Toshiba GRL100-701B Instruction Manual page 137

Mho element
The characteristic of the mho element is obtained by comparing the phases between signals S1 and
S2. If the angle between these signals is 90° or more, it means that the fault is within the mho
characteristic, and the mho element will operate.
S1 = V − IZs
S2 = Vp
where,
V = fault voltage
I = fault current
Zs = zone reach setting
Vp = polarizing voltage
Figure 2.16.3.5 is a voltage diagram, which shows that the mho characteristic is obtained by the
phase comparison if V and Vp are in-phase.
The mho characteristic on the impedance plane is obtained by dividing the voltage in Figure
2.16.3.5 by current I.
Both the phase fault mho element and earth fault mho element of the GRL100 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
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 dual-polarization improves the directional security when applied to heavily loaded lines or
weak infeed terminals.
X
S1 = V − IZs
IZs
V
Figure 2.16.3.5 Mho Element
 136 
6 F 2 S 0 8 5 0
S2 = Vp
R