SinglePhaseEnergyMeterIC
with Integrated Oscillator
BL0921
RF
CT
V1A
+
±660mV
GAIN
Rb
CF
-
IP
V1N
IN
AGND
-
±660mV
GAIN
Rb
CF
+
V1B
CT
RF
Phase Neutral
CF
Ra
Ra >> RF
Rb
Rb+Rv=RF
AGND
±660mV
Rv
V1A
V1N
V1B
+
-
AGND
IP
IN
AGND
-
±660mV
GAIN
Rb
CF
+
CT
RF
Phase Neutral
Figure 7.
Fault Detection
Typical Connections for Current Channels
ꢀ
The BL0921 incorporates a novel fault detection scheme that warns of fault conditions and allows
the BL0921 to continue accurate billing during a fault event. The BL0921 does this by
continuously monitoring both the phase and neutral (return) currents. A fault is indicated when
these currents differ by more than 12.5%. However, even during a fault, the output pulse rate on
F1 and F2 is generated using the larger of the two currents. Because the BL0921 looks for a
difference between the signals on V1A and V1B, it is important that both current transducers are
closely matched. On power-up the output pulse rate of the BL0921 is proportional to the product
of the signals on Channel V1A and Voltage Channel. If there is a difference of greater than 12.5%
between V1A and V1B on power-up, the fault indicator (FAULT) will go active after about one
second. In addition, if V1B is greater than V1A the BL0921 will select V1B as the input. The fault
detection is automatically disabled when the voltage signal on Channel 1 is less than 0.5% of the
full-scale input range. This will eliminate false detection of a fault due to noise at light loads.
If V1A is the active current input (i.e., is being used for billing), and the signal on V1B (inactive
input) falls by more than 12.5% of V1A, the fault indicator will go active. Both analog inputs are
filtered and averaged to prevent false triggering of this logic output. As a consequence of the
filtering, there is a time delay of approximately one second on the logic output FAULT after the
fault event. The FAULT logic output is independent of any activity on outputs F1 or F2. Figure 8
illustrates one condition under which FAULT becomes active. Since V1A is the active input and it
is still greater than V1B, billing is maintained on VIA, i.e., no swap to the V1B input will occur.
V1A remains the active input.
http://www.belling.com.cn
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3/15/2007
Total 13 Pages
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