The over-current protection of final voltage is the maintenance of over-current flow with compound working voltage locking. It is an engineering design of "over-current flow maintenance" related to working voltage. Locking means turning off and locking, that is, over-current protection controlled (turned off or turned on) by compound working voltage. Only when the current and working voltage need to meet a certain standard can they pose, and it is not easy to operate and maintain as long as there is a standard that does not meet the setting value. If it is only "over-current" maintenance, the maintenance will be run as long as the current exceeds a certain value.
The compound operating voltage components are composed of a filtered negative sequence voltage relay and a low voltage relay. The low-voltage relay is connected to the two-color working voltage through the normally closed contact of the negative-sequence voltage relay, so as to ensure the reliable posture of the protection device in the case of symmetrical three-phase short-circuit fault and improve the sensitivity of the low-voltage relay to the three-phase short-circuit fault. Because the negative sequence voltage may occur in a short time at the moment when the three-phase short-circuit fault occurs, the negative sequence voltage relay has a certain posture. After the negative sequence voltage subsides, the negative sequence voltage relay returns, and the low voltage relay is connected to the two-color working voltage. If the low-voltage relay is returned, it is stipulated that the residual voltage of the transformer bus duct must exceed the return working voltage of the automobile relay. Because the three-phase voltage decreases when the three-phase short circuit fails, the low-voltage relay is still in posture, and the maintenance work at this time is equivalent to the overcurrent maintenance of low-voltage operation.
In that case, I wonder if Xiaobian can understand it? Warmly welcome communication. Supplementary note: In the relay protection device, the final voltage includes low voltage and negative sequence working voltage, and the key to the maintenance of the final voltage locking overcurrent is the reserve maintenance of the transformer or the tn-s maintenance of the transformer. Why should this locking standard be added to overcurrent protection? Mainly in order to better avoid the misoperation of equipment caused by transformer load. When the transformer is loaded, the working voltage will decrease, and the current will certainly increase, so it is possible to achieve the overcurrent time constant, and the load condition always produces a short time. If there is no low-voltage locking standard, the transformer will be disconnected. Therefore, in order to better ensure the stability of the power supply system, the low-voltage locking standard is added. The standard of negative sequence working voltage locking is mainly to improve the sensitivity of three-phase short-circuit fault. Single-phase electricity and two-phase short-circuit fault will cause very large negative sequence working voltage, which need not be considered. In the case of three-phase short-circuit fault, the short-circuit capacity is also symmetrical, but at the moment of short-circuit fault, the three-phase voltage decreases and a certain negative sequence value (6~9V) ` will occur. This basic principle is selected for negative sequence working voltage locking. To sum up, the effect of final voltage locking overcurrent is to better avoid misoperation when transformer is loaded and improve the sensitivity of entrances and exits when three-phase short-circuit faults are common. In the protection equipment, the final voltage has two time constants, namely, the low voltage locking value and the negative sequence working voltage locking value, which are set by the customer himself.