P74x/EN OP/N
(OP) 5-
FIGURE 4: SENSITIVE EARTH FAULT BLOCKING CHARACTERISTIC
For an external phase fault the SDEF protection will be disabled by the blocking command
as long as the 'IbiasPh>Cur.' threshold remains exceeded.
It can be seen that for an internal phase to phase fault the bias current will be sufficient to
enable the SDEF blocking order. The SDEF protection is then blocked and no trip issued
from this element irrespective of SDEF setting thresholds being exceeded. As the main
phase differential protection is always active, it is able to react to the fault and issue a trip
command accordingly.
For an external phase to phase fault the SDEF will be disabled via the blocking order.
The sensitive differential earth fault protection is delayed by 20ms to prevent any
maloperation during CT saturation condition.
Important note: SDEF should only be used when all the CTs are of the same standard
1.2.4 Threshold coherency.
The measuring elements have several level detectors for differential current. Upon starting,
the protection reacts to any setting inconsistency in the detection of these levels' specific
order.
The differential protection is blocked until the thresholds [I
D
>1and I
D
>2] and [I
D
N>1and
I
D
N>2] are set in the correct sequence.
The thresholds must be set so that:
(I
D
>1) (I
D
>2) , (I
D
>1) (I
DCZ
>2) and (I
DN
>1) (I
DN
>2), (I
DN
>1) (I
DNCZ
>2)
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