SF6 Switch/breaker at very low gas pressure

[JJHorak]

I know you do not operate an SF6 breaker if gas pressure is low, but assume dead tank SF switch where there is at least a little positive pressure:
a) Is there a gas pressure level where there a risk of an internal line to ground fault?
b) If the breaker is open, is there a gas pressure where there is a risk of a flashover across open contacts?
If the pressure goes to 0, there is now a risk of air entering the tank. Again:
c) If there is a chance air has entered the tank, how large is the risk of an internal line to ground fault?
d) If there is a chance air has entered the tank, how large is the risk of flashover across open contacts?
Customer is reporting a 15kV SF6 padmount 5 way switch with very low gas pressure, and taking the switch out of service is going to be a major system disruption.

J.Horak

 

[cranky108]

Actually we specify three levels on our breakers. The first is alarm, the second is a trip, and the lowest is block trip.

 

[JJHorak]

Did you ever hear a discussion on whether there a point of low SF6 where internal flashover becomes a concern? I suppose in a well monitored breaker it is not realistic to go to 0 pressure without first getting an alarm and having time to isolate the breaker, but that is not the case here. Customer will lose loads if they take the switch out of service, and gauge shows a little pressure, so they are leaving it energized while they work to get the mfr to take a look.

J.Horak

 

[cranky108]

I think we have some of those distribution switches. And only a few have monitoring.
I think in some cases it becomes necessary to bite the bullet.
That said we only allow four way switches. When you get bigger, the problems get bigger.

For a breaker it could mean a bus outage, which is why we are moving from straight bus to ring, and breaker and a half.
But with higher voltage breakers, we have disconnects. With distribution gear all you have is elbows.

We moved to SF6 distribution switches, after we had problems with open air switches, and the alignment issues.
We are now looking at solid dielectric switches. Or in some cases reclosers.

Anyway the industry push is away from SF6.

 

[RRaghunath]

Generally speaking, the SF6 switchgear can withstand rated voltage even with SF6 gas pressure going down to zero.
The risk is when there is over voltage (due to single phase to ground faults in the system) or when there is a switching (due to Vacuum breaker or contactor switching etc.) or lightning surge.
Further, if the switchgear is left in service for quite sometime in service after the SF6 gas pressure came down to zero, there is risk of air entry and that can lead to partial discharges and flashovers.
How much time we have before any such thing happens depends on how big is the leak and how moist or polluted is the air in the area etc.
It is a calculated risk that should be taken after assessing the consequences versus benefits.