ade7762 Analog Devices, Inc., ade7762 Datasheet - Page 12

• Manufacturer Part Number: ade7762
• Description: Polyphase Energy Metering Ic With Phase Drop Indication
• Manufacturer: Analog Devices, Inc.

ADE7762
THEORY OF OPERATION
The six signals from the current and voltage transducers are
digitized with ADCs. These ADCs are 16-bit, second-order
∑-Δ devices with an oversampling rate of 833 kHz. This analog
input structure greatly simplifies transducer interface by
providing a wide dynamic range and bipolar input for direct
connection to the transducer. High-pass filters in the current
channels remove the dc component from the current signals.
This eliminates any inaccuracies in the active power calculation
due to offsets in the voltage or current signals (see the HPF and
Offset Effects section).
The active 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. To extract the active power component, the dc compo-
nent, the instantaneous power signal is low-pass filtered on
each phase. Figure 12 illustrates the instantaneous active power
signal and shows how the active power information can be
extracted by low-pass filtering the instantaneous power signal.
This method is used to extract the active power information
on each phase of the polyphase system. The total active power
information is then obtained by adding the individual phase
active power. This scheme correctly calculates active power
for nonsinusoidal current and voltage waveforms at all power
factors. All signal processing is carried out in the digital domain
for superior stability over temperature and time.
VBP
VCP
VAP
V × I
V × I
IAP
IAN
IBP
IBN
ICP
ICN
VN
2
TIME
ADC
ADC
ADC
ADC
ADC
ADC
p(t) = i(t) × v(t)
WHERE:
POWER SIGNAL - p(t)
v(t) = V × cos (ωt)
i(t) = I × cos (ωt)
p(t) = V × I
INSTANTANEOUS
MULTIPLIER
MULTIPLIER
MULTIPLIER
2
HPF
HPF
HPF
{1+ cos (2ωt)}
Figure 12. Signal Processing Block Diagram
LPF
LPF
LPF
Rev. 0 | Page 12 of 28
V × I
2
ACTIVE POWER SIGNAL
ABS
|X|
|X|
|X|
INSTANTANEOUS
The low frequency output of the ADE7762 is generated by
accumulating the total active power information. This low
frequency inherently means a long accumulation time between
output pulses. The output frequency is therefore proportional to
the average active power. This average active power information
can, in turn, be accumulated (for example, by a counter) to
generate active energy information. Because of its high output
frequency and, therefore, shorter integration time, the CF
output is proportional to the instantaneous active power. This
pulse is useful for system calibration purposes that take place
under steady load conditions.
POWER FACTOR CONSIDERATIONS
Low-pass filtering, the method used to extract the active power
information from the individual instantaneous power signal, is
still valid when the voltage and current signals of each phase are
not in phase. Figure 13 displays the unity power factor condition
and a displacement power factor (DPF) of 0.5, that is, current
signal lagging the voltage by 60° for one phase of the polyphase.
Assuming that the voltage and current waveforms are sinusoi-
dal, the active power component of the instantaneous power
signal (the dc term) is given by
This is the correct active power calculation.
⎛ ×
⎜
⎝
V
2
1
VA × IA + VB × IB +
⎞
⎟
⎠
×
DIGITAL-TO-FREQUENCY
DIGITAL-TO-FREQUENCY
cos
VC × IC
INSTANTANEOUS
TOTAL POWER
SIGNAL
2
(
60
°
)
F1
F2
CF
(2)