ADE7880 datasheet(43/104 Pages) AD

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Data Sheet

ADE7880

Rev. A | Page 43 of 104

Voltage RMS Offset Compensation

The ADE7880 incorporates voltage rms offset compensation

registers for each phase: AVRMSOS, BVRMSOS, and CVRMSOS.

These are 24-bit signed registers used to remove offsets in the

voltage rms calculations. An offset can exist in the rms calcula-

tion due to input noises that are integrated in the dc component

of V2(t). One LSB of the voltage rms offset compensation register is

equivalent to one LSB of the voltage rms register. Assuming that

the maximum value from the voltage rms calculation is 3,766,572

with full-scale ac inputs (50 Hz), one LSB of the current rms offset

represents 0.00045%

(

100

3767

128

3767

⎟

⎠

⎞

⎜

⎝

⎛

−

of the rms measurement at 60 dB down from full scale. Conduct

offset calibration at low current; avoid using voltages equal to zero

for this purpose.

VRMSOS

rms

128

(16)

where V rms0 is the rms measurement without offset correction.

As stated in the Current Waveform Gain Registers section, the

serial ports of the ADE7880 work on 32-, 16-, or 8-bit words

and the DSP works on 28 bits. Similar to registers presented in

Figure 43, the AVRMSOS, BVRMSOS, and CVRMSOS 24-bit

registers are accessed as 32-bit registers with the four most

significant bits padded with 0s and sign extended to 28 bits.

Voltage RMS in 3-Phase Three Wire Delta Configurations

In 3-phase three wire delta configurations, Phase B is

considered the ground of the system, and Phase A and Phase C

voltages are measured relative to it. This configuration is chosen

using CONSEL bits equal to 01 in ACCMODE register (see

Table 15 for all configurations where the ADE7880 may be

used). In this situation, all Phase B active, reactive, and apparent

powers are 0.

In this configuration, the ADE7880 computes the rms value of

the line voltage between Phase A and Phase C and stores the

result into BVRMS register. BVGAIN and BVRMSOS registers

may be used to calibrate BVRMS register computed in this

configuration.

ACTIVE POWER CALCULATION

The ADE7880 computes the total active power on every phase.

Total active power considers in its calculation all fundamental

and harmonic components of the voltages and currents. In

addition, the ADE7880 computes the fundamental active power,

the power determined only by the fundamental components of

the voltages and currents.

The ADE7880 also computes the harmonic active powers, the

active powers determined by the harmonic components of the

voltages and currents. See the Harmonics Calculations section

for details.

Total Active Power Calculation

Electrical power is defined as the rate of energy flow from source

to load, and 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. If an

ac system is supplied by a voltage, v(t), and consumes the current,

i(t), and each of them contains harmonics, then

sin

)

(

∑

∞

(kωt + φk)

(17)

()

∑

∞

sin

)

(

where:

Vk, Ik are rms voltage and current, respectively, of each

harmonic.

φk, γk are the phase delays of each harmonic.

The instantaneous power in an ac system is

p(t) = v(t) × i(t) =

∑

cos(φk – γk) −

cos(2kωt + φk + γk)

{cos[(k − m)ωt + φk – γm] – cos[(k + m)ωt + φk + γm]}

∞

k I

∑

∞

k I

∑

∞

≠

k I

(18)

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

given by the equation in Equation 19.

P =

()

∑

∫

∞

cos(φk – γk)

(19)

where:

T is the line cycle period.

P is referred to as the total active or total real power.

Note that the total active power is equal to the dc component of

the instantaneous power signal p(t) in Equation 18, that is,

∑

∞

k I

cos(φk – γk)

This is the equation used to calculate the total active power in the

ADE7880 for each phase. The equation of fundamental active

power is obtained from Equation 18 with k = 1, as follows:

FP = V1I1 cos(φ1 – γ1)

(20)

Figure 70 shows how the ADE7880 computes the total active

power on each phase. First, it multiplies the current and voltage

signals in each phase. Next, it extracts the dc component of the

instantaneous power signal in each phase (A, B, and C) using

LPF2, the low-pass filter.

If the phase currents and voltages contain only the fundamental

component, are in phase (that is φ1 = γ1 = 0), and they correspond

to full-scale ADC inputs, then multiplying them results in an

instantaneous power signal that has a dc component, V1 × I1,

and a sinusoidal component, V1 × I1 cos(2ωt); Figure 71 shows

the corresponding waveforms.

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