ABB REL670 2.2 IEC, Applications Manual

Page: 248 / 908

operation of load encroachment function is always activated. To deactivate the
function, setting
LoadEnchMode should be set off or the setting of RLdFw and
RLdRv must be set to a value much higher than the maximal load impedance.
8.4.2.5 Short line application
SEMOD154453-105 v6
The definition of short, medium and long lines is found in IEEE Std C37.113-1999.
The length classification is defined by the ratio of the source impedance at the
protected line’s terminal to the protected line’s impedance (SIR). SIR’s of about 4
or greater generally define a short line. Medium lines are those with SIR’s greater
than 0.5 and less than 4.
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
21 .
Table 21: Definition of short and very short line
Line category Un Un
110 kV 500 kV
Very short line 1.1-5.5 km 5-25 km
Short line 5-11 km 25-50 km
The use of load encroachment algorithm in Full-scheme distance protection, mho
characteristic function (ZMHPDIS) improves the possibility to detect high resistive
faults without conflict with the load impedance (see to the right of figure
124 ).
For very short line applications the underreaching zone 1 can not be used due to
that the voltage drop distribution through out the line will be too low causing risk
for overreaching.
Load encroachment is normally not a problem for short line applications so the
load encroachment function could be switched off meaning
LoadEnchMode = Off .
This will increase the possibility to detect resistive close-in faults.
8.4.2.6 Long transmission line application
SEMOD154453-127 v5
For long transmission lines the 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 end of a long line when the line is heavily loaded.
What can be recognized as long lines with respect to the performance of distance
protection is noted in table
22 .
Section 8
1MRK 506 369-UEN B
Impedance protection
242 Line distance protection REL670 2.2 IEC
Application manual