8.2.2.5 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 well known that it is difficult to
achieve high sensitivity for phase-to-earth fault at remote line end of long lines when
the line is heavy loaded.
What can be recognized as long lines with respect to the performance of distance
protection can generally be described as in table
26. Long lines have Source
impedance ratio (SIR’s) less than 0.5.
Table 26: Definition of long and very long lines
Line category
Un Un
110 kV 500 kV
Long lines 77 km - 99 km 350 km - 450 km
Very long lines > 99 km > 450 km
The IED's ability 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-earth 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), see figure 83.
8.2.2.6 Parallel line application with mutual coupling
General
Introduction of parallel lines in the network is increasing due to difficulties to get
necessary land to build new lines.
Parallel lines introduce an error in the measurement due to the mutual coupling
between the parallel lines. The lines need not be of the same voltage level in order to
experience mutual coupling, and some coupling exists even for lines that are separated
by 100 meters or more. The mutual coupling does influence the zero sequence
impedance to the fault point but it does not normally cause voltage inversion.
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 a common 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.
The different network configuration classes 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.
Section 8 1MRK 502 071-UEN -
Impedance protection
194 Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual
To Next Page
Loading...
Need help?
General
