Short-circuit impedance tolerance 2

[AntonioPerez]

IEC 60076-1 and other standards detail a concept of a tolerance in short-circuit impedance value. As I understand this tolerance must be taken into account during the process of manufacturing the transformer, as it is hard to define initialy the parameters to achieve the required impedance and there is an uncertainty about the real value. Nevertheless, when the transformes has been manufactured and tested, the impedance is already well known and this tolerance becomes useless and it must not be used in future short-circtuit current calculations. Do you agree?

Thanks in advance.

 

[krisys]

Yes, your above statement is correct. But what do you want to know here?

If you are looking for only an endorsement, yes, I would fully endorse the above argument.

 

[RRaghunath]

Tolerance is for the transformer design engineer.
For power system study, it is the tested value of transformer %Z that should be considered.
As the tested value is actual value for the subject transformer, no tolerance shall be applied.

 

[AntonioPerez]

Thanks. It is what I thought but I wanted to be sure.

I guess this thread should be closed but I don't know how or even if I'm allowed to do that.

 

[waross]

Be aware that the impedance is tested with the transformer at rated operating temperature.
Testing a cold transformer will yield a slightly lower impedance than the nameplate value due to the lower resistance of the cold windings.
Only the resistive component of the impedance is affected.
The offset portion of an asymmetrical current is reactive and is not affected by the resistance change.
The temperature effect on an actual offset fault current is less than would be indicated by the error shown by an Available Short Circuit Current test on a cold transformer.

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

 

[rcw retired EE]

Include a statement that replacement of any transformer may affect the results.

Most of our clients require using a 7.5% adjustment factor to the transformer impedance to cover replacements. Increase + 7.5% for voltage drop and motor starting studies and decrease -7.5% for short circuit calculations.

That 7.5% ANSI tolerance is related to the winding bracing. When a new transformer design is given a short circuit through fault test, the before and after impedance measurement can be within +/-7.5% for the test to be successful. No change means good bracing. Short circuit forces can deform the windings, changing the leakage flux that affects the %X. Including the 7.5% factor in calculations covers a decrease in transformer impedance after a through fault. (Too much information?) In my opinion, the 7.5% change is less than the "noise" in a lot of the other input data, but it satisfies the clients.

 

[cuky2000]

Below is a table from the IEEE Std Committee on Power Transformers comparing the Impedance tolerances allowed by the manufacturer per IEEE and IEC Standards.

.....

 

[prc]

1) waross- In case of Power Transformers, temperature correction is never applied on measured impedance. This is because the R part of impedance in pu is negligible compared to X in pu.

2)rcwilson- The tolerance of +-7.5 % in impedance is the tolerance permitted on contracted impedance to measured (tested )impedance. Experienced designers and good manufacturing practices will give much lower band width compared to design value. The allowable impedance increase after short circuit test in a high power lab is only 1% for Power Transformers above 100 MVA 3 phase and 2 % for transformers up to 100MVA(IEC 60076-5). For system calculations, better to take the max tolerance limits over the nominal specified impedance values.

3)In a transformer lot there can be variation between unit to unit of same design and production run. Some users specify a limit for this, say 2 %.