where:
● Vn = Nominal phase to neutral voltage
● I = Fault current
● Z = Reach setting for the zone concerned
Sub-cycle tripping is maintained for lower SIRs, up to a ratio of 2. The instantaneous operating time is increased by
about a quarter of a power frequency cycle at higher SIRs.
Transients caused by voltage dips, however severe, do not affect the protection’s directional measurement
because it uses voltage memory.
4.8
LOAD BLINDING
Load blinders are provided for both phase and earth fault distance elements, to prevent incorrect-tripping for
heavy load flow. A blinder envelope which surrounds the expected worst case load limits should be configured to
block tripping for any impedance measured in the blinded region. Only a fault impedance which is outside the area
bounded by the load blinders is allowed to cause a trip. The blinder characteristics are shown in the following
figure:
V00645
Blind area
Operate area
Blind area
Operate area
Z
β
jX
R
Figure 79: Load Blinder Characteristics
● Z denotes the Load/B Impedance setting. This sets the radius of the under-impedance circle.
● ß denotes the Load/B Angle setting. This sets the angle of the two blinder boundary lines - the gradient of
the rise or fall with respect to the resistive axis.
The protection can allow the load blinder to be bypassed any time that the measured voltage for the phase in
question falls below an undervoltage setting. Under such circumstances, the low voltage could not be attributed to
normal voltage excursion tolerances on load. A fault must be present on the phase in question, so it is acceptable
to override the blinder action and allow the Distance protection to trip for an in-zone measurement. The
advantage of bypassing the load blinders is that the resistive coverage for faults near to the protection location
can be higher.
Chapter 7 - Distance Protection P543i/P545i
174 P54x1i-TM-EN-1
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