Transformer Impedance questions

[SM0705]

Hello

I specified two transformers (5 MVA and 1 MVA) with a transformer impedance not to exceed 6.5 % and 5.75 %. The impedance we got on the two (2) transformers were 5.81 % and 5.09 % respectively, which is below the -7.5 % ANSI min tolerance level.

Is it correct to think that even though I specified the transformer impedance at 6.5 % (Max) and 5.75 % (Max), the vendor is still supposed to meet the minimum negative tolerance (-7.5%) per ANSI.

Thanks for your help in advance.

 

[cranky108]

On that small of a transformer, I would be asking if the vender read the specification, and just gave you an off the shelf model.

 

[RRaghunath]

The allowable tolerance is +/-10% for transformers with rated impedance <10%, i.e. a difference of 20% between max and minimum impedance.
Further, if max impedance is specified, the minimum impedance is allowed to be 20% below the max impedance(IEC 60076-1).
Not withstanding the above, IEC allows specifying narrower tolerances.

 

[Kiribanda]

SM0705,
You have specified as Z=6.5% for the 5MVA and Z=5.75% for the 1MVA.
What is the guaranteed impedance specified by the manufacturer in his quote?
The IEC & IEEE tolerances are applied on that guaranteed impedance(s) and not for the
impedance(s)you have specified in your data sheet. Similar for losses too.
Any comments?

 

[ppedUK]

If you specified that the impedance was not to exceed those values, then technically they have complied with your specification, regardless of what is in the ANSI standard.

 

[waross]

Was this the listed impedance or the result of acceptance testing?
The listed impedance is tested at operating temperature.
If you tested the transformers at a lower temperature the impedance will be noticeably lower.
I was caught out on this. Once!

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[prc]

Better to be clear in your specifications. Normally there is a +- % variation allowed . If you say no positive tolerance , then manufacturer will limit him selves to +0,- % tolerance. But if you say not to exceed or maximum only, supplier is at liberty to have any lower impedance or at least total of plus and minus range.