ABB RET670 Applications Manual page 296

Section 3
IED application
290
Long transmission line application
For long transmission lines the margin to the load impedance that is, to avoid load
encroachment, will normally be a major concern. It is difficult to achieve high
sensitivity for phase-to-ground fault at remote end of a long lines when the line is
heavy loaded.
The definition of long lines with respect to the performance of distance protection is
noted in table 63.
Table 63: Definition of long lines
Line category Vn
110 kV
Long lines 45-60 miles
Very long lines >60 miles
As mentioned in the previous chapter, the possibility in IED to set resistive and
reactive reach independent for positive and zero sequence fault loops and individual
fault resistance settings for phase-to-phase and phase-to-ground fault together with
load encroachment algorithm improves the possibility to detect high resistive faults at
the same time as the security is improved (risk for unwanted trip due to load
encroachment is eliminated).
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 parallel 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 (< 1-2%) of the self
impedance and it is practice to neglect them.
From an application point of view there exists three types of network configurations
(classes) that must be considered when making the settings for the protection function.
Those are:
1. Parallel line with common positive and zero sequence network
2. Parallel circuits with common positive but isolated zero-sequence network
3. Parallel circuits with positive and zero sequence sources isolated.
Vn
500 kV
200–250 miles
>250 miles
1MRK504116-UUS C
Application manual