Need help with procedure for testing impedance of a dist
- Thread starter RobertAL
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bdarnell
Senior Member
- Location
- Indianapolis, IN
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- Retired Engineer
Re: Need help with procedure for testing impedance of a dist
What is it that you are trying to match ? Do you need to verify the transformer impedance to arrive at available fault current ? The transformer Z can be calculated using simple math.
What is it that you are trying to match ? Do you need to verify the transformer impedance to arrive at available fault current ? The transformer Z can be calculated using simple math.
bdarnell
Senior Member
- Location
- Indianapolis, IN
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- Retired Engineer
Re: Need help with procedure for testing impedance of a dist
The transformer is rated 1000 kVA, 480 secondary volts, 5.75% impedance.
Rated full load amp output of the transformer is
1000 kVA / (480 x 1.732) = 1203 amps
The 5.75% impedance rating indicates that 1203 amps will flow in the secondary if
the secondary is short circuited line to line and the primary voltage is raised from
zero volts to a point at which 5.75% of 480 volts, or, 27.6 volts, appears at the
secondary terminals. Therefore, the impedance (Z) of the transformer secondary may
now be calculated:
Z = V / I = 27.6 volts / 1203 amps = .02294 ohms
The transformer is connected directly to the utility power lines which we will
assume are capable of supplying the transformer with an unlimited short circuit
kVA capacity. The utility company will always determine and advise of the short
circuit capacity available at any facility upon request.
With unlimited short circuit kVA available from the utility, the short circuit
amperage capacity which the transformer can deliver from its secondary is
480 volts / .02294 = 20,924 amps
This is for example only.
The transformer is rated 1000 kVA, 480 secondary volts, 5.75% impedance.
Rated full load amp output of the transformer is
1000 kVA / (480 x 1.732) = 1203 amps
The 5.75% impedance rating indicates that 1203 amps will flow in the secondary if
the secondary is short circuited line to line and the primary voltage is raised from
zero volts to a point at which 5.75% of 480 volts, or, 27.6 volts, appears at the
secondary terminals. Therefore, the impedance (Z) of the transformer secondary may
now be calculated:
Z = V / I = 27.6 volts / 1203 amps = .02294 ohms
The transformer is connected directly to the utility power lines which we will
assume are capable of supplying the transformer with an unlimited short circuit
kVA capacity. The utility company will always determine and advise of the short
circuit capacity available at any facility upon request.
With unlimited short circuit kVA available from the utility, the short circuit
amperage capacity which the transformer can deliver from its secondary is
480 volts / .02294 = 20,924 amps
This is for example only.
bdarnell
Senior Member
- Location
- Indianapolis, IN
- Occupation
- Retired Engineer
Re: Need help with procedure for testing impedance of a dist
The simple math is Z = E/I
I guess I'm confused as to why you are concerned about the exact impedance of the transformer. The manufacturer should be able to tell you and there should be a nameplate on the door with the impedance. Even if the actual measured is off by a 1/2 percent, your fault current at 480 volts is still going to be in the neighborhood of 20 kA at the secondary bushings.
Maybe I'm missing something ?
[ February 03, 2006, 05:15 PM: Message edited by: bdarnell ]
The simple math is Z = E/I
I guess I'm confused as to why you are concerned about the exact impedance of the transformer. The manufacturer should be able to tell you and there should be a nameplate on the door with the impedance. Even if the actual measured is off by a 1/2 percent, your fault current at 480 volts is still going to be in the neighborhood of 20 kA at the secondary bushings.
Maybe I'm missing something ?
[ February 03, 2006, 05:15 PM: Message edited by: bdarnell ]
Re: Need help with procedure for testing impedance of a dist
Well, the part that you're missing is that I work for a transformer repair shop. We have repaired the transformer and want to make sure it has the right percent impedance. I've lost a key person and am not sure how to connect the equipment to measure the impedance without applying rated current on the secondary.
Well, the part that you're missing is that I work for a transformer repair shop. We have repaired the transformer and want to make sure it has the right percent impedance. I've lost a key person and am not sure how to connect the equipment to measure the impedance without applying rated current on the secondary.
bdarnell
Senior Member
- Location
- Indianapolis, IN
- Occupation
- Retired Engineer
Re: Need help with procedure for testing impedance of a dist
Good luck.
If that is the situation you are in, I would recommend that you contact a NETA certified firm to assist. I can't take you any further than to explain how impedance values are obtained.
Good luck.
Re: Need help with procedure for testing impedance of a dist
Robert
The way its done is to short the secondary terminals. Then you gradually apply voltage to the primary until you have full load current on the secondary. Example assume 12.46 kv primary
480 secondary FLA on a 1000 kva = 1204 amps.
If you gradually increase he primary voltage to
say 623 volts and get 1204 amps on the secondary
then 623/12600= 0.05 or 5%.
Where are you located?
[ February 04, 2006, 11:53 AM: Message edited by: bob ]
Robert
The way its done is to short the secondary terminals. Then you gradually apply voltage to the primary until you have full load current on the secondary. Example assume 12.46 kv primary
480 secondary FLA on a 1000 kva = 1204 amps.
If you gradually increase he primary voltage to
say 623 volts and get 1204 amps on the secondary
then 623/12600= 0.05 or 5%.
Where are you located?
[ February 04, 2006, 11:53 AM: Message edited by: bob ]
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