2. The protection of DC traction sy

2. The protection of DC traction system varies with the operation of Metro traction power supply system. The initial metro power supply system is usually the third contact track power supply mode, but because the power supply voltage is 750V DC, the power supply distance is relatively short, but the loop resistance is relatively large, and the short circuit current is relatively small, it is sometimes difficult to distinguish short-circuit current from traction load current. Early DC protection systems lacked good protection, more relay protection, and usually set only rapid current interruption and overcurrent protection to eliminate the fault. When the vehicle density is high, the maximum load current may be higher than the terminal short-circuit current or the value of the two methods. Therefore, the effect of this protective device is often not ideal. To solve these problems, one is to increase the two-way joint defense protection. Because the DC traction system usually uses two-sided power supply, when the fault is found in the two-sided power supply circuit, it is relative to the side of the traction substation near-end fault, short-circuit current value is larger, and easy to carry out current protection operation, for the other side of the substation for Remote Fault, its short-circuit current often can not make DC circuit breaker trip. If double side trip protection is used, the other side of the DC circuit breaker will trip immediately. Another solution is to use bilateral joint trip protection and low-voltage protection, mutual cooperation. Because the short circuit will cause the DC voltage value to drop, so when the current is too high and overcurrent can not work, low-voltage protection can be used as a backup protection.