ABB REG670 Applications Manual page 248

Section 8
Impedance protection
242
1.0
0.8
0.6
0.4
0.2
0 20 40
IEC06000619 V1 EN
Figure 121: Underreaching safety factor K
compensation degree 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 122
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.
A
- jX
Permissive Zone
C
X
11
Permissive Zone
D
A
IEC06000620 V1 EN
Figure 122: Permissive overreach distance protection scheme
Negative IED impedance, positive fault current (voltage inversion)
Assume in equation 117
< < +
X X X X
11 C S 11
EQUATION1898 V1 EN
and in figure 123
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
B
the spark gap has flashed.
Distance protections of adjacent power lines shown in figure
this negative impedance. If the intermediate infeed of short circuit power by other
lines is taken into consideration, the negative voltage drop on X
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
60 80 100
K [%]
C
en06000619.vsd
in dependence on system
S
C
shows the permissive zones. The first underreaching zone
B
A
X
12
B
D
B
en06000620.vsd
1MRK 502 071-UEN -
(Equation 117)
123 are influenced by
is amplified and a
C
Application manual