Scheme Earth Characteristic Element - GE P741 Technical Manual

P74x/EN OP/Na7
(OP) 5-8
The check zone is the sum of all the current nodes entering and leaving the substation
(feeders).
Scheme differential current = sum of all differential current feeder nodes:
(t) CZ = Σ i
i
diff
The Check Zone will operate as the Zone element.
1.2.3

Scheme Earth characteristic element

The Earth characteristic is determined from the following protection settings:
I N>2 High-set zone differential current threshold setting which crosses the set slope of the
D
bias characteristic (k × I
I NCZ>2
D
set slope of the bias characteristic (kCZ × I
KN2 Percentage bias setting ("slope") for the zone
kNCZ Percentage bias setting ("slope") for the check zone
The current control and blocking matrix is shown in figure 3
OP
This element is automatically enabled/disabled via the load (flowing) current. The point at
which the sensitive earth fault protection is enabled/disabled (IbiasPh>Cur.) is settable in the
range. This threshold is usually set to be equal to the minimum phase to phase short circuit
current.
Under earth fault conditions the risk of CT saturation is minimal and therefore the slope of
the characteristic can be set low, however, should the fault evolve to a phase fault, it is
important that the normal characteristic be restored.

diff
)
bias
High-set check zone differential current threshold setting which crosses the
FIGURE 3: SENSITIVE EARTH FAULT CHARACTERISTIC
)
bias
Operation
MiCOM P741, P742, P743