Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

Register Log in
  • Congratulations waross on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

SF6 Switch/breaker at very low gas pressure

Status
Not open for further replies.
JJHorak

JJHorak

Electrical
Dec 19, 2000
68
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
 
Replies continue below

Recommended for you

Sort by date Sort by votes
Actually we specify three levels on our breakers. The first is alarm, the second is a trip, and the lowest is block trip.
 
Upvote 0 Downvote
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
 
Upvote 0 Downvote
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.
 
Upvote 0 Downvote
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.
 
Upvote 0 Downvote
Status
Not open for further replies.

Similar threads

J
  • Locked
  • Question
Utilizing an SF6 circuit breaker as a normally open point 1
Replies
14
Views
7K
stevenal
stevenal
NickParker
  • Locked
  • Question
Auto Transfer Switch in place of Low Voltage breaker
Replies
7
Views
528
wcaseyharman
wcaseyharman
simonfr
  • Question
LO/TO - remote circuit breaker 1
Replies
6
Views
5K
PersonalProfile
PersonalProfile
EEENGRX
  • Locked
  • Question
Tie feeder breaker connection
Replies
0
Views
1K
EEENGRX
EEENGRX
milwaukeebob
  • Question
120/240V 1P-3W, 100A Service to Very Remote Building 2
Replies
16
Views
6K
Amp Dylan
A

Part and Inventory Search

Sponsor

Back
Top Bottom