GE MiCOM P40 Agile, Technical Manual, Page: 188 / 962

6.8 LOAD BLINDING SETUP
We strongly recommend enabling the load blinder, especially for lines above 150km (90miles) and for any networks
where power swings might be experienced. This will prevent non-harmonic low-frequency transients causing load
encroachment problems.
The impedance radius must be set lower than the worst-case loading, and this is often taken as 120% overloading
in one line, multiplied by two to account for increased loading during outages or fault clearance in an adjacent
parallel circuit. Then an additional allowance for measuring tolerances results in a recommended setting typically
between a quarter and one third of the rated full load current:
Z <= (Rated phase voltage Vn)/(I
FLC
x 3)
When the load is at the worst-case power factor, it should remain below the beta (
b
setting. So, if we assume a
typical worst case 0.85 power factor, then:
b
>= Cos
-1
(0.85) + 15° margin >= 47°
and to ensure that line faults are detected:
b
<= (Line Angle -15°).
In practice, an angle half way between the worst-case leading load angle, and the protected line impedance
angle, is often used.
This product has a facility to allow the load blinder to be bypassed any time that the measured voltage for the
phase in question falls below an undervoltage (Load Blinder V<) setting. Under such circumstances, the low
voltage would not be explained by normal voltage excursion tolerances on-load. A fault must be present on the
phase in question, and it is acceptable to override the blinder action and allow the distance zones to trip according
to the entire zone shape. The benefit is that the resistive coverage for faults near to the protection location can be
higher.
The undervoltage setting must be lower than the lowest phase-neutral voltage under heavy load flow and
depressed system voltage conditions. The recommended Load Blinder V< setting is 70% Vn.
6.9 POLARIZING SETUP
You can choose how much memory polarization to mix with self-polarization using the Dist. Polarizing setting.
Some recommendations are:
Cable applications
Use 20% (0.2) memory. This results in minimum Mho expansion and keeps the protected line section well within the
expanded Mho, thereby ensuring better accuracies and faster operating times for close-up faults. This matches
the guidance previously provided for LFZP123 or LFZR applications for cable feeders
Series compensated lines
Use a mho with the maximum memory polarization (setting = 5). The large memory content ensures correct
operation even with the negative reactance effects of the compensation capacitors seen either within the zones,
or within the line impedance.
Short lines
For lines shorter than 10miles (16km), or with an SIR higher than 15, use the maximum memory polarization
(setting = 5). This ensures sufficient characteristic expansion to cover fault arc resistance.
General line applications
Use any setting between 0.2 and 1.
Chapter 7 - Distance Protection P446SV
156 P446SV-TM-EN-1