ABB RET670 - Page 234

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).
Underreaching and overreaching schemes
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
96.
en06000618.vsd
X
11
X
12
-
jX
C
A B
D
A
D
B
Zone 1
A
Zone 1
B
Zone 2
A
Zone 2
B
G
IEC06000618 V1 EN
Figure 96: 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
96. 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 (Equation 140)
Here K
S
is a safety factor, presented graphically in figure 97, 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
97 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 3 1MRK504116-UUS C
IED application
228
Application manual