ADE7854AACPZ View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
'ADE7854AACPZ' PDF : 96 Pages View PDF
Data Sheet
ADE7854A/ADE7858A/ADE7868A/ADE7878A
Voltage RMS Offset Compensation
The ADE7854A, ADE7858A, ADE7868A, and ADE7878A
incorporate voltage rms offset compensation registers for each
phase: AVRMSOS, BVRMSOS, and CVRMSOS. These 24-bit
signed registers 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). The voltage rms
offset register is multiplied by 128 and added to the squared
voltage rms before the square root is executed. Assuming that
the maximum value from the voltage rms calculation is
4,191,910 with full-scale ac inputs (50 Hz), one LSB of the
voltage rms offset represents the following value of the rms
measurement at 60 dB down from full scale:
0.00037%
=
41912 + 128
4191
− 1 × 100
Conduct offset calibration at low voltage; avoid using voltages
equal to zero for calibration purposes.
V rms = V rms02 + 128 ×VRMSOS
(21)
where V rms0 is the rms measurement without offset correction.
The serial ports of the ADE7854A, ADE7858A, ADE7868A,
and ADE7878A work with 32-, 16-, or 8-bit words, whereas the
DSP works with 28-bit words. Like the xIGAIN registers shown
in Figure 34, the 24-bit AVRMSOS, BVRMSOS, and CVRMSOS
registers are sign extended to 28 bits and padded with four 0s
for transmission as 32-bit registers.
Voltage RMS in 3-Phase, 3-Wire Delta Configurations
In 3-phase, 3-wire delta configurations, Phase B is considered
the ground of the system, and Phase A and Phase C voltages are
measured relative to it. Select this configuration using the CONSEL
bits equal to 01 in the ACCMODE register (see Table 16 for all
configurations where the ADE7854A, ADE7858A, ADE7868A,
and ADE7878A can be used). In this situation, all Phase B
active, reactive, and apparent powers are 0.
In this configuration, the ADE7854A, ADE7858A, ADE7868A,
and ADE7878A compute the rms value of the line voltage between
Phase A and Phase C and store the result in the BVRMS regis-
ter. BVGAIN and BVRMSOS registers can be used to calibrate
the BVRMS register computed in this configuration.
ACTIVE POWER CALCULATION
The ADE7854A/ADE7858A/ADE7868A/ADE7878A compute
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 ADE7878A 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, and it is given by the product of the voltage and current
waveforms. The resulting waveform is 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
∞
v(t) = ∑Vk 2 sin (kωt + φk)
(22)
k =1
∞
i(t) = ∑ Ik
2 sin(kωt + γk )
k =1
where:
Vk, Ik are the 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) = ∑Vk Ik cos(φk − γk) −
k =1
∞
∞
∑Vk Ik cos(2kωt + φk + γk) + ∑Vk Im {cos[(k − m)ωt +
k=1
k, m=1
k≠m
φk − γm] − cos[(k + m)ωt + φk + γm]}
(23)
The average power over an integral number of line cycles (n) is
given by the expression in Equation 24.
( ) P =
1
nT
nT
∫p
0
t
dt
=
∞
∑Vk Ik
k =1
cos(φk − γk)
(24)
where:
T is the line cycle period.
P is 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 23, that is,
∞
∑Vk Ik cos(φk − γk)
k=1
Use this expression to calculate the total active power in the
device for each phase. The expression of fundamental active power
is obtained from Equation 24 with k = 1, as follows:
FP = V1I1 cos(φ1 − γ1)
(25)
Figure 66 shows how the device 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 instanta-
neous power signal in each phase (A, B, and C) using LPF2, the
low-pass filter.
Rev. C | Page 47 of 96
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