increase the security but might also lower the dependability since the blinder might cut off
a larger part of the operating area of the circle (see to the right of figure 126).
It is recommended to use at least one of the load discrimination functions for long heavy
loaded transmission lines.
8.4.2.7 Parallel line application with mutual coupling
General
Introduction of parallel lines in the network is increasing due to difficulties to get
necessary area for new lines.
Parallel lines introduce an error in the measurement due to the mutual coupling between
the lines. The lines need not to be of the same voltage in order to experience mutual
coupling, and some coupling exists even for lines that are separated by 100 meters or
more. The reason to the introduced error in measuring due to mutual coupling is the zero
sequence voltage inversion that occurs.
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:
ANSIEQUATION14002 V1 EN (Equation 159)
ANSI13000253 V1 EN
Figure 128: Mutual coupling
Section 8 1MRK 506 369-UUS -
Impedance protection
260 Line distance protection REL670 2.2 ANSI
Application manual
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