Equation 66 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
93, 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
Figure 94: 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
95 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_ansi.vsd
X
11
X
12
-jX
C
A B
D
A
D
B
Permissive Zone
A
Permissive Zone
B
ANSI06000620 V1 EN
Figure 95: Permissive overreach distance protection scheme
Negative IED impedance, positive fault current (voltage inversion)
Assume in equation 67
Section 8 1MRK 506 369-UUS -
Impedance protection
196 Line distance protection REL670 2.2 ANSI
Application manual
To Next Page
Loading...
