Utilizing an SF6 circuit breaker as a normally open point 1

[jjs378]

Hi. New here. I have a question about utilizing an SF6 circuit breaker as a normally open point on a bus. Most utilities I have worked with have operating procedures that do not allow an SF6 circuit breaker to be utilized as an normally open point. (ie - Left energized at line voltage on both sides of the open contacts for long periods of time.) Other than preference, is anyone aware of any references that are available to support that position? IEEE C37.09-2018 Section 4.5.8 eludes to the likely hood of flash over from exposure to switching transients. I could have sworn I read a white paper on this topic years back, however the Google is failing to locate anything useful. I would appreciate any information that may be out there.

 

[bacon4life]

I have heard of limitations for synching across breakers, since having the sides 180 degrees out of phase puts 2x stress on the open contacts. Sorry I don't have a specific reference for that.

In my state the rules for clearances require a visual open, which in some cases gives a slight preference for normal open points to be air break disconnects rather than SF6 breakers.

 

[crshears]

bacon4life:
I have heard of limitations for synching across breakers, since having the sides 180 degrees out of phase puts 2x stress on the open contacts. Sorry I don't have a specific reference for that.

Hey Bacon, I don't have a reference for that either, but I know in Ontario's post-2003-blackout reviews the system design and operating people were horrified to find out that there were locations in the province where islands had been synchronized with breakers that weren't rated for 180° out of phase voltage. I don't recall there being any reports of breaker damage found post-event, but there definitely was discussion around replacing specific breakers with ones having said 2x rating along system restoration paths.

bacon4life:
In my state the rules for clearances require a visual open, which in some cases gives a slight preference for normal open points to be air break disconnects rather than SF6 breakers.

Hmmm, visual open points . . . I don't know of any breaker the contacts of which can be visually verified open, but the SF₆ disconnects in series with them for maintenance/isolation purposes most commonly have sight ports for this, and my utility routinely used these switches as visually verifiable open points for work protection purposes.

One non-utility generation site encountered a billing metering issue that caused them to choose to run with their SF₆ ring bus open [using breakers] between their two outgoing circuits until their units were generating > 100 MW; no impediments were found to constrain this practice.

There was a vintage of SF₆ breaker-and-a-half switchgear that had problems where the voltage measuring equipment could burn up if a line terminal's disconnect and breakers were left open due to the terminal bus having voltage applied to it via the breaker's grading capacitors; special operating procedures were developed to mitigate the risk of this.

All that being said, there may be other constraints out there that I never encountered . . .

 

[waross]

Given that the clearance required to extinguish an arc is many times the clearance to initiate an arc, I don't see a problem with 2x voltage across an open breaker.
Voltages to ground will be the same and the clearance, even at 2 x voltage will still be much below the voltage required to initiate an arc.
That said, if a switching failure elsewhere puts 2 x voltage across a closed breaker, there may be difficulty clearing the circuit with that breaker.
I am open to comments on this.

 

[stevenal]

We had lightning take out an open gas breaker pole. The damage would have been much less if it flashed over an open air disconnect rather than inside the breaker pole. (The powder formed is nasty stuff). A similar event was presented at Western Protective Relay Conference last October.

 

[System Protection]

I don't remember seeing a policy like that before.

Can really only think of one SF6 breaker that I've seen that was an intentional N/O and energized on both sides.

Its done all the time at distribution levels but those aren't SF6 breakers.

 

[stevenal]

I'd point you to the paper in the WPRC archive, but 2024 is not there yet.
Following the event I described above, we installed a gas breaker at a N/O point and paired it with a N/O MOD on the line side. The station side disconnect is N/C is but the station transformer has a primary side arrester.

 

[waross]

Hi Stevenal.
Respectfully;
I wonder if this was a result of the breaker being energized on both sides, or if this may happen to any SF⁶ breaker subject to a lightning strike.

 

[stevenal]

Waross,
It might be energized on one side. You need a closed path to the lightening strike (lightening hit the line in our case), and some AC follow through to ensure the gas is thoroughly converted to the nasty powder. In our case, we were energized on both sides. Line protection operated, attempting to open an already open breaker, while simultaneously operating bus protection. Bus protection also attempted to open the same open breaker, while clearing the rest of the bus. Fault was in the breaker pole between the two sets of CTs where the protection zones overlap.

 

[waross]

Thank you for the explanation Steven.

 

[stevenal]

My pleasure.
I found the WPRC paper at https://selinc.com/api/download/140361/.

 

[stevenal]

Apologies for my bad spelling day yesterday. Lightning of course, not lightening.

 

[wcaseyharman]

I’ll have to look, I know for a fact that my company is using an sf6 breaker for a NO on a 55kV system, but I’m not sure about the 115kV system.

 

[davidbeach]

Not breakers per se, but we have SF6 circuit switchers used as the NO (and the NC) for automatic transfer (auto throw-over) substations at both 57 and 115kV. Seems to work fine, certainly much better than MODs. We do, however, have very little lightning.

 

[stevenal]

David,
Little lightning here also. The one time was more than enough, though.
Switchers are live tank devices, so the path from the interruption chamber to a solid ground is a longer one than for a dead tank breaker.