Transformer Switching with Air Switches

[111R]

What is the recommended approach for energizing and de-energizing unloaded power transformers with group-operated or individual disconnect air switches? If the switch has arcing horns, do you de-energize the transformer's excitation current with the switch? What about the inrush current during energization?

The only reference I've found on this topic is IEEE C37.36b-1990. I've attached a screenshot from it. It states the maximum "Resistive or Transformer Excitation Current" based on voltage class. I'm confused by this wording since it seems like reactive excitation current would cause much higher voltage spikes than resistive current, but they're grouped together as a single rating. I assumed that the no load excitation current is very reactive.

 

[davidbeach]

That's why they build circuit breakers and circuit switchers. Can't imagine why anybody would want to close into a transformer with a switch.

 

[magoo2]

I agree with David. Years ago, when we routinely used motor-operated air-break (MOAB) switches instead of circuit switchers to energize our distribution substation transformers, we had GE do a TNA study on the effect of the energizing transients using MOABs. The conclusion was the relatively long closing span between poles of the MOAB switch led to ferroresonance. These were mostly delta wye connected transformers and were mostly 230 kV and 115 kV high side transformers ranging in size from about 20 MVA to 100 MVA. Transformer engineers at Pittsfield were concerned about possible damage to these transformers. As a result, we stopped this practice.

As David suggests, why would you want to do this?

 

[111R]

Thanks. I'm arguing against it, but trying to develop a solid argument on the reasons why and the calculations behind it.

What is the recommended practice on a fused transformer on a tapped line with other tapped loads? I've heard of these being re-fused manually which seems risky. Is it common to use circuit interrupters to de-energize the entire line and remove all loads (if bypass switch not available), re-fuse, and then re-energize the entire circuit?

 

[cuky2000]

Disconnectors do not have interrupting current ratings but, given that they have a contact breaking arrangement, they have a certain current interrupting capability. Standards recognize this fact and the IEC disconnector standard defines a negligible current interrupting capability at 0.5 A and a further bus-transfer (loop) switching capability of up to 1600 A for some disconnectors. Present IEEE standards have a similar negligible current definition but do not recognize bus-transfer as a switching duty. In an earlier version of the IEEE standard, transformer magnetizing and capacitive currents, and small load currents were identified in this context

QUESTION 1:
What is the recommended approach for energizing and de-energizing unloaded power transformers with

a) Group-operated disconnect switch: for HV medium size and large power transformer is a risky option to use disconnect switch even for switching unloaded transformers. If the budget allow, circuit switcher or better, circuit breaker is recommended. For MV applications, there is 3 pole load break disconnect switches available in the market.

b) Individual disconnect air switches: This is an standard utility practice for small MV distribution transformer using fuse cut-out. Occasionally this individual switching operation could create unwanted ferroresonance issues.

QUESTION 2:
If the switch has arcing horns, do you de-energize the transformer's excitation current with the switch? For HV application some US manufacturer have design with high speed whip that allow to interrupt transformer magnetizing and line charging current. Arcing horn are primarily designed to control the arc reach and somehow help with the Interruption of Inductive and resistive current.

 

[crshears]

"My" utility has been taking this risk for many, many years as both a capital and OM&A cost-saving measure; switching 115 kV and 230 kV transformers on and off potential with ganged disconnect switches is routine. 500 kV transformer disconnect switches, on the other hand, not being ganged but individually motor-operated due to physical distance considerations, could experience appreciable time displacements between phases and therefore cause cumulative single-phasing over the life of the transformer, and as such are always operated off potential only, with the transformer itself being switched on and off pot with breakers.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Mbrooke]

I know of several US utilities that used MODs for energizing and denergizing transformers (and still do in legacy substations). The scheme was usually accompanied with transfer tripping or suicide tripping (deliberate shorting via MO ground blade) to remove a faulted transformer or secondary side fault. In some cases the MOD would open after the remote breakers tripped for faulted transformer and then reclose. The key however being that all maintenance switching was done with no load on the secondary. This practice however is being phased out via circuit switchers in North America. Whether the MODs had no arcing horns or other special requirements I am unaware of. I know in loop splitting arcing horns are not recommended because the high current can over heat them.


As for the IEC (outside of North America) the trend has always been to use a circuit breaker.


Personally I would never use a MOD to protect a transformer. Not so much what the magnetizing make or break will do to the switch, but rather the fact you remove other elements (like transmission lines) out of service for transformer faults. Multiple transformer failures at a substation can remove multiple transmission lines with it. I believe this practice was routine in North America to save on breakers while still being able to have a ring buss, but personally its no longer worth it.

Disconnectors do not have interrupting current ratings but, given that they have a contact breaking arrangement, they have a certain current interrupting capability. Standards recognize this fact and the IEC disconnector standard defines a negligible current interrupting capability at 0.5 A and a further bus-transfer (loop) switching capability of up to 1600 A for some disconnectors. Present IEEE standards have a similar negligible current definition but do not recognize bus-transfer as a switching duty. In an earlier version of the IEEE standard, transformer magnetizing and capacitive currents, and small load currents were identified in this context

Question, not to side track, but why do IEEE disconnects not recognize loop splitting? Does the arc break/distorting mechanism play a role? And thank you regarding the change. Any particular reason why magnetizing currents are no longer identified in this context?

 

[MKFPE]

Google "Current interruption using high voltage air-break disconnectors"

You will find a Phd thesis (David Peelo) with lots of information regarding the issues.

I once watched a fellow at a steel plant drop a 75MVA 138kV transformer using an ancient gang operated vertical break switch with the manual gear-reduction style operator. Seems like it took an eternity and I can only imagine the restriking going on. Xfmr and arrestors survived intact though.

Early in my career I also got the opportunity to watch as a short 765kV station bus segment was dropped using a vertical break style disconnect switch. Arcs probably went 20 feet into the air.

Also, a Google search without the quotes should turn up an IPST paper (2005) discussing an ATP simulation of interruption with switches. This is the modern version of what we used to call a TNA study as mentioned by magoo2. I had one done in the 90's for a different problem, cost $20k. Now I can do the same thing in a few hours with ATP.

All very interesting stuff but these days on our system I use only breakers to energize and drop transformers and lines. Less worries.

 

[rockman7892]

Mbrooke


Its interesting that you mentioned this "suicide tripping" I was at a 138kV substation recently where the incoming switches to the yard also had a ground switch which was a switch used to deliberately short one of the phases to ground. I couldn't quite figure out the intent of this switch but believe it was somehow controlled by the fault protection devices in the substation to create a fault in order to trip an upstream remote breaker? Perhaps part of a breaker failure scheme if the breakers in the substation failed to clear a fault it would somehow trigger a larger fault that would be seen by upstream breakers?

 

[stevenal]

We've used trans-ruptors at a few 5MVA transformer locations. Since they must be cranked closed phase by phase into a dead line, the upstream gang operated air break (GOAB) is used to energize/deenergize the transformer. The GOABS are equipped with quick break whips in order to break the magnetizing current. Older mobile substations are still equipped with fuses and GOABS, although we are looking at refitting one of them with a circuit switcher with staged MOAB.

 

[crshears]

In "my" utility's past and in locations where installing communications facilities was problematic / inconvenient / expensive, "auto-grounds" [ a single-phase switch oriented phase by ground ] between the transformer and the transformer line disconnect were commonly used to apply a solid fault to the line upon transformer protection operations so the terminal breaker/s at the supply station would trip and the bank primary switch would open off potential; the circuit would auto-reclose after a suitable time delay.

When we refer to "suicide switches" we mean those that self-destruct upon operation, literally martyring themselves to protect other much more expensive system elements. Within our utility these suicide switches are very rare; the previously referenced auto-grounds would eventually need their contacts cleaned up, but could operate many, many times before this was required.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Mbrooke]

Its interesting that you mentioned this "suicide tripping" I was at a 138kV substation recently where the incoming switches to the yard also had a ground switch which was a switch used to deliberately short one of the phases to ground. I couldn't quite figure out the intent of this switch but believe it was somehow controlled by the fault protection devices in the substation to create a fault in order to trip an upstream remote breaker? Perhaps part of a breaker failure scheme if the breakers in the substation failed to clear a fault it would somehow trigger a larger fault that would be seen by upstream breakers?

Never heard of deliberate shorting being used as breaker failure scheme, but in theory it is doable and would actually be an easy low cost solution if communication did not exist between the two substations. I mean if it worked on transformers why not lines?

 

[stevenal]

Sounds a bit like folks are speaking of high speed grounding switches, or what Blackburn calls fault switches. In my experience they are sized for the duty, so they do not fail upon use. They are not motor operated, since motors are too slow; they are spring operated. I see no application in ordinary breaker failure, since you already have a fault present that will eventually clear by remote devices. They are used where a fault on one side of a transformer is used to initiate a fault the other side, since upstream devices are generally not set sensitively enough to see past the transformer impedance.Link

 

[ScottyUK]

In the IEC world we use the term 'fault thrower' for a device which is designed to short a circuit to earth in order to operate upstream protection in the event of a breaker fail condition. The fault thrower is expected to survive the event, albeit possibly needing inspection and maintenance after operation.

 

[MKFPE]

Well, this one has gotten a bit off track from the original question but I'll go ahead and add an automatic grounding switch related comment to this excellent discussion:

In the one case I am familiar with wherein one of these things actually operated in response to a transformer differential trip by the time the remote breakers opened (Zone 2 time) the transformer was damaged beyond economic repair. They saved some $$ by not buying a high-side breaker, not sure it worked out all that well since a local breaker may have been able to limit the damage. Five cycles clearing time is a lot better than 20 cycles or more when the fault is inside a transformer tank.

Getting back to 111R's original question small transformers with fuse only protection are switched with air breaks with interrupter attachments as mentioned by cuky2000. As he says the attachments help with dropping the transformer, but the attachments can't help with the pre-striking that happens during closing. I see more and more breaker and circuit switcher installations, perhaps because the transformers are larger these days. A customer recently built a small station to take service from our 69kV system and it is fused with only a disconnect switch. But we have arranged our transmission to be flexible (for our own purposes) and as a result can help this customer out by switching with station breakers at each end of the line he is tapped to and we are agreeable to do so. In fact his station was first energized by us closing a station breaker by SCADA with all personnel well out of harms way. Again, less worries.