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)
=
