Toshiba GRZ100 B Series, Instruction Manual

Page: 104 / 447

⎯
103
⎯
6 F 2 S 0 8 4 6
Both the phase fault mho element and earth fault mho element of the GRZ100 employ a dual
polarization (self-polarization plus cross-polarization). Its polarizing voltage Vp is expressed by
the following equations.
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 dual-polarization improves the directional security when applied to heavily loaded lines or
weak infeed terminals.
The polarizing voltage for the phase fault mho 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
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 2.5.1.6. 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 Z1X, Z2, ZF and Z3 do not have the modifying function mentioned above.
Figure 2.5.1.6 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
R
X