A series of engineering insightsby Analog Devices.

Changing the Way 3-Phase Meters Are Designedwith New Isolation Technology

ADIs high speed data isolation, iCoupler technology, and isoPower technology, combined with its energy metering ICs solution addresses communication speed and cost challenges in 3-phase electronic energy meters.

Today, the market for 3-phase energy meters is facing an inflection point where demand is accelerating, fueled by new deployments of 3-phase meters for residential buildings. The predominance of electronic solutions allows the market to support this surge in demand, but the cost of the solutions is not falling as quickly as the utilities would like. This situation means that electronic meter designers are searching for innovative ways to address the need for low cost 3-phase meters.

With more than 225 million energy metering ICs deployed worldwide and 17 products servicing this market, Analog Devices is the preferred IC solution provider of meter manufacturers and utilities. Analog Devices has also 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.

Challenges in Existing Solutions

Three-phase energy meters are typically 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 tis 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 tthe half-wave rectified waveform test of the IEC 62053-21 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 tutilities prefer to use simple testing equipment for 3-phase calibration and verification. This nonnormative requirement leads to the isolation between voltage and current specification.

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 (see Figure 1). Four types of technologies can achieve current sensing isolation: current transformer (CT), Rogowski coil, Hall effect, and 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 wide spread use of dc tolerant CT 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 needs 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 reach low cost systems. In summary, the technical advantages and disadvantages of a current sensor technology are currently largely overridden by the lack of competition in the supply of current sensing solutions.

Alternative Approach

Alternatively, Analog Devices single-phase AFE and system on a chip (SoC) combined with its high speed digital isolation technology, iCoupler technology, and low value shunt resistors can provide a new system solution that allows electronic meter designers to choose a new system architecture using technology not previously available (see Figure 2).

POWERSUPPLY

ADC

TEMPERATURESENSOR

ENERGYMEASUREMENT

DSP

VOLTAGEREFERENCE

ENERGY METERING AFE

ADCs

ADCs

ADCs

ADCs

ADCs

ADCsLOAD

LOAD

LOAD

VO

LTA

GE

SE

NS

INGP

HA

SE

C

NE

UT

RA

L

PH

AS

E B

PH

AS

E A

POWER GENERATION 3-PHASE ENERGY METER

BATTERY

BATTERYMANAGEMENT

IRINTERFACE

EEPROM

WATCHXTALRAM

LCD DRIVER

LCD DISPLAY

SPI/I2C

UART

FLASHMEMORY

RTC

MCU

Figure 1. Existing 3-phase energy meter system.

Shunt

The shunt resistor solves a number of current sensing issues. However, it has some limitations.

Advantages

Costt

Availabilityt

DC tolerancet

Linear ranget

Frequency ranget

No phase calibration neededt

Limitations

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