REF setting
REF setting
(OP)
I am trying to work out the setting for a restricted earth fault protection relay we have installed. I am trying to work out the primary side fault current (If) but the transformer impedance is rated in ohms and not %. How can I convert the ohms value to % or work out the If using the ohms value?






RE: REF setting
%Z = [100 x Z(ohms) x MVA(base)] / kV(base)^2
But why do you need the trsf impedance to calculate the primary fault current? You'll have to use the source-data to calculate it. Or do you want to calculate the through-fault current? (Secondary fault current seen from the primary side)
Regards
Ralph
RE: REF setting
If the CT primary is correctly sized according to the transformer ratings, I would just set it to 10%..15% of the rated current and it should be safe to include errors in CT ratio etc.
RE: REF setting
You are quite right, REF-protection is a typical diff-scheme, and doesn't need to coordinate with other protection schemes.
However, there are two types of REF-schemes, eg. the low-impedance REF-schemes (normally incorporated in the mordern electronic relays) and high impedance REF-schemes.
On older systems high-impedance REF-schemes are quite common, and it normally operate on voltage. To set a high-impedance voltage-operated scheme, you need the max fault-level (as seen on CT secondary) and the resistance of the loop (internal CT-resistance + wire resistance etc.) to calculate the setting.
Regards
Ralph
RE: REF setting