ABB REO 517, Applications Manual, Page: 71 / 440

61
3 zone impedance protection (Z(n)RW)
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Conventional earth return compensation is used for phase to earth fault in two phase sol-
idly earthed systems (example for a phase L1 to earh fault):
(Equation 16)
where
(Equation 17)
and I
N
is a phasor of the residual current in the relay point.
The calculation of apparent impedances for phase-to-phase faults follows the equation:
(Equation 18)
U and I is the corresponding voltage and current phasors in the respective phase.
The use of zero sequence compensator factor (K
N
) results in the same reach along the
line for all types of faults.
The apparent impedance is considered as an impedance loop with resistance R and re-
actance X, as presented in figure 32.
Z
ap
U
L1
I
L1
K
N
I
N
⋅
+
------------------------------ =
K
N
1
2
---
Z
0
Z
1
–
Z
1
--------------------
⋅
=
Z
ap
U
L1
U
L2
–
I
L1
I
L2
–
-------------------------
=