Study On Traveling Wave Protection Of Multi Terminal DC Transmission Line Under The Condition Of Double Pole/Single Pole Earth Return Line Co-existence

Multi terminal HVDC system can connect multiple AC systems,which can effectively solve the problem of increasingly tense transmission channels.At the same time,when the system adopts hybrid DC transmission technology,it can reduce the risk of commutation failure and improve the stability of the receiving AC system.Therefore,multi terminal Hybrid DC transmission technology is an important development direction of HVDC.The multi terminal DC transmission system has many operation modes,which can give full play to the flexible and economic advantages of DC transmission.The co operation mode of double pole / single pole earth return line is one of the most important.In this way,DC transmission system can guarantee the transmission power of the rest lines will not be reduced under the condition of single station and single pole outage.However,due to the outage of some lines,the topological structure of the lines will change.An asymmetric T-zone refraction interface will be formed in the DC transmission lines,which will cause the cross refraction of the originally independent propagation modulus of the lines,which will affect the fault transient line porter Performance of traveling wave protection based on performance.Considering that the traveling wave protection is generally the main protection of DC transmission lines,its performance is directly related to the safety of the system and equipment,therefore,it is of great significance to study the traveling wave protection under the co-existence operation mode of bipolar / monopole earth return lines,which can provide theoretical basis for the actual project:(1)According to the structure characteristics of DC transmission line under the coexistence operation mode of bipolar / monopole earth return line,combined with the phase mode transformation of lines on each side of the asymmetric T-zone refraction interface,the cross refraction coefficient matrix of voltage modulus travelling wave from different lines incident into the asymmetric T-zone refraction interface is obtained.(2)According to the cross refraction coefficient matrix,the propagation characteristics of the fault traveling wave in the two pole / monopole earth loop coexisting operation mode and the conventional two pole operation mode are compared and analyzed.The results show that the voltage modulus traveling wave has great difference after refracting in the T-zone,and the operation mode of the incident line will bring different effects.In addition,combined with the specific principle of modulus traveling wave protection,the response of each station traveling wave protection criterion is analyzed respectively: the linear modulus criterion is greatly affected by the refraction of T-zone,for bipolar operation converter station traveling wave protection,after the refraction of T-zone,the response of bipolar operation line fault criterion will increase,and the response of monopole operation line fault criterion will decrease;for monopole operation converter station traveling wave protection The results show that the fault criterion response of bipolar operation line will decrease,and the fault criterion response of monopole operation line will increase.(3)The hybrid multi terminal DC transmission line has multiple transmission lines,which will bring the contradiction between the attenuation of the transmission line to the ground modulus and the attenuation of the equipment at the end of the line to the ground modulus.Therefore,this paper modifies the setting principle based on the criterion function and verifies its applicability.In addition,this paper discusses the problems existing in the co-existence operation mode of bipolar / monopole earth return line and the common setting value of other conventional operation modes,and calculates the setting of travelling wave protection in bipolar / monopole operation mode.