ADE7932 datasheet(67/125 Pages) AD

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Data Sheet

ADE7978/ADE7933/ADE7932/ADE7923

Rev. D | Page 67 of 125

APPARENT POWER CALCULATION

Apparent power is defined as the maximum active 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

(41)

where:

S is the apparent power.

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

The ADE7978 computes the arithmetic apparent power on each

phase. Figure 89 illustrates the signal processing in each phase for

the calculation of the apparent power in the ADE7978. Because

V rms and I rms contain all harmonic information, the apparent

power computed by the ADE7978 is total apparent power. The

ADE7978 does not compute fundamental apparent power.

The ADE7978 stores the instantaneous phase apparent powers

in the AVA, BVA, and CVA registers (Address 0xE51E to

Address 0xE520). The equation for apparent power is

PMAX

xVA

(42)

where:

V and I are the rms values of the phase voltage and current.

VFS and IFS are the rms values of the phase voltage and current

when the ADC inputs are at full scale.

PMAX = 26,991,271, the instantaneous power computed when

the ADC inputs are at full scale and in phase.

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

any of the serial port interfaces. For more information, see

the Waveform Sampling Mode section.

The ADE7978 can also compute the apparent power by multi-

plying the phase rms current by an rms voltage that is introduced

externally. For more information, see the Apparent Power

Calculation Using VNOM section.

APPARENT POWER GAIN CALIBRATION

The average apparent power in each phase can be scaled by ±100%

by writing to the appropriate 24-bit phase gain registers: APGAIN,

BPGAIN, or CPGAIN (Address 0x4399 to Address 0x439B). The

same registers are used to compensate the other powers computed

by the ADE7978. For more information about these registers, see

the Active Power Gain Calibration section.

APPARENT POWER OFFSET CALIBRATION

Each rms measurement includes an offset compensation register

to calibrate and eliminate the dc component in the rms value (see

the Root Mean Square Measurement section). The voltage and

current rms values are multiplied together in the apparent power

signal processing. Because no additional offsets are created in the

multiplication of the rms values, there is no specific offset

compensation for the apparent power signal processing. The offset

compensation of the apparent power measurement in each phase

is accomplished by calibrating each individual rms measurement.

APPARENT POWER CALCULATION USING VNOM

The ADE7978 can compute the apparent power by multiplying

the phase rms current by an rms voltage introduced externally

in the 24-bit signed VNOM register (Address 0xE533).

When one of Bits[13:11] (VNOMCEN, VNOMBEN, or

VNOMAEN) in the COMPMODE register (Address 0xE60E) is

set to 1, the apparent power in the specified phase (Phase x for

VNOMxEN) is computed in this way. When the VNOMxEN

bits are cleared to 0 (the default value), the arithmetic apparent

power is computed.

The VNOM register value can be calculated as follows:

VNOM = V/VFS × 3,761,808

(43)

where:

V is the nominal phase rms voltage.

VFS is the rms value of the phase voltage when the ADC inputs

are at full scale.

The serial ports of the ADE7978 work with 32-, 16-, or 8-bit

words, whereas the DSP works with 28-bit words. Like the

SAGLVL register, the VNOM register is transmitted as a 32-bit

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