ADE7758 Analog Devices, ADE7758 Datasheet - Page 30

• Manufacturer Part Number: ADE7758
• Description: Poly Phase Multifunction Energy Metering IC with Per Phase Information
• Manufacturer: Analog Devices

ADE7758
Voltage RMS Gain Adjust
The ADC gain in each phase of the voltage channel can be
adjusted for the rms calculation by using the voltage rms gain
registers (AVRMSGAIN, BVRMSGAIN, and CVRMSGAIN).
The gain of the voltage waveforms before LPF1 is adjusted by
writing twos complement, 12-bit words to the voltage rms gain
registers. The expression below shows how the gain adjustment
is related to the contents of the voltage gain register.
For example, when 0x7FF is written to the voltage gain register,
the ADC output is scaled up by 50%.
Similarly, 0x800 = –2047d (signed twos complement) and the
ADC output is scaled by –50%.
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 instantaneous
power signal in an ac system.
where VRMS = rms voltage, IRMS = rms current.
The average power over an integral number of line cycles (n) is
given by the expression in Equation 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 instantaneous power
signal p(t) in Equation 5, that is, VRMS × IRMS. This is the
relationship used to calculate the active power in the ADE7758
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 (the low-pass
0x7FF = 2047d, 2047/2
Content
v
t i
Nominal
p
p
p
( )
( )
( )
(t)
=
t
t
=
nT
=
=
=
1
v
2
R I
of
( )
nT
2
∫
0
t
RMS
MS
VRMS
×
p
×
×
( )
t
VRMS
IRMS
Values
t i
×
dt
( )
Register
VRMS
=
×
VRMS
×
without
12
sin
sin
–
= 0.5
=
(
( )
IRMS
ω
ω
×
t
t
Gain
)
IRMS
×
VRMS
×
⎛
⎜
⎝
1
+
×
VRMSGAIN
cos
2
(2
12
ω
t)
⎞
⎟
⎠
Rev. A | Page 30 of 68
(3)
(4)
(5)
(6)
filter) to obtain the average active power information on each
phase. Figure 64 shows this process. The active power of each
phase accumulates in the corresponding 16-bit watt-hour
register (AWATTHR, BWATTHR, or CWATTHR). The input to
each active energy register can be changed depending on the
accumulation mode setting (see Table 17).
VRMS
Because LPF2 does not have an ideal “brick wall” frequency
response (Figure 65), the active power signal has some ripple
due to the instantaneous power signal. This ripple is sinusoidal
and has a frequency equal to twice the line frequency. Because
the ripple is sinusoidal in nature, it is removed when the active
power signal is integrated over time to calculate the energy.
0xCCCCD
0x19999A
0x00000
×
–12
–16
–20
–24
IRMS
–4
–8
0
1
Figure 65. Frequency Response of the LPF Used
INSTANTANEOUS
POWER SIGNAL
to Filter Instantaneous Power in Each Phase
CURRENT
i(t) = 2
Figure 64. Active Power Calculation
VOLTAGE
v(t) = 2
3
×
IRMS
FREQUENCY (Hz)
×
×
p(t) = VRMS
VRMS
sin(ωt)
8
10
×
sin(ωt)
×
IRMS – VRMS
30
ACTIVE REAL POWER
SIGNAL = VRMS
×
IRMS
×
100
cos(2ωt)
×
IRMS