TIDUEM8B March   2019  – February 2021

 

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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 Section 3.1.2.1, and the energy pulses connected to the reference meter.