Bushing Ampere Rating 2

[KnicksJets]

Our Transformer standard specifies that Bushing ampere rating shall be at least 20% higher than the respective transformer winding ampere rating at maximum transformer nameplate rating.
Does anybody have a basis for this standard? I checked c57.19 series on bushings but couldn’t find anything on this. Our vendor is committing to 19% over the LV winding ampere rating and we want to know if that is okay or not. The bushing in question is LV bushing – 25kV class, 150 kV BIL for a GSU transformer 25kV-230kV (750 MVA Transformer).
Thanks in advance.

 

[ScottyUK]

Transformers can usually survive a moderate overload for a reasonably long period without drastically reducing their service life. Bushings - especially modern ones - don't tend to have much 'spare' metal in their designs so if sized at 100% of transformer rating they would prevent operators accessing the overload capability of the transformer.

I personally don't think that 19% O/L capability is significantly worse that 20% O/L capability, but I live in a climate where we won't ever see +40°C ambient so in reality I would always have a little more in hand than indicated by the standard ratings. If you are in a hot location then maybe it would be a problem.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[KnicksJets]

Thanks Scotty.
Anybody has any idea if there is a standard that dictates the ampere (current) rating of the bushing as compared to the transformer winding? Like I mentioned, ours says it should be 120% compared to the winding.

 

[prc]

Yes .See clause 4.2 of IEC 60137-2008 for Bushings.AS Scotty said this is to take care of the overloading requirements of Transformers ( Table 4 of IEc60076-7)Generally GTs are not subjected to overloading and hence 1% reduction may not matter.But LV bushings of such large GTs are another fish and normally rating selected for such bushings will be much more than 120 % due to higher oil temperature in transformer and also busduct air temperature.Contact bushing manufacturer for selection.Higher ambient temperatures will be taken care by lower oil temperature rises specified for transformers.

 

[KnicksJets]

First of all thanks to PRC.
I’d have preferred a reference in IEEE or even NEMA document to IEC (specially since we don’t follow it and we don’t have access to those documents), but still I am thankful.
Wouldn’t this also depend upon what is the fault availability in the area? I mean, if the max short circuit current that the transformer sees is much less than this value (120% of winding ampere rating which is around 21kA, as opposed to the vendor proposed bushing which is 18KA) then we really shouldn’t be worried? I guess the short ckt current would be much much higher than the rated current, but 21kA is pretty high.

 

[prc]

The IEEE standards on bushings are as follows - C57.19.00-2004 General Requirements and tests C57.19.01-2000 Dimensional requirements C57.19.100-1995 Bushing application guide. Clause 7.0 of Application Guide gives the application of bushings inside busducts.

No, short circuit current has nothing to do with the current rating.Normally bushings are very strong to SC currents.

Being inside the busduct, rating may have to be derated.This depends on design of bushing etc. Manufacturer will be able to guide you.

 

[KnicksJets]

I had actually gone through all the standards you mentioned but didn't find the limits compared to the transformer winding. However, your answer on SC helped immensely.
Thanks for the help.

 

[stevenal]

See 4.1.1.1 and 4.4 of C57.19.100.