ABB RET670 Applications Manual page 172

Section 3
IED application
166
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 47.
Table 47: Definition of short and very short line
Line category Vn
110 kV
Very short line 0.75 -3.5mile
Short line 4-7 miles
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-ground fault together with load encroachment algorithm improves the possibility to
detect high resistive faults without conflict with the load impedance, see figure 49.
For very short line applications, the underreaching zone 1 can not be used due to the
voltage drop distribution throughout the line will be too low causing risk for
overreaching. It is difficult, if not impossible, to apply distance protection for short
lines. It is possible to apply an overreaching pilot communication based POTT or
Blocking scheme protection for such lines to have fast tripping along the entire line.
Usually a unit protection, based on comparison of currents at the ends of the lines is
applied for such lines.
Load encroachment is normally no problem for short line applications.
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-ground 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 48, long lines have Source impedance
ratio (SIR's) less than 0.5.
Table 48: Definition of long and very long lines
Line category Vn
110 kV
Long lines 45-60 miles
Very long lines >60 miles
Vn
500 kV
3-15 miles
15-30 miles
Vn
500 kV
200-250 miles
>250 miles
1MRK504116-UUS C
Application manual