4 Protection Functions and Specifications
D02706R02.30
L-PRO 4000 User Manual
4-5
Figure 4.3: Mho Characteristic Shapes
Load
Encroachment
(LE)
The load impedance may enter into the protection zones permanently or tem-
porarily due to system condition. This is observed in very long lines or heavily
loaded medium lines will have issue. This is a normal load conditions and it is
not an abnormal/fault conditions. Therefore, the relay should not initiate any
trip command during this condition. The relay should identify properly wheth-
er the impedance entered into the protection zone is normal load condition or
fault condition.
This is distinguished by monitoring all phase-to-phase impedance values (i.e.
Zab, Zbc & Zca).If all the three phase impedances enter into the protection
zones with the limited load angle area, it is declared as a loading condition and
21P function is blocked. If only one of the phase-to-phase impedance enters
into the protection zones with the limited load angle area then this can be de-
clared as fault condition.
Generally, the three phase fault will not have any additional resistance compo-
nent other than the conductor resistance. Therefore, three phase fault will al-
ways have the fault angle as close to line angle only and it will not enter into
the loading area.
Figure 4.4: Load Encroachment (LE) Logic on page 4-5 shows how the LE
function works. Phase-to-phase current monitoring has been added to the logic
to ensure stable operation.
Figure 4.4: Load Encroachment (LE) Logic
Line angle
Circle (90°)
Line angle
Tomato (<90°)
Line angle
Lens (>90°)
Summary of Contents for L-PRO 4000
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