ABB RELION RET670, Applications Manual

Page: 235 / 1016

Equation 71 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
104 , 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 105: 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
106 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-US
Figure 106: Permissive overreach distance protection scheme
Negative IED impedance, positive fault current (voltage inversion)
SEMOD168320-367 v3
Assume in equation 72
1MRK 504 163-UUS A
Section 8
Impedance protection
Transformer protection RET670 2.2 ANSI 229
Application manual