ade7753 Analog Devices, Inc., ade7753 Datasheet - Page 26

• Manufacturer Part Number: ade7753
• Description: Active And Apparent Energy Metering Ic With Di/dt Sensor Interface
• Manufacturer: Analog Devices, Inc.

ADE7753
CALIBRATING THE ENERGY METER
When calibrating the ADE7753, the first step is to calibrate
the frequency on CF to some required meter constant, e.g.,
3200 imp/kWh.
A convenient way to to determine the output frequency on CF
is to use the line cycle energy accumulation mode. As shown
in Figure 37, DFC generates a pulse each time a LSB in the
LAENERGY register is accumulated. CF frequency (before
the CF frequency divider) can be conveniently determined by
the following expression:
When the CYCMODE (bit 7) bit in the Mode register is set
to a logic one, energy is accumulated over an integer number
of half line cycles. If the line frequency is fixed and the
number of half cycles of integration is specified, the total
elasped time can be calculated by the following:
For example, at 60Hz line frequency, the elasped time for
255 half cycles will be 2.125 seconds. Rewriting the above in
terms of contents of various ADE7753 registers and line
frequencies (fl):
where f
Alternatively, CF frequency can be calculated based on the
average LPF2 output.
CF
Calibrating the Frequency at CF
When the frequency before frequency division is known, the
pair of CF Frequency Divider registers (CFNUM and
CFDEN) can be adjusted to produce the required frequency
on CF. In this example a meter constant of 3200 imp/kWh
is chosen as an appropriate constant. This means that under
a steady load of 1kW, the output frequency on CF would be,
Frequency
Assuming the meter is set up with a test current (basic
current) of 20A and a line voltage of 220V for calibration, the
load is calculated as 220V
expected output frequency on CF under this steady load
condition would be 4.4
Under these load conditions the transducers on Channel 1
and Channel 2 should be selected such that the signal on the
voltage channel should see approximately half scale and the
signal on the current channel about 1/8 of full scale (assuming
a maximum current of 80A). Assuming at line frequency of
60Hz, energy is accumulated over FFh number of half line
cycles, the resulting content of the LAENERGY register will
be approximately 2971.4 (decimal). CF frequency is there-
fore calculated to be:
CF
Elasped
CF
Frequency
Frequency
Frequency
l
Time
is the line frequency.
(
CF
=
)
LAENERGY[2
Average
Content
2
60
3200
1
LINECYC[15
f
min
l
LPF2
imp
of
number
60
LAENERGY[2
/
PRELIMINARY TECHNICAL DATA
kWh
2
Output
3
sec
20A = 4.4kW. Therefore the
Elasped
27
:
0]
0.8888Hz = 3.9111Hz.
of
:
0]
half
2
3200
3600
×
Time
CLKIN
fl
cycles
3
. 0
:
8888
0] Registe
Hz
(25)
(24)
r
–26–
Alternatively, the average value from LPF2 under this con-
dition is approximately 1/16 of the full-scale level. As
described previously, the average LPF2 output at full-scale
ac input is CCCCD (hex) or 838,861 (decimal). At 1/16 of
full-scale, the LPF2 output is then 52,428.81.
Digital to Frequency Conversion, the frequency under this
load is calculated as:
This is the frequency with the contents of the CFNUM and
CFDEN registers equal to 000h. The desired frequency out
is 3.9111Hz. Therefore, the CF frequency must be divided
by 2797/3.9111Hz or 357.5 decimal. This is achieved by
loading the pair of CF Divider registers with the closest
rational number. In this case, the closest rational number is
found to be 1/358 (or 1h/166h). Therefore, 0h and 165h
should be written to the CFNUM and CFDEN registers
respectively. Note that the CF frequency is multiplied by the
contents of (CFNUM + 1) / (CFDEN + 1). With the CF
Divide registers contents equal to 1h/166h, the output
frequency is given as 2797Hz / 358 = 3.905Hz. This setting
has an error of -0.1%.
Calibrating CF is made easy by using the Calibration mode
on the ADE7753. The critical part of this approach is that the
line frequency needs to be exactly known. If this is not
possible, the frequency can be measured by using the PE-
RIOD register of the ADE7753.
Note that changing WGAIN[11:0] register will also affect
the output frequency from CF. The WGAIN register has a
gain adjustment of 0.0244% / LSB.
Determine the kWHr/LSB Calibration Coefficient
The Active Energy register (AENERGY) can be used to
calculate energy. A full description of this register can be
found in the Energy Calculation section. The AENERGY reg-
ister gives the user both sign and magnitude information
regarding energy consumption. On completion of the CF
frequency output calibration, i.e., after adjusting the CF
Frequency divider and the Watt Gain (WGAIN) register, the
second stage of the calibration is to determine the kWh/LSB
coefficient for the AENERGY register. Equation 26 below
shows how LAENERGY can be used to calculate the calibra-
tion coefficient.
Once the coefficient is determined, the MCU can compute
the energy consumption at any time by reading the AENERGY
contents and multiplying by the coefficient to calculate kWh.
In the above example, at 4.4kW, after 255 half cycles (at
60Hz), the resulting LAENERGY is approximately 2971
decimal. The kWHr/LSB can therefore be calculated to be
8.74
Frequency
Frequency(
kWHr/LSB
Calibratio
3600
-7
n
kWHr/LSB using the above equation.
seconds/Hr
CF)
(CF)
Power
=
=
52428.81
(in
2
971
kW)
4 .
255
×
×
2
LAENERGY [2
2
3.579545MH
×
27
60
LINECY C[15
=
1398.3Hz
z
REV. PrF 10/02
=
3
:
1
0]
398
:
0]
Then using
3 .
2
Hz
fl
(26)