Analysis
One of the 220 KV feeder at 220/132Kv GIS system, suddenly tripped automatically indicating the cause of tripping being the leakage of SF6 and finally reaching of LOCK OUT condition.
In practice it is observed in the AIS scheme that SF6 monitoring is used for two different purposes (Alarm for the leakage to certain low level and Blocking of operational circuit after reduction of SF6 to next level to avoid flashing of the chamber).
But in this case of GIS system, the operational circuit of Breaker was studied and found with monitoring contact of SF6 in the scheme.
It was found with SF6 monitoring contact as used for two purposes. First contact being activated from the LOW level of SF6 has been used as the warning buzzer to take corrective action. In case corrective action could not be attended or leakage is severe, then before attaining to the danger limit of SF6, the next contact shall be used to trip out the system instead of causing blocking of the trip operation. But after the situation of final LOCKOUT condition as described in this case, for the safety of the system, the supply system used to be interrupted.
On inquiry about this concept to the grid personnel, it was advocated that in the case of GIS arrangement, SF6 gas is used for two purposes (1. For quenching of the arc during the operation of breaker on load. 2. For maintaining of statutory clearance with the metal body part and conductor to conductor).
So they have taken the decision for tripping of the system just before the allowable limit of the danger LOCK OUT condition.
Suppose an Example 6.0 bar if selected for SF6 low alarm and 5.2 as the actual danger limit of , then they have used the limit as 6.0 being the LOW ALARM and 5.6 as the emergency forced tripping of the system for safety of the GIS.
After attaining the limit of emergency forced tripping, the closure of the breaker cannot be attended till to attend the faulty SF6 compartment suffered with emergency forced outage of the system.
Conclusion
For GIS sub-station, the emergency forced outage of the main supply scheme has to be chosen.
The contact of the limit switch has to be selected just above the actual danger limit of breaker lock out condition.