Underreaching and overreaching schemes
SEMOD168320-348 v2
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
.
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 91:
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
. 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 63)
Here K
S
is a safety factor, presented graphically in figure
, which covers for
possible overreaching due to low frequency (sub-harmonic) oscillations. Here it
should be noted separately that compensation degree K
C
system reactance, inclusive line and source impedance reactance. The same setting
applies regardless MOV or spark gaps are used for capacitor overvoltage
protection.
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
, of the power line where no tripping occurs from either
end.
1MRK 506 369-UEN B
Section 8
Impedance protection
Line distance protection REL670 2.2 IEC
183
Application manual
Содержание REL670 2.2 IEC
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