Toshiba GRL100-701B Instruction Manual page 142

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
V pbc = 3 (V a − V 0 ) ∠ − 90° + V bc
For an A-phase earth fault element
V pa = 3 (V a − V 0 ) + V bc ∠90°
where,
V a = A-phase voltage
V 0 = zero-sequence voltage
V bc = B-to-C-phase voltage
The polarizing voltage for the phase fault element has a memory action for the close-up
three-phase fault. V a and V bc 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.
Offset directional element
The offset directional element is used only in Z4 for phase faults in the quadrilateral four zone
characteristics.
X
Figure 2.16.3.11 Quadrilateral characteristic
 141 
Reactance
Blinder
R
Directional
6 F 2 S 0 8 5 0