[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.
METERING FUNDAMENTALS Dating back to the 1890s, the electromechanical induction meter most of us grew up with is based on reluctance, or an eddy-current motor. A horizontal metallic disc rotates in a field supplied by a permanent magnet. Induced fields from ac voltage and the supplied current create a torque that will rotate the disk. Eddy currents from the disc's rotation through the permanent magnetic field retard that rotation.
The ac line voltage and the power being drawn both affect the disc's rotational velocity, which is therefore proportional to the power demanded by the circuits connected to the meter. The disc rotations increment a multidial clockwork mechanism that maintains a record of energy consumption. Commonly, one disc revolution represents 7.2 Watt-hours (Wh).
The international standard for electricity meters is IEC 61036. It specifies environmental requirements such as how much power the meter itself can dissipate and how much voltage it must tolerate, along with performance specs such as accuracy and electromagnetic compatibility.
IEC 61036 meters fall into Class 1 or Class 2, based on accuracy. Further subdivisions in these classes are based on the nominal operating voltage (UN), the base current used for most measurements (IB), and the highest current at which accuracy is guaranteed (IMAX) (see the tables 1, 2, 3: "IEC 61036 Electrical Requirements").
DIGITAL METER DESIGN
A great deal of design help is available from chip vendors that manufacture metering chips (Fig. 2). Plenty of information is online for engineers who want to evaluate their options. Analog Devices, Cirrus Logic, Maxim, Microchip, and Texas Instruments all offer chips expressly for digital metering. But it's also a business for fabless semi companies such as Teridian (Fig. 3).
To understand what's inside a basic metering chip, consider Analog Devices' ADE7752, which reads only active power. ADI calls it a "polyphase energy metering IC with pulsed output," meaning it's strictly a measurement device. Intended for wye- or delta-connected three-phase sources, the chip incorporates a total of six 16bit, second-order, delta-sigma analog-to-digital converters (ADCs). The bandwidth of the active power measurement is 14 kHz with an oversampling rate of 833 kHz.
Externally, a voltage sensor could be a potential transformer or a resistive divider (Fig. 4a and 4b). The transformer, of course, provides isolation from the main voltage. With the resistive divider, the chip input is biased around the neutral wire. The virtue of the divider approach is that adjusting the ratio of RA, RB, and VR offers an easy way to calibrate gain.
For current sensing, ADI recommends a current transformer for each channel (Fig. 4c). The common-mode voltage for the current channel can be derived by center-tapping the burden resistor (RB) to the metering chip's analog ground. The transformer turns ratio and the value of RB are selected to provide a peak differential voltage of 500 mV at maximum load.
After the voltages and currents are digitized, high-pass filters remove the dc component from the current signals, eliminating any offset effects. Instantaneous power is obtained by multiplying the current and voltage signals of each phase. To extract the active power component, the instantaneous power signal on each phase is low-pass filtered. The results are added to obtain the total active power.
The metering chip's output frequency, which is proportional to the average active power, is obtained by accumulating the total active power information. The average active power information then can be accumulated to obtain active energy information.
The ADE7752's low-pass filtering approach only records active power and ignores reactive and harmonic currents. Figure 5 shows the unity power factor on top and a condition with a purely reactive displacement power factor (DPF) = 0.5 (current lags voltage by 60 ) on the bottom. In the simple case where the voltage and current waveforms are sinusoidal, the active power component of the instantaneous power signal (the dc term) is one-half the voltage times current times the cosine of phase displacement.
Extending that to the case of any non-sinusoidal voltage and current waveforms and separating those waveforms into their Fourier components: where (see equation):
v(t) is the instantaneous voltage VO is the average value VN is the root mean square (rms) value of voltage harmonic N αN is the phase angle of the voltage harmonic.
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