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ELECTRICITY METERING
A comprehensive approach to electricity metering
 

The growing need for metering applications with high accuracy over a wide dynamic range of current, low power consumption, high reliability and robustness explains the trend of moving from the traditional mechanical solution to a solid state one.
A complete solution for energy meters has been developed, taking into account qualities such as accuracy, cost, ease of manufacture, cost of the components, easy calibration and of course reliability. This solution is discussed below, with a special focus on the most interesting products in ST’s portfolio.
ELECTRICITY METERING
The energy meter described allows new functionalities like Automatic Meter Reading (AMR), tamper proofing, multi-tariff billing and a prepayment module with a smart card reader.

Architecture

Looking at the general functional block diagram of a power metering application 3 different solutions are evident:

Low-End;
Medium-End;
High-End.

General functional block diagram
General functional block diagram


A complete ST portfolio for meters
A complete ST portfolio for meters

The low-end solution is built only with Voltage and Current sensors, AFE (Analo Front End), ADC (Analog Digital Converter) DSP (Digital Signal Processor) and a stepper display to show the value of the active energy measured in kWh.
The medium-end solution allows the use of an LCD display and is able to calculate also reactive and apparent power. Other possible features are anti-tampering and statistical information including dates and times. The implementation of such a solution basically requires, in addition to the component used in the low-end solution, a MCU together with RTC, Reset IC and Memory (EEprom, Flash or RAM).
The high-end solution adds other optional features like automatic meter reading, pre-paid billing and multi-tariffing. For this reason a connectivity section is requested (RF, GSM, PLM, IrDA) as well as a smart card reader.

AFE-ADC-DSP Section

This section is implemented by the STPM01 device that is an IC integrating the analog front-end, the ADC and the DSP. It can be defined as a single chip energy meter IC compliant with IEC 62052, IEC62053-2x specification for class 0.5AC watt meters with antitampering features. It is able to measure the active, reactive and apparent value of energy as well as the RMS values of the voltage and current. It supports all the available sensing methods like the Rogowski
coil, current transformer and shunt resistor.
This device can be used as a single chip for the implementation of a 1-phase energy meter or as the measurement peripheral in a microcontroller based 1-phase or 3-phase energy meter.
The tamper detection feature is implemented by monitoring the current of both the live and the neutral wires. The calibration is fast and simple (single point) consequently bringing lower production costs. Its integrated OTP memory prevents modification of the data and calibration program. A complete reference design is available upon request.

Metering reference design
Metering reference design

Power Supply Section For 1-Phase Meters

With regards to the power supply section of 1-phase meters, both a non-isolated
and an isolated solution are available. Considering the power needs of the application are very low (usually less than 0.5W), the SMPS topologies used are the buck (for the non-isolated solution) and the flyback (for the isolated solution). The ideal solution is a combination of the VIPer12, STTHxxx Rectifiers, and the TLxxx Voltage Reference for the feedback loop, and a positive voltage regulator to get the 5V needed to supply the other devices used inside the application.

See application notes:
AN1357 VIPower: low cost power supplies using VIPer12A in non isolated applications.
AN1642 VIPower: 5V buck smps with VIPer12A.
A reference design is aviable upon request.
For the 3-phase meters also the discrete solution made with high voltage Power MOSFETs (SuperMESH “NK” series or MDmesh “MD” series) or the new ESBT, Emitter Switched Bipolar Transistors STC03DE170 and STC03DE150 can be used.

Power supply reference design
Power supply reference design

Smart Card Reader Section

For high-end solutions, pre-paid functionality with smart card readers could be implemented. In such a case ST's wide range of microcontrollers (ST7xxx family) and the ST8004 and ST890 smart card reader interface represent a possible solution for such a section of a power meter.
Similar considerations can be made for the automatic meter reading where one possible solution consists in using ST7538, ST’s Power Line Modem.

Smart card reader section using ST8004

Smart card reader section using ST8004

Other Sections

Several other smaller functions should also be taken into consideration. For instance the implementation of the “breaker” that blocks the supply of energy after overload, could be made with ST’s high voltage Power MOSFETs (SuperMESH "NK" family of 600V, 1A or 800V, 3A devices) connected with an electromechanical switch.


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