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page 8 of http://www.analog.com/UploadedFiles/Datasheets/97400678ADE7752_pri.pdf
THEORY OF OPERATION
The six ADCs digitize the voltage signals from the current
and voltage transducers. These ADCs are 16-bit second order
sigma-delta with an oversampling rate of 833 kHz. This analog
input structure greatly simplifies transducer interface by
providing a wide dynamic range for direct connection to the
transducer and also simplifying the antialiasing filter design.
A high-pass filter in the current channel removes the dc component
from the current signal. This eliminates any inaccuracies in the
real power calculation due to offsets in the voltage or current
signals—see HPF and Offset Effects section.
The real power calculation is derived from the instantaneous
power signal. The instantaneous power signal is generated by a
direct multiplication of the current and voltage signals of each
phase. In order to extract the real power component (i.e., the dc
component), the instantaneous power signal is low-pass filtered
on each phase. Figure 2 illustrates the instantaneous real power
signal and shows how the real power information can be
extracted by low-pass filtering the instantaneous power signal.
This method is used to extract the real power information on each
phase of the polyphase system. The total real power information is
then obtained by adding the individual phase real power. This
scheme correctly calculates real power for non-sinusoidal current
and voltage waveforms at all power factors. All signal processing is
carried out in the digital domain for superior stability over tem-perature and time.
The low frequency output of the ADE7752 is generated by accu-mulating
the total real power information. This low frequency
inherently means a long accumulation time between output
pulses. The output frequency is therefore proportional to the
average real power. This average real power information can,
in turn, be accumulated (e.g., by a counter) to generate real
energy information. Because of its high output frequency and
therefore, shorter integration time, the CF output is propor-tional
to the instantaneous real power. This is useful for
system calibration purposes that would take place under steady
load conditions.
Power Factor Considerations
The method used to extract the real power information from the
individual instantaneous power signal (i.e., by low-pass filtering) is
still valid when the voltage and current signals of each phase are
not in phase. Figure 3 displays the unity power factor condition
and a DPF (Displacement Power Factor) = 0.5, i.e., current signal
lagging the voltage by 60 for one phase of the polyphase. If we assume the voltage and current waveforms are sinusoidal, the real
power component of the instantaneous power signal (i.e., the dc
term) is given by:
(V*I/2)*cos(60)
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