TIDUEM8B March   2019  – February 2021

 

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1.1.1 Electricity Meter

An increase in utility providers' desired functionality for an electricity meter has driven a need for more features from electricity meters. Advanced features, such as harmonic analysis, are increasingly being required from meters. To meet these requirements, the processing and accuracy requirements often also have to evolve. As an example, adding harmonic analysis capabilities to an electricity meter may require an increase in the sample rate of the meter to capture the desired frequency range. The increase in sample frequency many times has to be done without compromising on accuracy or even while simultaneously increasing accuracy. The high sample rate, in turn, also requires more processing.

As the accuracy and amount of processing expected from electricity meters increases, it becomes more difficult to find a metrology SoC that fulfills both the processing and accuracy requirements of an electricity meter. To address this limitation, a standalone ADC can be used with a host microcontroller (MCU) to simultaneously overcome the processing and accuracy limitations of electricity meter SoCs. Using an accurate standalone ADC typically has the following advantages:

  • It enables meeting the most stringent of accuracy requirements
  • It enables meeting minimum sample rate requirements(without compromising on accuracy) that may not be obtainable with applications specific products or metrology SoCs
  • It enables flexibility in selecting the host microcontroller since you are not limited to selecting host microcontrollers that have accurate ADCs. The host microcontroller can be selected solely based on application requirements, such as processing capability, minimum RAM and Flash storage for logging energy usage, and microcontroller security features for ensuring meter data security.

To properly sense energy consumption, voltage and current sensors translate mains voltage and current to a voltage range that an ADC can sense. To sense the energy consumption when a split-phase distribution system is used, it is necessary for the current sensors to be isolated so they can properly sense the current drawn from the two different lines without damaging the ADC. As a result, current transformers, which inherently have isolation, have historically been used for the current sensors for split-phase, two-phase, and three-phase electricity meters.

In this reference design, Class 0.1 split-phase CT-based energy measurement is implemented by using a standalone ADC device. The standalone ADC senses the Mains voltage and current. When there are new ADC samples available, the host MCU communicates to the standalone ADC via SPI to get the new samples. The host microcontroller uses the new ADC samples from the standalone ADC to calculate metrology parameters. In addition to calculating the metrology parameters, the host MCU also drives the liquid crystal display (LCD) of the board and communicates to a PC GUI through either the isolated RS-232 circuitry or isolated RS-485 circuitry on the board. As an additional safeguard, an external SVS device is added to the design to reset the host MCU when the supplied voltage to power the host MCU is not sufficient. In general, using an external SVS provides more security than the internal SVS on a host microcontroller.

In this design, the test software specifically supports calculation of various metrology parameters for split-phase energy measurement. These parameters can be viewed either from the calibration GUI or LCD. The key parameters calculated during energy measurements are:

  • Active, reactive, apparent power and energy
  • RMS current and voltage
  • Power factor
  • Line frequency
For testing the active energy % error when using an ADC sample rate of 32 ksps, the test software is stripped down to support a subset of these parameters for 1-phase operation.

The design also enables adding external radio or radio modules for communication. The rail for these external radio modules is current limited in this design to prevent any shorting issues with the communication modules from affecting the metrology.