Vacuum Breaker Prestrike Blowing Fuses?

[big john]

Very unusual problem on a 25MVA transformer 34.5 wye:13.8 delta.

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 upstream feeder fuses.

This substation has been tested 6 ways from Sunday:

Transformer PF, TTR, excitation, winding Ω, leakage X, and oil sampled.
All bushings PF tested.
All high and low side arrestors PF tested and AC overpotential.
Breaker PF tested and resistance tested - No timing test yet.
Overhead structure hipot.

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 reenergize and it stays energized. Add an artestor 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?

 

[Jraef]

The fault may have compromised the vacuum bottles. If there was even a hint of an arc because the fault involved a voltage spike exceeding the vacuum bottle dielectric, the inside of the bottle can get coated with contact material, so it arcs again before the contacts fully close. It's rare, and I've never seen it but I have seen papers on the phenomenon. Joslyn Clark used to publish one back before they were bought by Danaher, but it might be difficult to find now.

Edit: I found this, not the same one I remember, but similar.
https://www.specialtyproducttechnol...cum-contactor-application-issues.pdf?sfvrsn=2

 

[big john]

We didn't hipot those bottles, but this was a design with a dual series of interrupters, and each of those 2 bottles was PF tested at 10kV and passed with flying colors.

Even assuming we didn't find a fault by over potentialing each one, we still appear to have a measured 20kV interrupting ability on a 19.9kV line.

But hypothetically if the bottles were leaking across, where is my overcurrent coming from?

 

[mbrooke]

Where are the fuses exactly? Most POCOs would not put fuses on the 34.5kv line in series with a 25MVA unit. Please forgive me if I don't understand.

 

[big john]

This is a private sub, so the POCO set fused cutouts at the transition to customer overhead. That's what keeps blowing. Trying to find out what the POCO is fusing it at, haven't heard yet.

 

[mbrooke]

This is a private sub, so the POCO set fused cutouts at the transition to customer overhead. That's what keeps blowing. Trying to find out what the POCO is fusing it at, haven't heard yet.

Look at the pole, if there is something like 100T or 140K on the side of it, that might be it. Personally I'd never fuse a 25MVA with typical off the shelf line fuses due to inrush. A recloser with a specific time current curve would be more appropriate and that is exactly what most POCOs do in their own subs when stepping 34.5kv to distribution at that MVA level.

 

[big john]

Yeah, we talked about that, but it doesn't begin to explain why it has worked reliably for decades prior and why it will energize and hold without high-side arrestors.

 

[mbrooke]

Anyway, in case the POCO wishes to stay with fuses:


(give it time to load, its a hefty file but gold imo)



http://www.sandc.com/edocs_pdfs/edoc_025854.pdf

 

[mbrooke]

Yeah, we talked about that, but it doesn't begin to explain why it has worked reliably for decades prior and why it will energize and hold without high-side arrestors.

I don't know to be honest, unless there is some low level ferroresonance thats causing the arrestors to spark over. Its a pure guess, I have no idea. When the unit is switched on, is is loaded or unloaded?

 

[big john]

I don't know to be honest, unless there is some low level ferroresonance thats causing the arrestors to spark over. Its a pure guess, I have no idea. When the unit is switched on, is is loaded or unloaded?

Totally unloaded during all attempts, so it would be excitation current only.

I didn't think about ferroresonance because this is a wye high-side, and I thought that all-but-eliminated the FR concerns. It's an idea, though. Maybe put a stable secondary load on to change the impedance the high-side was seeing during closing?

It's counter-intuitive, because if I'm already popping fuses, adding more current doesn't seem like the solution.

 

[mbrooke]

Totally unloaded during all attempts, so it would be excitation current only.

I didn't think about ferroresonance because this is a wye high-side, and I thought that all-but-eliminated the FR concerns. It's an idea, though. Maybe put a stable secondary load on to change the impedance the high-side was seeing during closing?

It's counter-intuitive, because if I'm already popping fuses, adding more current doesn't seem like the solution.

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.


As for ferrorrsonance, it can happen on some ultra high efficiency (amorphous metal) wye grounded primary transformers from what I have read. Another thing that pops into my mind are capacitor banks. Over compensated lines and those where a cap bank has blown a fuse but the other two phases remain compensated can cause over voltage, and I've heard heard of line resonant conditions also occurring under those circumstances. Its possible (theory) that when loaded the caps are doing their job, voltage within spec, ect and when the load is shed the line is over compensated (improper settings or fixed bank). When closed unloaded the over compensated line resonates with the transformer's inductance causing an over voltage and in turn the arrestors to flash over blowing fuses. Its a guess, but it is a plausible theory imo.


I know I keep subverting the conversation away from those arrestors but fuses and a wye delta are two immediate red flags for me, because its likely those fuses will continue to blow every time a fault occurs on the feeder (or even other feeders) even if the lightning arrestor condition is solved.

But, that does not mean you should stop probing further. My gut right now is to talk to the POCO about cap banks and how that line is compensated under load. Good time to ask about voltage regulators (those need to have their line drop impedance settings set correctly in order to provide the right voltages) Id also have the POCO verify all the banks have fully functional fuses and see if they can obtain voltage, reactanace, ect conditions with your line unloaded. My gut (and its just an educated guess) that there is some type of over voltage condition causing those arrestors to flash over.

 

[mbrooke]

And oh, yes I would try energizing that transformer with arrestors under load just to see what happens. If its a resonant condition it might actually hold (assuming the fuses can handle cold load inrush + transformer inrush). If tThe facility has ridden through momentary (5 second) trip and reclose without those fuses poping I see why not.

 

[zog]

The fault may have compromised the vacuum bottles. If there was even a hint of an arc because the fault involved a voltage spike exceeding the vacuum bottle dielectric, the inside of the bottle can get coated with contact material, so it arcs again before the contacts fully close. It's rare, and I've never seen it but I have seen papers on the phenomenon. Joslyn Clark used to publish one back before they were bought by Danaher, but it might be difficult to find now.

Edit: I found this, not the same one I remember, but similar.
https://www.specialtyproducttechnol...cum-contactor-application-issues.pdf?sfvrsn=2

The insides of the bottles do get coated with contact material every time you operate them, then (most) of the material eventually works it's way back to the contact surface. We see this often when we cut open failed VI's, which we do all the time.

 

[zog]

Big John, a hipot test or a PF test will really only tell you when your bottle has already failed, and testing repeatedly can give you a false passing test. We MAC test every VI that comes in our shop and all the utilities in my area are now requiring this test. http://www.vacuuminterruptersinc.co...fe-of-Vacuum-Interrupters-in-the-Field_v2.pdf

 

[big john]

If we did have a failed pole assrmbly, any suggestions on how that would explain the fuses blowing?

Also, approaching this from another angle: If the arrestors are popping the fuses, why? Even clamping an overvoltage, I would not expect them to pass enough current for that. They would have to be seeing power follow-through and I thought MOV arrestors prevented that?

 

[mbrooke]

If we did have a failed pole assrmbly, any suggestions on how that would explain the fuses blowing?

Also, approaching this from another angle: If the arrestors are popping the fuses, why? Even clamping an overvoltage, I would not expect them to pass enough current for that. They would have to be seeing power follow-through and I thought MOV arrestors prevented that?

My guess is that the fuses are already at their limit, and the arrestor is passing just enough to blow them. I still believe that one way or another fuses should not be present at the riser.

 

[big john]

We see a lot of fuse protected transformers, it may not be ideal, but I think I'd be going down the wrong path focusing on that, especially given that this has run for a long time as designed.

I agree it appears that the arrestors are pushing the fuses over budget, but if that's the case there's a much bigger problem here that I'm missing because where is the overvoltage coming from?

 

[mbrooke]

We see a lot of fuse protected transformers, it may not be ideal, but I think I'd be going down the wrong path focusing on that, especially given that this has run for a long time as designed.

Even if the arrestor issue is mitigated, every time the line has a fault those fuses are at potential risk of blowing.

I agree it appears that the arrestors are pushing the fuses over budget, but if that's the case there's a much bigger problem here that I'm missing because where is the overvoltage coming from?

The million dollar question. It certainly is possible, just need to find what conditions are coming together to create the perfect over voltage. Id start asking the POCO a lot of questions.

 

[big john]

An update:

We enegized an alternate feeder that had OCBs as protection instead of fuses and the customer went online and stayed on despite having all the arrestors in place.

We found a low-side ground fault in the facility that may have accounted for the original fuse failure that single-phased the plant: There was a flashover in a 15kV disconnect.

During a final round of unsuccessful testing prior to trying the alternate feeder we put a PQ meter on the transformer breaker CTs and recorded a serious current imbalance during inrush on the unloaded transformer. One phase was 5A, the other two phases were 2.5A. No arrestors connected during this test. We went back through historic data for excitation tests to see if we could spot a trend and they all looked fine.

There was talk of core magnetization being the root cause due to the through-fault, but I can't swear and the customer chose not to do any more investigation now that they are online.

The only thing I am reasonably sure of at this point is that a problem still exists, but unfortunately it's out of my hands.

Thanks much for the help so far.

 

[mbrooke]

How are they running the 13.8kv system? Grounded, ungrounded or other?