I have been hearing problems of new TP-1 xfmrs (low voltage distribution type 15-225kva 480/208Y-120v) having inrush currents up to 30X the primary full load and breakers tripping out on the xfmr, inrush, one manufacturers information listed a 35.6X multiplier and another listed 6.41 for the same size TP-1 xfmr. All other characteristics were pretty close (x%, r%, z%, losses etc.) just curious if there is a formula to calculate the inrush in amps or estimate it, I cannot figure out why the significant difference anyone else come across such an issue or have seen such large variations of the multiplier? Thanks.
Tp-1 xfmr.'s
- Thread starter Mike01
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I know that most, generally published, transformer data comes from designers that model the transformer as an air core inductor. Actual testing of the product as an iron core inductor tend to have lower inrush values than the models predict. Another issue is that some values are reported as Peak Inrush instead of RMS Inrush (plotted at .1s)
Some TP-1 transformers even have lower inrush values than the older designs. Temperature rise also affects the inrush value, so an 'energy efficient' TP-1 unit may have drastically higher values than a standard design.
For example: a Square D standard EE75T3H, 75kVA 480-208Y/120, has a peak inrush of 14.2X but has a tested RMS inrush of 10.04X; and their 80?C rise unit EE75T3HB has peak = 25.70X and RMS = 18.17X.
Some TP-1 transformers even have lower inrush values than the older designs. Temperature rise also affects the inrush value, so an 'energy efficient' TP-1 unit may have drastically higher values than a standard design.
For example: a Square D standard EE75T3H, 75kVA 480-208Y/120, has a peak inrush of 14.2X but has a tested RMS inrush of 10.04X; and their 80?C rise unit EE75T3HB has peak = 25.70X and RMS = 18.17X.
RMS = .707 X Peak
I would think the RMS value to be of concern since all rated values are RMS values.
confused
confused
I might be way off base here but I am not quite getting something if the published data indicates for example inrush = 14.4 x (rated primary current) and you calculate the rated primary current by ( Kva x 1000) / (V*sqrt(3) ) you would get the rms current correct then multiply that by the 14.4 to get the xfmr inrush current. This would already be in rms and not peak correct?
confused
I might be way off base here but I am not quite getting something if the published data indicates for example inrush = 14.4 x (rated primary current) and you calculate the rated primary current by ( Kva x 1000) / (V*sqrt(3) ) you would get the rms current correct then multiply that by the 14.4 to get the xfmr inrush current. This would already be in rms and not peak correct?
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Protective devices react to RMS current. We normally perform transformer inrush coordination at the .1s point on a TCC.
??confused??
??confused??
The calculated current based on the information would be the rms correct or are you saying the multiplier is to achieve the peak current therefore to find the rms you need to take the results and multiply by .707 to find the rms current and use that number or take the multipler and multiply it by .707 to get the multiplier for the rms current and use that set at the 0.1 sec.?
??confused??
The calculated current based on the information would be the rms correct or are you saying the multiplier is to achieve the peak current therefore to find the rms you need to take the results and multiply by .707 to find the rms current and use that number or take the multipler and multiply it by .707 to get the multiplier for the rms current and use that set at the 0.1 sec.?
Jim:
Where do you find these numbers at? Are the manufacturers actually putting this data in the equipment sheets?
Is anyone trying to use the actual transformer inrush for coordination studies? It doesn't do much good to have selective coordination for all the circuit breakers in an Article 700 system when the breakers trip from transformer inrush. But it is a PITA to get a breaker to hold 20x inrush current, and still trip before the upstream circuit breaker.
Steve
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I ask the manufacturers for their actual data. The Square D values came from their on-line tech library.
I do not use peak current values, as protective devices respond to RMS values.
I do not use peak current values, as protective devices respond to RMS values.
thanks
thanks
Jim,
Interesting thanks for the info but I could not find where is indicates ?peak? also I cannot find out why they are so high speaking to other manufacturers max inrush is approx. 8-10 times for most applications and less in a lot of cases closer to 6-7, also one manufacturer indicated that was max. (I am not sure if that means peak) also that the practical value was closer to 3 (2.6) for one specific xfmr. But could not publicly release the information.
thanks
Jim,Interesting thanks for the info but I could not find where is indicates ?peak? also I cannot find out why they are so high speaking to other manufacturers max inrush is approx. 8-10 times for most applications and less in a lot of cases closer to 6-7, also one manufacturer indicated that was max. (I am not sure if that means peak) also that the practical value was closer to 3 (2.6) for one specific xfmr. But could not publicly release the information.
gar
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Peak inrush current results from core saturation. If at turn off of excitation the residual core flux density is high in the positive direction, and when next turned on the voltage phase is such as to create further magnetization in the same direction, then the core is driven far into saturation. This requires a lot more magnetizing current and therefore large inrush current. Whether this occurs and its magnitude is a random function dependent on where in the cycle the transformer is turned off, and where it is turned on.
On my web site at
http://www.beta-a2.com/EE-photos.html
I have photos of inrush current for a typical iron core transformer. See photos P6 thru P8. This transformer uses E-I laminations and probably uses a very standard transformer steel.
Edit.
The full load RMS input to this transformer is about 1.5 A. Thus, peak inrush to RMS full load is about 26 to 1.
End edit.
Materials with a more square loop hysteresis curve would show a higher peak current, especially wound on a toroid.
Magnetizing current is very non-sinusoidal and therefore 1.732 is not a good predictor of the peak to RMS ratio.
A magnetically tripped breaker may be more peak sensitive than RMS. An electronically tripped breaker can have almost any desired trip characteristic. In work I did for Mechanical Products in the early 1960s we developed an electronic breaker that could tolerate any reasonable inrush current and yet trip at, for example, 1% above the set-point. Further, the set-point could be compensated for line voltage or any other desired input.
.
Peak inrush current results from core saturation. If at turn off of excitation the residual core flux density is high in the positive direction, and when next turned on the voltage phase is such as to create further magnetization in the same direction, then the core is driven far into saturation. This requires a lot more magnetizing current and therefore large inrush current. Whether this occurs and its magnitude is a random function dependent on where in the cycle the transformer is turned off, and where it is turned on.
On my web site at
http://www.beta-a2.com/EE-photos.html
I have photos of inrush current for a typical iron core transformer. See photos P6 thru P8. This transformer uses E-I laminations and probably uses a very standard transformer steel.
Edit.
The full load RMS input to this transformer is about 1.5 A. Thus, peak inrush to RMS full load is about 26 to 1.
End edit.
Materials with a more square loop hysteresis curve would show a higher peak current, especially wound on a toroid.
Magnetizing current is very non-sinusoidal and therefore 1.732 is not a good predictor of the peak to RMS ratio.
A magnetically tripped breaker may be more peak sensitive than RMS. An electronically tripped breaker can have almost any desired trip characteristic. In work I did for Mechanical Products in the early 1960s we developed an electronic breaker that could tolerate any reasonable inrush current and yet trip at, for example, 1% above the set-point. Further, the set-point could be compensated for line voltage or any other desired input.
.
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