ABB RET670 - Page 177

( )
A L ph N 0 Nm 0pV p Z1 I K 3I K 3I= × + × + ×
EQUATION1278 V3 EN (Equation 62)
One can also notice that the following relationship exists between the zero sequence
currents:
( )
0
3 0 3 0 0 2
L L
I Z I p Z p× = × -
EQUATION1279 V2 EN (Equation 63)
Simplification of equation 63, solving it for 3I0p and substitution of the result into
equation 62 gives that the voltage can be drawn as:
3 0
1 3 0
2
A L N m
I p
V p Z Iph K I KN
p
æ ö
×
= × + × + ×
ç ÷
-
è ø
EQUATION1280 V1 EN (Equation 64)
If we finally divide equation 64 with equation 59 we can draw the impedance present
to the IED as
0
0
3
3 0
2
1
3
m
L
I p
Iph KN I KN
p
Z p Z
Iph I KN
é ù
æ ö
×
+ × + ×
ê ú
ç ÷
-
è ø
ê ú
= ×
ê ú
+ ×
ê ú
ë û
EQUATION1379 V2 EN (Equation 65)
Calculation for a 400 kV line, where we for simplicity have excluded the resistance,
gives with X1L=0.48 Ohm/Mile, X0L=1.4Ohms/Mile, zone 1 reach is set to 90% of
the line reactance p=71% that is, the protection is underreaching with approximately 20%.
The zero sequence mutual coupling can reduce the reach of distance protection on the
protected circuit when the parallel line is in normal operation. The reduction of the
reach is most pronounced with no current infeed in the IED closest to the fault. This
reach reduction is normally less than 15%. But when the reach is reduced at one line
end, it is proportionally increased at the opposite line end. So this 15% reach reduction
does not significantly affect the operation of a permissive underreaching scheme.
Parallel line out of service and grounded
1MRK504116-UUS C Section 3
IED application
171
Application manual