What is Auto Reclosure?
Autoreclosing is a feature which is provided in the Line Circuit Breaker so that Single Pole of a Breaker may trip and close when command is given to do so.
- Transient Fault
- Semi-Transient Fault
- Permanent Fault
Philosophy of Auto Reclosure:
It has been found that 80% of the fault in Power System are Transient in nature, 12% are Semi-transient and remaining 8% are only permanenet fault. Semi-transient fault are those fault which take some finite time to clear by itself. For example, suppose a bord spanning the two lines sit then it will cause a fault which will clear by itself after the burning of cause of fault,after some time say 1sec. Thus we will expect, Autorecloser to take place for 1 sec i.e. Breaker shall close after a time delay of 1 sec. Here note that the time after which fault clears by itself is called DEAD TIME. Therefore, in our example DEAD TIME = 1 sec.
But for Permanent Fault, Autorecloser will not help as the cause of fault continuously exists so if we incorporate the Autorecloser the Breaker will again trip after the Autorecloser. So how many attempts will the Realy take to Autoreclose and after how much time it will take another consecutive attempt to Autoreclose?
“Here we come to another concept, called RECLAIM TIME. RECLAIM TIME is the time after which Relay will take another consecutive attempt to Autoreclose. This RECLAIM TIME is typically set at 25 sec. The number of attempt for Autorecloser is set in the Relay which is 4 for MiCOM P444 Distance Protection Relay. This means that Relay will take four Autorecloser shots and at the end of fourth shot, if still fault is existing, the Line will be taken out.” In figure below, a Numerical Relay is shown.
Thus we see that for permanent fault Autorecloser won’t help as we need to attend the fault and rectify it.
Autorecloser can be Single pole or Three pole. Here Pole means Breaker of any of the three phase i.e. either R, Y or B phase. Single Pole Autorecloser take place during Line to ground fault. It shall be noted that Autorecloser facility is provided only in Line Breaker and that to by Distance Protection Relay.
Single Autorecloser take place in the following conditions:
Zone-1 protection operated AND Carrier Channels are healthy AND Three Pole Tripping has not taken place.
Zone-2 protection operated with Carrier Signal Received.
This seems surprising that only a single pole of Breaker trips during Zone-1 fault. But it’s true. The phase, say B phase, in which Line to ground fault has taken place will only trip and reclose after the DEAD TIME. If within the RECLAIM TIME, another fault take place then Three Phase trip will occur. During the DEAD TIME period, power is fed to the system via the two healthy phases.
In case of Three Pole Autorecloser, all the three phases are opened independently irrespective of type of fault be it Single L-G or L-L or L-L-L fault and reclosed after the DEAD TIME. During the DEAD TIME period, no power can be transmitted and therefore system is liable to operate unstably.
Hope you enjoyed the topic. Thank you!