Fuse Sizing for 3.75/4.68 MVA Transformer 23/4.16kV. 1

[Uquresh1]

I have no experience with fuses (other than some fuses on radial distribution lines); I need some help coming up with the fuse size to protect a 3.75/4.68 MVA distribution Transformer. The fuses obviously are on high side (Delta 23kV) and the low side is Wye-grounded 4.16kV. I have looked into using 125E, 150E, and 175E S&C fuses, but don't know what the actual criteria is for coming up with the fuse size based on Transformer data. Transformer Z=6%

Since there is no other form of protection (breakers, relays, etc) on this transformer, I am a little more concerned. Your help would be really appreciated.

 

[7anoter4]

In my opinion, well interpreting NEC Art.450.3:
According to NEC Art.450.3 [Table 450.3(A)]:
If it is a unsupervised locations if Z<=6% the primary fuse rating has to be not more than 3*Irated[300%].Here is a note:
"1. Where the required fuse rating or circuit breaker setting does not correspond to a standard rating or setting, a higher rating or setting that does not exceed the next higher standard rating or setting shall be permitted."
The rated current has to correspond to S[rated apparent power]used to state the short-circuit impedance Z.I think S has to be 3.75 MVA and 300 A has to be the fuse rating.

 

[VTer]

I would plot the transformer damage curve, inrush current, cable damage curve, short-circuit current, downstream and upstream protection devices on the TCC and size and coordinate your fuse accordingly. Do not exceed 300% of the transformer rated current as 7anoter4 stated.

Also do some searching on the S&C site, they have great references on fuse sizing.

Check with your utility for coordination and possibly the building insurance company if the transformer will be indoors. I believe some insurance companies such as FM Global require current-limiting devices in order for the liquid-filled transformer to contain the FM label.


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[unclebob]

Nominal current at 23 kV and 4680 KVA is 118A. I would not go below 125 E. Fuses don't like being loaded at 100%. Just make sure it clears the inrush current. Primary fuses protect against short-circuit, secondary protection devices, if any at 4.16 kV, protect against overloads. In your case, you need to make sure your monitoring the windings and or oil temperatures and send an alarm or trip signal to operation, as this is your only mean of overload protection.

 

[davidbeach]

Does the transformer have a listed facility rating in excess of the top rating? Most utility transformers have a winter emergency in excess of the top nameplate rating.

 

[waross]

I have always considered the purpose of primary fuses on distribution transformers to be to disconnect the transformer and so protect the network in the event of a transformer failure. Generally when the primary fuses operate, the transformer has failed or an animal has contacted the circuit between the fuse and the transformer bushing.
If you wish to have some type of overload protection on the transformer, add some type of protection to the fuses.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Uquresh1]

Thanks for the input guys. David, I don't think there is any other rating on this Transformer that I know of. Again, this is just the planning phase and we haven't ordered anything yet.

Also, this TXfmr is feeding only one feeder which has a recloser on it, so I think overload protection will be done by that feeder on the 4.16kV side. The only other protection that I proposed for this unit other than fuses were 63FP and 71Q. Are those enough or do I need to add some more?