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68
3 zone impedance protection (Z(n)RW)
•
The effect of a load transfer between the terminals of the protected line. When the
fault resistance is considerable, the effect must be recognized.
•
Zero-sequence mutual coupling from parallel lines.
Usually, these errors require a limitation of the underreaching zone (normally zone 1)
to 85-90% of the protected line. For the same reason, it is necessary to increase the reach
of the overreaching zone (normally zone 2) to at least 120% of the protected line - to
ensure that the overreaching zone always covers a complete line. The zone 2 reach can
be even higher, but in general it should never exceed 80% of the following impedances:
•
The impedance corresponding to the protected line, plus the first zone reach of the
shortest adjacent line.
•
The impedance corresponding to the protected line, plus the impedance of the max-
imum number of transformers operating in parallel on the bus at the remote end of
the protected line.
The back-up overreaching zone (normally zone 3) must never exceed 90% of the short-
est zone 2 reach of any of the lines connected to the remote end bus. It must be at least
2 times the zone 1 reach.
The reverse zone is applicable for purposes of scheme communication logic. The same
applies to the back-up protection of the busbar or power transformers. It is necessary to
secure, that it always covers the overreaching zone, used at the remote line terminal for
the telecommunication purposes.
In the case of a long line followed by a short line, or by a large bank of low impedance
transformers, the mandatory 120% setting might overreach zone 1 of the adjacent line,
or reach through the transformer bank at the other line end. In such cases, one must in-
crease the zone 2 time delay and thus secure the selectivity. The zone 2 reach must not
be reduced below 120% of the protected line section. It must be covered under all con-
ditions.
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A measurement loop with single-phase to earth fault consists of three impedances,
which are shown in figure 35, where:
Z
:
impedance in phase conductor
1
R
:
fault resistance
F
Z
:
earth return impedance
N
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