7.9.2.5
Long transmission line application
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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 a long line 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
, long lines have Source impedance ratio (SIR’s) less
than 0.5.
Table 26: Typical length 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
.
7.9.2.6
Parallel line application with mutual coupling
M17048-417 v2
General
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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 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 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
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.
One example of class 3 networks could be the mutual coupling between a 400kV line and rail
road overhead lines. This type of mutual coupling is not so common although it exists and is
not treated any further in this manual.
For each type of network class, there are three different topologies; the parallel line can be in
service, out of service, out of service and earthed in both ends.
Section 7
1MRK 505 343-UEN B
Impedance protection
272
Application manual
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