The Hybrid of PLC and RF Maximizes Reliability in Smart Grid Networks
An unholy trio of impedance variations, attenuation on selective frequencies and noise interference is at war with PLC communication, but the Smart Grid is fighting back with an strategy that makes PLC the backbone superhero of the network with low-power RF as a trusty sidekick.
Intelligent self-configuring fully adaptable networks that connect the power producer with the consumer form the Smart Grid. Smart grids create a platform of robust data networks that enable bi-directional exchange of data for all kinds of power supplies and electrical devices that are 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 and outages.
The use of power line communication (PLC) and low-power RF (Radio Frequency) as the communications media for smart grids has many advantages over traditional twisted pair RS-485 networks. With the absence of data cabling between nodes, the team of PLC and RF is easier and less costly to install and provides better communications security.
PLC Network Technology
PLC is 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. This has attracted the attention of power producers and users. Like RF wireless modules, PLC modules can be easily embedded into electrical meters. With mesh networking technology, data collection units (DCUs) are able to exchange data with all the electrical meters within their network of control. Power lines go through floors and walls in the building. Therefore, theoretically, as long as there are power lines, communications over power line may be achieved. However, power lines are constructed with the primary objective of delivering electricity. Its complex distribution network and noisy environments may cause interference, resulting in unstable communications. The factors causing interference can be summed up as:
- Huge load impedance variations impact PLC signals: Load impedance changes will affect PLC signal voltages coupled onto the power lines, and this has a direct impact on 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 varies randomly.
Low-Power RF Networking Technology
Low-power RF networking refers to the use of 315M/433M/780M/2.4GHz frequencies with transmit power equal to or less than 50mW. 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. Low-power RF modules can be embedded directly into the meter during production and installed on site without laying cables when deploying. Mature wireless mesh networking technology allows the concentrator to communicate with all the meters within its network control. This kind of low-power RF network is most suitable for deployment within a restricted range where there is a concentration of a large number of low-power communications modules, for example, 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 are easily embedded into electrical meter, DCUs and electrical appliances.
As low-power RF communications make use of publicly available radio frequencies, other devices that make use of the same frequencies will inevitably cause signal interference. Also, RF signals are vulnerable to obstructions such as walls, which cause signal instability and result in shorter effective communication distances. Signal interference can be alleviated with frequency hopping.
However, when other devices also use frequency hopping to counter interference, this in itself introduces more interference. Hence, the problem of mutual interference cannot be easily resolved. RF signals’ vulnerability to obstructions also limits their use in smart grid applications. For example, wireless communications between different floors, for instance, between the basement and the ground floor, are often impeded by thick walls resulting in unstable or no communications at all. On the other hand, a PLC network could easily resolve such problems.
PLC devices, like RF devices, may be networked, thus boosting 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 50kHz to 500kHz. With this ability to use the entire spectrum comes the wherewithal to counter the three major problems notes earlier: impedance, attenuation and interference.
First, depending on different load impedance situations, transmitter output power needs to 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 for fixing these problems is the use of single frequency hopping. PLC OFDM technology, which uses multiple carrier frequencies, is effective in countering selective carrier frequency attenuation. However, its inherent Peak-to-Average Power Ratio issue presents another set of problems, which results in signal power being averaged down as compared to using a single carrier frequency. An effective approach that is similar to frequency hopping in OFDM is the use of a single carrier frequency to automatically change to the next best carrier frequency when the current carrier frequency encounters interference. This single-frequency hopping has the advantage of ensuring that there is sufficient power coupled to the power line while effectively addressing signal interference issues due to load impedance variations and selective carrier frequency attenuation.
By changing the transmit output power and the carrier frequencies between two nodes in point-to-point communications, load impedance, line attenuation and noise interference may be generally overcome. This effectively improves the reliability and distance for point-to-point communications thus providing a layer of robustness to mesh networks.
Hybrid Implementation Achieves the Best of Both Worlds
While the above measures are effective, they still cannot guarantee a foolproof PLC network in all situations and at all times. To best maximize the reliability, gridComm has developed a hybrid approach with an integration of low-power RF wireless networking technology with ever-robust PLC technology.
The approach is to make 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 this in places of overly strong signal interference or where the power lines are physically separated, or on different phases. In addition, RF with reduced power to avoid mutual interference may be used in wide-open places with a high concentration of electrical equipment.
Smart grids deployed using the hybrid approach will form a highly robust network that should counter most of the issues of signal obstruction, interference and attenuation, maximizing Smart Grid networking reliability.
Li Zhou Dai is the CTO of gridComm, a company that provides fully adaptable self-configuring PLC solutions for smart grid, smart street lighting and all other forms of M2M. Dai, who architected the GC2200 OFDMA PLC transceiver, has several years of research and development experience on PLC at the Harbin Institute of Technology. Dai designed the first PLC-based AMI system in China in Langfang city of Heibei province that automatically controls load balancing and consumption of households besides performing normal AMR. This is accomplished by integrating the Medium-Voltage network with the Low-Voltage network using the GC2200 as the underlying PLC device. More than 30,000 units of GC2200 are currently deployed between household low-voltage meters and the medium-voltage distribution system. Dai graduated from Harbin Institute of Technology in China with a Master of Engineering in Electrical Control and Automation.
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