For the “best” power product, I chose Maxim Integrated Power’s “Petaluma” reference design, which measures and reports on three-phase power as it travels across the grid (Fig. 1). In terms of applicability, it crosses the generation, transmission, and distribution functions of the smart grid. A number of companies, including Maxim Integrated, jumped on the smart-meter bandwagon. The difference with Maxim, though, was that it went on to apply design resources on the other 90% of the equation.
Measuring electrical power as it’s handed off between all entities in the smart grid is not a trivial issue. Compared to making measurements at the home user’s meter, i.e., collecting current and voltage data across a mere two phases 180° apart is relatively simple. Synchronously measuring Steinmetz’ phasors1 is a good deal more challenging.
In Electronic Design’s “Smart Grid Design Opportunities Extend From The Meter To The Mercantile Exchange”, I wrote of seven smart-grid domains: Bulk-Generation, Transmission, Distribution, Operations, Service Providers, Customers, and the “Market” domain. Precise real-time measurements are critical across this whole spectrum.
Essentially, power engineers these days talk less about a “smart” grid (“smart” being an overworked term anyway) than they do about a “distributed” grid. In the latter, energy flows in various directions among large- and small-scale generating stations, microgrids, residential and industrial solar sites, and other non-traditional sources of electricity, such as natural-gas-powered Bloom Energy servers and even parked electric vehicles that buy energy cheap and sell it back when it’s at a premium.
With that image in mind, what is Maxim Integrated’s Petaluma? The company describes it as: “A subsystem reference-design that provides eight high-speed, 250-ksample/s, 16-bit, simultaneous-sampling, analog input channels that accept ±10-V input signals.”
Continuing, “The Petaluma design utilizes two quadruple ultra-precision, ultra-low-noise input buffers (the company’s MAX44252); an 8-channel, 16-bit simultaneous-sampling MAX11046B analog-to-digital converter; a MAX6126 high-precision 4.096-V voltage reference; and regulated +10-V, -10V, and +5-V power rails (Fig. 2).
The company’s literature explains operation this way: “Three-phase power measurement applications require multiple analog inputs for voltage and current measurements on each line of the three-phase system. A power monitoring system must sample all of the analog inputs simultaneously to accurately calculate the instantaneous power consumption.”
This file type includes high resolution graphics and schematics when applicable.
A pair of “quadruple ultra-precision, low-noise op amps” (U1 and U2 in the figure) attenuate and buffer the ±10-V input signals to match the input range of the ADC. The devices are set up in the inverting configuration, so the polarity of the signal is reversed at the input of the ADC.
That ADC, a MAX11046 (U3 in the figure), is a low-cost, 8-channel, 250-ksample/s, 16-bit, single-supply, bipolar, simultaneous-sampling device. Although the ADC itself has an internal 4.096-V voltage reference, Petaluma uses an external voltage reference (a MAX6126 (U4) to provide the highest possible accuracy (the ADC has an initial accuracy of 0.02% and a 3-ppm/°C maximum temperature coefficient).
The point of all this, of course, is to give an external FPGA or micro some numbers to crunch. The Petaluma reference design accomplishes the feat by interfacing via a standard FMC connector. FMC is an ANSI standard for mezzanine-card, field-programmable gate-array/microcontroller development boards.
Maxim’s Earlier Work on the Smart Grid
The company already has considerable experience with products aimed at the consumer/energy-supplier aspect of the smart grid. In addition to components for home and business smart meters, Maxim has long maintained a relationship with the makers of automatic teller machines (ATMs), developing communications security products.
Another range of products developed by the company targets power-line communication (PLC) using sub-500-kHz frequency-shift keying technology below the AM broadcast band. PLC is the preferred mode for smart-meter communications in Europe and Asia, while energy suppliers in North America tend to use RF technology on some of the lower cellular communications frequencies.
1. “In 1893, Charles Proteus Steinmetz presented a paper on simplified mathematical description of the waveforms of alternating current electricity. Steinmetz called his representation a phasor. With the invention of phasor measurement units (PMUs) in 1988 by Dr. Arun G. Phadke and Dr. James S. Thorp at Virginia Tech, Steinmetz’s technique of phasor calculation evolved into the calculation of real time phasor measurements that are synchronized to an absolute time reference provided by GPS. Early prototypes of the PMU were built at Virginia Tech, and Macrodyne built the first PMU in 1992. (Wikipedia)