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Section 4
1MAC050144-MB C
Protection functions
154
615 series ANSI
Technical Manual
4.1.4.2
Function block
Figure 68:
Function block
4.1.4.3
Functionality
The ground-fault function 51N/50N or 51G/50G is used as non-directional ground-fault
protection for feeders.
The function picks up and trips when the measured (IG) or calculated (IN) ground current
exceeds the set limit. The trip time characteristic for low stage 51N/G and high stage
50N/G-1/2 can be selected to be either definite time (DT) or inverse definite minimum
time (IDMT). The instantaneous stage 50N/G-3 always trips with the DT characteristic.
In the DT mode, the function trips after a predefined trip time and resets when the fault
current disappears. The IDMT mode provides current-dependent timer characteristics.
The function contains a blocking functionality. It is possible to block function outputs,
timers or the function itself, if desired.
4.1.4.4
Operation principle
The function can be enabled and disabled with the
Operation
setting. The corresponding
parameter values are Enable and Disable.
The operation of non-directional ground-fault protection can be described by using a
module diagram. All the blocks in the diagram are explained in the next sections.
Figure 69:
Functional module diagram. I
0
represents the residual current.
Level detector
The measured ground current is compared with the set
Pickup value
. If the measured value
exceeds the set
Pickup value
, the level detector sends an enable-signal to the timer module.
If the
ENA_MULT
input is active, the
Pickup value
setting is multiplied by the
Pickup value
Mult
setting.
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