AN939 Datasheet, PDF (11/18 Pages) Microchip Technology – Designing Energy Meters with the PIC16F873A

AN939
Design Considerations with Shunt
Measurement
When substituting a shunt for current measurement,
the value of K_i is calculated as the reciprocal of the
product of the shunt resistance and the amplifier gain.
For example, a shunt of 0.005 ohm used with the same
amplifier circuit will also produce approximately the
same value of K_i (around 9.3).
However, IEC specifications call for a power loss of
less than 2W in the measurement circuit. At the design
voltage and current, this would mean using a shunt
smaller than 0.0001 ohm. To compensate, the amplifier
gain would have to be increased by reducing R7, which
would create different gain requirements for the CT and
shunt circuits. Since a single amplifier is used for both
current channels, combining shunt and CT
measurement may not be possible in this design.
Tamper Proofing and Current
Measurement
As already noted, the energy meter design measures
both the phase and neutral currents. Measuring two
current channels allows for tamper Fault detection. If
tampering is indicated, the meter will automatically
measure and calculate usage from the leg with the
higher load current. The threshold for detection is set in
the firmware at an 8% difference between the two
channels.
It is possible to configure the meter to use only one CT
and a single current measurement channel. Doing this
will disable tamper detection. Provisions are made in
the firmware to bypass the tamper detection feature.
If a shunt is used for current measurement, only one
current measurement channel is available. As before,
doing this disables tamper protection.
CALIBRATING THE METER
The firmware contains a default set of values for the
meter calibration constants. These values assume the
use of CTs to measure current and are based on the
prototype design. Using different current sensors
(shunts or Hall sensors), in addition to component vari-
ations in individual devices, may require individual
meter calibration to account for intolerance variations
and phase error from sensors. To perform calibration, it
will be necessary to use the Meter Calibration Software
provided with the firmware. Users must also have
access to a watthour meter test stand, or other system
that can provide power at fixed current levels and
power factors.
The calibration software uses inputs provided by the
user to calculate the gain calibration constant, K_i, for
both current channels, for both low-gain and high-gain
settings of the current amplifier. In addition, it calculates
the phase error and required timing correction from the
same inputs. The calibration data is then downloaded
to the meter through a serial (RS-232) interface.
The software is designed to run on desktop or laptop
computers under any 32-bit version of Microsoft^®
Windows^® operating system. The computer must also
have an available serial port.
FIGURE 8:
CALIBRATION SCREEN FOR THE METER CALIBRATION SOFTWARE
© 2005 Microchip Technology Inc.
DS00939A-page 11