Transformer Protection

[Inder76]

Hi,..
Heard a lot about this website and trying my luck for the first time....I am an still in Training and trying to learn things. Still some time to go to be a P.E....


Just a quick question;

I have to coordinate a 150E(44 KV Primary Delta-10 MVA )s&C fuse with a ABB breaker(F35 Multinlin feeder relay- Secondary. 13.8 KV -Star). I know what I will see on the curve will be three things:

1. Transformer Damage Curve
2. S&C Fuse Curve
3. F35 Relay (Which I have to move around)
For now there is nothing down stream.It will come later so for now just two things Fuse and CB.

I was wondering, any reasonable time difference I can assume between fuse and CB for a secondary fault after the CB. I have already calculated three phase and single phasae ground fault current from short cicuit study.

I would appreciate, if any one of you be kind enough to advise me.
Regards,

 

[DanDel]

One other important item is the transformer inrush point, which should be 0.1s and 10X FLA for <2500kVA and 12x FLA for >/=2500kVA transformers, per IEEE 242.

Typically, the primary device (fuse) is used for fault clearance. You must make sure that this device will not interfere with the inrush point.

The secondary device (relay) should be used for overload conditions. Frequently, the 50 (IOC) function is not used here. It is also acceptable for the two device curves to overlap somewhere in the high-current region, (also per IEEE 242).

 

[jghrist]

I would use a coordinating interval of 0.3 seconds plus the breaker clearing time between the relay curve and the fuse minimum melt curve. The curves must be shifted to take into account the turns ratio and fault type.

If you have reclosing on the secondary, you need to account for fuse heating during the first trip. Electrical Distribution-System Protection by Cooper Power has a chapter on fuse-relay coordination that explains the methods used.