Toshiba GRL100-701B Instruction Manual page 138

The polarizing voltage for the phase fault mho 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
mho element has no memory action.
When a three-phase fault occurs within zone 1, the phase fault mho element for zone 1 is modified
to an offset mho characteristic as shown in Figure 2.16.3.6. 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.
Figure 2.16.3.6 Offset of Z1 in Three-phase Fault
Offset mho element
Three independent offset mho elements are used for Z1 for phase faults, reverse zone ZR2 and Z4
for phase faults.
The characteristics of each offset mho element are obtained by comparing the phases between
signals S1 and S2.
If the angle between these signals is 90° or more, the offset mho element operates.
S1 = V − IZs
S2 = V + IZso
where,
V = fault voltage
I = fault current
Zs = zone reach setting
Zso = offset zone reach setting
Figure 2.16.3.7 is a voltage diagram showing the offset mho characteristics obtained by the phase
comparison between S1 and S2.
The offset mho characteristic on the impedance plane is obtained by dividing the voltage in Figure
2.16.3.7 by current I.
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