ADE7878 datasheet(52/92 Pages) AD

52 / 92 page

ADE7878

Rev. 0 | Page 52 of 92

The ADE7878 stores the instantaneous phase apparent powers

into the AVA[23:0], BVA[23:0], and CVA[23:0] registers. Their

expression is

APPARENT POWER CALCULATION

Apparent power is defined as the maximum power that can be

delivered to a load. One way to obtain the apparent power is by

multiplying the voltage rms value by the current rms value (also

called the arithmetic apparent power) as follows:

PMAX

xVA

(40)

S = V rms × I rms

(39)

where:

U, I are the rms values of the phase voltage and current.

where:

UFS, IFS are the rms values of the phase voltage and current when

the ADC inputs are at full scale.

S is the apparent power

V rms and I rms are the rms voltage and current, respectively.

PMAX = 33,516,139, the instantaneous power computed when

the ADC inputs are at full scale and in phase.

The ADE7878 computes the arithmetic apparent power on each

phase. Figure 67 illustrates the signal processing in each phase

for the calculation of the apparent power in the ADE7878.

Because V rms and I rms contain all harmonic information,

the apparent power computed by the ADE7878 is a total apparent

power. The ADE7878 does not compute a fundamental apparent

power because it does not measure the rms values of the funda-

mental voltages and currents.

The xVA[23:0] waveform registers may be accessed using

various serial ports. Refer to the Waveform Sampling Mode

section for more details.

The ADE7878 can compute the apparent power in an alternative

way by multiplying the phase rms current by an rms voltage

introduced externally. See the Apparent Power Calculation

Using VNOM section for details.

AVAHR[31:0]

AVAGAIN

DIGITAL SIGNAL PROCESSOR

AIRMS

ACCUMULATOR

AVRMS

VATHR[47:0]

32 BIT REGISTER

Figure 67. Apparent Power Data Flow and Apparent Energy Accumulation

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