Datasheet

Data Sheet ADE7854/ADE7858/ADE7868/ADE7878
Rev. G | Page 57 of 100
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)
S = V rms × I rms (39)
where:
S is the apparent power.
V rms and I rms are the rms voltage and current, respectively.
The ADE7854/ADE7858/ADE7868/ADE7878 compute the
arithmetic apparent power on each phase. Figure 73 illustrates
the signal processing in each phase for the calculation of the
apparent power in the ADE78xx. Because V rms and I rms con-
tain all harmonic information, the apparent power computed by
the ADE78xx is total apparent power. The ADE7878 does not
compute fundamental apparent power because it does not measure
the rms values of the fundamental voltages and currents.
The ADE7854/ADE7858/ADE7868/ADE7878 store the instan-
taneous phase apparent powers into the AVA, BVA , and CVA
registers. Their expression is
4
2
1
×××= PMAX
I
I
V
V
xVA
FSFS
(40)
where:
V, I are the rms values of the phase voltage and current.
V
FS
, I
FS
are the rms values of the phase voltage and current when
the ADC inputs are at full scale.
PMAX = 33,516,139, the instantaneous power computed when
the ADC inputs are at full scale and in phase.
The xVA[23:0] waveform registers may be accessed using
various serial ports. Refer to the Waveform Sampling Mode
section for more details.
The ADE7854/ADE7858/ADE7868/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
DIGITA
L SIGNAL
PROCESSOR
AIRMS
ACCUMUL
ATOR
AVRMS
VATHR[47:0]AVA
2
4
32-BIT REGISTER
08510-048
Figure 73. Apparent Power Data Flow and Apparent Energy Accumulation