Toshiba GR200 Series Instruction Manual page 51

where,
Va = Voltage in phase-a
V 0 = Voltage in zero-sequence
Vbc = Voltage in phase-ab
The dual-polarization can improve the directional security when applied to heavily loaded
lines or weak infeed terminals. If a close-in fault in three-phase occurs, the polarizing voltage
is memorized in the relay (i.e., memory action). That is, voltage information (Va and Vbc) just
before the occurrence of the fault is memorized; the memory action will be held for two-cycles
after the fault.
Figure 2.1-5 Offset action in Mho characters for a three-phases fault
Figure 2.1-5 shows that an offset Mho characteristic is applied when the relay detects a
fault in three-phase. That is, the ZS element having the memory action is able to issue a trip
signal with a time delay; the ZS element is able to issue an instantaneous tripping for the
close-in fault in three-phase.
Note that the memory action is not performed in the ZG element; the offset action is only
available for the ZS elements.
Element equations for offset Mho characteristic
(ii)
The element in offset Mho characteristic is obtained by comparing the angle between the S1
and S2 phases, which are denoted in Equation (2.1-19) and (2.1-20). If the angle is greater than
90°, the element in the offset Mho characteristic detects a fault.
where
V = fault voltage
X
o
S1 = V − I × Z
s
S2=V+IZ
SO
- 30 -
Mho element
R
Offset Mho element
(2.1-19)
(2.1-20)
GRZ200 (Soft 031 & 032)
6F2S1915 (0.46)
S2=Vp
IZs