Transformer impedance

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steve66

Senior Member
Location
Illinois
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Engineer
Coopers SPD book gives more details. Just in case you really want to know how it is calculated:

The transformer secondary is short circuited, and Voltage on the primary is increased until the full load secondary current flows. The applied voltage divided by the rated primary voltage is the impedence.

For example, for a 480V transfomer, if 16.8 volts applied to the primary causes full load secondary current, the impedence is 16.8/480 = 3.5%.

Steve
 

ramsy

Roger Ruhle dba NoFixNoPay
Location
LA basin, CA
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Service Electrician 2020 NEC
steve66 said:
For example, for a 480V transfomer, if 16.8 volts applied to the primary causes full load secondary current, the impedence is 16.8/480 = 3.5%.

Isn't this measured-Z formula for SCC = E / [Z*Sqrt(?)], not for the calculated formula SCC = E / [(%Z*E) / (VA/E)*Sqrt(?)], which uses %Z (Nameplate value)?
 
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steve66

Senior Member
Location
Illinois
Occupation
Engineer
ramsy said:
Isn't this measured-Z formula for SCC = E / [Z*Sqrt(?)], not for the calculated formula SCC = E / [(%Z*E) / (VA/E)*Sqrt(?)], which uses %Z (Nameplate value)?


Don't know, you totally lost me. Are you saying there are two different types of Z??

Steve
 

ramsy

Roger Ruhle dba NoFixNoPay
Location
LA basin, CA
Occupation
Service Electrician 2020 NEC
Yes, when calculating SSC there are different formulas for the Xfmr's %Z vs measured impedance(Z). I don't believe these values are interchangeable.
 

wirenut1980

Senior Member
Location
Plainfield, IN
per unit conversion?

per unit conversion?

ramsy said:
Isn't this measured-Z formula for SCC = E / [Z*Sqrt(?)], not for the calculated formula SCC = E / [(%Z*E) / (VA/E)*Sqrt(?)], which uses %Z (Nameplate value)?


I believe the first formula you posted is using %Z to find short circuit current, and the second is using %Z and converting it to per unit impedance to find short circuit current. I'm not sure if the formula is correct though. I would need to look it up tomorrow, but that is what it looks like to me.
 
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