This application report describes the implementation of a three phase electronic electricity meter using the Texas Instruments MSP430F471xx system-on-chip (SOC) processor. This application report includes the necessary information with regard to metrology software, hardware and calibration procedures for this single chip implementation. Results are included at the end, which show performance of this device for three phase using current transformers as sensors.
The MSP430F471xx devices belong to the MSP430F4xx family of devices. These devices find its application in energy measurement and have the necessary architecture to support it. The MSP430F471xx devices have a powerful 16 MHz CPU with MSP430CPUx architecture. The analog front-end consists of up to seven analog to digital converters (ADC) based on a 2nd order sigma-delta architecture that supports differential inputs. The sigma-delta ADCs (SD16) that have a resolution of 16-bits can be configured and grouped together for simultaneous sampling of voltages and currents on the same trigger. Each SD16 supports a common mode voltage of up to -1 V and enables all sensors to be referenced to ground. In addition, it also has an integrated gain stage to support gains up to 32 for amplification of low-output sensors. A 32-bit x 32-bit HW multiplier on this chip can be used to further accelerate math intensive operations during energy computation. The SW supports calculation of various parameters for total three phase and for each individual phases. The key parameters calculated during energy measurements are: RMS current and voltage, Active and reactive power, power factor and frequency. The entire operations take about 1/3rd of the processing power and use about a tenth of resources. The application note has complete metrology source code provided as a zip file.
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