ADE7754 datasheet(24/44 Pages) AD

24 / 44 page

REV. PrG 01/03

PRELIMINARY TECHNICAL DATA

ADE7754

– 24 –

No Load Threshold

The ADE7754 includes a selectable “no load threshold” or

“start up current” feature that will eliminate any creep effects

in the active energy measurement of the meter. When

enabled, this function is independently applied on each

phase’s active power calculation. This mode is selected by

default and can be disabled by setting to logic one bit3 of the

GAIN register (Address 18h) - see Table X. Any load

generating an active power amplitude lower than the mini-

mum amplitude specified, will not be taken into account

when accumulating the active power from this phase.

The minimum instantaneous active power allowed in this

mode is 0.005% of the full scale amplitude. As the maximum

active power value is 13,743,895d with full scale analog

input, the no-load threshold is 687d. For example, an energy

meter with maximum inputs of 220V and 40A and Ib=10A,

the maximum instantaneous active power is 3,435,974d

assuming that both inputs represent half of the analog input

full scale . As the no-load threshold represents 687d, the start

up current represents 8mA or 0.08% of Ib.

Mode selection of the sum of the three active energies

The ADE7754 can be configured to execute the arithmetic

sum of the three active energies, Wh = WhφA + WhφΒ + WhφC,

or the sum of the absolute value of these energies, Wh =

modes can be made by setting bit2 of the GAIN register

(Address 18h) - see Table X. Logic high and logic low of this

bit correspond respectively to the sum of absolute values and

the arithmetic sum. This selection affects the active energy

accumulation in the AENERGY, RAENERGY, LAENERGY

registers as well as for the CF frequency output.

When the sum of the absolute values is selected, the active

energy from each phase is always counted positive in the total

active energy. It is particularly useful in 3-phase 4-wire

installation where the sign of the active power should always

be the same. If the meter is misconnected to the power lines

i.e. CT connected in the wrong direction, the total active

energy recorded without this solution can be reduced by two

third. The sum of the absolute values assures that the active

energy recorded represents the actual active energy delivered.

In this mode, the Reverse Power information available in the

CFNUM register is still detecting when negative active

power is present on any of the three phase inputs.

LINE ENERGY ACCUMULATION

The ADE7754 is designed with a special energy accumula-

tion mode which simplifies the calibration process. By using

the on-chip zero-crossing detection, the ADE7754 accumu-

lates the Active Power signal in the LAENERGY register for

an integer number of half cycles, as shown in Figure 29. The

line active energy accumulation mode is always active.

Important: It is recommended to use this mode with only

one phase selected. If several phases are selected, the amount

accumulated can be smaller than it is supposed to be.

Each one of three phases zero-crossing detection can contrib-

ute to the accumulation of the half line cycles. Phase A, B and

C zero crossings are respectively taken into account when

counting the number of half line cycles by setting to logic one

bits 4-6 of the MMODE register. Selecting phases for the

Zero crossing counting has also the effect of enabling the

Zero-crossing detection, Zero-crossing Time-Out and Pe-

riod Measurement for the corresponding phase as described

in the Zero-crossing Detection paragraph.

The number of half line cycles is specified in the LINCYC

ADE7754 can accumulate Active Power for up to 65535

combined half cycles. Because the Active Power is integrated

on an integer number of line cycles, the sinusoidal compo-

nent is reduced to zero. This eliminates any ripple in the

energy calculation. Energy is calculated more accurately

because of this precise timing control. At the end of an energy

calibration cycle the LINCYC flag in the Interrupt Status

LPF1

FROM VA

ADC

ZEROCROSS

DETECT

CALIBRATION

CONTROL

LINCYC[15:0]

Power Phase A

Power Phase B

Power Phase C

LPF1

FROM VB

ADC

ZEROCROSS

DETECT

LPF1

FROM VC

ADC

ZEROCROSS

DETECT

ACCUMULATE ACTIVE POWER DURING

LINCYC ZERO-CROSSINGS

LAENERGY[23:0]

WDIV

MMODE register

bit 6

MMODE register

bit 4

MMODE register

bit 5

Figure 29 - ADE7754 Active Energy Calibration

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