ADE7762_15 datasheet(12/28 Pages) AD

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ADE7762

Rev. 0 | Page 12 of 28

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.

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

()

⎟

⎠

⎞

⎜

⎝

⎛ ×

cos

(2)

This is the correct active power calculation.

TIME

IAP

IAN

VAP

HPF

LPF

IBP

IBN

VBP

ICP

ICN

VCP

INSTANTANEOUS

ACTIVE POWER SIGNAL

INSTANTANEOUS

POWER SIGNAL - p(t)

VA × IA + VB × IB +

VC × IC

ABS

|X|

LPF

|X|

p(t) = i(t) × v(t)

WHERE:

{1+ cos (2ωt)}

v(t) = V × cos (ωt)

i(t) = I × cos (ωt)

p(t) = V × I

V× I

V × I

MULTIPLIER

HPF

ADC

INSTANTANEOUS

TOTAL POWER

SIGNAL

DIGITAL-TO-FREQUENCY

Figure 12. Signal Processing Block Diagram

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