CO2 Overcurrent Formula

[Jabrone41]

Hello,

I am looking for the formula that describes a Westinghouse CO2 (Short Time Inverse). I have the manual that shows the pictures of the curves and it accurately describes the time per my test of 5 times the min. pu.

I searched these boards and found the .pdf attached below that gives a CO2 formula, but this does not synch with what the graph in the CO2 manual shows.

My settings are a min. pu of 4A, a TD of 3.5, and I am applying a 5X min pu. I should be getting 14 cycles (.233s)

Per the formula in the attached document, it says I should be getting about 0.202

Thoughts?

 

[davidbeach]

No formula. The best that can be done is to curve fit. Another possibility is to look and see what the formula is that Basler uses to emulate the CO2 curve in their relays, but that is also just going to be somebody's idea of a best fit.

 

[DTR2011]

ABB did publish a curve according to IEEE C37. It works best on time dial 5 and is less accurate at TD 1& 10. I have tested numerous Co relays with the OMICRON, which uses this formula.

I'll look it up later and post it.

The Basler best fit is also quite accurate.

 

[OliverJanke]

Hi Jabrone,

I found this formula:

t(s) = 105.2ms * Td / (M^0.8 - 1) + 26.2ms * Td

with M = multiples of PU.

With your values (Td = 3.5 and M = 5) I get 232ms, so it seems to fit to the values from your manual.

OMICRON customers are supported in testing this relay by using the PTL (free library of test templates) which also makes use of this formula.

Hope I could help a little,
Oliver Janke
OMICRON Project engineer