Rainbow Electronics AT73C502 - Manual (Page 22)

AT73C500
22
scale values of the powers. The nominal full-scale values
are:
The valid range for the offset calibration factors is -128 to
+127.
The scale of offset calibration for active and reactive power
is different, 89W versus 98VAr in nominal conditions of
270V maximum phase voltage and 80A maximum phase
current. Typically, a small offset factor of a few watts is
enough to compensate the non-linearity of current sensing.
It should be noted that offset calibration will also affect the
starting current level of a meter. If the full scale current or
voltage is changed to a non-default value, the range for off-
set calibration will be scaled accordingly.
The same offset value is used independent of phase angle.
However, as default (OFFMOD=0), the sign of power is
taken into account in the calculations so that positive offset
factor will always increase the absolute power value and
negative coefficient will decrease absolute results. This
guarantees that current sensor non-linearity is corrected in
the same way even though the current flow is reversed.
It is possible to change this default condition by program-
ming value one to OFFMOD coefficient. In this case, offset
coefficient will be always added to power result without
checking the sign of the power. Positive coefficient will
increase the absolute value of positive power results and
decrease the absolute value of negative result.
Phase Calibration
The phase difference between voltage and current channel
is compensated with three 8-bit phase calibration figures.
The displacement is usually due to the phase shift in cur-
rent transformers. Based on the calibration values, the
DSP interpolates new current samples with sample instants
coinciding with the corresponding voltage samples. The fol-
lowing formula is used to determine the phase offset to be
used in the interpolation. One 8-bit phase calibration value
is stored for each of the three phases.
where PO
N
is the sample phase offset of channel N, mea-
sured as phase(U) - phase (I). The allowed range for phase
calibration factor, PC
N
, is -128 to +127.
Starting Current Adjustment
The meter IC is designed to fulfill IEC 1036, class 1 specifi-
cation. This specification is based on a certain basic cur-
rent, I
b
. As a default, AT73C500 operates with 5A basic
current. The chipset has a preprogrammed starting current
level of
where I
FS
is the full scale current of the meter, i.e. 80A in
nominal conditions. The default startup current corre-
sponds to 0.4% of the 5A I
b
, assuming that the full-scale
range is 80A. When the phase current is below the starting
level, the calculated cycle power results are replaced by
zeros and no energy is accumulated.
It is possible to adjust the start-up level in the range of 0.2
to 10 compared with the nominal value. This is performed
with a special calibration factor. The following formula is
used to determine the current:
where STUPC is the starting current calibration factor,
allowed to vary in range -4 to +45, only. Care should be
taken that the STUPC is correctly programmed and is not
beyond -4 to 45 range. Also, it should be noted that low
starting thresholds may force the device to a level where
accuracy is restricted due to a finite resolution of converters
and mathematics.
Adjustment of Display Pulse Rate
An 8-bit byte is provided for adjustment of the impulse rate
of display pulses. This coefficient will only affect the display
pulse rate of active and reactive energy but not to the meter
constant rate. The content of all measurement registers will
remain unchanged.
The impulse rate can be scaled in the range of 1 to 6 com-
pared to the nominal value. In default conditions (U
max
=
270V, I
max
= 80A) the LSB of energy registers REG12-15
(See “Status Information” on page 17.) corresponds to
0.4Wh. This means that accumulated 25 LSBs of energy
will generate one pulse to the display pulse output (25 x
0.4Wh/impulse = 10 Wh/impulse = 100 impulses/kWh).
By using MCC calibration coefficient, the nominal figure 25
can be changed in the range of 25 to 152. MCC may range
from 0 to 127, only. The following formulas are used to cal-
culate the impulse rate.
and
P
FS
270V 80A
×
21.6kW = =
Q
FS
270V 80A
×
21.6VAr = =
PO
N
PC
N
128
------------ 5.625
° ×
=
I
SU
1
4000
------------- I
FS
×
=
I
SU
1
4000
------------- I
FS
(1 0.2 STUPC)
Ч
+
Ч Ч
=
IMP (25 MCC) E
LSB
×
+ =
PR
1000
(25 MCC) E
LSB
Ч
+
-------------------------------------------------- =