padmount t-form shorted?

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Polarcat

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on a 3 phase utilty transformer padmount if one of the secondary wires (4 parallel feed) is cross phased creating a dead short between the b and c phase, wouldnt the primary cutout fuses blow imediatly upon energizing. or would it buck heads and run for a day or so.
 
Rick,
I'm not so sure about that. The utility does not really size the primary OCPD to protect the transformer. There real concern is to keep the power on.
Don
 
Rick, here is an old post from one of the moderators that works for the utility.

In it he talks about how the utility may protect a 25 kva transformer.

In this case the primary voltage is 7.62 kV and they will protect it with a '15 T fuse that will carry 150% indefinitely'

Now the transformers rated primary current is about 3 amps. (25,000 / 7,620)

The fuse is a 15 amp that can carry about 22 amps forever. (Or a staggering 167 kva 'protecting' a 25 kva transformer)
 
If memory serves the pad mount TX's had secondary fuses in them. They were called bayonet fuses & the fuses were from 3 amp to a common 8 amp on the residential tx's & up on the commercial tx's. The bayonets were located in the primary compartment of the TX. The bayonet fuse was not a class 3 switch & was not to make or break primary loads of 7.6 or 13.2. It's been about 10 years since I worked for FPL. I had an accident & some of my skills were lost up stairs. Hope this helps, I can go find my FPL book some where & look @ the primary schematics but like I said, I would have to look for it.
 
Rollbar said:
If memory serves the pad mount TX's had secondary fuses in them. They were called bayonet fuses & the fuses were from 3 amp to a common 8 amp on the residential tx's & up on the commercial tx's. The bayonets were located in the primary compartment of the TX. The bayonet fuse was not a class 3 switch & was not to make or break primary loads of 7.6 or 13.2. It's been about 10 years since I worked for FPL. I had an accident & some of my skills were lost up stairs. Hope this helps, I can go find my FPL book some where & look @ the primary schematics but like I said, I would have to look for it.
Click to expand...

You are remembering bayonet fuses installed in the transformer primary compartment that are installed under oil. My company installs a 6 Amp bayonet fuse for a 25 kva single phase transformer, and also for a 75 kva 3 phase (6 Amp fuse for each winding).

If these fuses blow, they cannot be replaced in the field. The transformer must be taken out of service.
 
OK.
We also had the bayonets (in oil) for the single phase w/3, 6 & 8 amp fuses depending on what was needed. Most of the time when the TX failed the 3amp fuse would still be good but the TX would be moved off the pad. Secondary dig-ins were common here.
 
We had a 1500 kva that the contractor marked a couple of wires wrong and when we hooked it up it blew a primary (pole mount) fuse and at the same time blew an elbow arrestor on one of the B bushings. We thought we had a bad arrestor and tried it again, same thing. Then I stopped the job and had our people check the wiring even after the contractor told me it wasn't on him and we found it to be two phase wires crossed on the secdy side. I think we had 65 K fuses in the dip pole and the transf. manuf. put 140s in the bayonets. Needless to say the bayonets held just fine.
 
let me further explain my question.
300kvx 3phase pad mount transformer with (4) sets of secondary parallel conductors.
(2) conductors (from one set) where swapped (b to c).
what would happen when they energized it?
would it run for an extended period of time?, generating heat enough to fry everything including the insulation and wire.
would it matter if they did not ground the transformer?
 
A transformer will continue to run even if there is a bolted line-line fault on the secondary terminals. The transformer is "stupid" and will try to supply all of the current for the short circuit based on the source voltage and the impedances of: the source, the primary conductors, the transformer itself, and any secondary conductor impedance.

The winding arrangement of the transformer will affect how the fault current is seen by the primary protective devices. The time vs. current settings of the protective device will dictate how long the fault current will flow. But the transformer will provide the current until it "melts" or "burns".
 
don_resqcapt19 said:
Rick,
I'm not so sure about that. The utility does not really size the primary OCPD to protect the transformer. There real concern is to keep the power on.
Don
Click to expand...
I suppose that it also is because it is in free air.I can't imagine a shorted coil lasting very long at all, but with that kind of fusing, you never know.
Rick
 
I once witnessed a three phase arcing fault on the secondary of a 1.5MVA 115kV-480V three phase poweer transformer. It was a result of some switching that was performed with isolation switches. The resulting arc burned away the blades, continued to arc across the switch terminals for a while, then went phase to phase across all three phases. The arc continued for what seemed like a minute or so. The primary fuses finally blew, but only after the primary windings failed. The primary fuses had been sized at 300% of FLA to avoid nuisance failures. Granted, it was not a hard bolted fault, but it is an example of how a "stupid" transformer will perform.
Vern
 
I hope this is a hypothetical question, cause if not, it would make whomever energized it, as "Stupid" as the transformer. In all my years in the trade, I have never seen a POCO not check the wiring prior to energizing. But, S$$T Happens.

Just my thoughts,

R
 
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