Inrush Current Characteristics Analysis And Differential Protection Research Of Converter Transformer In UHVDC Transmission System

In order to realize the transformation of China ’s energy structure,China is actively promoting UHVDC transmission technology.Converter transformer is a key component in UHVDC transmission system,so it is very important to ensure its safe and stable operation.Because the UHVDC transmission system has a high voltage level and is an AC/DC hybrid system,its coupling relationship is closer,so the parameters of the converter transformer are quite different from those of the ordinary transformer.Therefore,whether the traditional differential protection can be applied to UHV converter transformers remains to be studied.This paper conducts a detailed analysis of the inrush current characteristics of UHV converter transformers,and a new inrush current identification method is found.The primary research conducted in this paper can be summarized as follows.Firstly,this paper models the UHVDC transmission system and analyzes the mechanism and parameters of each component in the UHV transmission system.The control principle and various operation modes of the UHV transmission system are introduced in detail,which lays a foundation for the follow-up research.Secondly,the characteristics of inrush current and internal fault of the converter transformer in the UHVDC transmission system are analyzed,and the adaptability of traditional differential protection is analyzed.It is concluded that the traditional differential protection cannot accurately distinguish the inrush current and no-load switching on inter-turn fault.Because the trend characteristics of inrush current and internal fault waveform are quite different,the Spearman coefficient is introduced to distinguish the waveform.Given the obvious inrush current characteristics of the waveform characteristics when the no-load closing is in the inter-turn fault,the time coefficient of the zero sequence current on the grid side is used as an auxiliary criterion.According to the simulation results obtained from PSCAD,it can be concluded that the proposed method is capable of accurately discriminating between inrush current and internal faults.By implementing this method,it is possible to enhance the safety of the converter transformer.Once again,the characteristics of the sympathetic inrush current of the UHV converter transformer are analyzed,and on this basis,an analysis was conducted on the suitability of both the traditional differential protection and the new differential protection method.The findings indicate that it is difficult to accurately distinguish between these two methods under conditions where CT saturation occurs due to sympathetic inrush current.According to the characteristics that the amplitude of the sympathetic inrush current differential current is much smaller than that of the internal fault,the proposed new differential protection method incorporates a criterion that takes into account the amplitude of the differential current.This serves to further refine the discrimination process between sympathetic inrush current and internal faults,following the initial assessment,the internal fault is further judged by Spearman and the zero sequences current time coefficient.For the case where the amplitude of the differential current is small and the no-load closing is in the slight inter-turn,the time coefficient of the zero sequence current can be used to judge.Finally,an analysis was conducted on the fault inrush current characteristics of UHV converter transformers,and an assessment was made regarding the suitability of both traditional differential protection as well as an improved new method of differential protection.It can be concluded that due to the obvious inrush current characteristics of the fault inrush current,the traditional differential protection and the new method of differential protection improvement cannot be accurately judged.According to the obvious inrush current characteristics of the valve side current of the fault inrush current,the second harmonic content analysis can be obtained.The second harmonic content of the valve side current of the fault inrush current is high.Therefore,an auxiliary criterion based on the second harmonic content of the valve side current can be added to effectively ensure the safe operation of the component.