ADE7878 datasheet(41/92 Pages) AD

41 / 92 page

ADE7878

Rev. 0 | Page 41 of 92

Voltage RMS Offset Compensation

The ADE7878 incorporates voltage rms offset compensation

registers for each phase: AVRMSOS[23:0], BVRMSOS[23:0], and

CVRMSOS[23:0]. These are 24-bit signed registers used to

remove offsets in the voltage rms calculations. An offset can

exist in the rms calculation 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 4,191,400 with full-scale ac inputs

(50 Hz), one LSB of the current rms offset represents 0.00037%

(

)

100

4191

128

4191

−

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

Vrms

128

(15)

where V rms0 is the rms measurement without offset correction.

As previously stated, the serial ports of the ADE7878 work on

32-, 16-, or 8-bit words, and the DSP works on 28 bits. Similar

to registers presented in Figure 32, 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.

ACTIVE POWER CALCULATION

The ADE7878 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. The

ADE7878 also computes the fundamental active power, the

power determined only by the fundamental components of the

voltages and currents.

Total 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. 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)

(16)

()

∑

∞

sin

)

(

where:

Vk, Ik

= rms voltage and current of each harmonic.

φk

, γk are the phase delays of each harmonic.

The instantaneous power in an ac system is

(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

(17)

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

given by the expression in Equation 18.

()

∑

∫

∞

cos(φk – γk)

(18)

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 17, that is,

cos(φk – γk). This is the expression used to calculate the

total active power in the ADE7878 for each phase. The expression

of fundamental active power is obtained from Equation 18 with

k = 1, as follows:

∑

∞

k I

FP = V1I1 cos(φ1 – γ1)

(19)

Figure 58 shows how the ADE7878 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.

ADE7878 数据表(PDF) 41 Page - Analog Devices

ADE7878 数据表(HTML) 41 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92

数据表下载