Causes of Transformer Explosion in 152 Hectares University Campus

[young e.]

hi pf folks..

currently i am the electrical engineer incharge in a 152 hectares university campus in our place. commonly our transformer are rated 7620/240 volts, 1p with capacity of 25 KVA, 50 KVA and 100 KVA.. for our three phase loads we used to bank 3- 1p transformer
wye delta...all in all we have a total of 8 transformer substation in our campus.. now few months past we experienced succesive transformer explosion in our campus in different substations, we check the secondary line there's no fault in it,,, being a neophyte in this field of practice (electrical engg.) now my question is:

1. aside from overloading, what are the other causes of transformer explosions?

2. i suspect that the succesive explosion came from the primary side since we had already made an investigation in all of our loads and secondary lines, and we found that everything is all right, now what are the factors affecting transformer explosion with regards to the primary side..

tnx in advance!

young engineer

 

[Tom.G]

Just some background information since it's not my speciality.
There is always some leakage current between the winding layers. Over time, and especially with high internal temperature, both the insulation and oil fill degrade and become slightly conductive. This of course increases the leakage currents a little. Eventually the insulation becomes carbonized and rather conductive, then you are on the way to a run-away short circuit and explosion if there ar not any fuses in the primary circuit.

To monitor the internal conditions, a chemical analysis of the oil is periodically done to check for breakdown by-products.

Others here, perhaps @jim hardy or @anorlunda, may have more details.

Cheers,
Tom

 

[jim hardy]

Monitoring oil is a must.

Are primaries 4 wire connected? In other words, is there a neutral wire?
If so,
Measure the neutral current on WYE side.

Why do i say that ?
It is possible to overload a (4 wire WYE)-DELTA connected transformer with third harmonic current by powering it from a much larger source that has just a few % third harmonic content in its voltage .

Look up "Zero Sequence Current".

old jim

 

[Asymptotic]

Given that they have been providing trouble-free service until just recently.

1. Has the nature of their loads changed?
Has anything unusual (an earthquake, etc.) happened?

2. How old are the transformers?
Date of manufacture will be on their nameplates. If they were all installed at about the same time, and are closing in on expected service life that could explain the rash of failures.

3. Is your area particularly prone to lightning strikes?
Metal oxide, and/or spark gap arresters are popular for transformer/switchgear protection. MO arresters can fail shorted (which is hard to ignore - loud explosion, and likely to take out the primary fuse), but they may also fail open, which isn't easily detectable (special tests are required), and afterward, provides no lightning and surge protection.

A good course of action is look into transformer and switchgear testing services in your area, and have a reputable firm assess your equipment.

 

[anorlunda]

Others mentioned oil. That is the first place to look.

High harmonic content can also cause transformer explosions. Especially if you had several in the same area during the same period, I would check harmonics.

An arc furnace in a steel recycling plant can be a big source of harmonics.