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Microcontroller-Based Energy Metering using AD7755
energy metering industry converts from electromechanical meters more-accurate solid-state meters, power-system designers have chance incorporate features that weren't previously possible. demand solid-state meters that measure energy more accurately than electromechanical meters, incorporate multiple-rate billing, capable being read remotely utility company. This article describes AD7755 Energy Meter1 integrated circuit could used three-phase energy metering with power outage detection measurement backup, remote, automated, multiple-rate metering. AD7755 accurate 0.1%) single-phase energymeasurement accepts pair voltage inputs that represent voltage current power line. Internally, these signals converted digital domain with oversampling converters. fixed-function digital signal processor continuously multiplies signals; their product proportional instantaneous power. After being low-pass filtered, digital signal then converted frequency-scaled according selectable settings-to generate frequency outputs terminals signals used drive electromechanical counter (typically full-scale rates from Hz), while higher-frequency signal suitable calibration. frequency rate) pulse outputs proportional instantaneous real power being monitored meter. Accordingly, given interval, total number pulses generated these outputs proportional energy transferred load. reverse-polarity logic signal indicates when measured instantaneous power goes negative (i.e., load returning power line).
Figure shows three AD7755s-one each phase-are used with microcontroller make three-phase energy meter.
SOURCE
POWER SUPPLY
POWER DOWN SEGM RESET CURRENT SENSING VOLTAGE SENSING
AD7755
COMM ENABLE
REVP
3-to-8 DECODER
CURRENT SENSING VOLTAGE SENSING
CLEAR
DISPLAY
MODE ENTER
AD7755
REVP
CURRENT SENSING VOLTAGE SENSING
RESET ENABLE
AD7755
REVP
EEPROM
LOAD
Figure Functional block diagram threephase microcontroller-based energy meter microcontroller serves "brains" system, performing required housekeeping tasks interacting with other components-the energy meter ICs, power supply, EEPROM, display, buttons operate meter-to view energy power, calibrate phases, clear reading. Besides cost, basic microcontroller requirements are: *Sufficient drive display. display used driver required. incorporated into MCU, display easily controlled with 3-to-8 decoder. *Interrupts. avoid missing energy-indicating pulses, system configured trigger interrupts MCU. power supply monitor generate interrupt when detected brownout condition initiate emergency energy measurement backup. *EEPROM Serial Interface. simple serial interface created using only three lines. with built-in serial interface makes design even easier. *Timers. There main time intervals that need maintained. First, display update rate must about seconds. Also, display used, timer must cycle through digits sufficient rate minimize flicker. Additionally, calibration routine must carefully timed, implemented with interrupt postscalers. added feature, second serial interface could used communicate with host system remote/automated metering. Also, either external internal clock could used implement multi-rate metering. Reference Design: three-phase energy-meter reference design (Figure been implemented demonstrate multiple AD7755s interfaced microcontroller. uses Microchip PIC16C67 microcontroller2, serial EEPROM, 8-digit display, current transformers current sensing, resistor dividers voltage sensing. Power furnished transformerbased supply incorporating power-loss detection. analog interface AD7755s instrumented with voltage divider resistors voltage channels current transformers
AVDD
AGND
AC/DC
DVDD
DGND
AD7755
POWER SUPPLY MONITOR PHASE CORRECTION
.110101.
SIGNAL PROCESSING BLOCK
MULTIPLIER 2.5V REFERENCE DIGITAL-TO-FREQUENCY CONVERTER RESET
.11011001.
REFIN/OUT
CLKIN CLKOUT
REVP
Figure Block Diagram AD7755 frequency output pulse train proportional F1,F2 outputs, with full-scale output rates 21.76 43.52 5.57 kHz, inputs. well suited interfacing microcontroller that performs calculations makes decisions.
www.analog.com www.microchip.com
Analog Dialogue 33-9 (1999)
current channels. current transformers provide degree electrical isolation eliminate need current-sensing shunts within meter. microcontroller code written programming language been programmed into PIC16C67. particular compiler that used also includes instructions interface Microchip serial EEPROM, which stores energy measurement digits calculation limits obtained calibration mode. display consists digits multiplexed 3-to-8 decoder. power supply uses three transformers, rectifier, regulator convert 220-V, 3-phase voltage capable powering meter even phase goes out. reference design data sheet/application note available3. Since meter determines cost energy user, most important requirements energy meter reliability accuracy over time. energy measured fairly simple way-by pulse-limit comparison. this method, microcontroller counts number pulses phase until total reaches calibrated limit. this point, energy reading incremented smallest unit within range display this case, 0.01 kWh). This technique implies that display register need only updated when necessary also avoids complex numerical operations that could make meter operate inefficiently. maximum output frequency slow mode 43.52 156,672 pulses/h. Allowing headroom, 220-volt, 60ampere system could calibrated that 0.01 measured energy produces approximately pulses Calibration done high-frequency mode, which maximum frequency times faster, 5.57 kHz. During calibration, fixed power line value that yields 1/128 over calibration interval. Because scaling factor 128, number pulses counted during calibration time equivalent when AD7755 returned low-frequency mode.
Suppose 10287 pulses been counted during this interval. Then display would have incremented 0.01 every 102.87 pulses, rather than every pulses. This fractional-N count could accomplished several ways. example, during time required advance display increments (i.e., accumulate 1.00 0.01 steps), those steps could produced count pulses, other steps would require count pulses. alternative that been starting with pulses step, pulses every 10th step, more pulses every 100th step. high frequency setting AD7755s yields better results shorter calibration time (about seconds). possible variations system, historical practices, each phase three-phase meter calibrated independently. Conclusion: basic solid-state meter design like that described above will most likely more accurate, more reliable, cheaper than electromechanical meters, will allow added features that benefit both customer utility company. near future, utility company will monitor your energy consumption remotely bill according peak- off-cycle usage-or even Motorola4 system) allow keep track your usage. only will measurement more accurate, but, implemented over entire electrical network, solid-state metering allows more efficient energy management.
References
Three-Phase Energy Meter reference design data sheet (REFAD7755-3). Analog Devices. AD7755 Energy Meter with Pulse Output data sheet. Analog Devices. Daigle, Paul. "All Electronic Power Energy Meters," Analog Dialogue. Volume Number February, 1999.
Analog Dialogue 33-9 (1999)

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