Transformer Protection

[Humble2000]

Dear All,
in a transformer protection graph with a fuse.
I have seen in most cases that transformr damage curve crosses fuse curve around 50 sec.

What's the concept behind this?
I have reviewd some articles,but couldn't find anything regarding this matter.

Thanks

 

[dpc]

Because that's the best a fuse can do while still allowing you to energize the transformer. It isn't because they want the curves to cross - just a fundamental limitation of trying to protect transformers with fuses.

 

[DanDel]

Per IEEE 242, this is acceptable, since the primary fuse is designed to protect the transformer from faults (the lower/high current part of the damage curve), while still allowing inrush (typically 10-12x FLA for 0.1s). The secondary device should be set up to protect from an overload (the upper/low current part of the curve).

 

[Humble2000]

Thank you for all answers. We are going to change a 2000KVa, oil-filled transformer with a 1500KVA transformer, to my understanding since both size transformers are in same category and load or secondary or primary cables are not changing, the secondary relay settings also should not change.

A colleague suggested changing the breaker trip setting to 2000A from 3000A and so we need to change the CT ratio too!

I appreciate your feedback on this.

 

[DanDel]

Humble2000, I wouldn't be so sure about not having to change the secondary protection settings when going from a 2000kVA to a 1500kVA transformer. The damage curve will change, as will the overcurrent protection requirements.