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 92: 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 service. Figure
93 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 93: Permissive overreach distance protection scheme
Negative IED impedance, positive fault current (voltage inversion)
SEMOD168320-367 v3
Assume in equation
64
EQUATION1898 V1 EN-US
(Equation 64)
and in figure 94
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.
Distance protections of adjacent power lines shown in figure
94 are influenced by
this negative impedance. If the intermediate infeed of short circuit power by other
lines is taken into consideration, the negative voltage drop on X
C
is amplified and a
Section 8 1MRK 506 369-UEN B
Impedance protection
184 Line distance protection REL670 2.2 IEC
Application manual
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