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 between short-circuit current and traction load current. Early DC protection systems lacked good protection, more relay protection, and usually only set current quick break and over-current 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 the above-mentioned problems, one is to increase the protection of two-way joint defense. Because the DC traction system usually adopts bilateral power supply, when a fault is found in the bilateral power supply circuit, it is a near-end fault relative to the side of the traction substation, the short-circuit current value is larger, and it is easy to carry out the 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 short circuit will cause DC voltage value to drop, so when the current is high and overcurrent can not work, low-voltage protection can be used as a backup protection.