Transformer core was not grounded by a mistake! 1

[pwrtran]

Hi all

Does any of you have some comments on what if a transformer core was not grounded by a repair factory;s mistake? What are the potential risks if leave it as it was shipped and put into service? The xfmr is a 20/33/37MVA, Y-Y, 27.6-13.8kV unit.

Thanks in advance!

 

[unclebob]

For small dry-type transformers < 2000 kva, it is common to use a core without a grounding strap.

For bigger transformers (all types), I don't know. I'm hoping to get some anwsers here like you.

 

[electricpete]

A few thoughts.
1 - If core not grounded, the first fault that may occur upon weakening of insulation would be phase to phase fault instead of phase to ground fault. Fault current associated with phase to phase can be higher. It may be harder to protect against (although differential should work). And it causes more damage.
2 - Grounded core results in known voltage stress distribution. You can put the stress where you want it.. mostly in the insulation. Ungrounded core can result in more unknowns in the voltage stress distribution, especially during transients.
3. That's the way it's always be done (?)


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[electricpete]

Another thought related to item 2 - remember transformers used to use electrostatic shields? Something to do with controlling transient voltage stress distribution. I think grounding the core has a similar function.

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[Marmite]

Any conducting metal parts inside the transformer will acquire a potential during normal operation unless solidly connected to earth. Metalwork which is inadequately bonded can create arcing which will breakdown the insulating oil producing gassing. This can cause Buchholz relay operation, if you have one, and can also confuse DGA results and mask other more serious internal faults. I would get the repair factory to recify their mistake. In my experience the core earthing link is usually reasonably accessible (ie under a removable cover plate) to facilitate removal for testing. This of course depends on the transformer design.
Regards
Marmite