Function blocking guidelines
The operation of the underimpedance protection must be restrained if the voltage in one
or more phases suddenly drops close to zero without any significant change in the current
observed at the same time. This situation is considered as a fuse failure or a miniature
circuit breaker operation in the voltage transformer secondary circuit. The voltage drop
could cause an unwanted operation of the function block since the calculated impedance
could fall below the set operating limit even if there is no actual fault in the primary
system.
The blocking operation is provided by an external function block, the fuse failure
supervision 60, whose output is connected to the BLOCK input of 21G.
4.7.3.6 Signals
Table 614: 21G Input signals
Name Type Default Description
I_A SIGNAL 0 Phase A current
I_B SIGNAL 0 Phase B current
I_C SIGNAL 0 Phase C current
I
1
SIGNAL 0 Positive sequence current
V_A_AB SIGNAL 0 Phase-to-ground voltage A or phase-to-phase voltage
AB
V_B_BC SIGNAL 0 Phase-to-ground voltage B or phase-to-phase voltage
BC
V_C_CA SIGNAL 0 Phase-to-ground voltage C or phase-to-phase voltage
CA
V
1
SIGNAL 0 Positive phase sequence voltage
BLOCK BOOLEAN 0=False Block signal for activating the blocking mode
Table 615: 21G Output signals
Name
Type Description
TRIP BOOLEAN Trip
PICKUP BOOLEAN Pickup
4.7.3.7 Settings
Table 616: 21G Group settings (Basic)
Parameter
Values (Range) Unit Step Default Description
Percentage reach 1...6000 %Zn 1 7 Radius of the origin centric circle
Trip delay time 40...200000 ms 10 200 Trip delay time
1MAC059074-MB A Section 4
Protection functions
615 series ANSI 715
Technical Manual
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