Lose one phase of 3 phase transformer

[ccov]

Quick Question someone asked me .....
If a source phase is lost on an oil filled three phase transformer (NO LOAD CONNECTED), will this eventually damage the transformer?
I say no, the transformer can remain energized indefinately with no concerns

 

[slavag]

You are right, NO.
Regards.
Slava

 

[bacon4life]

It is fed from cables? If so, would ferroresonance be a concern?

 

[Bronzeado]

It depends on the transformer winding connection and configuration of the core.
In a three-leggeg core, Y-Y connected transformer, supplied by two phases will be damaged.
The magnetic flux produced will leak the core and heating the tank.

H. Bronzeado

 

[Skogsgurra]

Heating the tank, yes. But to the extent that damages occur before the temperature switch cuts it out? No.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Bronzeado]

Gunnar,

If you talk about (big) transmission power transformer, you are right. In general there is protection that will trip it out when one phase is lost.
However, in the case of distribution transformers, there is no temperature protection. So, when one phase is opened due, say, a fuse blow up, after some time, the transformer oil may boil due to tank heating.
That is one of the reasons that three-phase, three-legged core distribution transformers should have a delta connected winding.

H. Bronzeado

 

[Skogsgurra]

Thanks Bronzeado. I lived under the impression that there are thermal protection (and also sometimes Bucholz protection) also in distribution transformers (not pole transformers). But that may have been in older times. Don't see many distribution transformers any more.

Hey ccov, what kind of transformer did you ask about?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[slavag]

Bronzeado, Gunnar.
1.Please see again OP: NO LOAD CONNECTED.
2.In my area lot of disribution transformer DGPT equiped.
3. Temp. trip at the big trafos, I assume, is not common practic.
Best Regards.
Slava

 

[Bronzeado]

Slava,

1. With NO LOAD, there is NO PROBLEM. Sorry!

2. Our transformers, 33MVA and above, all have a temperature gauge with contacts that control fans, give a alarm and, eventualy trip.

H. Bronzeado

 

[ppaya]

Guys,
in North America distribution transformers are 10MVA and 5MVA most of the time. Mome of them is equiped with IEEE 49 protections. Big transformers YES. Also distribution transformers are always Delta-Wye. Very few exceptions

 

[slavag]

Hello.
I would like remind Alehman's thread:
thread238-201894
Bronzeado, I think it will be intresting for you.
BTW, I check my papers:
All industrial trafos in my area with thermal trip connected.
Utilities disconneted all thermal trip.
Same for all size of trafos.
Regards.
Slava

 

[stevenal]

Distribution transformers are generally rated in kVA rather than MVA. I think ppaya is talking about substation transformers. From IEEE 100:

"distribution transformer (power and distribution transformers) A transformer for transferring electrical energy from a
primary distribution circuit to a secondary distribution circuit
or consumer’s service circuit. Note: Distribution transformers
are usually rated in the order of 5–500 kVA."

 

[ppaya]

Stevean, thanks for correcting me. I meant Substation transformers ( I am not sure how IEEE calls this class of transformers - Power Tx?)