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112
Two step definite and inverse time-
delayed residual
overcurrent protection (TEF1 and TEF2)
The polarizing voltage for directional earth-fault overcurrent protection is obtained by
internal calculation (U
N
ics, especially when the protection is connected to CVTs.
Due to the bandpass filtering a polarizing voltage down to 1 percent of the rated voltage
will provide correct directional functionality. This is also valid when the protection is
connected to CVTs.
The minimum polarizing voltage to the protection (Umin) is calculated from the formu-
la:
⋅
U
I
Z
=
min
F min
n
where:
I
is the minimum primary operate fault current
F min
Z
is the minimum zero-sequence impedance seen from the relay
n min
U
, U
are the rated phase voltages of connected CVTs (VTs)
sec
prim
To even secure operation in unfavorable cases, Umin must be equal to at least 2 volt
plus the maximum network frequency false voltage, due to measuring errors in the VT
circuits.
=UL1+UL2). The voltage contains a certain amount of harmon-
U
⋅
sec
-------------- -
min
U
prim
&KDSWHU
&XUUHQW
(Equation 56)
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