Transformer protection Curve

[NJWVUGrad]

I am in the middle of a discussion with an equipment supplier who is saying my fusing for transformer protection is undersized per his tables. I have modeled the curve in ETAP using the FLA of the transformer and plotting it against the magnetizing inrush and the damage curve.

Is the 1.4 FLA suggested by the manufacturer just a very high safety margin?

 

[NJWVUGrad]

Forgot to insert picture. As Homer would say, "Its my first day!"

 

[krisys]

The upstream fuse is basically to offer transformer primary side main protection and the back up for the secondary side faults. Assuming that there is a protective device existing in the secondary side.

From the TCC curve you posted transformer it is clear that the transformer is protected for the through fault current. Hence the fuse rating appears to be fine.

Please remember that the veiws may vary from person to person, as there is no hard and fast rule on this.

 

[distributionPlanner]

I'm a little confused by where the FLA of ~3000A was plotted. Your x axis is current x10, but it appeared to be plotted in the 300A range. Same comment applies to the inrush dot @ 12x for 6 cycles.
Is the primary voltage 8.3kV, or is that just the rating of the fuse?

Something seems a little off here.

 

[Parchie]

@distributionPlanner,
The plot is on the 4.16kV level. FLA at primary is 346.97 A. Equivalent X 12 = 4163.64 A.
@NJWVUGrad,
IDK how it is with other engineers but I prefer sizing fuses to cover for short-circuit and allow the secondary breaker protection relay to cover for the transformer overload and short-time duration loading.

Your fusing ratio is around 2.59, which seems okay (=>300%). From your fuse TCC, at 12 X FLA = 4163 A, your 400A fuse will clear at around 0.15 seconds, just a bit higher than the criterion of 0.1 seconds (6 cycles) of inrush. At 25 X FLA = 8674 A, your fuse will clear in around 0.023 seconds, higher than the 0.01 seconds (3/5 cycle).