EVAL-ADE7858EBZ datasheet(37/76 Pages) AD

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Preliminary Technical Data

ADE7858

Rev. PrA | Page 37 of 76

line cycles, as shown in Figure 44. The number of half line

cycles is specified in the LINECYC[15:0] register.



AWATTOS



ACCUMULATOR

WTHR[47:0]

32 bit register

AWATTHR[31:0]

OUTPUT

FROM LPF

ZERO CROSSING

DETECTION

(PHASE B)

ZERO CROSSING

DETECTION

(PHASE C)

ZERO CROSSING

DETECTION

(PHASE A)

ZXSEL[1] in

LCYCMODE[7:0]

ZXSEL[0] in

LCYCMODE[7:0]

ZXSEL[2] in

LCYCMODE[7:0]

CALIBRATION

CONTROL

LINECYC[15:0]

AWGAIN



Figure 44. ADE7858 Line Cycle Active Energy Accumulation Mode

The line cycle energy accumulation mode is activated by setting

bit 0 (LWATT) in the LCYCMODE[7:0] register. The energy

accumulation over an integer number of half line cycles is

written to the watt-hr accumulation registers after

LINECYC[15:0] number of half line cycles are detected. When

using the line cycle accumulation mode, the bit 6 (RSTREAD) of

the LCYCMODE[7:0] register should be set to Logic 0 because

the read with reset of watt-hr registers is not available in this

mode.

Phase A, Phase B, and Phase C zero crossings are, respectively,

included when counting the number of half-line cycles by

setting bits 5, 4, 3 (ZXSEL) in the LCYCMODE[7:0] register.

Any combination of the zero crossings from all three phases can

be used for counting the zero crossing. Only one phase should

be selected at a time for inclusion in the zero crossings count

during calibration.

The number of zero crossings is specified by the

LINECYC[15:0] 16-bit unsigned register. The ADE7858 can

accumulate active power for up to 65535 combined zero

crossings. Note that the internal zero-crossing counter is always

active. By setting bit 0 (LWATT) in LCYCMODE[7:0] register,

the first energy accumulation result is, therefore, incorrect.

Writing to the LINECYC[15:0] register when the LWATT bit is

set resets the zero-crossing counter, thus ensuring that the first

energy accumulation result is accurate.

At the end of an energy calibration cycle, the bit 5 (LENERGY)

in the STATUS0[31:0]register is set. If the corresponding mask

bit in the MASK0[31:0] interrupt mask register is enabled, the

IRQ pin also goes active low. The status bit is cleared and

IRQ pin is set back high by writing STATUS0 register with the

corresponding bit set to 1.

Because the active power is integrated on an integer number of

half-line cycles in this mode, the sinusoidal components are

reduced to 0, eliminating any ripple in the energy calculation.

Therefore, total energy accumulated using the line-cycle

accumulation mode is























cos

(26)

where nT is the accumulation time.

Note that line cycle active energy accumulation uses the same

signal path as the active energy accumulation. The LSB size of

these two methods is equivalent.

REACTIVE POWER CALCULATION

The ADE7858 computes the total reactive power on every

phase. Total reactive power integrates all fundamental and

harmonic components of the voltages and currents.

A load that contains a reactive element (inductor or capacitor)

produces a phase difference between the applied ac voltage and

the resulting current. The power associated with reactive elements

is called reactive power, and its unit is VAR. Reactive power is

defined as the product of the voltage and current waveforms when

all harmonic components of one of these signals are phase

shifted by 90°.

Expression (29) gives an expression for the instantaneous

reactive power signal in an ac system when the phase of the

current channel is shifted by +90°.



sin

)

(















(27)



sin

)

(















(28)































sin

)

(

 

i

is the current waveform with all harmonic components

phase shifted by 90°.

Then the instantaneous reactive power q(t) can be expressed as

 







(29)





















































































sin

)

(

Note that q(t) can be rewritten as

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