ABB RELION 670 Series Applications Manual page 181

1MRK 504 152-UEN B
separately that compensation degree K
inclusive line and source impedance reactance. The same setting applies regardless MOV or
spark gaps are used for capacitor overvoltage protection.
Equation
capacitor. It is possible to remove X
but it is still necessary to consider the safety factor K
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 91, of the power line where no tripping occurs from either end.
1.0
0.8
0.6
0.4
0.2
0
IEC06000619 V1 EN-US
Figure 92: Underreaching safety factor K
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
service. Figure
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.
A
IEC06000620 V1 EN-US
Figure 93: Permissive overreach distance protection scheme
Negative IED impedance, positive fault current (voltage inversion)
Assume in equation
<
X X
11
EQUATION1898 V1 EN-US
and in figure
a three phase fault occurs beyond the capacitor. The resultant IED impedance seen from the
D IED location to the fault may become negative (voltage inversion) until the spark gap has
B
flashed.
Application manual
78 is applicable for the case when the VTs are located on the bus side of series
20 40 60
degree K
C
93 shows the permissive zones. The first underreaching zone can be kept in the
- jX
Permissive Zone
C A
X
11
Permissive Zone
B
D
A
79
< +
X X
C S 11
94
in figure 92 relates to total system reactance,
C
from the equation in cases of VTs installed in line side,
C
.
S
80 100
K [%]
C
en06000619.vsd
in dependence on system compensation
S
B
X
12
D
B
en06000620.vsd
Section 7
Impedance protection
SEMOD168320-367 v3
(Equation 79)
175