Low SF6 action 8

[Mbrooke]

What are the typical choices of action when dealing with low SF6 and an SF6 cutout? Is an automatic relaying action initiated or is the decision left to operators via remote SCADA?

 

[davidbeach]

We alarm on low pressure and block action on low low pressure.

 

[Mbrooke]

As is standard, but once in low or low low pressure do you have relaying do an automatic buss trip or have SCADA take the breaker out of service? Also does a breaker in low low still hold dielectric strength?

 

[111R]

Once in low low, you're under the assumption that the breaker can not be counted on for interrupting current...hence the block trip. So, the only action possible is tripping adjacent breakers as you would during breaker fail.

The action would most likely depend on the criticality of the load. It should be addressed soon, but you may not want to automatic trip if you can provide the customer some advance notice to shut down their process, etc.

 

[Mbrooke]

This could be avoided by opening the breaker at the remote end (removing load and looped power flows from the line) and opening the air break switches?

 

[submonkey]

Hi Mbrooke,

There is a good argument (in my opinion) for automatically tripping
the breaker when the stage 2 (low low) density is reached.

Regardless of what action is taken for low gas, the manufacturer must
ensure that the breaker can safely interrupt at the stage 2 density.

All practical gas leaks are slow leaks, which means that the moment when
the densimeter announces stage 2 is the last moment to safely open the
breaker. When the densimeter reaches stage 2, two actions are possible:

(1) Lock out all trips, because tripping after *passing* stage 2 is dangerous.
(2) Trip immediately.

I favour option (2) because:

- It avoids low gas blocking circuitry which must affect both A and B trip
circuits. Consider how much expense and effort goes into redundant
protection, only to have both protections blocked by a (usually) single
densimeter and cheap auxiliary contactor. Densimeters are often not
maintained, tested, etc.

With a tripping strategy, you can just operate the B trip circuit and
leave A clean and independent.

- Recovering from a "locked out" condition often means de-energising the
bus. In some cases (bus section breakers) it may be necessary to interrupt
two buses. Where I work, the network arrangement allows tripping of one
breaker without major consequence. Opening several is often not convenient.

Note that the stage 2 density is only ever reached when a stage 1 alarm has
been ignored. Automatic tripping action leaves the breaker in a safe state.

Breaker manufacturers like to lock out on low gas because that is the safest
thing for their circuit breaker (and it's associated warranty). I argue that
tripping is usually the best option for the network owner.

Thanks,
Alan

 

[Mbrooke]

But how do you trip immediately on stage two when it locks out the trip coils?

 

[stevenal]

Since Metcalf made the news, I challenge the assumption that all leaks will be slow.

 

[bwen08]

We employ the same methodology as submonkey's comment on our SF6 breakers.

Mbrooke: The factory control scheme provided two somewhat parallel SF6 lockout aux contacts in the trip circuit; one to block any remote trip from relays and a second that closes upon the SF6 lockout scenario and operates the trip coil.

 

[Mbrooke]

Basically a single pole double throw or double pole double throw type deal?


Also, when a breaker does trip on low low, as the leak continues does the breaker still hold its dielectric ability?

 

[slavag]

Standard:Alarm by SF6 Low alarm
Lock-out by SF6 Low Low alarm
trip blocked by breaker internal mechanism only, signal is send to breaker failure protection for zone fast tripping in case of fault on this feeder.

 

[DTR2011]

@Mbrooke

I think we should clarify Live tank breaker vs. dead tank, as the volume of SF6 would be quite different in each application.

Also - it is my understanding that an SF6 (dead tank) can still open normal load current @ 0 PSIG, as SF6 density is greater than air, so some will still remain in the tank at Low Low.

 

[submonkey]

Hi Mbrooke,

We negotiate with the breaker manufacturer to provide direct
access to the breaker trip coils without a blocking contact
in the way. A lot of them don't like it much, but they usually
provide it.

The densimeter initiates a binary input on one of the protection
relays. The relay trips the breaker and raises a "low gas trip"
target.

Thanks,
Alan

 

[submonkey]

Hi Stevenal,

Do you have any examples of "fast" SF6 leaks?

Would the cause of such a leak have lead to destruction
of the breaker anyway (e.g. pressure release disk operated
due to internal fault)?

We had a lot of internal debate on the topic, and I'd
be really interested in a real world example.

Thanks,
Alan

 

[Mbrooke]

This is a dead tank application.


As for fast leaks, how does trip on low low work out? And why can't the denismeter do a direct trip?

 

[Mbrooke]

Page 11 has the circuit diagram for a typical breaker, but it looks like as you say the factory will need to configure it for automatic trip on low-low. Or can this be done in the field?

http://www.energy.siemens.com/us/po...er-circuit-breakers-dead-tank/sps2updated.pdf

 

[stevenal]

submonkey,

Sorry, I have no real world examples, I believe transformers were targeted at Metcalf. Yes, I believe that when subjected to heavy gunfire, breaker replacement is probably in order. However, if the breaker is set to trip on low gas, and there is insufficient gas to interrupt the load; the resulting outage will be an unnecessary one.

 

[Mbrooke]

If a breaker was shot at, it might not even be able to open or open safely. Under such a condition Id imagine SF6 pressure would be the least concern.

 

[stevenal]

Mbrooke,

My point exactly, it cannot open safely. A simple trip at a certain threshold ignores the possibility that the leak may be severe enough that interruption is impossible. The tripping spring and coil are most likely still available, so the contacts will separate and arc over creating a large outage as upstream devices clear the fault that was intentionally produced by this policy. If you don't try to trip on low gas, even atmospheric air is likely to be sufficient to prevent arcing, and switching can be performed to remove the breaker from service. Perhaps a smarter device could detect the gas density rate of change, but I don't think it's really needed. Just do as Davidbeach does (alarm and block) and call it good.

 

[davidbeach]

We get a steady stream of low gas alarms, all of which get attended to post haste. To my knowledge we've never had a breaker fail to trip due to the low-low block. An automatic trip at low would be highly disruptive, possibly dropping load. The switching necessary to isolate a breaker to add SF6 can be done without dropping load. Any breaker that goes from low to low-low faster than we can get gas added to it is probably loosing gas too fast to safely trip below the point at which it is no longer safe to allow the breaker to trip.

A clean breaker failure trip following a breaker at low-low having the trip blocked will be recovered from much quicker than the mess following a catastrophic breaker failure. Do what you will, but we'll continue to not trip on gas alarms.