Transfromer Primary fuse opening 2

[buzz53]

I need help in deteriming what causes the primary fuses of a 2500KVA, 13.8KV (Delta)- 4160/2400V Wye transformer to open during motor start-up.
The transformer primary fuses are 125A and the motor is 2000HP, 4160V, 242FLA. A reduce motor start is used as a starting method. It is set for 350% reduce start.

We can get one start but the second start opens the primary fuse for the transfromer. There is no other load connected (only the 2000HP). Start current of 1045A for 4.5 seconds was captured using a recorder.

Any suggestions would be very greatful.

 

[waross]

That looks like 315 amps for 4.5 seconds, on a 125 amp fuse.
Check the time/current curve of your fuses.
Have you changed to a different type or rating of fuse lately?
Have you changed fuse manufacturers?
I get 104.6 amps rated current for the transformer primary.
It looks like your fused for 120% of full load. Have you considered larger fuses as per NEC table 450-3(a)(1), 300% or NEC (2)a, 250%
That would be about 300 Amps and 250 Amps respectively.
If 125 Amps gives you a start and a half, 250 Amps will probably do the job.
yours

 

[davidbeach]

The cause of the fuse blowing is that it is doing exactly what it is asked to do. The first start is likely enough to damage the fuse and the second start is enough to finish it off. A 300E, or even a 350E fuse would be a much better rating than the 125A (A for Amperes, but medium voltages don't use that, they use an E, K, T, or other letter for fuse characteristic) you mention; what type of fuse is it? Where does 315A at 4.5 seconds appear on the fuse curve; my guess would be that it is between minimum melt and maximum clear, closer to minimum melt and it isn't a non-damaging fuse (one where the melt process is non-reversible once it starts).

waross, if you are going to quote the NEC, please get a current version or mention which version you are using each time. Per the 2005 edition, the only relevant table is 450.3(A). I happen to remember you mentioning the 1996 edition (2005 is the third since then) but others might not and might be confused trying to find your references; thanks.

 

[waross]

davidbeach
Your point is well taken and I apologise.
I will in future quote my edition. I am a little far from home to upgrade just now.
yours

 

[mykh]

E-rated fuses are typically sized as low as 133% of transformer rated current (1.33 x 105A = 140A in the discussed case). So, 150E rated fuse (closest next larger size) could solve the problem without need for replacement of the existing holder / clips (150E sizewise is same as 125E).