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ElecDan (Electrical) (OP)
6 Jan 12 2:13
When you calculate CT's ANSI/IEEE standard burden, do you have to include CT's internal registance?

This is to calculate Per Unit CT burden (Zb)for the final calculation of CT's performance in terms of saturation at the time if there is a (severe) fault.

For instance, for a CT C400 the ANSI standard burden is 2 ohms. And you may want to calculate CT remanence (20-80%)and this further downgrades your C400 CT to, say, C350 (depends on your % used).

But I'm not sure whether I still have to add CT's internal registance in order to calculat the final standard burden. Note that this internal registance has already been taken into consideration when CT's normal burden (i.e., internal registance + CT lead registnace + relay burden)is calculated.

Thanks for any comments.
burntcoil (Electrical)
7 Jan 12 6:41
Refer to the CT data sheet, the total burden on the CT will be the

1. CTs secondary windings resistance
2. Resistance of the wires from CTs to the relays
3. Burden of the relays

The sum of all above you will be using to calculate the knee point voltage at a given fault current to check the CT for saturation.
ElecDan (Electrical) (OP)
23 Jan 12 0:55
Thanks burntcoil. I think CT's standard burden is different from the CT's total burden. In oder to calculate CT's per unit burden ratio, one needs to calculate both burden. ANSI C400 CT will give us a burden of 4 ohms. I'm not sure whether I need to add anything else to the ANSI standard burdrn for the PU ratio calculation.

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