Operation of the SF6 Circuit Breaker with a gas leak 1

[silkwormfrog]

Hi

I am trying to write up a procedure in a event of a SF6 gas leak. My understanding is the circuit breaker will still be able to operate.

Can anyone confirm if this is the case, and does the operation of the Circuit Breaker been affected at all with low or no SF6 gas present? Thanks

 

[rasevskii]

Of course you cannot. The SF6 is the insulation and arc-quenching medium. At low gas pressure, in two or more stages, there is an alarm and a trip or blocking of operation. The very concept of SF6 gear is that it is possible, due the high density compared to air, to build such compact gear and GIS HV stations in a footprint a fraction of the size of conventional air-insulated outdoor substations.

rasevskii

 

[DTR2011]

Most SF6 breakers will have a 2 stage pressure - alarm / cut out scheme. If the pressure falls below a set level (temperature compensated), then the "low" (63G) contacts will operate. These contacts are generally wired to SCADA to provide an alarm. In this condition, the breaker can be operated. If the pressure falls below the " low - low" setting, there are generally contacts (63Gx), wired to block trip and close internal to the breaker. Some use low Sf6 contacts as an extra input to a breaker failure relay logic as well.

Since Sf6 has a higher density than air, there will not likely be a case where all of it leaks out and depending on the breaker type / Manufacturer, the breakers can be operated (not for fault interruption) at 0 PSIG. My experience with dead tank SF6 breakers and GIS is that this holds true.

With no Gas present, the breaker should not be operated under any circumstances - energized or not. This is generally spelled out explicitly in the breaker manual.

 

[Marmite]

The default answer would be not to operate the breaker with reduced gas pressure. In reality it depends on the type of breaker and the voltage and is manufacturer specific. Some medium voltage switchgear can be operated safely with SF6 pressure reduced down to atmospheric. Sometimes the elevated SF6 pressure is used to increase the BIL, rather than relying on it for arc quenching.
Regards
Marmite

 

[oldfieldguy]

The 'safe' answer is 'no'. A better answer is 'yes' if the breaker is in low pressure alarm but not yet below cut-out. Below cut-out, the electrical controls should be disabled anyway, but there's always the manual option.

I would not relish the idea of finding out if my breaker is one that will successfully open and interrupt line charging or transformer excitation current, nor would i like to be the author of the procedure that is being reviewed after an accident.

old field guy

 

[bacon4life]

One of the manuals I have handy states: Operations below the 64 PISG limit can result in reduced back pressure in the puffer cylinder causing damage from excessively high interrupter speeds.