BL6502A
Single Phase Energy Meter IC
with Integrated Oscillator
pulses. The output frequency is therefore proportional to the average real power. This average real
power information can, in turn, be accumulated (e.g., by a counter) to generate real energy
information. Because of its high output frequency and hence shorter integration time, the CF
output is proportional to the instantaneous real power. This is useful for system calibration
purposes that would take place under steady load conditions.
Offset Effect
The dc offsets come from the input signals and the forepart analog circuitry.
Assume that the input dc offsets on the voltage channel and the current channel are U
offset
and I
offset
,
and PF equals 1 (φ=0).
p
(
t
)
=
[
U
cos(
ω
t
)
+
U
offset
]
×
[
I
cos(
ω
t
+ Φ
)
+
I
offset
]
=
UI
UI
+
I
offset
U
cos(
ω
t
)
+
U
offset
I
cos(
ω
t
)
+
cos(2
ω
t
)
2
2
Figure 3.
Effect of Offset
As can be seen, for each phase input, if there are simultaneous dc offsets on the voltage channel
and the current channel, these offsets contribute a dc component for the result of multiplication.
That is, the offsets bring the error of U
offset
×I
offset
to the final average real power. Additionally,
there exists the component of U
offset
×I+U×I
offset
at the frequency of
ω.
The dc error on the real
power will result in measure error, and the component brought to the frequency of
ω
will also
affect the output of the average active power when the next low-pass filter can’t restrain the ac
component very completely.
When the offset on the one of the voltage and the current channels is filtered, for instance, the
offset on the current channel is removed; the result of multiplication is improved greatly. There is
no dc error, and the additional component at the frequency of
ω
is also decreased.
When the offsets on the voltage channel and the current channel are filtered respectively by two
high-pass filters, the component at the frequency of
ω
(50Hz) is subdued, and the stability of the
output signal is advanced. Moreover, in this case, the phases of the voltage channel and the current
channel can be matched completely, and the performance when PF equal 0.5C or 0.5L is improved.
In BL6502A, this structure is selected. Though it is given in the system specification that the
ripple of the output signal is less than 0.1%, in real measure of BL6502A, the calibration output is
very stable, and the ripple of the typical output signal is less than 0.05%.
Additionally, this structure can ensure the frequency characteristic. When the input signal changes
from 45Hz to 65Hz, the complete machine error due to the frequency change is less than 0.1%. In
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3/15/2007
BELLING [ SHANGHAI BELLING CO., LTD. ]
