Finding the right combination for optimized power line communications
Communications over the power grid presents several challenges including noise, attenuation, and distortion. Engineers must make the right trade-offs in defining and configuring power line communications systems to provide optimal performance.
Power Line Communications (PLC) enables intelligent Machine-to-Machine (M2M) communication in the evolving power grid to establish greater efficiency and productivity, streamlining energy delivery to the world’s increasingly integrated and demanding economy.
Given that the power grid is an established ubiquitous network, connectivity via PLC is potentially the most cost-effective, scalable interconnectivity approach for utility-side as well as consumer-side applications. Utility-side applications include automatic meter reading, dynamic rate management, load management, load profiling, and street light control/management systems. Consumer-side applications include smart appliance connectivity and home automation. Additional applications include other electrically connected devices requiring monitoring and control, such as vending machines, solar panels, electrical vehicle charging, and other data-gathering and control systems.
Narrowband PLC has been around for many years, but with recent advancements in technology, increasing needs in M2M connectivity, and awareness of better resource management, it is gaining momentum as the most ubiquitous network medium available.
Due to harsh noise and variations in equipment and standards, communications over the power grid is difficult. Engineers using PLC must optimize their designs to achieve the best combination of distance, data throughput, error rates (packet retries), and overall reliability. To meet PLC systems requirements, engineers must configure modulation, select modulation frequencies, determine the level of packet redundancy, and select appropriate signal levels.
Difficulties of communications over the power grid
The power line network presents a number of roadblocks for data transfer, as the primary design goal of the power line network is electric power distribution. It was not originally designed as a communication channel. Signals propagating along the power line are subjected to very large amounts of noise, attenuation, and distortion that make them erratic and frequently variable over time. As a result, data communications requires a modem whose configuration changes to match the variations that result over power lines.
Power line noise
Many noise sources on power lines result in interference, cross-chatter, and signal distortion. Electrical devices connected to the power mains can inject significant noise back onto the network. The characteristics of the noise from these devices vary widely and are classified as impulse noise or tonal noise. Impulse noise sources include light dimmers, motors, power line-based intercom modules, and impulses that result from power utility equipment switching. Tonal noise sources include switching power supplies (PCs and electronic fluorescent ballasts).
Attenuation results from the wiring topology of the power line infrastructure within buildings, between buildings and concentrators (low-voltage network), and connectivity to utility/substation equipment like transformers and switches (medium-voltage network). Attenuation levels vary greatly within a given location and across power line installations. It is not uncommon to observe attenuation variances from 2x to 500x (3 dB to 30 dB) in outlets of the same building and even in close proximity to each other. Note that adjacent outlets do not guarantee a short wiring path or less noise.
Attenuation is also caused by loss from transformer coupling, distribution wire cross-coupling, and multiphase load impedance. This results in attenuation in the range of 25 dB to 60 dB near 100 kH within the low-voltage network. PLC is also used across much larger distances (as far as 15 km on medium-voltage utility lines), leading to greater signal attenuation. Transformers and DC-DC converters attenuate the input frequency signal almost completely. Bypass devices become necessary for the signal to be passed to the receiving node.
So why not simply increase the transmit signal level to compensate for attenuation loss? While this is exactly what is done in a configurable PLC modem, signal amplitudes and total power on the channel are regulated. For Europe, the CENELEC standard limits signals to a maximum amplitude of 116 dBμV, with a frequency band from 9 kHz to 145.5 kHz divided into specific bands for electricity suppliers and consumers. In the United States, the FCC similarly limits signal and maximum power spectral density with a maximum frequency of 550 kHz.
Signal distortion resulting from power line impedance
Varying power line impedance distorts the communications signal. Varying impedance results from transformers, filters, and coupling devices connected to the power line and switched in and out at different times.
PLC design trade-offs
Table 1 shows the trade-offs PLC engineers make when implementing PLC designs.
To both enable data communications with the channel variations of PLC and make the appropriate trade-offs of communications distance, data throughput, reliability, and network cost requires real-time PLC modem configurability. What complicates the problem is the general nature of PLC experience – high variability, variable attenuation, noise, and phase distortion – that leaves only a few good channels available for reliable communications. Consequently, it is critical that the power line modem support multiple configurable data channels, as illustrated in Table 2.
Configuring PLC for maximum performance
Selectable modulation: Example trade-off of noise immunity and data throughput
Triple-carrier Binary Phase-Shift Keying (BPSK) enables 3x the data rate on a given channel when compared to single-carrier BPSK. For channels at frequencies with more noise, lower data rate and higher noise immunity BPSK modulation can be used at the expense of reduced data rate. For less noisy channel frequency, higher data throughput 3PSK is used. This allows trade-offs of data rate and performance in noisy environments.
Multiple individually configurable data channels: Example trade-off via number of channels and amplitude
As shown in Figure 1, selecting fewer data channels enables higher amplitude per channel to address attenuation issues. Additional channels allow more data through and possibly increase reliability due to data redundancy at the expense of lower signal-to-noise and increased error rate. Configuring each channel amplitude individually enables an amplitude boost of channels with the most attenuation while still meeting requirements that regulate maximum channel power. Real-time selection of the modulation frequency allows the PLC modem to avoid impulse noise that might prevent communications.
Example trade-off via data redundancy
Hardware data redundancy allows for reduced error rates resulting from noise. The same data packets can be transmitted over multiple channels, or different data packets can be used on each channel (see Figure 2).
Achieving reliable, high-throughput communications
Data communications over the power grid is difficult, mainly due to extreme variations caused by noise, attenuation, and signal distortion. To meet PLC system requirements, engineers must configure modulation, select modulation frequencies, determine the level of packet redundancy, and select appropriate signal levels. This challenging communications environment requires the PLC modem to be configurable in real time. With multiple configurable data channels, selectable modulation, data redundancy, configurable network protocol, and configurable automatic gain control, reliable, high-throughput narrowband communications over long distances is achievable.
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