Solid-state electricity meters – trends and technologies
The demand for solid-state electricity meters is growing worldwide. Today’s solid-state meters exceed the performance and reliability of their electromechanical counterpart. Production costs are decreasing continuously, and the metering industry is now at the point where a solid-state meter is becoming more cost-effective than the Ferraris meter.
A modern meter with features can be divided into two major functions – the metering circuit itself and the feature part. The feature part is usually based on a standard micro-controller.
The metering section calculates the power consumption from the voltage and current values supplied to the load. The current information is usually taken through a ‘current sensor’, which is often a shunt resistor or a current transformer or a Hall sensor-based element. The voltage information is usually taken from the mains voltage through a voltage divider. The calculated power is integrated over time, which then represents the energy consumed.
The first energy meter circuits were based on discrete components solutions. Most were based on time division multiplication. Expensive passive and active components were required to keep the stability and drift requirements within required limits. Multiple trim points were necessary to achieve the desired accuracy.
Soon after, the first integrated circuits for metering were developed. These components were not freely available, because they were ASICs (application specific integrated circuits) and belonged to the metering company which paid for the development.
The situation changed at the beginning of the last decade, when the first metering integrated circuit was developed independently and made available to anybody who wanted to enter the market. Within two years a complete range of metering circuits was available, covering most applications from single-phase to three-phase.
Worldwide, many companies took advantage of this situation. In a few years, the organisations supplying electricity meters grew from a few to several hundred large and medium sized companies. The growing market justifies more specific solutions for different requirements, and today these meters can be divided into categories.
The Fully Electronic Meter
Fully electronic meters consist of the metering electronics and a micro-controller, which usually drives an LCD display. Electricity meters where the micro-controller also performs the power calculation are not common, mainly because of the higher cost/performance ratio and component count required to implement the metering function.
The micro-controller allows for flexibility and for different features. The calibration is usually done in the micro-controller software. The calibration constant and other meter parameters are stored in EEPROM, allowing the manufacturer to offer a range of meters which differ only in software. The metering functions are integrated in a specific integrated circuit, the metering chip.
The metering chip does not contain any intelligence – it just calculates the power or energy consumption and makes this information available to a micro-controller. Metering chips for tri-vector meters are able to calculate additional parameters such as current, voltage and kVar.
Metering circuits presently available for these applications cover the complete range, from the simple single-phase kWh metering IC in an 8-pin package to the integrated circuits for tri-vector meters. The newest integrated circuits include all critical components, including the crystal for the system clock. Figure 1 shows the metering front end (power supply not shown) based on a state-of-the-art integrated metering circuit.
However, the fully electronic meter may not be suitable for countries where visual meter reading on site is common, and uninterrupted power supply is not guaranteed.
The Electronic Meter with Electromechanical Display
This type of meter is popular in many countries. A mechanical counter provides the display and storage function for the energy consumed. Meter reading can take place even if the display is not powered.
A single chip solution is the most cost-effective for this type of meter. In this case, the manufacturer must be able to configure the integrated circuit for different rated conditions, eliminating the need for electromechanical displays with different gearbox-ratios. Calibration is done by means of external resistor networks or potentiometers.
First samples of new generation integrated circuits are available, which are able to configure themselves from an external EEPROM. Meters based on these new ICs allow for fully automated configuration and calibration during the calibration stage, which reduces costs and increases reliability.
Some countries require anti-tamper features, and integrated circuits for this application provide for two current sensors, one for live and the other for neutral. Metering is taken from the wire where the highest power consumption is measured. The different tamper conditions are indicated on outputs connected to indicator LEDs.
Figure 2 shows the dual current sensor meter, which uses a low-cost EEPROM to store calibration and configuration data. The kilowatt-hours are accumulated into an electromechanical register, which acts as display and memory. (Power supply not shown.)