ADE7752B datasheet(13/27 Pages) AD

13 / 27 page

Preliminary Technical Data

ADE7752B

Rev. PrA | Page 13 of 27

THEORY OF OPERATION

The six signals from the current and voltage transducers are

digitized with ADCs. These ADCs are 16-bit second-order ∑-Δ

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. In order to extract the active power component, the dc

component, the instantaneous power signal is low-pass filtered

on each phase. Figure 15 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 ADE7752B 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 would 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 16 displays the unity power factor

condition and a displacement power factor (DPF) = 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 sinusoidal, the active power component of the instantaneous

power signal (the dc term) is given by

()

⎟

⎠

⎞

⎜

⎝

⎛ ×

cos

(1)

TIME

IAP

IAN

VAP

HPF

LPF

IBP

IBN

VBP

ICP

ICN

VCP

DIGITAL-TO-

FREQUENCY

DIGITAL-TO-

FREQUENCY

INSTANTANEOUS

ACTIVE POWER SIGNAL

INSTANTANEOUS

POWER SIGNAL - p(t)

INSTANTANEOUS

TOTAL

POWER SIGNAL

VA × IA + VB × IB +

VC × IC

ABS

|X|

LPF

|X|

p(t) = i(t) × v(t)

WHERE:

{1+ cos (2

ωt)}

v(t) = V × cos (

ωt)

i(t) = I × cos (

ωt)

p(t) = V × I

V× I

V × I

MULTIPLIER

HPF

ADC

Figure 15. Signal Processing Block Diagram

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