ABB RELION RET670 Applications Manual page 275

1MRK 504 163-UUS A
Transformer protection RET670 2.2 ANSI
Application manual
Simplification of equation 121, solving it for 3I0p and substitution of the result into
equation 120 gives that the voltage can be drawn as:
æ
V Z 1 ç I K I
= × + ×
p 3
A L è ph N 0
EQUATION1280 V2 EN-US
If we finally divide equation
present to the IED as

I KN
+

ph

Z p ZI
= ⋅
L



EQUATION1379 V3 EN-US
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
A
OPEN
CLOSED
21
ANSI05000222 V1 EN-US
Figure 127: The parallel line is out of service and grounded
ö
I
×
3 p
K 0 ÷
+ ×
Nm ø
-
2 p
122 with equation 117
3 I p
⋅
0
I KN
3
⋅ + ⋅
0 m
2 p
−
I 3 I KN
+ ⋅
ph 0
B
OPEN
Z0m
CLOSED
21
en05000222_ansi.vsd
Section 8
Impedance protection
we can draw the impedance






(Equation 122)
(Equation 123)
M17048-479 v7 M17048-477 v3
269