ADE7761A
Fault with Active Input Greater than Inactive Input
Calibration Concerns
If V1A is the active current input (that is, being used for billing),
and the voltage signal on V1B (inactive input) falls below 93.75%
of V1A, the fault indicator becomes 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 3 sec on the logic output FAULT after the fault
event. The FAULT logic output is independent of any activity on
outputs F1 or F2. Figure 28 shows one condition under which
FAULT becomes active. Because V1A is the active input and it is
still greater than V1B, billing is maintained on V1A, that is, no
swap to the V1B input occurs. V1A remains the active input.
Typically, when a meter is being calibrated, the voltage and
current circuits are separated, as shown in Figure 30. This
means that current passes through only the phase or neutral
circuit. Figure 30 shows current being passed through the phase
circuit. This is the preferred option because the ADE7761A
starts billing on the input V1A on power-up. The phase circuit
CT is connected to V1A in Figure 30. Because there is no current
in the neutral circuit, the FAULT indicator comes on under
these conditions. However, this does not affect the accuracy of
the calibration and can be used as a means to test the functionality
of the fault detection.
FAULT
FILTER
AND
V
R
1A
F
V
V
1A
1A
IB
CT
V
COMPARE
A
B
1B
V
V
TO
RB
RB
1A
C
C
V
F
F
1A
MULTIPLIER
0V
AGND
V
V
1N
V
V
AGND
< 93.75% OF V
1N
1B
IB
0V
TEST
V
1B
1A
1B
CURRENT
CT
FAULT
<0
R
F
1B
1
RA
C
F
1
1
RB
>0
ACTIVE POINT – INACTIVE INPUT
V
V
2P
VR
6.25% OF ACTIVE INPUT
R
F
2N
Figure 28. Fault Conditions for Active Input Greater than Inactive Input
V
C
T
240V rms
Fault with Inactive Input Greater than Active Input
1
RB + VR = RF.
Figure 29 illustrates another fault condition. If the difference
between V1B, the inactive input, and V1A, the active input (that
is, being used for billing), becomes greater than 6.25% of V1B,
the FAULT indicator becomes active and a swap over to the V1B
input occurs. The analog input V1B becomes the active input.
Again, a time constant of about 3 sec is associated with this
swap. V1A does not swap back to the active channel until V1A is
greater than V1B and the difference between V1A and V1B—in
this order—becomes greater than 6.25% of V1A. However, the
FAULT indicator becomes inactive as soon as V1A is within
6.25% of V1B. This threshold eliminates potential chatter
between V1A and V1B.
Figure 30. Conditions for Calibration of Channel B
If the neutral circuit is chosen for the current circuit in the
arrangement shown in Figure 30, this may have implications for
the calibration accuracy. The ADE7761A powers up with the
V1A input active as normal. However, because there is no
current in the phase circuit, the signal on V1A is zero. This
causes a fault to be flagged and the active input to be swapped
to V1B (neutral). The meter can be calibrated in this mode, but
the phase and neutral CTs may differ slightly. Because under
no-fault conditions all billing is carried out using the phase CT,
the meter should be calibrated using the phase circuit. Of
course, both phase and neutral circuits can be calibrated.
FAULT
FILTER
AND
COMPARE
V
V
1A
1A
A
V
MISSING NEUTRAL MODE
1B
V
V
TO
1A
The ADE7761A integrates a novel fault detection that warns
and allows the ADE7761A to continue to bill in case a meter is
connected to only one wire (see Figure 31). For correct
operation of the ADE7761A in this mode, the VDD pin of the
ADE7761A must be maintained within the specified range (5 V
5%). The missing neutral detection algorithm is designed to
work over a line frequency of 45 Hz to 55 Hz.
MULTIPLIER
0V
V
V
1N
AGND
< 93.75% OF V
B
1B
V
1A
1B
1B
FAULT + SWAP
<0
>0
ACTIVE POINT – INACTIVE INPUT
6.25% OF INACTIVE INPUT
Figure 29. Fault Conditions for Inactive Input Greater than Active Input
Rev. 0 | Page 18 of 24
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