In fact, nearly 90% of the faults in transmission and distribution systems are transient faults. Whenever a transient fault occurs, the protective relay detects the fault. It immediately sends a trip command to the circuit breaker. The circuit breaker opens to disconnect the faulty line from the source. Since the fault is transient in nature, it usually disappears almost instantly. The line becomes healthy again within a very short time.
Now imagine that we restore the line manually. An operator has to inspect the system first. Then he closes the circuit breaker manually. Naturally, this process takes time. This delay is not desirable. It affects the stability of the power system. The longer the interruption, the greater the disturbance to the network. Therefore, the system should restore the line automatically. It should do so as quickly as possible. This is the purpose of the auto-reclosing mechanism.
Let us understand this with a few examples.
Transient Tree Faults
Suppose a tree branch touches a high-voltage transmission line. Sometimes, the branch does not even touch the conductor. It only comes very close to it. In such a situation, the air between the conductor and the tree branch may break down. It produces an electric arc. The protective relay senses this arc as a fault. It immediately trips the circuit breaker.
However, the arc itself produces a very high temperature. Because of this heat, the tree branch may burn away. Alternatively, wind may move the branch away from the conductor. As a result, the arc extinguishes naturally. The fault disappears automatically.
Transient Bird Faults
Let us take another example. Suppose a bird creates a short circuit on an overhead line. This is common in distribution systems. The fault current is extremely high. Therefore, the body of the bird burns almost instantly. The arc is extinguished. The line becomes healthy again.
Transient Lightning Fault
Lightning can also create transient faults. A lightning strike can produce a temporary fault. A nearby lightning strike can create a transient overvoltage. This sudden voltage disturbance may initiate a transient fault. However, such faults usually disappear within a very short time because of lightning arresters.
Auto Reclosure Mechanism
That is why nearly 90% of overhead line faults are transient. Now let us see how the auto-reclosing sequence works. Whenever the relay detects a fault, it trips the circuit breaker. In a transmission line, the circuit breakers at both ends usually trip simultaneously. In a radial feeder, only the source-end circuit breaker trips.
Dead Time
After tripping, the circuit breaker remains open for about 0.3 second, or 300 milliseconds. We call this time dead time. After the dead time expires, the auto-reclosing relay sends a closing pulse to the circuit breaker.
Suppose the fault has already disappeared during these 300 milliseconds. The arc ends. The air in the fault path has also become completely de-ionised. In that case, the circuit breaker closes successfully. This restores the transmission line. Normal power supply resumes.
But suppose the fault is still present. The moment the circuit breaker closes, the relay detects the fault again. It immediately sends another trip command. The circuit breaker opens instantly. Now a question arises.
Why does the autoreclosure provide a 300-millisecond delay?
The answer is simple.
Immediately after the circuit breaker opens, the air in the fault path remains ionised. Ionised air is electrically conductive. Therefore, it cannot provide proper insulation. The air needs some time to de-ionise and regain its insulating strength. The 300-millisecond dead time allows this de-ionisation process to complete. If the circuit breaker is reclosed too early, the arc may restrike. The fault will continue. Therefore, the relay intentionally waits for about 300 milliseconds before sending the closing command.
Now suppose the fault is still present even after the first reclose. The relay trips the circuit breaker immediately. For transmission lines, this trip is generally permanent. The autoreclosure relay does not issue any further closing commands. This prevents repeated switching under permanent fault conditions.
Autoreclosing for Distribution Systems
The situation is different in distribution systems. Distribution utilities generally allow multiple auto-reclosing attempts for 11 kV networks. This is because some temporary faults require a longer time to clear completely.
Therefore, after the second trip, the relay waits for about three minutes. After this waiting period, it sends another closing pulse.
Why is this three-minute delay provided?
There are two reasons. The first reason is complete fault clearance and de-ionisation. The second reason is spring charging.
During the first trip operation, the tripping spring becomes discharged. During the closing operation, the closing spring also becomes discharged. Therefore, the tripping spring becomes charged again during the discharging of the closing operation for the first closing. If the circuit breaker trips immediately after reclosing, the tripping spring becomes discharged again. Hence, now both springs are in the discharged condition.
Before the next closing operation, the closing spring must be fully charged again. The spring-charging motor performs this operation. Naturally, it requires some time. Therefore, the relay waits for about three minutes. During this period, the spring becomes fully charged. At the same time, the fault path also gets sufficient time to recover.
After three minutes, the relay again sends a closing pulse. If the fault has disappeared, the circuit breaker remains closed. The feeder returns to normal service.
If the fault still persists, the relay trips the circuit breaker immediately. The relay again waits for another three minutes. Then it performs another reclosing operation. If the fault still does not clear, the circuit breaker trips once again. This becomes the final trip or lockout trip.
After this lockout, the auto-reclosing relay does not issue any further closing commands. The circuit breaker remains open. The utility personnel have to locate and clear the fault through proper maintenance. Then, they energize the line.