Toshiba GR200 Series Instruction Manual page 50

Blind zone protection
2.1.7
Protection in blind zone
(i)
In bus-coupler and bus-section bays, it is normal to arrange two CTs, one on either side of the
breaker, so that the discriminating zone protections overlap.
Figure 2.1-13 Normal CT arrangement in Bus-coupler or Bus-section
If CTs are arranged on one side of the breaker, or one CT is used for both zones, the
protection zones do not overlap; so the zone between the CTs and the breaker remains as a
blind zone. One of the discriminating zone protections (DIFZA in the example shown below)
operates for the fault in the blind zone, but the other (DIFZB) does not; the fault is not cleared.
I1
I2
Figure 2.1-14 CT arrangement leaving blind zone
Differential current(Id) of DIFZA:
Differential current(Id) of DIFZB:
The DIF function provides a countermeasure for blind zone faults. It controls the current
of the bus-coupler/section to zero ampere in the discriminating zone protection after the
breaker is tripped, which ensures that the protection of the other zone operates. This counter
measure is called "Zero ampere control function".
Since Zero ampere control function is controlled by the position of the circuit breaker, it
is necessary to connect an auxiliary contact to the GRB200 relay. If an auxiliary contact is not
connected, the function is disabled.
(ii)
Protection in operating busbar with coupler/section CB open
If the busbar is operated with a bus-coupler/section CB open, as shown in Figure 2.1-15, DIFZA
Bus-A
Coupler
Ic2
CB
Bus-A
Ic2
DIFZA
I -ZA = | I1+I2+Ic1 | ≠ 0
d
-ZB = | I3+I4+Ic2 | ≈ 0
I
d
-ZB = | I3+I4+Ic2(*) | ≠ 0
I
d
(*)Ie2 controlled to 0A by CB open
- 36 -
Bus-B
Ic1
Bus-B
I3
Ic1
I4
DIFZB

DIFZA operates

DIFZB does not operate
DIFZB operates
Centralized GRB200
(Soft: 030, 031, 032, 033, 034)
6F2S1931 (0.20)
(2.1-10)
(2.1-11)
(2.1-12)