Research On The Method Of UHVDC Line Protection Setting Considering The Transfer Characteristics Of DC Transformers

Ultra high voltage direct current(UHVDC)transmission has the advantages of large transmission capacity,long transmission distance and high stability,has been widely used in Chinese power grid construction.As an important part of UHVDC protection system,DC line protection plays a crucial role in the stable operation of the entire DC system.The DC line protection mainly uses the high-frequency variation signals of voltage and current for fault diagnosis,and the generation of fault characteristic quantities in the protection criterion is based on the measured value of the DC transformer.However,DC transformers are of various types and complex structures.Affected by many factors,it is difficult for the DC transformer to accurately transmit the signals of each frequency band on the primary side,especially the high frequency signals.Therefore,the transfer characteristics of the DC transformer will have an impact on the performance of the DC line protection.If the influence of the transfer characteristics of the DC transformer on the protection performance of the DC line is not considered when setting the protection,it will inevitably result in an unreasonable selection of the DC line protection setting,which will lead to a decrease in the protection reliability.This paper studies the above issue and mainly includes the following contents:1)According to the actual engineering parameters,the DC voltage divider and DC current transformer models are established and their transfer characteristics are analyzed.Firstly,the transfer function of each component of the DC transformer is deduced to obtain the transfer function of each part.secondly,obtain the overall transfer function of the DC transformer.then,the frequency response test and step response test of the DC transformer transfer function are carried out,obtain the transfer characteristics of DC transformers in time and frequency domains.finally,the UHVDC transmission system model is built according to the actual engineering parameters,and the influence of the DC transformer transfer characteristics on the line voltage and current transfer is verified by simulation.2)The influence of DC transformer’s transfer characteristics on UHVDC line protection is analyzed.Firstly,the DC line protection criterion and protection logic are analyzed,and the DC line protection is modeled on this basis.Then,the influence of DC transformer’s transfer characteristics on DC line protection under fault conditions inside and outside the area is simulated and analyzed.finally,the influence of DC transformer’s transfer characteristics on DC line protection under disturbance conditions is simulated and analyzed,and the DC transformer’s transfer characteristics are obtained.Influence regular pattern of characteristics on DC line protection.3)A setting method of UHVDC line protection based on fault characteristic quantity and considering the transfer characteristics of DC transformer is proposed.Firstly,the protection setting principle is determined according to the characteristics of different DC line protection,reset the value for the criterion which has a great influence on the result of the protection action,and verify the feasibility of the existing value for the criterion that has a small influence on the result of the protection action.secondly,determine the simulation conditions during fault simulation,such as line fault location,transition resistance size,and DC transformer type.then,the fault simulation is carried out and the simulation results are screened,and the setting value is calculated.According to the different transformers used,the protection Different setting values are used.finally,by setting the fault conditions inside and outside the typical area,the reliability of the obtained setting values is verified by simulation.