ABB REL670 2.2 IEC - Underreaching and Overreaching Schemes

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
209.
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-US
Figure 209: 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 209. 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 369)
Here K
S
is a safety factor, presented graphically in figure 210, 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
210 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.
Equation
369 is applicable for the case when the VTs are located on the bus side of
series capacitor. It is possible to remove X
C
from the equation in cases of VTs
installed in line side, but it is still necessary to consider the safety factor K
S
.
If the capacitor is out of service or bypassed, the reach with these settings can be
less than 50% of protected line dependent on compensation degree and there will
be a section, G in figure
209, of the power line where no tripping occurs from
either end.
Section 8 1MRK 506 369-UEN B
Impedance protection
386 Line distance protection REL670 2.2 IEC
Application manual