IEC 60076-5 (Recognized minimum values of short-circuit impedance for transformers with two separate windings)
For rated power from 25 to 630kVA, Minimum short-circuit impedance : 4%
Little correction,
replace : IEC 60076-5 (Recognized minimum values of short-circuit impedance for transformers with two separate windings)
By : IEC 60076-5 (Power transformers – Part 5: Ability to withstand short circuit / Table 1 : Recognized minimum values of short-circuit impedance for transformers with two separate windings)
I don't believe there is a minimum value stated in IEC 60076, or at least I couldn't find one. A typical value for a transformer of that rating would be around 4% - 5%.
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If we learn from our mistakes I'm getting a great education!
60076-5 gives recognized minimal values (not mandated),and states :
'Commonly recognized minimum values for the short-circuit impedance of transformers at the rated current (principal tapping) are given in Table 1. If lower values are required, the ability of the transformer to withstand short circuit shall be subject to agreement between the manufacturer and the purchaser.'
I agree with k0412d there is a recommended minimum value for short-circuit impedance of transformers. But I also agree with ScottyUK that actually there is no restriction for uk% [and Zk in turn].
Usually for 630 KVA uk%=4 but I encountered uk%=6 for short-circuit limitation reason. The accuracy is about [ +/-] 7-10% and is stated by manufacturer. Also X/R ratio is about 5 but this
depends upon copper losses[short-circuit losses].
If you intend to order a transformer "tailor made" you may require a suitable uk%. If the uk% is too small the short-circuit current will be high and the limit will be the breaker rating, I suppose. If the uk% will be high the voltage drop across the transformer could be a problem.
See a specification model, for instance:
It is true that "recognised minimum values of impedance" is 4 %.But we cannot say that IEC is asking for a minimum value of 4 % impedance for trfs of rating 25-630 kVA.In India there are utilities who specify 2.5 % impedance for single phase units of ratings 10,16 &25 kVA. They also get done Short circuit withstand test as type test on these units.There are some who ask 3.75 % for 3 phase 10 kVA and 3.5 % for 5 kVA single phase units.
To know generally specified impedance values for transformers around the world, refer to IEC TR60909-2 ed2.0(2008-11) Data for electrical equipment for short circuit current calculations.
At the end of 19th centuary when transformer engineering was taking shape,impedance values for transformers were extremely low. As SC level went up and transformers started failing, users started asking for a minimum impedance value of 4 % .
