Power Transformer Inrush Calculation
Power Transformer Inrush Calculation
(OP)
I'm looking at the existing protection for a bank of three single-phase, 2500 KVA, 44 KV to 13.2/7.62 KV transformers, connected delta-wye, at a rural substation.
Protection for this transformer bank was originally supplied by 125 Amp Southern States Type PF fuses. About 2 years ago (before I began working here), protection was changed to 100 Amp Type PF fuses at the recommendation of an outside engineering firm.
The 100 Amp fuse curves are to the right of the traditional inrush points, 12x FLA at 0.1 second and 25x FLA at .01 second. However, the sofware I'm using (Milsoft LightTable) plots a transformer inrush curve instead of just plotting inrush at two points. This curve seems to indicate that proper coordination is not achieved between between inrush and the fuses in the range of about 200 thru 500 amps (2x thru 5x FLA).
I'm not finding any documentation describing the software's inrush assumptions, but it does appear that the curve may be a creation designed to cover both inrush and cold-load pickup. The curve passes through 25x FLA @ .01s, 12x @ .1s, 6x @ 1s, 3x @ 10s, and 2x at 100s.
At 2x FLA, fuse minimum melt is about 20 seconds and 'inrush' curve is 100 seconds. At 3x FLA, fuse min melt is 5.25 seconds and 'inrush' is 10 seconds. At 6x FLA, fuse min melt is about 1.2 seconds and 'inrush' is 1 second.
So, here's my attempt to get some of you to help me make a judgement call... Given that the minimum melt curve passes to the right of the inrush curve as plotted between 2x and 5x FLA, would you replace the 100 Amp fuses with 125 Amp fuses or just let it go. Curves for both fuses lie entirely to the left of the transformer damage curve. Peak load on this substation is approximately equal to the OA rating of the transformer bank.
Thanks,
Robert
Protection for this transformer bank was originally supplied by 125 Amp Southern States Type PF fuses. About 2 years ago (before I began working here), protection was changed to 100 Amp Type PF fuses at the recommendation of an outside engineering firm.
The 100 Amp fuse curves are to the right of the traditional inrush points, 12x FLA at 0.1 second and 25x FLA at .01 second. However, the sofware I'm using (Milsoft LightTable) plots a transformer inrush curve instead of just plotting inrush at two points. This curve seems to indicate that proper coordination is not achieved between between inrush and the fuses in the range of about 200 thru 500 amps (2x thru 5x FLA).
I'm not finding any documentation describing the software's inrush assumptions, but it does appear that the curve may be a creation designed to cover both inrush and cold-load pickup. The curve passes through 25x FLA @ .01s, 12x @ .1s, 6x @ 1s, 3x @ 10s, and 2x at 100s.
At 2x FLA, fuse minimum melt is about 20 seconds and 'inrush' curve is 100 seconds. At 3x FLA, fuse min melt is 5.25 seconds and 'inrush' is 10 seconds. At 6x FLA, fuse min melt is about 1.2 seconds and 'inrush' is 1 second.
So, here's my attempt to get some of you to help me make a judgement call... Given that the minimum melt curve passes to the right of the inrush curve as plotted between 2x and 5x FLA, would you replace the 100 Amp fuses with 125 Amp fuses or just let it go. Curves for both fuses lie entirely to the left of the transformer damage curve. Peak load on this substation is approximately equal to the OA rating of the transformer bank.
Thanks,
Robert






RE: Power Transformer Inrush Calculation
Transformer inrush is notoriously difficult to predict and seems to vary even for very similar transformers.
BTW, when looking at transformer damage curves for this delta-wye transformer, be sure to consider the impact of a secondary line-to-ground fault - the damage curve needs to be shifted to the left - 58% of the "normal" damage curve. You're probably aware of this, but just in case....
RE: Power Transformer Inrush Calculation
If the FLA current of a trf is 100 A,what rated fuse is to be used? 100 A or higher rated?
RE: Power Transformer Inrush Calculation
For a line to ground fault on the secondary side of a delta-wye transformer bank, the fault current will be split between two phases on the primary side of the transformer.
Fault current seen by each fuse will be {1/sqrt(3) = 0.58} times the fault current in a wye-wye connection.
Robert
RE: Power Transformer Inrush Calculation
The IEEE Red Book covers this pretty well.
RE: Power Transformer Inrush Calculation
dpc,can I have your expert view on my second question?
RE: Power Transformer Inrush Calculation
The fuse will see only 58% of the current that the secondary winding is seeing so it will not trip as quickly as might be believed by simply looking at the standard damage curve. By shifting the damage curve, this is compensated for.
RE: Power Transformer Inrush Calculation
My second query was if transformer rated line current is 100A ,what should be the rating of fuse-100A or higher ?
RE: Power Transformer Inrush Calculation
I thought already answered that question. If the 100 A fuse clears 12X @ 0.1 sec and has never caused a nuisance trip on inrush, I wouldn't bother taking an outage just for changing it.
I did not really evaluate the adequacy of the 100A fuses to carry load current. That's another issue.
RE: Power Transformer Inrush Calculation
I think the confusion stems from looking at a line to ground fault on a wye secondary and the resulting primary current. Only one phase is loaded with a wye secondary ground fault. Hence the 58% factor.
With a three phase secondary fault, the 100% factor is of course correct.
Bill
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"Why not the best?"
Jimmy Carter
RE: Power Transformer Inrush Calculation
Some software programs allow you to put a "setback" allowance on the minimum-melt curve to see how it's really going to respond. Also the 12X multiplier is pretty conservative, and it's likely to be much smaller due to source impedance and transformer size.
In short, it appears that the 125A fuses were the proper size for this application.
RE: Power Transformer Inrush Calculation
Check the 125A fuse against the transformer damage curve. If it is OK, then have some available to use as replacements if the 100A fuse ever blows on inrush.