Vacuum Breaker Prestrike Blowing Fuses? 1

[BigJohn1]

Very unusual problem on a 25MVA transformer 34.5kV Y : 13.8kV Δ.

Has been in service for many years. Recent unexplained feeder fault blew a fuse. Now every single time the transformer is energized via the vacuum breaker it blows those same upstream feeder fuses. The transformer has so far always been energized unloaded.

After the initial fault, this sub was extensively tested:
[ul]
[li]Transformer PF, TTR, excitation, winding Ω, leakage X, and oil sampled.[/li]
[li]All bushings PF tested.[/li]
[li]All high and low side arrestors PF tested and AC overpotential.[/li]
[li]Breaker PF tested and resistance tested - No timing test yet.[/li]
[li]Overhead structure hipot.[/li]
[/ul]
No problems whatsoever. But every time the transformer is energized with any of the high side arrestors in place it blows fuses. Remove arrestors and re-energize and it stays energized. Add an arrestor and it blows a fuse on that phase.

Here's the kicker: All these affected arrestors are in the load-side of the VCB. There is an identical set of arrestors on the line side that have never blown any fuses: If the VCB is open the line side will remain happily energized for hours, as soon as the VCB closes, fuses pop.

I am at a lose except maybe to say that a new malfunction in the breaker is causing transformer overvoltage due to prestrike?

Any ideas at all?

 

[chuckd83]

How far from the breaker is the transformer?

 

[BigJohn1]

There might be 50 feet of overhead conductor.

 

[magoo2]

Sounds like you have conflicting info. If the arresters test good and you put them back in, it sounds like you might still have a bad arrester.

Can you supply a simple sketch? It might help.

Do you have any capacitor bank nearby?

 

[BigJohn1]

We know that historically it can be very difficult to find a bad arrestor, I have seen them pass a 10kV power factor and fail in service.

Knowing that, we actually took a PT and backfed these things at their 24kV MCOV to try and prove a fault, but they were all passing less than 5mA ay their working voltage. They did eventually begin to clamp at 30kVRMS, and that matches nameplate specs.

Further, any single arrestor will blow a fuse even with the arrestors dosconnected on the other two phases, so that would mean all three went bad simultaneously and yet they pass every test we can perform.

The only thing that does concern me about the arrestors is why we appear to have power follow-through each time they clamp (assuming the arrestors are the reason the fuses are blowing)?

I will put together a 1-line tomorrow morning.

 

[chuckd83]

Could be caused by a switching transient. VCBs are known for that. Especially at such a short distance (rule of thumb is 200 ft) and being switched into no load (highly inductive). Surge arrestors will only clamp the magnitude, not the rate of rise or DC offset. You may need a surge capacitor or snubber.

I'd study it using EMTP before doing anything. Continually switching onto that transformer, especially without arrestors, may cause damage.

 

[BigJohn1]

I am very willing to believe it is a switching transient, and would be willing to try switching in under moderate load.

But that raises a huge flag: Why?

Barring a utility change that isn't showing uo in the voltage readimgs, what about this substation could've changed to cause this voltage?

 

[cuky2000]

Check for potential ferroresonace with unloaded or light loaded transformer.
VCB are could generate a very fast transient (VFT) coupling with a short line.

Just a curiosity, how the arrester is reconnected after energize the transformer?

 

[waross]

Has been in service for many years. Recent unexplained feeder fault blew a fuse. Now every single time the transformer is energized via the vacuum breaker it blows those same upstream feeder fuses

Have you considered that the specs of the replacement fuses may be different than the specs of the original fuses?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[BigJohn1]

Check for potential ferroresonace with unloaded or light loaded transformer.
VCB are could generate a very fast transient (VFT) coupling with a short line.

The only solution I have for checking for ferroresonance would be to put on as many single phase loads as possible and then try energizing into that to see if it drops the circuit impedance enough to prevent this. What say you?

As far as fast switching causing it, the operation of the VCB would have had to fundamentally change, because it has worked hundreds of times previously. If anything, I would expect the travel time to have slowed due to lubrication deterioration.

Have you considered that the specs of the replacement fuses may be different than the specs of the original fuses?

Definitely, and am trying to find out what the POCO has been putting in. But that doesn't explain how the arrestors appear to figure in.

Here's the best 1-line MS Paint is capable of producing:

 

[waross]

I suspect that the transient has been present from day one.
The original fuses passed the transient but newer fuses are acting on the transient.
Whatever the cause of the transient the arrestors appear to have been acting to protect the transformer.
I would be concerned that energizing without the arrestors in the circuit will be subjecting the transformer to excessive over voltage transients.
Solutions:
1> Mitigate the transients.
2> Return to the original spec fuses.
Notwithstanding I am interested to learn the cause of the transients.
Will the next higher voltage rating arrestors protect the transformer?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

Arrestors. My theory as why the arrestors are making a difference is that they are indeed passing an over voltage through them. Just enough current on top of things to blow the fuses. What do others think?

 

[magoo2]

Your earlier description was a wye delta connection for the 34.5 kV to 13.8 kV transformer but your drawing shows a grounded-wye delta connection. Which is it?

If it's a grounded-wye delta, then its a grounding bank which would explain why you get fuse blowing when there's a feeder fault ("Recent unexplained feeder fault blew a fuse"). It would also explain why some of the energizing tries blow fuses as long as there's enough load imbalance because a grounding bank will try to correct the unbalance.

Are there other feeders adjacent to this one?

I have never seen fuses on the source side of a breaker either.

None of this would explain why you see a change with the arresters in versus out.

 

[BigJohn1]

our earlier description was a wye delta connection for the 34.5 kV to 13.8 kV transformer but your drawing shows a grounded-wye delta connection. Which is it?

Grounded. Floating wyes are rare as hens teeth up here so I assumed folks would assume.

If it's a grounded-wye delta, then its a grounding bank which would explain why you get fuse blowing when there's a feeder fault...

But if there is we can't find it. The line section passed a hipot and also will stay energized when the arrestors are disconnected. Further, I would think a fault would only blow fuses in the affected phase: Whatever this is has blown fuses on each phase at least once.

It would also explain why some of the energizing tries blow fuses as long as there's enough load imbalance because a grounding bank will try to correct the unbalance.

Can you expand on this? I'm not familiar with that.

There is a related-sounding piece of the story where the POCO claims to have measured 34.5 between all phases of the wye even with one feeder phase open after the initial fault. Can an open wye manufacture a phase? I didn't think that was a workable connection.

Are there other feeders adjacent to this

The only other parallel feeder is grounded down.

 

[Mbrooke]

Can you expand on this? I'm not familiar with that.

I probably explained it better in this post then I will here, but I'll try my best:

Grounded why delta is a very poor choice because any fault on the feeder will blow those fuses regardless of the arrestors because a wye grounded-delta transformers are nothing but a giant grounding transformer. They are the ultimate zero sequence source and will "absorb" any neutral/ground current. They are used where a neutral is needed like on a ungrounded delta system, and in fact where grounded wye primary - delta secondary transformers are applied to substations (115-34.5kv) engineers actually have to consider an increase in fault current at the primary 115kv station bus and adjust protective relaying/arc flash studies to take this into account.

http://forums.mikeholt.com/showthread.php?t=176830&page=2&p=1739581#post1739581

A grounded wye delta is what is nick-named as a grounding transformer. You could take a 115kv-13.8kv delta delta transformer and then add a separate grounded wye delta transformer to the buss it feeds into and the system will actually behave as a grounded wye secondary. Any neutral current will actually go up into the primary neutral and be transferred into the phases. When this happens, the current through one or more phases rises. Engineers call this a zero sequence source.

In short in your application the fuses not only see normal load current, but also neutral current from other transformers using your transformer's primary neutral to get back on the phases. Ditto for a fault, current some current will go back to the substation while some of it goes through the transformer's primary neutral to get back on the phases. A delta or ungrounded wye primary is absolutely key in stopping this.

There is a related-sounding piece of the story where the POCO claims to have measured 34.5 between all phases of the wye even with one feeder phase open after the initial fault. Can an open wye manufacture a phase? I didn't think that was a workable connection.

It could in theory due the secondary delta back feeding the primary. Even more interesting is that a single blown fuse is unlikely to be noticed on the secondary as open wye grounded open delta is connection that can still give 3 phase power.

 

[Mbrooke]

And oh, just to add, if they transformer has a 3 limb core inductive tank heating is a real possibility during an open phase condition.

 

[waross]

But if there is we can't find it. The line section passed a hipot and also will stay energized when the arrestors are disconnected. Further, I would think a fault would only blow fuses in the affected phase: Whatever this is has blown fuses on each phase at least once.

Perusing old textbooks we see reference to this phenomena. Historically much of North America was electrified with delta:delta transformer connections. In times of rapid industrial expansion a quick solution to the need for greater capacity was to run a neutral conductor and reconnect the transformers from line to line to line to neutral. Of course at the same time the voltage would be raised by a factor of 1.73. This resulted in most industrial transformer banks being connected wye:delta.
Several textbooks noted that a fault at one location may result in fuses blowing at various locations across the system.
A solution that was tried was to float the wye point. This avoided the circulating currents and fuse blowing issues, but gave rise to switching transient overvoltages. A solution to that issue was to install a fused cutout in the neutral connection to the transformer bank. The bank would have four fused cutouts rather than the usual three. When the bank was being returned to service a fuse would be installed in the neutral. Then the phase cutouts could be closed without overvoltage issues. Once the bank was energized, the neutral fuse would be remmoved and the neutral left floating.
About 12 or 14 years ago I saw a legacy transformer bank, wye:delta with the four fused cutouts. The neutral cutout empty. I have since searched for that installation on Google street view without being able to locate it. A picture would have been nice.
That's one of the reasons that I love to hate that connection.
The grounded wye delta connection will try to correct any voltage or phase angle errors in the primary circuit. This connection will transfer power from healthy phases to a faulted phase. A ground fault or a phase to phase fault anywhere on distribution circuits fed from the same source transformer may blow any of the fuses on that bank. Circulating currents and transformers running hotter than expected are normal any time the primary parameters are less than perfect. Because of this the fuses may not be carrying the same currents. One fuse may be hot and as a result may blow before a fuse with less current that is cool.
On the other hand, the neutral ground connection may not be in good condition. If a poor ground connection is allowing the neutral or wye point to float, then you will not have circulating currents or transfer of power between phases. However with a floating neutral you may be seeing switching overvoltage transients when the bank is energized. As far as I know these transient are of issue on the secondary, but may be reflecting back to the primary.
Are the arrestors rated for line to neutral or for line to line voltages?
Does your ground connection provide a good connection back to the source neutral?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[BigJohn1]

What you describe sounds like a chain-grounded: Used to close in an ungrounded wye when no gang switch is available. I'm only familiar with it being used to prevent resonance failures.

The arrestors are only rated L-N. If this were a delta, I could absolutely explain the overvoltage and blown fuses because the arrestors would try to clamp L-L voltage. But I assumed there couldn't be a phase overvoltage with the reactor grounded wye. We did test the reactor, I didn't think to try and confirm the connection to the MGN. Maybe the wye is floating the whole substation grid? Seems unlikely, what symptoms could I be looking for to prove that one way or thw other?

 

[waross]

I had the adventure of spending several years in an area where grounded wye delta was common. There are a lot of failure modes depending on the relative size of the transformer bank, the loads on a shared primary circuit, and the source impedance of the primary circuit.
The best way I have found is to consider the two energized phases/transformers as an open delta. Full secondary voltage will be developed across the open delta.
But, we have a transformer secondary connected across the open delta being back fed with full voltage. The primary of that transformer will develop full voltage and will backfeed into the missing phase.
A single phase ground fault on a primary phase will drop to zero volta to ground. This will be back fed by the wye:delta bank with power from the healthy phases. It is common for a fuse to blow on the wye:delta bank due to a ground fault on one primary phase. The failed fuse may not be on the faulted phase.
I became the system engineer for a small island utility.
Power failures were common.
We had a customer with a wye:delta transformer bank.
Things were pretty basic with mostly single phase switching with fused cut-outs and a telescoping hot-stick.
When A phase was closed, all the customers would have full voltage.
The other two transformers in the wye:delta bank would be connected in series across the energized secondary. "B" phase and "C" phase would each get about 50% voltage and would backfeed about 50% voltage into the primary lines. All the customers on "B" phase and "C" phase would get about 50% voltage.
Back at the power plant, two men would be out at the pole. One would be holding the flashlight and the other man would be manning the hotstick. It was not a fast procedure getting the hotstick from one cut-out to the next. If they were looking up into a rainy night it took longer.
Then "B" phase would be energized. Now customers on "A" phase and "B" phase would have full voltage. "C" phase customers would have almost full voltage. With one wye:delta bank backfeeding the load on "C" phase there would be a noticeable voltage dip.
Eventually "C" phase would be energized and "C" phase customers would have full voltage.
A lot of refrigerators and freezers died over the years.
My hotel room, with a single incandescent bulb was on "C" phase. Many times I sat out a power failure and then saw my light bulb come on at half voltage. Then the brightness would increase as "B" phase went in. Finally "C" phase would be closed and my bulb would be at full brilliance.
You mentioned a reactor. If your wye point is impedance grounded you may well be getting switching transients which are putting line to line voltage across the arrestors.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

If the ungrounded primary route is taken (IMHO) I Personally would place a microprocessor recloser with 3 VTs at the riser and set a source sensationalizing response on the control as is done with loop schemes. The recloser would provide 3 phase switching and 3 phase over current protection, and any time the supply is threatened from a single phase condition (broken conductor) the recloser would open.


As for the feeder itself, how it grounded? 4 wire MGN?