The New Research On Protection Of DC Feeder In Urban Rail Transit

Urban rail transit is considered as the prefferred solution to solve traffic congestion in large and medium cities, which has many advantages compared with other transport:fast, punctual and large capacity. It is very important to the protection for DC traction power supply system in urban rail transit can detect abnormality and remove the fault current timely.This paper analyzed the principle of protections applied in power supply system in the rail. we found that:the existing main protection methods may not be able to detect the fault when it takes place in a long distance, as the fault current is lower than the overloaded current. On the other hand, the transient nature of the fault current also makes it hard to detect compared with the load starting current, resulting in great difficulties in the value setting of the current transient component protection (rate of current rise protection and current increment protection). This paper proposes to analiye a protection which is simple in threshold value setting, reliable in operation, and independent of the locomotive operation and transition resistance.Based on the characteristics of the DC power supply system we analyzed two solution:application of traveling wave for directional protection; application of power balance protection. The principle of the two protections as followed:the first one proposes to base the subway protection decisions on the fault traveling wave information that is free from the impact of transition resistance. Specifically, a fault current can be detected by analyzing the traveling wave signal through Wavelet transformationl. The later proposes to apply the power balance protection to the urban rail transit system. According to the special nature of load in the urban rail transit system, we analyzed the way of transmission to achieve the function of protection and the characteristics of power variation when the locomotive started and line loss in order to set up a reasonable threshold for protection.In order to testify the feasibility of the principle of the above protections, building a model of DC traction power supply system in PSCAD/EMTDC. Modeling work includes traction substation rectifier system; locomotive traction load system; metro vehicle electric drive system as well as the rail system. Finally, we apply the current and voltage datas from simulation to prove the two protection can make up the shortage of the existing protection.