Troubleshooting Help... what would you do next?

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angrymonkey

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One of our facilities had several fuses fail on the Control Power Transformer in the High Voltage Line-up last week. (A&C Phase)

13.8KVBUS--------->FUSE DRAWER------>CPT XFMR------->120/208PanelBoard

We tested the transformer for a fault, and it tested out good.
We checked the panelboard that it feeds for a fault, and it tested out good.

We isolated the load from the Control Power Transformer by shuting off the main in the panelboard.

We reloaded the fuses into the CPT fuse drawer/rack and rolled the drawer/rack back in.

Both A&C fuses failed again. (No load on the CPT transformer)

We called in a contractor to test everything on the system.
They tested everything and could not find a fault on the system.
TTR, Power Factor, Megger tested the transformer. (Good)
Megger tested the line & load cables (Good)
Megger tested the 120/208V panelboard. (Good)
Megger tested the CPT Fuse rack holders/insulators. (Good)
Looked into the shutter windows that the fuse rack mates into (bus stabs looked good)

We verified that we have 3-phases of 13.8KV power on the main primary bus. We re-loaded the fuse into the CPT fuse drawer/rack and rolled the drawer/rack back in.

Both A&C fuses failed again.
We all spent time scratching our tails on this one.

What would you look at next?

Any time is greatly appreciated.
 
I am not sure I quite understand the setup. It sounds like you are using the term "control power transformer" to describe the step-down transformer that takes 13.8KV from the bus and provides power to a 120/208V panel. Is that right? If so, CPT is not the correct term for that transformer. If not, what does the CPT serve? Does it provide power to the protective relaying for the 13.8KV bus, or perhaps power to the breaker operating mechanisms?

My other question is whether something changed recently. Was this system operating normally for a year or two, and suddenly the two fuses failed? Did someone perform a maintenance or repair or inspection task recently, and did the fuses fail shortly after that task was finished? Was a new load recently installed on the 120/208V panelboard, and did the fuses fail shortly after that load was first turned on? What happened recently; what happened the day (or the day before) the fuses first failed?
 
charlie b said:
I am not sure I quite understand the setup. It sounds like you are using the term "control power transformer" to describe the step-down transformer that takes 13.8KV from the bus and provides power to a 120/208V panel. Is that right? If so, CPT is not the correct term for that transformer. If not, what does the CPT serve? Does it provide power to the protective relaying for the 13.8KV bus, or perhaps power to the breaker operating mechanisms?

?
Click to expand...

I have the same ?.

Control Power Transformer term does not seem to apply in this case.
 
How about removing every thing, and then adding it back 1 piece at a time.

For example,
Disconnect the transformer primary from the fuse truck. Do the fuses hold now?

Reconnect the transformer primary with the secondary open. Do the fuses hold now?

How are your fuses sized compared to the transformer inrush current (open secondary transformers often have more inrush than partially loaded units)?
 
angrymonkey said:
One of our facilities had several fuses fail on the Control Power Transformer in the High Voltage Line-up last week. (A&C Phase)

13.8KVBUS--------->FUSE DRAWER------>CPT XFMR------->120/208PanelBoard

We tested the transformer for a fault, and it tested out good.
We checked the panelboard that it feeds for a fault, and it tested out good.

We isolated the load from the Control Power Transformer by shuting off the main in the panelboard.

We reloaded the fuses into the CPT fuse drawer/rack and rolled the drawer/rack back in.

Both A&C fuses failed again. (No load on the CPT transformer)

We called in a contractor to test everything on the system.
They tested everything and could not find a fault on the system.
TTR, Power Factor, Megger tested the transformer. (Good)
Megger tested the line & load cables (Good)
Megger tested the 120/208V panelboard. (Good)
Megger tested the CPT Fuse rack holders/insulators. (Good)
Looked into the shutter windows that the fuse rack mates into (bus stabs looked good)

We verified that we have 3-phases of 13.8KV power on the main primary bus. We re-loaded the fuse into the CPT fuse drawer/rack and rolled the drawer/rack back in.

Both A&C fuses failed again.
We all spent time scratching our tails on this one.

What would you look at next?

Any time is greatly appreciated.
Click to expand...

Suspect turn-to-turn short. Isolate the secondaries and

1./ if you can isolate the individual windings on the primary side of the transformer you can check the individual resistance of the coils with a microohm meter, should be within 5-10% maximum.
2./ megger with 5 kV between the individual primary windings.
3./ if you can't isolate the individual phases, read the ohms between the phases 1-2, 1-3, 2-3. Should be close to each other.
4./ Smell test, literally put your nose close to the windings and scan the entire area for burnt smell.

If your primary fuses were marginally sized, it is possible that as the transformer ages the inrush current increases and the fuse no loinger can take it.


Look for short, green, moldy looking gremlins...........:D
 
zog said:
And do a TTR test
Click to expand...

I did not suggested that because it seemed beyond the expertise and capability of the OP. It usually requires more equipment than your regular V/A/Megger and if they would have the equipment the question would have never arose.(Unless of course you're old school and have your AM radio setup withe low voltage rigged transformer.):D
 
weressl said:
I did not suggested that because it seemed beyond the expertise and capability of the OP. It usually requires more equipment than your regular V/A/Megger and if they would have the equipment the question would have never arose.(Unless of course you're old school and have your AM radio setup withe low voltage rigged transformer.):D
Click to expand...


OP states TTR was already performed.
 
I had nearly and identical thing happen on a 21kV switchgear. Metering PT fuses kept blowing. I can't remember if it was one phase or two that kept blowing. Turned out to be the insulators that were used in the fuse holders, but it seems like you have ruled that out......
 
Thanks for the info thus far guys...


We are planning an outage in about 2 weeks, I will let you know what kind of results we get out of it.


Again, I appreciate the time.
 
I once had installed a transformer just a small one 75KVA 480v pri 120/208 sec. Fed by a properly sized breaker, and wire, engineer was right on.

I switched it on hummed along great. Tunrned it off to snug everything down and check all the connections before turning the project over to the owner.
Turned it back on At the panel and BZZZZZ RATTTTTLE POP.... I go outside and nothing's wrong.. Poped the breaker.

Take off the covers check everthing, megger the wires ect. Nothing

Put back the covers cycled the breaker and. Pop the whole school is out. Walked clear across the school to the main service panel and one of the mains were tripped. Repeated this a couple of times found no smoke or smell.

Checked the transformer it seemed ok, contacted somone at the local POCO I knew , it was a holiday weekend we spent almost the full day scrating our heads. Called a plant manager friend in San Diego state Univ. He offerd to haul a spare transformer to us.

Anyone have a guess as to what the problem was ??????????
Bet ya no one can guess:D
 
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