Are Wireless Links killing Power Line Communication?
IoT, sub-GHz, LPWAN… Everyone who is aware of contemporary trends in telecommunication technology and its evolution could not have missed these ‘magical’ abbreviations. All prominent manufacturers of microcontrollers, sensors, communication chipsets and their components offer some ‘IoT ready’ components. At first, it may seem the bells have tolled for PLC (power line communication) and that the future belongs to wireless connection. But is that really the case?
LPWAN
Huge range, low price, easy to implement, negligible consumption – that is what these networks promise us. We can distinguish between two main types:
Unlicensed bands
Most frequently it is 868 MHz band for European applications and 900-915 MhZ for the US. Some networks also use 433 MHz 2.4 GHz or the spectrum originally established for TV broadcasting.
Typical members of this family are Sigfox, LoRa, Weightless, Nwave and others. Working in the unlicensed frequency bands requires following a rather strict set of rules given by the local regulator.
To give an example, for 868.8 MHz spectrum the transmit power is restricted to 25 mW ≈ 14dbm and transmit duty cycle to 0.1% (without LBT – listen before talk). This restriction is the reason, for example, the end point of the Sigfox network allows 140 sent messages maximum a day, and only 4 received messages.
Licensed frequency bands
These include LTE-M or NB-IoT standards. These are mostly providers of mobile networks who already have GSM and LTE infrastructure and own a license to it. Using these ‘old-new’ technologies, with this type of network, massive rollouts are expected over the next few years.
Distribution of electricity and grid control
The most commonly used Power Line Communication application is remote electricity meter reading. With the massive upswing of renewable energy use, distributors are facing serious problems.
The main goal is to get the immediate volume of generated and consumed energy in balance. Simple meter reading then changes into a more complex issue as the energy grid needs to be regulated accordingly. Regulation of this sort places high demands upon communication infrastructure across the whole electrical network – from electric meter to substation and then to distributor head-end system.
The task is to keep the defined availability, throughput and latency across the entire route. Experience shows that it poses an unsolvable problem for the technology currently being used (GPRS, 3g or LTE network).
Data from field
Using LPWAN for remote reading of electricity meters and controlling and regulating the distribution grid may seem to be convenient at first. Tracking vehicles and shipments, monitoring systems, controlling public lighting and many other applications could follow.
Can we fit all that into the unlicensed spectrum? What will happen during LPWAN network overload? What will happen, especially in big cities, to throughput and latency which are crucial for electrical grid regulation?
Electricity meters can be (and frequently are) located in the basement/cellar of the house or in a utility room without any windows, which means no wireless service. If an external antenna is not installed, LPWAN are practically useless in unlicensed frequency bands.
Another factor that can influence the selection of appropriate technology for the strategic role of electrical grid control is dependence on other subjects. It is common that one provider has a monopoly of a given wireless standard in the country. Many standards require the use of a cloud backend operated by the network provider to ensure data collection from the end points or using a chipset only one licensed manufacturer supplies.
There are cases in which it is impossible to use PLC. For instance, meters in remote locations, out of PLC mesh network reach. GSM connection is used in these situations, despite the higher costs. But these are the instances when the LPWAN might be a more interesting alternative.
Communication between substations
Having transferred the data via PLC network from the field to data concentrator in the secondary (MV-LV) substation, the data path is far from over. It is necessary to create a connection between the data concentrator and primary/central substation (more precisely primary transformer station MVHV). Experience shows that more than 12% of secondary substations are located out of mobile network service.
Apart from the data measured and commands for the electric meter, this channel is also used for substation telemetry and remote control (RTU). To avoid being the bottleneck, it should have better features than subordinate low-voltage PLC communication. Modern interoperable standards such as PRIME version 1.4 or G3-PLC work with speed around 1Mbps in optimal conditions, therefore connection between DC and the head-end system should never be slower. That, unfortunately, excludes most of the LPWAN standards.
Standard WIMAX can be considered appropriate technology as it provides a sufficiently sturdy connection. It operates in the licensed frequency band where channels are allocated by the national regulator. The result is a state where the frequency band is basically ‘sold out’ in perspective areas (for example, used by local internet providers). Another factor is the high price of WIMAX access point (AP).
High-voltage lines are always present in a transformer and compared to low-voltage lines, high-voltage lines are barely influenced by unwanted interference. It allows the use of modulation and higher order coding to achieve sufficient transmission speed. Even in this area PLC is still at least a competitive alternative and its operation is not dependent on any private provider
In-home applications
PLC does not concern only industry or power engineering. Probably the most well-known PLC application is “powerline networking” usually using home plug technology based on IEEE1901 (BPL) standard. Using this method of LAN/WAN transmission is marginal due to contemporary prices, simplicity and accessibility of WI-FI connections. It is only used in areas problematic in terms of WI-FI service.
Multi-utility systems and home automation (measuring values, control of appliances, alarm systems) are another area where it is convenient to use simple and cheap PLC modems as they typically work in CENELEC B and C bands characterised by relatively little interference.
Compared to wireless communication in no-license frequency bands (434 MHz, 868 MHz, 2.4 GHz), they can offer higher range without any restrictions caused by obstacles, such as reinforced concrete walls. It is an easy way to extend the range of Wireless M-Bus, Zigbee and other wireless connections in problematic areas, such as blocks of flats.
Conclusion: The future of PLC technology
Based on the information presented, we can see that even in the time of wireless technology boom, PLC technology has still a lot to offer not only in the area of smart grid.