16 SIPROTEC 7SJ81/7SK81 V4.70, Product Information
E50417-K1150-C462-A2, Edition 02.2012
Sensitivity for the Ground Current Measurement
If you want to measure very small ground currents, you have to use an
appropriate low-power current transformer with the corresponding ratio.
The measuring signal applying at the device should not become smaller
than 2 mV. With a ground current of only 0.1 A, a low-power current
transformer with 10 A/225 mV for example ensures that a sufficiently large
signal of 2.25 mV is generated for the measurement. If you want to use a
sensitive low-power current transformer, you have to communicate this to
the protection device by setting the parameter 132 Earth CT corre-
spondingly.
The use of a sensitive low-power current transformer requires the value of
the characteristic quantity Vrated, ground to range between 2250 mV and
225 000 mV.
You can specify Vrated, ground by setting the 3 parameters 312 Ignd-
PRIM ref., 313 Ignd-LPCT PRIM, and 315 Ugnd-LPCT SEC.
We obtain the value for Vrated, ground from the following equation:
Value range of Vrated, ground Setting for Earth CT
45 mV ≤ Vrated, ground ≤ 2250 mV Normal
2250 mV < Vrated, ground ≤ 225 000 mV Sensitive
Vrated, ground < 45 mV
or
Vrated, ground > 225 000 mV
The acquisition of the meas-
ured value is not possible.
The device is blocked and the
indication 233 CT gnd
mismatch is output.
Vrated ground,
Ignd-PRIM ref. Ugnd-LPCT SEC•
Ignd-LPCT PRIM
----------------------------------------------------------------------------------------------------- (in mV)=
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