TCC curves shifted by total fault current 1

[pwrengrds]

I am reviewing a coordination study and all the TCC's and pickup points are shifted by varying amounts. The explanation given is

All of the graphs have the curves aligned to the “total fault current”.

Have any of you heard of this? Is it a common practice? I haven't and have been doing this for 20 years.

David

 

[pwrengrds]

Attached is an example of a TCC. Has anyone seen TCC's shown like this?

 

[davidbeach]

When you have relays on both sides of a transformer you have to adjust on side to the base of the other side. Otherwise the TCC plot is meaningless.

 

[stevenal]

Besides the above, it shifts for the delta wye connection affect on the various fault types. Also consider multiple sources with protection on each source, and a fault past a downstream device. The downstream device sees the full fault current while each upstream device sees only its contributing portion. Aligning the curves one to one will not give an indication of how things coordinate. Shift the curves of the upstream devices to account for the split, and the coordination becomes evident. Yes it is common practice. Also common was manually shifting the curves on a light table in the olden days.