Transformer popping noise 1

[farmape]

Recently, a 1MVA 12470-4160 oil filled pad mount transformer was energized after being out of service for about 1 month (no changes or maintenance, just de-energized). The unit was energized (no load, just secondary cables) and soon started to "pop" or "thump". This sounded odd so the unit was de-energized within 5 minutes. Has anyone experienced this, as odd as it sounds (to me), it was similiar to the sound of metal thermally expanding or contracting, but coming from inside the transformer. A PI was immediately performed on both windings, both >2. Any ideas what would cause this noise?
Thanks.

 

[Zogzog]

Moisture perhaps? I suggest taking an oil sample, including DGA.

 

[farmape]

Thank you sir, yes that is my initial course of action (an oil analysis complete with DGA).
On a side note, curious if an intermitant fault on the transformer secondary (i.e. cables/switchgear), (too low to operate the protective device or before the pd), have the ability to cause a disturbance such as this inside the transformer.

 

[Zogzog]

I am not a guru on transformer construction but I would not be not suprised if a core at ambient being brought up to operating tempature would make some noises like that.

I was in submarine service, and when we dove the hull would make popping noises like that, scared the crap out of the new guys everytime.

 

[stevencress]

The transformer was probably manufactured in Mississippi. What you are hearing is a redneck fire alarm. They enclose a bag of popcorn inside the transformer, when it pops, the transformer is overheated.

 

[prc]

Source of abnormal sound from transformers can be any of the following(my experiences) :

1) Overfluxing - Check the incoming breaker for any arcing contacts.If possible use another breaker.

2) Any loose mechanical part eg tie-rods

3) Core clamping going down resulting in loose core laminations

4) Moisture getting inside insulation and partial discharge occurring inside press board.

 

[farmape]

Thanks for the replies. The oil analysis (DGA, moisture, and popcorn residue) and electrical tests checked OK. Circuit was re-energized. Same noises started occuring, this time let it go ..... eventually the secondary MV CB tripped on TD. A faulted secondary cable about a mile away was the cause. I can only assume incipient fault current was causing the noises in the transformer,

 

[Marmite]

I suspect this is the case. I have experienced this numerous times in distribution substations in the UK whilst hunting LV cable faults. As the fault "pecks" you often hear the effect on the transformer. The peck turns into a loud growl when the fault goes solid.
Regards
Marmite

 

[rbulsara]

Learn something new every day. Thanks for the feedback.

Rafiq Bulsara

http://www.srengineersct.com

 

[itsmoked]

Ditto!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[prc]

farmape,Thank you for the feed back.But I dont think the sound was due to fault current. I think the sound is from the overfluxing that will occur when a partial fault with arcing creates transient surges which will reach transformer terminals. It was the same case of item 1 of my earlier posting.Arcing across the breaker contacts( due to improper contact making) was sending surges to transformer winding which inturn creates overfluxing in core,sending out abnormal sound.

 

[farmape]

Interesting. Could you elaborate, what type of transient surges and the relationship to overfluxing?
p.s. the faulted cables were replaced and the transformer sounds fine now.

 

[PHovnanian]

I think "overfluxing" is a rather inexact description of what is going on.

When a transformer is energized, there is an inrush current due to the difference in core residual magnetization and the newly applied system voltage for a few cycles before the core flux and back e.m.f. stabilize. The initial change in magnetizing current and resulting core flux may occur more rapidly than the final steady state magnetizing flux and so will the mechanical stresses (magnetostriction, etc.) experienced by the core. This can produce a 'pop' or thump sound.

Normally a transformer experiences this effect once at energization. But in the case of a faulty contact, cable or other system problem that momentarily removes system voltage from the winding, its reapplication will behave in a similar manner to a normal inrush event.

 

[prc]

When an arcing occurs,momentary overvoltages are created which increases the flux density in core.When flux density exceeds saturation level (2.0T) -normal flux density at working voltage is around 1.7 T - high booming sound will result due to overfluxing.Flux density in core is directly proportional to applied voltage and inversely proportional to frequency.V/f sensing relays are used for the overfluxing protection of transformers.

This is the situation when geo-magnetically induced DC currents enter transformer during sun storms,resulting in booming sound from transformers.