possibility to detect high resistive faults without conflict with the load impedance, see
figure 97.
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. 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.
8.3.2.5
Long transmission line application
GUID-AD1A1A5B-B4F5-4B48-9DBD-59D28D71BA9C v2
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
28, long lines have Source impedance
ratio (SIR’s) less than 0.5.
Table 28: Definition of long and very long lines
Line category
Vn Vn
110 kV 500 kV
Long lines 45-60 miles 200-250 miles
Very long lines >60 miles >250 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 at the same time as the security is improved
(risk for unwanted trip due to load encroachment is eliminated), see figure 98.
Section 8 1MRK 502 071-UUS A
Impedance protection
234 Generator protection REG670 2.2 ANSI and Injection equipment REX060, REX061, REX062
Application manual
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