3.2.1.3.2.2 Calibration
Calibration is key to any meter performance, and it is absolutely necessary for every meter to go through this
process. Initially, every meter exhibits different accuracies due to silicon-to-silicon differences, sensor
accuracies, and other passive tolerances. To nullify their effects, every meter must be calibrated. To perform
calibration accurately, there must be an accurate AC test source and a reference meter available. The source
must be able to generate any desired voltage, current, and phase shifts (between V and I). To calculate errors in
measurement, the reference meter acts as an interface between the source and the meter being calibrated. This
section discusses a simple and effective method of calibration of this three-phase design.
The GUI used for viewing results can easily be used to calibrate the design. During calibration, parameters
called calibration factors are modified in the test software to give the least error in measurement. For this meter,
there are six main calibration factors for each phase: voltage scaling factor, active power offset (erroneously
called voltage AC offset in the GUI), current scaling factor, reactive power offset (erroneously called current AC
offset in the GUI), power scaling factor, and the phase compensation factor. The voltage, current, and power
scaling factors translate measured quantities in metrology software to real-world values represented in volts,
amps, and watts, respectively. The power offset is used to subtract voltage to current crosstalk, which appears
as a constant power offset and causes greater inaccuracies at lower currents. Note that offset calibration was
not used for testing this specific design. The last calibration factor is the phase compensation factor, which is
used to compensate any phase shifts introduced by the current sensors and other passives. Note that the
voltage, current, and power calibration factors are independent of each other. Therefore, calibrating voltage does
not affect the readings for RMS current or power.
When the meter SW is flashed on the MSP432 devices for the first time, default calibration factors are loaded
into these calibration factors. These values are modified through the GUI during calibration. The calibration
factors are stored in INFO_MEM, and therefore, remain the same if the meter is restarted.
Calibrating any of the scaling factors is referred to as gain correction. Calibrating the phase compensation
factors is referred to as phase correction. For the entire calibration process, the AC test source must be ON,
meter connections consistent with
, and the energy pulses connected to the reference meter.
3.2.1.3.2.2.1 Gain Calibration
Usually, gain correction for voltage and current can be done simultaneously for all phases. However, energy
accuracy (%) from the reference meter for each individual phase is required for gain correction for active power.
Also, when performing active power calibration for any given phase, the other phase must be turned OFF by
turning off the current but leaving the other voltages still enabled.
3.2.1.3.2.2.1.1 Voltage and Current Gain Calibration
To calibrate the voltage and current readings, perform the following steps:
1. Connect the GUI to view results for voltage, current, active power, and the other metering parameters.
2. Configure the test source to supply desired voltage and current for all phases. Ensure that these are the
voltage and current calibration points with a zero-degree phase shift between each phase voltage and
current. For example, for 120 V, 10 A, 0° (PF = 1). Typically, these values are the same for every phase.
3. Click on the
Manual cal.
button that
shows. The following screen pops up from
Hardware, Software, Testing Requirements, and Test Results
TIDUEM8B – MARCH 2019 – REVISED FEBRUARY 2021
High-Accuracy Split-Phase CT Electricity Meter Reference Design Using
Standalone ADCs
41
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