What’s the Difference Between PLC and RF for Smart-Meter Backhaul?
Smart, self-configuring, fully adaptable networks that connect the power producer with the consumer represent the essence of smart grids. Smart grids create a platform of robust data networks that enable bidirectional exchange of data for all kinds of power supplies and electrical devices plugged into the power grid. This enables remote and active monitoring of operation and fault conditions of the electricity network, thereby delivering the benefits of a highly efficient power network that automatically regulates and controls the distribution and consumption of electricity—without failures or outages.
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The use of power-line communication (PLC) and low-power radio frequency (RF) as the communications media for smart grids holds many advantages over the twisted-pair RS-485 network. Due to the absence of data cabling between nodes, PLC and RF are easier and less expensive to install, and provide better communications security over RS-485.
Low-Power RF Networking Technology
Low-power RF networking refers to the use of 315-MHz/433-MHz/780-MHz/2.4-GHz frequencies with transmit power equal to or less than 50 mW. Low-power RF modules may be embedded within electrical meters, to enable the use of wireless data communications in automatic meter reading (AMR) for power-consumption monitoring and data collection. Such modules can be embedded directly into the meter during production and installed on-site without laying cables when deploying.
Matured, wireless mesh networking technology allows the concentrator to communicate with all of the meters within its network control. This kind of low-power RF network is best suited for deployment within a restricted range that has a concentration of a large number of low-power communications modules (e.g., within a single floor of a building or a room of networked electrical meters).
Low-power RF networking also features low power consumption, auto-routing networks, two-way real-time communications, and mobility. RF modules can easily embed into electrical meters, data concentrator units (DCUs), and electrical appliances.
Because low-power RF communications use publicly available radio frequencies, other devices that utilize the same frequencies will inevitably cause signal interference. In addition, RF signals are vulnerable to obstructions, such as walls, which cause signal instability and result in shorter effective communication distances.
Frequency hopping can alleviate that signal interference. However, when other devices also use frequency hopping to counter interference, this in itself introduces more interference. Hence, it’s difficult to resolve the problem of mutual interference.
The fact that RF signals are vulnerable to obstructions limits their use in smart-grid applications, too. For example, thick walls often impede wireless communications between different floors (i.e., between the basement and the ground floor), resulting in unstable or no communications at all. PLC networks can easily resolve such problems.
PLC Network Technology
PLC offers a unique means of communication for a power-supply system, which takes full advantage of the wide coverage of power-line installations without having to lay dedicated cables. The technology has attracted the attention of power producers as well as users. Like RF wireless modules, it’s easy to embed PLC modules into electrical meters.
Thanks to mesh networking, DCUs can exchange data with all of the electrical meters within its network of control. Power lines go through floors and walls in the building. Therefore, theoretically, as long as there are power lines, it’s possible to achieve communications over them.
However, power lines are constructed with the primary objective of delivering electricity. Electricity’s complex distribution network and noisy environments may cause various forms of interference to PLC, resulting in unstable communications. Interference-inducing factors include:
• Huge load-impedance variations: Load-impedance changes will affect PLC signal voltages coupled onto the power lines, which directly impacts the transmission distance. Changes in power factor and location of power loads will change load impedances dynamically over time.
• Attenuation on selective PLC carrier frequencies: The random switching of electrical devices on a power distribution network may lead to changes in power parameters, resulting in attenuation on PLC signals on selective frequencies. At the same location and instance, this impact may vary across different PLC carrier frequencies. When certain frequencies are unsuitable for PLC, changing to different frequencies for communication might yield better results.
• Strong noise interference: Electrical equipment on the power grid, such as switched-mode power supplies and inverters, can produce significant amounts of interference on multiple frequencies that vary randomly.
PLC devices, like RF devices, can be networked, which boosts effective communication distances between the DCU and its meters. However, the realization of reliable long-distance communications between two points should be the basis of any PLC network. Unlike low-power RF, PLC may often enjoy exclusive use of the entire power-line-communication frequency spectrum from 50 to 500 kHz, thus triggering the above three issues and subsequently affecting the ability to address the reliability of PLC effectively.
There are two ways to tackle the above issues. First, depending on different load-impedance situations, transmitter output power must be automatically adjusted. This would boost the signals coupled onto the power line when required and maximize the transmit distance as much as possible.
The second method involves the use of single-frequency hopping. PLC orthogonal frequency-division multiplexing (OFDM) technology, which employs multiple carrier frequencies, effectively counters selective carrier frequency attenuation. However, its inherent peak-to-average power ratio issue presents another set of problems, resulting in signal power being averaged down as compared to using a single carrier frequency.
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Another method similar to frequency hopping in OFDM is to use a single carrier frequency to automatically change to the next, better carrier frequency when the current carrier frequency encounters interference. The advantage of this type of single-frequency hopping is that it ensures sufficient power is coupled to the power line, while addressing signal interference issues caused by load-impedance variations and selective carrier-frequency attenuation.
Changing the transmit output power and the carrier frequencies between two nodes in point-to-point communications helps overcome load impedance, line attenuation, and noise interferences. In turn, it improves the reliability and distance for point-to-point communications, thus providing a layer of robustness to mesh networks.
Best of Both Worlds
While these measures are effective, they still can’t guarantee a foolproof PLC network in all situations and at all times. To achieve this goal, it’s best to integrate both low-power RF wireless networking technology and PLC technology.
One proven solution is the use of PLC as the backbone of the network supplemented with low-power RF technology. As a backbone, PLC easily works between different rooms or between different floors. Low-power RF then supplements the backbone in places of overly strong signal interference, or where the power lines are physically separated, or on different phases. Furthermore, RF that has reduced power to avoid mutual interference may be used in wide-open places with a high concentration of electrical equipment.
Smart grids deployed in this manner will form a highly robust network that should counter most of the issues of signal obstruction, interference, and attenuation.
Li Zhou Dai is CTO of gridComm.