Toshiba GR200 Series Instruction Manual page 245

element for phase-A (OC-A) uses the phase-B-to-phase-C voltage rotated by 90° (V
the leading direction, this reference signal is illustrated in Figure 2.15-2. The direction of the
current in the protected circuit, in this case the phase-A current (I
angle measured for I
A
is so that the reference voltage (V
because the directional-sensing element requires a reference quantity that is reasonably
constant against which the current in the protected circuit can be compared. As a result, when
a fault occurs, the fault current I
element (OC-B) where the object current is (I
phase-C directional element (OC-C) uses the same approach as the OC-A and OC-B elements.
Table 2.15-2 shows the relationship between each current and its respective reference voltage.
Figure 2.15-2 Object current (I
Table 2.15-2 Directional polarizing signal
Directional element
OC-A
OC-B
OC-C
For a close-in three-phase fault, the three reference voltages V
V 90° may fall below the minimum threshold voltage for the directional OC element. In
AB
order to cover this case, a voltage memory function is used to provide a polarizing signal. The
polarizing signals are provided during three-phase faults so that the OC element is able to
determine the direction of the fault. In addition, the directional element function determines
its output based on the loss of voltage memory and is effective while the fault current flows in
accordance with Figure 2.15-3.
against V 90°. The purpose of using V
BC
90°) takes the same direction as the phase-A voltage (V
BC
will lag V
A
V
BC
) and reference voltage (V
A
Object current
I
A
I
B
I
C
- 222 -
BC
90°. Similarly, for the phase-B directional OC
BC
) the reference voltage (V
B
Leading 90°
V
BC
Reference voltage
90
V
BC
V 90
CA
90
V
AB
6F2S1914 (0.49)
90°) in
BC
), is determined from the
A
in the 90° leading position
90°) is adopted. The
CA
90°)
BC
90°, V 90° and
BC CA
GRL200 (Soft 033 & 037)
)
A