ABB REO 517 Applications Manual page 69

3 zone impedance protection (Z(n)RW)
In figure 31, the measuring characteristics for two impedance measuring zones (phase-
to-earth fault) in the forward direction are shown. Here the earth impedance measuring
loop, Z , consists of phase impedance Z
loop
sistance R .
F
)LJXUH  &KDUDFWHULVWLFV IRU SKDVH WR HDUWK LPSHGDQFH PHDVXULQJ ORRS ]RQH  DQG

The earth return impedance follows for each particular zone the expression:
1
⋅ ( )
-- -
Z = z – Z
N 0 1
2
with
, the earth return impedance Z
1
jX
X
2
R
F
Z
N
X
Z
1
loop
Z
1
RFPE
&KDSWHU 
/LQH LPSHGDQFH
and the re-
N
R
RFPE
1
2
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