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3 zone impedance protection (Z(n)RW)
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):
U
L1
----------------------------- -
Z
=
ap
⋅
I
K
I
+
L1
N
N
where
Z
Z
1
–
⋅
0
1
K
-- -
------------------- -
=
N
2
Z
1
and I
is a phasor of the residual current in the relay point.
N
The calculation of apparent impedances for phase-to-phase faults follows the equation:
U
U
–
L1
L2
-- ---------------------- -
Z
=
ap
I
–
I
L1
L2
U and I is the corresponding voltage and current phasors in the respective phase.
The use of zero sequence compensator factor (K
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.
/LQH LPSHGDQFH
) results in the same reach along the
N
&KDSWHU
(Equation 16)
(Equation 17)
(Equation 18)
61
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