separately that compensation degree K
C
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
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
, of the power line where no tripping occurs from either end.
en06000619.vsd
K
S
1.0
0.8
0.6
0.4
0.2
0
20
40
60
80
100
K
C
[%]
IEC06000619 V1 EN-US
Figure 107: Underreaching safety factor K
S
in dependence on system compensation
degree K
C
For that reason permissive underreaching schemes can hardly be used as a main protection.
Permissive overreaching distance protection or some kind of directional or unit protection
must be used.
The overreach must be of an order so it overreaches when the capacitor is bypassed or out of
shows the permissive zones. The first underreaching zone can be kept in
the total protection but it only has the feature of a back-up protection for close up faults. The
overreach is usually of the same order as the permissive zone. When the capacitor is in
operation the permissive zone will have a very high degree of overreach which can be
considered as a disadvantage from a security point of view.
en06000620.vsd
X
11
X
12
- jX
C
A
B
D
A
D
B
Permissive Zone
A
Permissive Zone
B
IEC06000620 V1 EN-US
Figure 108: Permissive overreach distance protection scheme
Negative IED impedance, positive fault current (voltage inversion)
SEMOD168320-367 v3
11
11
<
<
+
C
S
X
X
X
X
EQUATION1898 V1 EN-US
(Equation 85)
and in figure
a three phase fault occurs beyond the capacitor. The resultant IED impedance seen from the
D
B
IED location to the fault may become negative (voltage inversion) until the spark gap has
flashed.
1MRK 505 343-UEN B
Section 7
Impedance protection
175
Application manual
Содержание Relion 670 series
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