Possible overheating of THHN/THWN-2 Cable

[tsiengineer]

A new 1000 KVA step-up (208 -> 480/277) (220 deg C insul sys & 150 deg C rise) dry type power transformer was installed about 2 months ago to feed an existing 1600A busway.

Fused Switches on primary and secondary
Primary - 4000 A fuses
Secondary - 1600A fuses

It is located in a switchgear room which has a low ceiling (~7'). (More ventilation would help).

The primary cables B phase (10 sets of 3-600MCM THHN/THWN-2) are heating up to about 73 deg C (taken with thermal camera).
A and C phase are about 63 deg C (expected b/c B phase cables are directly above transformer coils).
Secondary cables are all much cooler (50 deg C) but are located in the back of the transformer (further from coils).

Spot load measurements taken
Primary - ~1800A
Secondary - ~830A

There is also a slight smell of burning insulation and the cover of the transformer is too hot to keep your hand on for more than a couple seconds.

Does the smell have anything to do with the transformer being brand new?
Are the 10 sets of wires at 73 deg C, a major concern? Are there any code requirements for wires to be rated above the typical 90 deg C?

p.s - first post, really enjoying the site.
Thanks

 

[augie47]

Thank you for joining in .......

Based on other installs, a few questions come to mind.
(a) Is there and XO on the 208 side and is it connected ?
(b) What is the actual load ?
(c) was there any mention in the mfg. literature on on the case referencing conductors above a certain height ?

 

[jim dungar]

There is also a slight smell of burning insulation and the cover of the transformer is too hot to keep your hand on for more than a couple seconds....

Does the smell have anything to do with the transformer being brand new?

If you can touch it at all, it is not one of the hottest transformers i have encountered.

New electrical equipment, especially dry type transformers, can have an amount of volatile organic compounds that can take a fairly long time to dissipate. The aroma from these compunds is often mistaken for 'burning'.

 

[steve66]

Thank you for joining in .......

Based on other installs, a few questions come to mind.
(a) Is there and XO on the 208 side and is it connected ?
(b) What is the actual load ?
(c) was there any mention in the mfg. literature on on the case referencing conductors above a certain height ?

To expand on Augie's third comment, many transformers have a line marked inside the transformer, and conductors aren't supposed to be installed above that line. Since heat rises, the top of the transformer is hotter than the bottom, and the top may be too hot for conductor insulation like THHN/THWN-2.

 

[tsiengineer]

I spoke with the manufacturer (MGM Transformer Company,) they do not make recommendations on wire temperature.
The wires are at the height of the manufactuers terminals.

augie47
There is no X0 terminal on the 208 side, its delta to wye.
Its delivering ~700 kVA around 10-11AM. Most likely goes up a bit higher afternoon / late afternoon. Mostly mechanical loads (A/C units and such).

Good news about the smell though.

Does anyone know of a situation where the cables on the primary or secondary had to be rated for higher than 90 deg C on a similar transformer?

 

[templdl]

I spoke with the manufacturer (MGM Transformer Company,) they do not make recommendations on wire temperature.
The wires are at the height of the manufactuers terminals.

augie47
There is no X0 terminal on the 208 side, its delta to wye.
Its delivering ~700 kVA around 10-11AM. Most likely goes up a bit higher afternoon / late afternoon. Mostly mechanical loads (A/C units and such).

Good news about the smell though.

Does anyone know of a situation where the cables on the primary or secondary had to be rated for higher than 90 deg C on a similar transformer?

Just to confirm that you transformer was actually built as a 208d-480y/277 step up transformer.
Also, termination are 75degC as such wire is applied at no more that 75degC at the terminals.
If this is a dry type transformer the modern transformer has 220degC insulation.
A 150degC rise transformer is designed as 150degC + 40degC ambient + a30degC hot spot allowance = 220degC.
220degC is very hot. Even if you drop the 30segC hot spot and back off the 40degC ambient to 25degC you still have 175degC. That's the transformer windings. As such the max winding temp at full load could be between 175degC to a max of 190degC with an internal max hot spot allowance of an additional 30degC. That's sort of hot you would think. As Jim Dungar said and residue left from the manufacture of the transformer could give off an unpleasant aroma.
That's the transformer. If you can justify that the transformer is within its design rating then confirm that the wire is sized and rated correctly if the wire isn't being overloaded then do you have a termination issue?
Unless the transformer mfgr directs you to use a higher rated insulation common UL486, 75degC terminals and starting with 75degC rated cable would be used.