformer residual magnetism,AC system strength and HVDC transmission power have great influence on the inrush current characteristics.Finally,the simulation results of inrush current influence boundary of typical HVDC project is carried out,it shows that the effect of no-load inrush current on 50 Hz protection in NC HVDC project is more serious than that of JZ HVDC project,which is consistent with the fact of HVDC project inrush coincidence in recent years.In view of the fact that actual HVDC project "set value,delay time" strategy to reduce the sensitivity of 50 Hz protection is not conducive to the safety and stability of HVDC system and the 50 Hz protection in actual HVDC project does not have a specific latching criterion for inrush current problems,optimization and improvement strategy of 50 Hz protection based on waveform recognition is proposed and custom modeling for simulation verification through PSCAD/EMTDC.The results show that 50 Hz protection latching criterion can be effectively identified the inrush current and lockout can be realized,which can effectively prevent the 50 Hz protection from malfunctioning.The results of this study provide the theoretical method and model for the research of the inrush current problem,and provide the optimization and improvement basis for the DC protection design researchers,which is of great significance to safety and stability of HVDC transmission system.This work has been funded by National Natural Science Foundation of China(51377059、51677073)、Power Dispatching and Communication Center of CSG and Ultra-High Voltage Transmission Company of CSG.The correctness and validity of some research results have been verified in actual HVDC PSCAD/EMTDC control and protection model and been tested in the actual HVDC system.