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Section 8
Impedance protection
8.13.2.4
8.13.2.5
364
Short line application
In short line applications, the major concern is to get sufficient fault resistance
coverage. Load encroachment is not so common. 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 30.
Table 30:
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,
see figure 186.
For very short line applications, the underreaching zone 1 cannot be used due to the
voltage drop distribution throughout the line will be too low causing risk for
overreaching.
Load encroachment is normally no problem for short line applications.
Long transmission line application
For long transmission lines, the margin to the load impedance to avoid load
encroachment is a major concern. 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 31, long lines have source
impedance ratio (SIR's) less than 0.5.
Table 31:
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 187.
GUID-7FA1B585-C979-4D51-A502-9C4070B99F0E v2
Un
500 kV
5-25 km
25-50 km
GUID-AD1A1A5B-B4F5-4B48-9DBD-59D28D71BA9C v3
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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