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Section 8
Impedance protection
8.12.2.4
8.12.2.5
338
Short line application
In short line applications, the major concern is to get sufficient fault resistance
coverage. Load encroachment is not such a common problem. The line length that
can be recognized as a short line is not a fixed length; it depends on system
parameters such as voltage and source impedance, see table 28.
Table 28:
Definition of short and very short line
Line category
Un
110 kV
Very short line
1.1-5.5 km
Short line
5.5-11 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 without conflict with the load impedance.
For very short line applications, the underreaching zone 1 can not be used due to
the fact that the voltage drop distribution throughout the line will be too low
causing risk for overreaching.
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 29. Long lines have Source
impedance ratio (SIR's) less than 0.5.
Table 29:
Definition of long and very long lines
Line category
Un
110 kV
Long lines
77 km - 99 km
Very long lines
> 99 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 172.
GUID-331451C9-EA93-481A-BA2E-D729EDB98828 v4
Un
500 kV
5-25 km
25-50 km
GUID-0C99A197-06CD-4668-A017-3CA9E5E01ED2 v4
Un
500 kV
350 km - 450 km
> 450 km
Line distance protection REL670 2.2 IEC
1MRK 506 369-UEN B
Application manual
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