REF setting

[andross]

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?

 

[RalphChristie]

The actual trsf impedances in ohms are quite different on the two sides of a trsf, with different voltage levels.

%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

 

[digitrex]

In my opinion, REF setting should be set to as low as practically possible. We don't need to calculate the fault level or to grade this setting with other OC/EF relays because it serves as an equivalence to a differential protection for one side of transformer winding(delta or star).

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.

 

[RalphChristie]

Digitrex:

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

 

[RRaghunath]

In high impedance schemes too, we calculate through fault current of the transformer to estimate the required stabilising resistor value (for the current setting selected) that would prevent relay becoming unstable (and maloperate) during through fault conditions. This applies to voltage setting based relays as well, applied slightly differently.