ABB RET670 Applications Manual page 204

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IED application

= Z0m/(3 · Z1L)

The second part in the parentheses is the error introduced to the measurement of the

line impedance.

If the current on the parallel line has negative sign compared to the current on the

protected line that is, the current on the parallel line has an opposite direction

compared to the current on the protected line, the distance function overreaches. If the

currents have the same direction, the distance protection underreaches.

Maximum overreach occurs if the fault infeed from remote end is weak. If we consider

a single phase-to-ground fault at "p" unit of the line length from A to B on the parallel

line for the case when the fault infeed from remote end is zero, we can draw the

voltage V in the faulty phase at A side as in equation 107.

EQUATION1278 V3 EN

Notice that the following relationship exists between the zero sequence currents:

EQUATION1279 V2 EN

Simplification of equation 108, solving it for 3I0p and substitution of the result into

gives that the voltage can be drawn as:

EQUATION1280 V1 EN

If we finally divide equation

present to the IED as

EQUATION1379 V2 EN

with equation

1MRK504116-UUS C

we can draw the impedance

Application manual

(Equation 107)

(Equation 108)

(Equation 109)

(Equation 110)

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