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It can be shown from analytical calculations of line impedances that the mutual impedances
for positive and negative sequence are very small and it is a practice to neglect them.
Mutual coupling effect
The mutual coupling is based on the known induction law, that a current induces a longitudinal
voltage in the parallel circuit.
The induced voltage is:
M
U
Z
I
=
×
IECEQUATION14002 V1 EN-US
(Equation 177)
E
Z
M
I
U
IEC13000253-1-en.vsd
IEC13000253 V1 EN-US
Figure 141: Mutual coupling
Mutual induction on three-phase transmission lines
In case of three phase lines, mutual coupling in the positive and negative sequence
components is relatively weak and can be neglected. These are of the order of 5% of the
related self-impedance for non-transposed and lower than 3% for transposed lines. The zero-
sequence currents are equal and in phase in all the three conductors of the three-phase line. In
this case, the three-phase conductors can be replaced by a representative single conductor for
each parallel line and the mutual coupling between lines and their earth return wires is then
reduced to a single-phase problem.
E
I
U
IEC13000254-1-en.vsd
O
I
O
I
O
O
O
IEC13000254 V1 EN-US
Figure 142: Mutual coupling of two three-phase lines
0
0
0
3
M
m
Z
Z
U
I
=
= ×
IECEQUATION14003 V1 EN-US
(Equation 178)
Where Z
M
is the mutual impedance between two conductors with earth return as defined
above. This zero sequence mutual impedance can be as high as 70% of the self-zero sequence
impedance of a protected line when the parallel lines are mounted on the same tower. The
mutual coupling effect has therefore a strong impact on the earth-fault relaying.
Section 7
1MRK 505 343-UEN B
Impedance protection
226
Application manual
Содержание Relion 670 series
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