ADE7754 datasheet(19/44 Pages) AD

19 / 44 page

REV. PrG 01/03

PRELIMINARY TECHNICAL DATA

ADE7754

– 19 –

ACTIVE POWER CALCULATION

Electrical power is defined as the rate of energy flow from

source to load. It is given by the product of the voltage and

current waveforms. The resulting waveform is called the

instantaneous power signal and it is equal to the rate of energy

flow at every instant of time. The unit of power is the watt or

joules/sec. Equation 5 gives an expression for the instanta-

neous power signal in an ac system.

v(t)

2V sin(

)

(3)

i(t)

2I sin(

)

(4)

where V = rms voltage, I = rms current.

p(t)

v(t)

i(t)

p(t)

2 t

= VI - VI cos(

)

(5)

The average power over an integral number of line cycles (n)

is given by the expression in Equation 6.

P =

p(t)dt=VI

∫

(6)

where T is the line cycle period.

P is referred to as the Active or Real Power. Note that the

active power is equal to the DC component of the instanta-

neous power signal p(t) in Equation 5 , i.e., VI. This is the

relationship used to calculate active power in the ADE7754

for each phase. The instantaneous power signal p(t) is

generated by multiplying the current and voltage signals in

each phase. The DC component of the instantaneous power

signal in each phase (A, B and C) is then extracted by LPF2

(Low Pass Filter) to obtain the active power information on

each phase. This process is illustrated graphically on Figure

20. In a polyphase system, the total electrical power is simply

the sum of the real power in all active phases. The different

solutions available to process the total active power are

discussed in the following paragraph.

Voltage

Current

Instantaneous

Power Signal

Active Real Power

Signal = V x I

V. I.

00000h

1A36E2Eh

D1B717h

)

sin(

i(t)

)

sin(

v(t)

)

cos(

p(t)

−

Figure 20– Active Power Calculation

Voltage RMS Gain Adjust

The Voltage Gain register (AVGAIN[11:0], BVGAIN and

CVGAIN) have an effect on the Apparent Power and voltage

rms values. It is not recommended to calibrate the voltage

rms measurements with these registers. The conversion of

the voltage rms registers values to Volts has to be done in an

external Micro-controller with a specific Volt/LSB constant

for each phase - see Calibration of a 3-phase meter based on the

ADE7754. Due to gain mismatches between phases, the

calibration of the Volt/LSB constant has to be done for each

phase separately. One point calibration is sufficient for this

calibration. The Voltage Gain registers are aimed to ease the

calibration of the apparent energy calculation in MODE 1

and 2 of the VAMODE register.

If the VGAIN registers are used for Apparent Power calibra-

tion (VAMOD bits in VAMode register = 1 or 2), the voltage

rms values are changed by Voltage Gain register value as

described in the expression below:

Voltage RMS

gister Phase A

RMS

AVGAIN

=×





















For example, when 7FFh is written to the Voltage Gain

2047d, 2047/2

12 = 0.5. Similarly, 800h = -2047 Dec (signed

2’s Complement) and ADC output is scaled by –50%. These

two examples are illustrated graphically in Figure 19.

Voltage RMS offset compensation

The ADE7754 incorporates a voltage RMS offset compen-

sation for each phase (AVRMSOS, BVRMSOS and

CVRMSOS). These are 12-bit 2-complement signed regis-

ters which can be used to remove offsets in the voltage RMS

calculations. An offset may exist in the RMS calculation due

to input noises and offsets in the input samples. The offset

calibration allows the contents of the VRMS registers to be

maintained at zero when no voltage is applied.

n LSB of the Voltage RMS offset are equivalent to 64 x n LSB

of the voltage RMS register. Assuming that the maximum

value from the Voltage RMS calculation is 1,898,124d with

full scale AC inputs, then 1 LSB of the voltage RMS offset

represents 0.07% of measurement error at -26dB down of full

scale.

VRMSOS

rms

where Vrmso is the RMS measurement without offset correc-

tion.

The voltage rms offset compensation should be done by

testing the rms results at two non-zero input levels. One

measurement can be done close to full scale and the other at

approximately Full scale/10. The voltage offset compensa-

tion can then be derived from these measurements - see

Calibration of a 3-phase meter based on the ADE7754.

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