Opening of 132kV Breaker with Very Low SF6 Pressure

Don't you think you would be better asking someone in your own organisation rather than a bunch of strangers on the internet? It's you that's going to be at risk, not me sat 1000's of miles away.

Sure, why not (if the switch is faulty you need to disconnect it for maintenance somehow), and you are hardly going to do it with either end live.

Best you also recheck what your site safety and control procedures are in the event of a low gas pressure warning (is there an automatic safety trip programmed anywhere?)

Your logic looks good. The real danger would be from opening this breaker under load. By removing the load remotely, this danger is eliminated.

I am curious, though, as to what would happen if the breaker were to open automatically under the low SF₆ condition. Is there a scheme that can replenish SF₆ while the breaker is closed?

Thanks to all discussants.

I am now a bit more confident. Opening a completely isolated SF6 breaker with very little or no SF6 gas (as if it were in a testing lab) would pose no threat to the health of the breaker.

There is no auto-trip feature. I think auto-trip at such low gas pressure may not be desirable because arcing might start across the open contacts of breaker due to very less insulation, before the isolators are opened to achieve complete isolation.

Just for info: We have to actually make the bus dead also, because we are not sure the isolators are rated for breaking the line-charging current.

(as regards discussion within the organization, I am supposed to have knowledge of every aspect of electrical system at our small Plant which, unfortunately, I don't. Hence, I have to look outside and such forums are very useful to give insight in problem-solving)

If I remember right, the SF6 breakers can interrupt load current even with zero SF6 gas pressure. You may like to check with the manufacturer.

Notwithstanding the above, I would think it prudent to open the remote end first (and make the load current zero).

Another important point from safety point of view is not operating the breaker from local (open it from control room).

Hi raghun

That's why they lock the breaker out under low SF6 gas pressure, so you don't blow yourself and everyone near you pixie dust

Hi AF

The low pressure lockout on the SF6 breaker is to prevent you opening (or closing) the breaker under those conditions. Even with no load you will still be in danger.

Isolate live and load side of the breaker in question before attempting to operate it.

SF6 breakers cannot normally be recharged under energized conditions, this would be an exercise in futility as the leak which caused the low pressure will still be present.

Regards

After lock-out the breaker fails to operate in transient stage,so it's better to top up the gas pressure from 5 or 5.3kg/cm2 to 6.8kg/cm2 by filling the SF6 gas and periodic checking of gas pressure is must.

Best to divert this circuit as the only circuit at the reserve busbars and then disconnect it via the bus section CB.

The circuit will now be safe to trip, as it does not carry any load, hence it does not require SF6 gas arc quenching .

The faulty circuit breaker may be topped up to correct SF6 gas pressure and then tripped, or otherwise tripped through the mechanical trip facility.

Hi AF, My experience is on 400kV GIS breakers, however I comment from instinct. It may be worth noting, that SF6 will insulate up to 400kv at atmospheric pressure - ie total loss. You won't find this assurance in writing from the OEM, but I attended a recent SF6 awareness course, & this was discussed. Of course, once at atmospheric pressure, the gas would begin to mix with air, become less insulative, & eventually flashover. This situation is extreme & emergency switching upstream should take place without delay to avoid flashover. Firstly I assume you are a generator. In this situation, I would arrange an in service SF6 top up to normal working pressure. I know you may be dealing with density units rather than pressure. No matter. The top up connectors should be self sealing quick fits, Dilo are popular manufacturer. Make sure you know the gas bottle history or have a gas quality certificate. The oxygen content should be as near to zero as possible, the dew point should be better than minus 40 deg C. The acidity can be checked with Drager tubes. Your gas man will know all this, & must check before administering the new gas. Also be aware of N Gas regs, & fulfil your duty under them by recording your gas movements. From bottle number, to gas zone number, how many kg, & analysis results. If top up is not an option, on load, Open generator breaker & unit off or island mode. You will now be back feeding your generator transformer, and maybe unit Auxillary transformer for domestic load on site. Configure your domestic load to the other feed, taking all load off the 132kv circuit you need out of service, with the exception of magnetising current for the gen & unit transformers. Switch your HV sub, so that your low pressure breaker is on its own bar. Isolate the bar & earth up under your site HV safety rules & cross boundary safety document with grid You can now do what you want with the breaker. Maintenance override to allow opening. I hope this helps. CTB

Thanks to all for useful comments.

Though the target was to provide complete electrical isolation under emergency condition of lockout, thanks to CTB for info about maintenance procedure.

CTB, you're right. We are a small generating station with single 132kV bus. There is density monitor and the gas filling equipment is of DILO.

Under such condition, after opening of remote-end breaker, we would be forced to go to islanding mode for few min, to open bus-side isolator of the affected breaker.

Regards