[Technology Report]
A Measure Of Opportunity Awaits In Electric Meters
Move over consumer and telecom worlds. Recent developments turn electric metering into an unexpectedly fertile field for engineers of all types.
POWER FACTOR Metering gets more interesting when it's necessary to consider power factor (PF), as it is in the European Union as well as in China and parts of India. Europe's power-factor-correction (PFC) standard is EN 61000-3-2, with Amendment A14.
Before switching power supplies became common, power factor was associated with reactive loads. Power companies dealt with it by installing big capacitors at switching yards and substations. Switching supplies changed that situation by introducing currents on the power line that are out of phase with the voltage and consist of multiple harmonics of the power-supply switching rate.
Electrical utilities care about power factor for two reasons. First, the out-of-phase power component represents system capacity that isn't available to do real work. Second, utility regulations prevent the supplier from charging for the out-of-phase "imaginary" component.
EN 61000 PFC's objective in switching supplies is to limit the magnitude of the individual harmonic currents up to the 39th harmonic. Amendment A14 relaxes some requirements, but not for personal computers, monitors, and television sets.
POWER FACTOR AND METERING For a look at how energy-metering chips separate reactive power from active power and report it, consider Cirrus Logic's CS5467, which comprises a four-channel ADC and a computation engine. The chip has two current channels and two voltage channels.
As with ADI's ADE7752, external voltage- and current-sensing elements generate signals that are amplified and applied to delta-sigma modulators (second order in the case of voltages, fourth order in the case of currents) inside the Cirrus chip. A cascade of Sinc3 and third-order infinite impulse-response (IIR) filters lies on the outputs of the data converters. The Sinc3 decimates, and the IIR compensates for the 5-magnitude roll-off of the low-pass decimation filter.
The Cirrus chip adds a calculation of apparent power, based on rms calculations on multiple instantaneous voltage and current samples. Apparent power is the combination of active and reactive power. Power factor is the active power divided by the apparent power. The active power determines the sign of the power factor.
Also, the CS5467 calculates the reactive power as the square root of the difference between the square of the apparent power minus the square root of the apparent power. Active, apparent, reactive, and fundamental power get updated every computation cycle.
To achieve average reactive power, the chip averages the voltage and multiplies that value by the current measurement with a 90 phase difference between the two. The 90 phase shift is created by another IIR digital filter in the voltage channel. The filter provides exactly 90 phase shift across all frequencies and uses the ratio of the input line frequency to the sample frequency to achieve unity gain at the line frequency.
Subsequently, the instantaneous quadrature voltage and current samples are multiplied to obtain the instantaneous quadrature power. The product is then averaged over N conversions. For details, see the CS5467 data sheet at www.cirrus.com/en/pubs/proDatasheet/CS5467_A1.pdf.
METERING TRENDS What's new in meters? According to Microchip, which makes standalone meter chips as well as chips that work with its PIC microcontrollers, utilities are primarily driven by the need to eliminate the costs associated with periodic meter readings by employees who record data meter by meter.
In environments that invite power theft, such as apartment buildings, the meters no longer are located on the outside wall of the building. Instead, they're in a locked closet inside the building. Human "meter readers" plug their apparatus into a box on the side of the building to capture data from all of the meters.
Plug-in remote reading isn't just for large residential structures. Easily accessible but tamper-proof interface boxes are appearing in single-home construction as well. They make the human meter reader's job somewhat more efficient and eliminate much of the potential for interactions with guard animals, overgrown rosebushes, and paranoid homeowners. The next step is to make the process wireless and keep the meter-reading human on the street or sidewalk.
Microchip says that as theft of services becomes more of a concern, some meters are including a separate ADC to continually monitor the line voltage to detect incidents. For instance, it could detect if the meter is temporarily disconnected or has its terminals swapped (to make it run backward).
Meters that use transformers, rather than shunts, for current measurement are susceptible to core saturation using external magnets. If that's perceived to be a potential problem, the meter may be given a shunt and a transformer, and the two can be checked against each other.
Microchip's meter-reading products aren't just adaptations of PICs with standard microcontroller peripherals. The company's latest replaces the customary RS-485 interface with a serial peripheral interface. Figure 6 shows a three-phase reference design based on three of these MCP3909 ICs, plus a PIC18F2520 and a PIC18F4550 microcontroller.
The PIC18F2520 performs the power calculations, while the 4550 provides a USB interface to desktop software. A software package that comes with the reference design enables meter calibration.
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