Transformer Switching with Air Switches
Transformer Switching with Air Switches
(OP)
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.
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.






RE: Transformer Switching with Air Switches
RE: Transformer Switching with Air Switches
As David suggests, why would you want to do this?
RE: Transformer Switching with Air Switches
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?
RE: Transformer Switching with Air Switches
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.
RE: Transformer Switching with Air Switches
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
RE: Transformer Switching with Air Switches
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.
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?
RE: Transformer Switching with Air Switches
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.
RE: Transformer Switching with Air Switches
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?
RE: Transformer Switching with Air Switches
RE: Transformer Switching with Air Switches
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]
RE: Transformer Switching with Air Switches
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?
RE: Transformer Switching with Air Switches
RE: Transformer Switching with Air Switches
RE: Transformer Switching with Air Switches
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.