The pre reclosing acceleration protection is generally referred to as "pre acceleration protection". Assuming that overcurrent protection is installed on each line, its action time limit is coordinated according to the stepped principle. Therefore, the time limit near the power terminal protection 3 is very long. In order to accelerate the removal of faults, a forward acceleration method can be used at protection 3, which means that when a fault occurs on any line, the first time it is removed by the instantaneous action of protection 3. If the fault is outside of line A-B (such as point d), then the action of protection 3 is non-selective. However, after circuit breaker 3 tripped, the reclosing was initiated to restore power supply, thus correcting the above non selective action. If the fault at this time is instantaneous, the power supply is restored after reclosing. If the fault is permanent, it is cut off by protection 1 or 2. When protection 2 refuses to operate, protection 3 will trip the second time according to the selective time limit t3. In order to ensure that the non-selective action range does not extend too long, it is generally stipulated that protection 3 should not operate when the low-voltage side of the transformer is short circuited. Therefore, its starting current should also be set to avoid short circuits (d2) on the low-voltage side of adjacent transformers

The advantage of using forward acceleration is:

a. It can quickly remove transient faults B. It may prevent transient faults from developing into permanent faults in time, thereby improving the success rate of reclosing C. Ensure that the rated voltage of the busbars in power plants and important substations is above 0.6-0.7 times, thereby ensuring the quality of auxiliary power and important users' electricity D. Less equipment is needed, only one set of reclosing device needs to be installed, which is simple and economical The disadvantage of using pre acceleration is:

a The working conditions of the circuit breaker are harsh and the number of actions is high When it overlaps with a permanent fault, the fault removal time may be longer If the reclosing device or circuit breaker refuses to close, the power outage range will be expanded. Even in the event of a fault on the last level of the line, it will cause a power outage for all users connected to this line. The front acceleration protection is mainly used on direct distribution lines led from power plants or important substations below 35KV, in order to quickly remove faults and ensure bus voltage. Generally, only simple current protection is installed on these lines

2, post acceleration protection

Reclosing post acceleration protection is generally referred to as "post acceleration". The so-called post acceleration refers to the selective action of the protection when the line fails for the first time, and then reclosing. If it coincides with a permanent fault, after the circuit breaker is closed, accelerate the protection action and instantly remove the fault, regardless of whether the first action has a time limit. The combination method of post acceleration is widely used in networks above 35KV and power transmission lines for important loads. Because protective devices with relatively complete performance are generally installed on these lines