Overload through-fault protection curve for a zig-zag trsf 1

[RalphChristie]

Hi

I want to draw the overload through-fault protection curve for a NEC. (zig-zag transformer, earthing transformer) The continious line current rating of the NEC is 60A, the short time line current rating is 400A for 30 seconds. Is there any way how I can plot this curve for values more than 400A? (Like the method describe in ANSI/IEEE transformer overload-through-fault standard - IEEE C37.91)
I have the nameplate details of the NEC - Voltage, Line/neutral currents and the measured Zo/phase - but don't know how to use it (if it is possible) to draw the curve for higher currents.

Thanks

RCC

 

[dpc]

I think the assumption is that if the transformer is specified with 400A rating, the current has to be limited to that value by external impedance.

I'm not sure what the standards say regarding short-time capability of zig-zag xfmr.

 

[jbartos]

Suggestion: Average commercial electrical protective coordination software will output a transformer damage curve. A transformer ignition curve is above to the right of the transformer damage curve.

 

[RalphChristie]

dpc:
The zig-zag xfmr is without an external impedance (older installation), thus no current limitation, that is why I want to draw the overload through-fault protection curve.

jbartos:
Just explain transformer ignition curve, I'm not familiar with it?

Many thanks

RCC

 

[dpc]

I'm not sure the ANSI C57 standards are fully applicable to grounding transformers. If you can, I'd try to contact the transformer manufacturer. If the rating is 400A, they must have been assuming some type of grounding impedance to limit fault current. It may not be designed to operate solidly-grounded.

 

[busbar]

400 amperes for 30 seconds is 4,800,000 A²∙sec. To some degree, can’t you extrapolate from that for other times?

 

[RalphChristie]

dpc: The NEC was manufactured by South Wales - I do not think the company exist anymore.

busbar: I'll give you a star for that one - I should have thought about it.

Thanks

RCC