Enphase M250 microinverter
Power line communication (PLC) is one of those technologies that is rediscovered every few years, for better or worse. The reason is that the wires carrying the voltage/currents being measured usually go to the same places where the data is needed. The allure of piggybacking that data on the power wiring essentially for free is enticing.
Most PLC schemes inject a modulated carrier signal at some frequency above the AC fundamental directly into the line and retrieve it at the other end via a high pass filter. If you look at the statutory rules governing the use of PLC it’s not difficult to spot their roots going back to a time when low power AM radio signals in the broadcast band were injected into the line for campus radio stations. Some folks still refer to PLC by the older name, “carrier current” and it apparently worked pretty well for local area broadcasting.
Digital PLC technology today generally splits into two types, narrowband and broadband. Narrow band PLC tends to have data rates from a few to a few hundred bits/second and generally operates in the frequency bands below a few 100 KHz. There are a plethora of commercially available integrated circuits on the market implementing a PLC transmitter and receiver plus digital/analog interfaces in a single chip. Simple systems have a reach determined by the transmit power, the in-band noise and the natural attenuation of the transmitted signal as it propagates down the wire, typically less than 500 feet.
It’s worth pointing out at least one exceptional contemporary application of narrowband PLC and that is the M215/M250 solar microinverters from Enphase Energy. The problem is to haul the operating data from a group of individual solar panels back to a small central aggregator box which then forwards it over to internet to be processed and ultimately made available to the end user. The data load is relatively small but this application demands a many-to-one point communication system so Enphase designed a proprietary PLC protocol to handle it. In addition to good engineers Enphase must have also hired at least one English major who came up with a superb name for the system, “ Enlighten.”
Broadband PLC is another matter entirely. These systems are as sophisticated as any modern wireless link and achieve 100’s of megabits of data throughput operating over the wires in the frequency band between about 2 MHz and 30 MHz. Channel sounding, bit loading and error correction algorithms are combined with OFDM modulation and state-of-the-art MAC layers into inexpensive IC solutions. The flexible MAC layers also allow independent networks of many stations to securely communicate over the same wires and can extend range almost indefinitely through repetition.
So, what’s not to like about broadband PLC? The fundamental problem is that the power line channel is noisy and not reliable. There is an old joke in the PLC community that optical fiber was invented for a reason and that is very true – it is reliable channel with highly favorable and stable characteristics. It is impossible to guarantee that any segment of power wiring with 100’s to 1000’s of volts on it will act as a stable channel for an intricate communications signal that’s riding along for free.
There is, however, one broadband PLC application worth singling out in the electric utility market and that is an advanced metering infrastructure (AMI) system revolving around de-facto standards called PRIME and G3. These are highly-repeated systems based on OFDM methods and complex MAC layers operating in the CENELEC bands below about 400 KHz. So far these systems have found the most commercial traction in Europe and despite the inherent poor quality of the power line as communications channel they do work well enough to be commercially viable.
No discussion of PLC technology is complete without a few words about RF interference. The bottom line is that just because PLC systems are compliant with the various statutory regulations for the countries where they operate does not mean they are undetectable or even particularly good neighbors to nearby in-band radio receivers. RF signals running along on unshielded wires radiate into the environment per the complexities of antenna theory and the physics of RF propagation. Who gets bothered by this and how much is determined largely by the operating bands and the noise floor.
PLC can definitely be useful but perhaps more than anywhere else in the general field of AC power metrology you need an expert on your team if you’re going to get it to work for you. That’s DJA Engineering.