Short-circuit impedance tolerance of Power Transformers
- Thread starter Joao
- Start date
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Probably not without spending an inordinate amount of money
Tolerances exist partly because %Z is theoretical until after the transformer is manufactured and actual measurements can be performed.
D. Castor
Senior Member
- Location
- Port Angeles, Wash
- Occupation
- Electrical Engineer
You can specify a minimum impedance without too much additional cost, but there will still have to be an acceptable tolerance above the specified minimum.
Yes, we have raised this issue with one Vendor and feedback was that tolerance could be decreased like for example +-5% instead +- 7,5%
D. Castor
Senior Member
- Location
- Port Angeles, Wash
- Occupation
- Electrical Engineer
Reducing the tolerance range will certainly increase your transformer cost. Why do you feel you need to do this?
Because we are facing issues with Short circuit current levels in 33 kV Switchgears. We want to keep below 31,5 kA. In this project unfortunelly we have start the project from the ROOF.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
Question: if the manufacturer typically supplies transformers that have a +-5% tolerance, would it really cost much more to get a -0% +10% tolerance.
It seems that the OP's situation could handle a typical range of possible impedance values, but with a hard minimum to that range.
It seems that the OP's situation could handle a typical range of possible impedance values, but with a hard minimum to that range.
JoeStillman
Senior Member
- Location
- West Chester, PA
- Occupation
- Electrical PE
I think it's easier for the manufacturer to hold to a specific impedance with their standard tolerance than a tighter tolerance on standard impedance. Just calculate the impedance you need and add the standard tolerance, then give that to the manufacturer as the minimum.
So 31.5 kA @ 33 kV = 1,800 MVA. For a 100 MVA transformer, you would need 100/1800 = 5.6% minimum impedance. If the tolerance is ±7.5%, then for the low end of the tolerance to be 5.6%, the nominal impedance needs to be 6%.
So 31.5 kA @ 33 kV = 1,800 MVA. For a 100 MVA transformer, you would need 100/1800 = 5.6% minimum impedance. If the tolerance is ±7.5%, then for the low end of the tolerance to be 5.6%, the nominal impedance needs to be 6%.
I do not know. What I can inform you is Vendor told me that they could decrease the tolerance, but never DEVIATE that tolerance.
Yes in general if you request some like: NEGATIVE TOLERANCE IS NOT ALLOWED, they will DEVIATE IT.