Today's electricity companies are demanding more information from meters in the residential sector, where metering has typically been limited to kilowatt-hours. New features such as multi-tariff billing, reactive energy measurement and power quality monitoring are desirable to improve generation, distribution, customer service and billing. In order to accommodate the advanced requirements not available in electromechanical meters, manufacturers have begun adopting all-electronic solutions. New energy measurement ICs (integrated circuits) are enabling accurate, dependable and robust meters with all the bells and whistles.

Figure 1: Block diagram of all-electronic energy meter using ADE7753

METERING ICS

Analog Devices has extended its expertise in high performance analogue to digital converter (ADC) design to revolutionise electronic energy meters. The new ADE (Analog Devices Energy) measurement ICs combine high accuracy ADCs with fixed function digital signal processing (DSP). The products interface directly with current and voltage sensors and perform the calculations necessary for metering. A Serial Port Interface (SPI) on the ADE products enables communication with a small microprocessor to control the functionality, perform digital calibration, and read the results of the measurements. Meter designers can now spend far less time developing algorithms in their microprocessors, and solutions with higher-level functionality can be deployed in shorter time.

ADVANCED FEATURES

Analog Devices' newest energy measurement IC is the ADE7753, which combines proprietary energy measurement technology with high performance ADCs to offer the most extensive feature set available today in a metering IC.

FLEXIBLE SENSOR INTERFACE

The ADE7753 gives designers freedom to choose from a variety of current sensors: shunts, current transformers (CT) or Rogowski coils. For low current meters, the programmable gain amplifier (PGA) allows a small, low-cost shunt to be connected to the IC. In higher current meters and where line isolation is required, CTs with burden resistors are most common. The phase calibration register can be set up to correct for phase shift introduced by the CT.

Rogowski coils output a voltage signal proportional to the time derivative of the current (di/dt). The ADE7753 has a selectable digital integrator for use with this sensor type to convert the signal back to I(t) for further processing.1

ACTIVE, REACTIVE, AND APPARENT ENERGY MANAGEMENT

Household loads are not exclusively resistive and utilities want to recover revenue lost due to the inductive and capacitive loads unaccounted for in KWh-only billing. Utilities are increasing metering requirements to include VA and VAR-hour outputs, four-quadrant billing and power factor information. The ADE7753 addresses these advanced needs, by providing signed registers for active, apparent, and reactive energies.

Figure 2: Block diagram of ADE7753

Accurate reactive energy measurement is achieved through a low pass filtering method. Traditional time-delay and power triangle methods for calculating reactive energy rely on assumptions that the current and voltage signals are perfect sinusoids at the fundamental line frequency. The low-pass filtering method reduces the error in the reactive energy calculation when harmonics are present on the line. Additionally, frequency compensation using the ADE7753's period measurement feature lessens error due to variations in line-frequency.2 Apparent energy is calculated inside the ADE7753 by multiplying the Vrms and Irms measurements. The RMS values of the current and voltage signals are also available in registers.

The ADE7753 has an optional accumulation mode, which will accumulate energy only when the sign of the power is positive. This ‘positive only accumulation mode' is required in the UK, and results in billing only for imported energy and not for energy exported by the customer. Additionally, a selectable indication shows when the direction of power flow changes.

In noisy environments, meters can register additional kilowatt-hours by accumulating the small noise resulting in ‘creep' and over-billing. The ADE7753's internal ‘No-Load threshold' prevents the accumulation of this noise.

The ADE7753 has sag detection, which monitors the line voltage. When the power supply begins to drop, the IC provides a warning to the micro-processor, so that the last value of the energy registers can be stored in the non-volatile memory. In this way, energy accumulation is not lost when power outages occur.

ACCURACY AND CALIBRATION

The ADE7753 exceeds the IEC61036 Class 1 and ANSI Class 0.2 meter specifications with 0.1 % linearity error for active energy accumulation over current dynamic range of 1000:1 (typical). The design also accommodates the IEC61268 for Class 2 VAR-hr meters.

The ADE7753 takes advantage of automated digital calibration for excellent long-term stability. This calibration does not rely on mechanical gears that wear out over time. The one-time digital calibration can be automated by programming the microprocessor to calculate the calibration constants. Once the gain, offset and divider constants are determined, they are stored in the meter’s non-volatile memory. When power outages or voltage sag events occur and are resolved, the meter can reload the calibration constants to the ADE7753 from the memory.

Figure 3: ADE product selection guide

A calibration pulse output (CF) displays a frequency proportional to active energy. During production, CF may be monitored under a test load and compared to the expected impulses/kWh to verify calibration. A similar test can be performed in the field to prove a meter's accuracy.

Meters that must operate in harsh environments can take advantage of the ADE7753's temperature sensor for additional compensation.

POWER AND QUALITY INFORMATION

Power quality monitoring has traditionally been reserved for industrial customers. Today even residential customers are sensitive to changes in power quality. With the ADE7753, a low-cost residential meter can also monitor power quality events. This additional information allows utilities to make more informed decisions about generation, distribution and customer services. The ADE7753 records peak current and peak voltage levels, voltage sag events, line period, and missing (voltage) zero crossings. Additionally, the waveform register contains the digitized waveform that can be captured and reconstructed for analysis.

CHOOSING A METERING IC

Many meters do not require the extensive features that the ADE7753 offers. Analog Devices has an extensive portfolio of high performance metering ICs to accommodate a variety of single and polyphase meter architectures. Simple kilowatt-hour, stepper-motor counter designs can be achieved using ADE7751, ADE7755, or ADE7757 for single phase and ADE7752 for polyphase meters. For advanced feature, LCD display meters, designers may choose the ADE7756, ADE7759, or ADE7753 for single phase and ADE7754 for polyphase systems.

  1. For more information about Rogowski coils, see Smart Energy International Issue 3: 2001
  2. For more information about Reactive Energy Measurement Methods, see Smart Energy International Issue 1: 2002