The most important consideration in the application of auto-reclose to EHV transmission lines is the maintenance of system stability and synchronism.
The problems involved are dependent on whether the transmission system is weak or strong.
With a weak system, loss of a transmission link may lead quickly to an excessive phase angle across the Circuit Breaker used for re-closure, thus preventing a successful re-closure.
In a relatively strong system, the rate of change of phase angle will be slow, so that delayed auto-reclose can be successfully applied.
Auto Reclose protection
Auto reclose operate when fault occurred on transmission line. Different types of fault occurred in transmission line.
Types of faults: 1.Transient faults 2. Semi permanent faults 3. Permanent faults
Transient faults
Disappear after Short dead time Various Transient Faults
Lightning strokes resulting in flashovers
Conductor swinging due to high winds
Bird fault
Temporary contact with foreign objects like tree etc.
80%-90% of faults on any overhead line network are transient in nature. The remaining 20%-10% of faults are either semipermanent or permanent.
Advantages
Decreases outage time,Improves reliability, Improves system stability, Reduces maintenance time
What is Dead and reclaim time of Auto reclose
Dead time:
The time between the auto reclosing scheme being energised and the operation of the contacts which energise the circuit breaker closing circuit.
Range: 0.3- 1.2 sec ; 1 Sec (NRPC & CBIP-274 for both 220 kV and 400kV)
Reclaim time:
The time between 1st and 2nd Auto reclosure.
Range: 10-30 sec ; 25 Sec (NRPC)
Single phase , Single pole Auto reclosing is recommended.(CBIP)
Auto reclose function should be kept for Zone-1.(CBIP)
The recommended operating cycle as per IEC publication 56.2 is O-0.3s-CO-3min-CO
Auto Reclose Out/Block Protection
CB internal problems
Local Breaker protection relay operates
Bus bar protection operates
Permanent fault in Transmission line
Single-phase Auto-reclosing
Single phase to earth faults are the majority of faults in overhead lines.
When three-phase auto-reclosing is applied to single circuit interconnectors between two power systems, the tripping of all three phases may cause the two systems to drift apart in phase resulting no interchange of synchronising power during the dead time.
If only the faulty phase is tripped, synchronising power can still be interchanged through the healthy phases leading to the reduction in the disturbance on the system when the circuit breaker recloses.
On the occurrence of a phase-earth fault, single-phase auto-reclose schemes trip and reclose only the corresponding pole of the circuit breaker.
Modern numerical relays often incorporate the logic for combined single and three-phase auto-reclosing where single phase to earth faults initiate single-phase tripping and reclosure, and phase-phase faults initiate three-phase tripping and reclosure.
Advantages of Single Phase Auto-reclosing
The maintenance of system continuity.
The particular faulty phase is restored by maintaining the stability and synchronism.
Disadvantages of Single Phase Auto-reclosing
The main disadvantage is the longer de-ionisation time resulting from capacitive coupling between the faulty and healthy lines. This leads to a longer dead time being required.
