141
6
F
2
S
0
8
5
0
The characteristic of the directional element is obtained by the following equation.
I
・
Vp cos (
θ
−
φ
)
≥
0
where,
I = fault current
Vp = polarizing voltage
φ
= lagging angle of I to Vp
θ
= directional angle setting
The
polarizing voltage Vp is the same one as employed in the mho element.
For B-to-C-phase phase fault element
Vpbc = 3 (Va
−
V0)
∠
−
90
°
+ Vbc
For an A-phase earth fault element
Vpa = 3 (Va
−
V0) + Vbc
∠
90
°
where,
Va = A-phase voltage
V0 = zero-sequence voltage
Vbc = B-to-C-phase voltage
The polarizing voltage for the phase fault element has a memory action for the close-up
three-phase fault. Va and Vbc mentioned above are the memorized pre-fault voltages. This
memory is retained for two cycles after a fault occurs. The polarizing voltage for the earth fault
element has no memory action.
When a three-phase fault occurs within zone 1, the phase fault element for zone 1 is modified to an
offset characteristic as shown in Figure 2.16.3.11. This, together with voltage memory action,
enables zone 1 to perform tripping with a time delay as well as instantaneous tripping for the
close-up three-phase fault.
The Z2 and Z3 do not have the modifying function mentioned above.
X
R
Reactance
Blinder
Directional
Figure 2.16.3.11 Quadrilateral characteristic
Offset directional element
The offset directional element is used only in Z4 for phase faults in the quadrilateral four zone
characteristics.
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Summary of Contents for GRL100-701B
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