Changing the way three-phase meters are designed with new isolation technology


By Etienne Moulin

With more than 175 million energy metering ICs deployed worldwide and 17 products servicing this market, Analog Devices is the preferred IC solution provider to meter manufacturers and utilities. Analog Devices also has a portfolio of more than 10,000 parts featuring innovative and high performance technologies, some of them applicable to electronic energy meters such as ISM RF transceivers, high speed digital isolation, voltage references, temperature sensors and RS-485 drivers. This breadth of technology puts Analog Devices in an excellent position to meet this new low cost 3-phase market demand.

Three-phase energy meters are typically of high performance. Although the trend toward low cost 3-phase meters is accompanied with simpler specifications, some basic electrical requirements and utility specifications have a large effect on its cost:

  • Electrical isolation between phases: A 3-phase system is comprised, by definition, of more than one phase, with each phase representing an independent voltage and current source. In a 3-phase meter, the energy measurement per phase is generally combined in one result, requiring energy information exchange between phases – thus the requirement of electrical isolation between phases.
  • Tolerance to DC current: This requirement is initiated from the half-wave rectified waveform test of the IEC62053-21 standard and MID CENELEC European standard and is critical to cover the extensive use of simple half wave rectifiers in electrical equipment. This need led to using sensing technology that can tolerate DC current.
  • Isolation between the voltage and current inputs: Meter manufacturers and utilities prefer to use simple testing equipment for 3-phase calibration and verification. This non-normative requirement leads to the isolation between voltage and current specification.

Figure 1 – Existing 3-phase energy meter system diagram

This combination of constraints has led to a design consensus on isolating the phases with the current sensor and implementing the signal processing and data management isolated from the power lines (Figure 1). Four types of technologies can achieve current sensing isolation: current transformer (CT), Rogowski coil, Hall effect, or current shunt with voltage transformer. However, due to patent protections, the CT is the only solution that can be implemented cost effectively with open market components, leading to the widespread use of DC tolerant CTs by many newly formed electronic meter manufacturers. Analog Devices has a portfolio of 3-phase Analog Front Ends (AFE) for Watt only and 4-quadrant energy measurements. These solutions serve the 3-phase market need where energy measurement is interfaced with isolated current sensors.

Unfortunately, due to the uniqueness of the technology, DC tolerant CTs are also priced at a high level and therefore are not the panacea to achieve low cost systems. In summary, the technical advantages and disadvantages of a current sensor technology are currently largely over-ridden by the lack of competition in the supply of current sensing solutions.

Alternatively, Analog Devices’ single phase AFE and System On Chip (SOC) combined with its high speed digital isolation technology, iCouplerTM, and low value shunt resistors can provide a new system solution, which allows electronic meter designers to choose a new system architecture using technology not available to them before (Figure 2).


Figure 2 – Alternative 3-phase energy meter system diagram

The shunt resistor solves a number of current sensing issues while also having some limitations. Advantages are cost, availability, DC tolerance, linear range, frequency range and no phase calibration needed. Limitations are maximum current <100 A, signal level of tens of mV at full scale, isolation required, and bipolar input around line voltage.

Energy metering ICs
Although the shunt current sensor limitations are few, Analog Devices’ AFE and SOC can uniquely solve the low signal level and bipolar input challenges with:

  • Large analogue internal gain to interface to low shunt value down to 200 µâ„¦
  • Patented analogue input structure to allow bipolar input in unipolar ICs
  • Accuracy better than Class 1
  • Best in class reliability
  • High quality and ease of manufacturing.

Additionally, the meter designer can choose the right AFE depending on the energy measurements needed (Watt only, Watt + rms or 4-quadrant) combined with the industry acclaimed ADE7569 SOC. The ADE7569 is part of the ADE75xx family of products which feature a high accuracy energy measurement engine with high precision voltage reference, power management with low power sleep modes and internal battery switching, low current real time clock, level adjustable LCD driver, and an 8-bit single-cycle 8052 micro-controller core with embedded flash memory and RAM.

Digital isolation ICs
Data isolation between the energy metering AFE and the SOC can be achieved using standard opto-couplers for Watt-only pulsed output AFEs. However, as the 3-phase energy meters become more complex, more information such as rms values, reactive power, apparent power or instantaneous waveforms for THD measurements is required from the AFE. To collect that information from the AFEs, the communication speed with the SOC needs to increase significantly, making the opto-coupler technology inadequate from performance and price perspectives. However, the iCouplerTM technology from Analog Devices solves this issue and allows the meter manufacturer to use this shunt-based architecture. The ADuM2401 family of products provides 5 kV of isolation per UL1577 test for 4 digital channels and can be used for the SPI interface.

Furthermore, the development of the IsoPower technology, as presented in the ADuM5242, integrates an isolated DC/DC power supply with digital channel isolation to provide not only the isolation of the AFE serial interface but also the AFE power supply, thus removing the need for the power supplies #2 in Figure 2. This additional feature is another reason for using the new solution proposed.

The combination of Analog Devices’ energy metering AFE, SOC and data isolation technology allows ADI to propose an innovative solution to solve the issue of low cost multifunction 3-phase electronic energy meters. While meeting the standards requirements in term of isolation, this solution allows more information to be transferred between AFEs and SOC and leverages the inherent benefit of the shunt current sensor.

While electronic energy metering is seen by many as a low cost market with little room for innovation and the development of value added solutions, Analog Devices continues to discover ways of challenging this belief. In 2007, it introduced the ADE7569 that integrates battery switching, power management features and solved the battery operation problem. The ADE7569’s innovative system architecture enables key functionality (including temperature measurement, time keeping, LCD display and UART) to be maintained in battery mode while consuming low power. At the same time, the ADuM5242, a breakthrough isolation technology combining high speed digital data isolation and an isolated power supply, was brought to market. This product enhances Analog Devices’ offering for 3-phase energy meter system solutions by proposing an innovative solution to solve energy meter manufacturers’ most critical design issues.