ABB REC 561, User Manual

Page: 529 / 643

ABB Network Partner AB
Direct current measuring unit
Version 1.0-00
1MRK 580 154-XEN
Page 7 - 10
Fig. 4 Block diagram of measuring channel
The digital information is filtered by the digital low-pass filter with the
(sinx/x)
3
response. The filter notch frequency automatically follows the
selected sampling frequency. The relation between the frequency corre-
sponding to the suppression of -3 dB and the filter notch frequency corre-
sponds to the equation:
Using optocouplers and DC/DC conversion elements that are used sepa-
rately for each measuring channel, the input circuirity of each measuring
channel is galvanically separated from:
• The internal measuring circuits
• The control microprocessor on the board
A microprocessor collects the digitized information from each measuring
channel. The microprocessor serves as a communication interface to the
main processing module (MPM).
All processing of the measured signal is performed on the module so that
only the minimum amount of information is necessary to be transmitted to
and from the MPM. The measuring module receives information from the
MPM on setting and the command parameters; it reports the measured
values and additional information—according to needs and values of dif-
ferent parameters.
Each measuring channel is calibrated very accurately during the produc-
tion process. The continuous internal zero offset and full-scale calibration
during the normal operation is performed by the A/D converter. The cali-
bration covers almost all analogue parts of the A/D conversion, but
neglects the shunt resistance.
Each measuring channel has built in a zero-value supervision, which
greatly rejects the noise generated by the measuring transducers and other
external equipment. The value of the measured input current is reported
equal to zero (0) if the measured primary quantity does not 0.5%
of the maximum measuring range.
DC
DC
A
D
U
Udc
I
(x80154-4)
f
3dB –
0 262
,
f
notch
⋅
=