hhsting
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- Glen bunie, md, us
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I know their is code ANSI or something else that tell typical transformer % impedance range values based on its size. Does anyone know which code I would find that in?
Transformer impedance
- Thread starter hhsting
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- Illinois
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- retired electrician
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- New Jersey
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- Journeyman Electrician
Is there particular reason you need to know this? We see specs that say no higher than a certain level say 5%.
IEEE.
impedance is generally specified by the end user.
here an acceptance guide.
Im in agreement with infinity here.. why would you possibly need this?
ANSI is generally used by utilities. Except that under around 500 kVA nobody follows ANSI and it gradually gets lower. In the tens of MVA and higher not only is it more expensive to follow ANSI (which uses one of two numbers depending on voltage) but short circuit current gets so high that it is advantageous to raise it. DOE has altered the lower end especially.
Contrary to popular belief %Z is a specification BUT it is a target for the transformer manufacturer, not a specification as such. So you build one then measure the result. With a fairly large history of designs they can reasonably predict the %Z but it is definitely not a controlled value. Depending on their confidence the transformer builder is going to quote you a range that they guarantee they can meet or a target with no guarantees. Generally as you’d expect some obvious design decisions like copper vs aluminum windings or 3 vs 5 legs or rectangular vs round wire have obvious effects on %Z as far as direction but not the final value. Due to the nonlinear nature of magnetic materials, especially siliconized steel, you can’t just feed it all into even a high end finite volume system.
DOE, like ANSI, gives specifications that are little more than targets. Because of DOE edicts %Z on certain transformer designs has a guaranteed upper limit but not a target or lower limit.
%Z by the way dramatically drives short circuit current. It has an effect on losses as well but the gains are minuscule. I once ran the calculations on a 50 MVA transformer with 12% Z. Lowering it to 5% would save something like $50,000 a year on an electric bill that was in the millions and the ROI would have completely disappeared (if it ever had one) not only on the transformer cost but the cost of upgrading all the switchgear to handle the AIC that goes up over 200%. And before anyone starts chiming in how cheap it is, the transformers in question are 230 kV:23 kV.
Now all that being said, DOE does not dictate autotransformers, LTCs, AVRs, or even extended tap range transformers (9 taps or more). So there are many ways to avoid it if it is necessary (manipulating short circuit). I want to caution you about going down that path though. Decreasing %Z drives short circuit (and switchgear costs) up. Raising it improves this but it can cause starting issues with large motors.
Contrary to popular belief %Z is a specification BUT it is a target for the transformer manufacturer, not a specification as such. So you build one then measure the result. With a fairly large history of designs they can reasonably predict the %Z but it is definitely not a controlled value. Depending on their confidence the transformer builder is going to quote you a range that they guarantee they can meet or a target with no guarantees. Generally as you’d expect some obvious design decisions like copper vs aluminum windings or 3 vs 5 legs or rectangular vs round wire have obvious effects on %Z as far as direction but not the final value. Due to the nonlinear nature of magnetic materials, especially siliconized steel, you can’t just feed it all into even a high end finite volume system.
DOE, like ANSI, gives specifications that are little more than targets. Because of DOE edicts %Z on certain transformer designs has a guaranteed upper limit but not a target or lower limit.
%Z by the way dramatically drives short circuit current. It has an effect on losses as well but the gains are minuscule. I once ran the calculations on a 50 MVA transformer with 12% Z. Lowering it to 5% would save something like $50,000 a year on an electric bill that was in the millions and the ROI would have completely disappeared (if it ever had one) not only on the transformer cost but the cost of upgrading all the switchgear to handle the AIC that goes up over 200%. And before anyone starts chiming in how cheap it is, the transformers in question are 230 kV:23 kV.
Now all that being said, DOE does not dictate autotransformers, LTCs, AVRs, or even extended tap range transformers (9 taps or more). So there are many ways to avoid it if it is necessary (manipulating short circuit). I want to caution you about going down that path though. Decreasing %Z drives short circuit (and switchgear costs) up. Raising it improves this but it can cause starting issues with large motors.
hhsting
Senior Member
- Location
- Glen bunie, md, us
- Occupation
- Junior plan reviewer
Utility want give so approximate worst case fault current based on transformer size and %z
- Location
- Illinois
- Occupation
- retired electrician
For other than incident energy calculations, the worst case current is what you want to see.
hhsting
Senior Member
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- Glen bunie, md, us
- Occupation
- Junior plan reviewer
Correct in other words other than Arc flash study the worst case is what I want. This lowest possible impedance %z for given sized utility xfmr. So then which code would have that DOE or ANSI? If ANSI then which version?
- Location
- Illinois
- Occupation
- retired electrician
You get that from the utility ....it is based on the transformers that they use. In fact, normally you get the service fault current from the utility and that is based on the worst case for the types of transformers that they use in their system.
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- Wisconsin
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- PE (Retired) - Power Systems
For first step design, I use 1.2% up to 150kVA, 3.5% up to 300kVA, and 5.0% for larger utility transformers.
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- New Jersey
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- Journeyman Electrician
Please in the future mention in the OP that this is a utility transformer.
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