ABB RELION RET670, Applications Manual

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compensated and adjacent lines are concentrated on finding some parallel ways, which
may help eliminating the basic reason for wrong measurement. The most known of
them are decrease of the reach due to presence of series capacitor, which apparently
decreases the line reactance, and introduction of permanent memory voltage in
directional measurement.
Series compensated and adjacent lines are often the more important links in a
transmission networks and delayed fault clearance is undesirable. This makes it
necessary to install distance protection in combination with telecommunication. The
most common is distance protection in Permissive Overreaching Transfer Trip mode
(POTT).
8.13.3.6 Underreaching and overreaching schemes
GUID-3090702F-508B-480C-9AC7-2D5A82B9AD1E v1
It is a basic rule that the underreaching distance protection zone should under no
circumstances overreach for the fault at the remote end bus, and the overreaching zone
should always, under all system conditions, cover the same fault. In order to obtain
section selectivity, the first distance (underreaching) protection zone must be set to a
reach less than the reactance of the compensated line in accordance with figure
213 .
en06000618.vsd
X
11
X
12
-
j X
C
A B
D
A
D
B
Zone 1
A
Zone 1
B
Zone 2
A
Zone 2
B
G
IEC06000618 V1 EN-US
Figure 213: Underreaching (Zone 1) and overreaching (Zone 2) on series
compensated line
The underreaching zone will have reduced reach in cases of bypassed series capacitor,
as shown in the dashed line in figure
213 . The overreaching zone (Zone 2) can this way
cover bigger portion of the protected line, but must always cover with certain margin
the remote end bus. Distance protection Zone 1 is often set to
( )
1 11 12
= × + -
Z S C
X K X X X
EQUATION1914 V1 EN-US
(Equation 339)
Here K
S
is a safety factor, presented graphically in figure 214 , which covers for
possible overreaching due to low frequency (sub-harmonic) oscillations. Here it should
be noted separately that compensation degree K
C
in figure 214 relates to total system
reactance, inclusive line and source impedance reactance. The same setting applies
regardless MOV or spark gaps are used for capacitor overvoltage protection.
Section 8
1MRK 504 163-UUS A
Impedance protection
426 Transformer protection RET670 2.2 ANSI
Application manual