Only 5% of the faults are of the Permanent nature, rest are transient faults and so auto reclosing EHV Transmission Lines has become a generally accepted practice in Power System.
What is Auto-reclosing in EHV Lines ?
Ans: Auto-reclose is automatically reclosing a Circuit Breaker in a set time after it has been opened due to operation of a protection element.
It helps avoid delay due to human intervention, by automatically attempting CB recloser.
What are the advantages of Auto-reclosing?
Ans:
1) It helps maintain system stability during a transient fault condition. It is mainly applied to Radial Feeders, where system stability problems do not generally arise.
2) Interruption time is reduced, especially in the case of unmanned substations.
What is Dead Time?
Ans:
The time starting from the Tripping to the AR Shot.
It should exceed the de-ionizing time.
The time required for the de-ionizing of the fault path depends on: Arcing time, fault duration, wind conditions, circuit voltage, capacitive coupling to adjacent conductors, etc.
Single phase Dead Time of 1 sec is recommended for both 400 kV & 220 kV System.
What is Reclaim Time?
Ans:
It is the time between 1st Successful AR and CB ready for the next AR Cycle.
During this time, a new start of the Auto-reclosing is blocked.
This is the time required for the Breaker to fully Charge the Spring or build up the required air pressure for Tripping & then Closing.
If reclosing shot has been carried out and the line is energized and a new fault occurs before the reclaim time has elapsed, the auto-reclosing is blocked and a signal for definite tripping (3 Phase Trip) of the breaker is obtained and the Breaker goes into Lockout.
After the Reclaim time has elapsed, the AR Equipment returns to the starting position and a new reclosing sequence can occur.
The reclaim time should not be set to a low value that the intended operating cycle of the breaker is exceeded, when two fault incidents occur close together.
According to IEC Publication 56.2, a Breaker must be capable of withstanding the following Operating Cycle with full rated breaking current:
O + 0.3 s + CO + 3 min + CO
Where 0.3 sec is the Minimum Dead Time
3 Min is the Maximum Reclaim Time
Reclaim time is Generally set at 25 Sec.
Explain the AR Sequence?
Ans:
- Consider a Pre-fault or a Healthy Condition where the CB of an EHV Line is ON.
- Let’s say there is a fault on the Line. We Auto-reclose the Line only for two types of below mentioned faults:
Single Phase Zone 1 or Single Phase Zone 2 plus Carrier Received. - Fault is being detected by the Distance Relay.
- On detecting the fault, the Distance Relay gives command to Single Phase Self Reset Trip Relay, which inturn gives command to the CB.
At the same time a signal called ‘AR Initiation’ is being generated inside the Distance Relay , if following four conditions are satisfied:
AR is Enabled, CB is ON, CB is ready and 86/96 are not operated.. - The Breaker Operates in around 30 ms which includes Opening time (Trip Coil energization and Contacts separation) and Arcing Ting where Arc gets extinguished.
- After Initiation, Dead time ( time starting from the Tripping to the AR Shot) starts in the Relay which covers CB Operating time (Opening time + Arcing time), time for the contacts to fully open plus some margin. Generally we consider a Dead time of about 1 Sec.
- After the Dead time, Distance Relay issues the first AR Shot i.e. Closing Pulse to the CB.
- CB is Switched ON and lets say its a Transient Fault, so the CB will remain ON i.e. AR is successful.
- If it’s a Permanent Fault, CB Closes, but trips immediately (Reclose on to Fault) which is called as AR Unsuccessful and the Relay locks out for AR (AR Lockout) i.e. No more AR signal is given by the Distance Relay and a Three Phase Trip command is issued.
- After the Dead time, Reclaim time starts which is the time between 1st Successful AR and CB ready for the next AR Cycle.
- If a New Fault occurs after the Reclaim time, a new reclosing sequence starts.
- If a new fault occurs before the reclaim time has elapsed, the auto-reclosing is blocked and a signal for definite tripping (3 Phase Trip) of the breaker is obtained and the Breaker goes into Lockout.
