Fig 1. Semitech’s SM2200, a power-line communications packet-data modem transceiver, uses OFDMA to achieve up to 175-kbit/s data rates on low and medium power lines.
Fig 2. OFDM simply splits one fast carrier into many slow carriers. By spreading data across three carriers, one bit is three times the length for roughly the same bandwidth. Each bit is much longer so it is more immune to noise.
Power-line-communication (PLC) semiconductor devices can transform the electricity grid into the Smart Grid by turning it into a communications network. Such a network connects utilities to their customers, making homes energy-aware (“smart homes”) and able to react to conditions on the grid. This includes connecting smart meters, Smart Grid monitors, and street lighting.
Due to harsh noise, changing conditions, and variations in equipment and standards, communications over the power grid become difficult. Reliable operation in this challenging environment, as well as successful interoperation with previously installed equipment, requires new PLC approaches.
Defining PLC
Also known as power-line carrier, power-line communications encompasses systems that use electric power lines to carry information. All PLC systems operate by impressing a modulated carrier signal on the wiring system.
Different types of PLC use different frequency bands, depending on the signal-transmission characteristics of the power wiring. Since the power-wiring system’s original intent is to transmit ac (or dc) power, the power-wire circuits are limited in terms of propagating higher frequencies on the wiring system.
Data rates over a PLC system vary widely. Higher data rates generally imply shorter ranges. A local-area network (LAN) operating at megabits per second may only cover several meters.
Narrowband PLC (NB PLC) is a generic term for power-line communications transmission below 500 kHz. Specifically, Europe’s CENELEC has authorized frequencies of 148.5 kHz and less for broadly deployable PLC systems.
Within this range, high-voltage transmission lines may carry data to distances ranging from hundreds of meters to a few kilometers. The resulting data rates are modest, ranging from 1 kbit/s to 200 kbits/s. These rates are appropriate for telemetry, data gathering, and control applications.
Narrowband PLC Apps
NB PLC can be applied anywhere there’s a need to communicate to and from equipment connected to the power line. It has become more popular than ever today, due to vigorous energy-conservation efforts that create new investments in power distribution and management. This phenomenon is generally known as the Smart Grid.
NB PLC isn’t new. But only with the recent advances in technology, increasing needs in machine-to-machine (M2M) connectivity, and the awareness for better resource management has it finally gained momentum.
Today, NB PLC is most often used to connect consumers to utilities for automatic meter reading (AMR) and load control. Many utilities have long preferred these systems because they allow data to move over an infrastructure that they control. Other rapidly emerging applications include street light control (SLC) and smart appliances.
NB PLC also is starting to find its way into a number of other applications with electrically connected devices that require monitoring and control. Examples include vending machines, solar panels, and electrical vehicle charging, among other potential applications:
- The Smart Grid: The Smart Grid would employ the power-line advanced meter infrastructure/automated meter reading (AMI/AMR) method. Electronic data is transmitted over power lines back to the substation, then relayed to a central computer in the utility’s main office. This would be considered a type of fixed network system—the distribution network built and maintained by the utility to deliver electric power. Such systems are primarily used for electric meter reading. Some providers have interfaced gas and water meters to feed into a PLC-type system. The power-line AMI/AMR system remotely reads customer meters in real time and then transfers the data into the billing system. AMI/AMR reduces the need for meter readers to manually gather utility meter readings each month.
- Smart light control: Street lights are among a city’s most important assets, providing safe roads, inviting public areas, and enhanced security in homes, businesses, and city centers. Streetlights are usually very costly to operate, though, and they consume a lot of energy (almost 40% of a city’s electricity spending). Connecting streetlights with PLC reduces the number of onsite operations and lowers electricity consumption while increasing overall light efficiency and average lamp lifetime.
- Smart homes and appliances: Home automation may include centralized control of lighting, HVAC (heating, ventilation, and air conditioning), appliances, and other systems to provide improved convenience, comfort, energy efficiency, and security. Since the goal of a home-automation system is to connect all of a home’s electrical devices with each other, PLC becomes the ideal approach.
- Solar energy: Photovoltaic panels (solar panels) must be carefully managed to deliver optimal performance. This involves communications to enable remote control and real-time monitoring. Remote control is used to control the degree of tilt to maximize sun exposure as well as control individual panels or an entire field. Real-time monitoring facilitates maintenance monitoring, detection of silicon-degradation/cell-replacement needs, weather conditions, theft detection, and power output and efficiency.
- Vehicle to grid: Following on the heels of Smart Grid deployments, rising fuel costs, and more cost-effective electric vehicles will be a greater demand for electric-vehicle charging stations. This vehicle-to-grid (V2G) infrastructure deployment requires communications between charging stations and billing and management systems. PLC offers an ideal solution because it utilizes installed power lines, provides strong security, and enables large number scalability.
Smart Grid Network Characterization
Many variables contribute to the communication characteristics of a Smart Grid network. Perhaps the two most critical are the network topology and the loads connected to the network. This variability means that no two power-line networks have identical transmission properties.
Enhanced data-transfer reliability though the power-grid communications channel requires an advanced communications scheme to cope with the noise. Such a scheme also contends with the many frequencies that are temporarily or permanently blocked to communication.
To adapt to noise variability, PLC devices must be able to estimate in-band noise level and received signal strength for each carrier frequency. Then, they must adapt communications frequencies and modulation to ensure the best possible data transmission. By measuring the in-band noise level and received signal strength on each frequency, the best frequencies can be chosen for the communication.
PLC Techniques
Low-voltage (LV) and medium-voltage (MV) networks primarily employ one of three narrowband communication techniques:
- Single-carrier modulations, such as binary phase-shift keying (BPSK) and frequency-shift keying (FSK)
- Orthogonal frequency division multiplexing (OFDM)
- Direct-sequence spread spectrum (DSSS), together with code division multiple access (CDMA)
The most widely deployed narrowband PLC solutions use relatively simple, but extremely cost-effective, FSK and BPSK modulation techniques. These techniques form the basis of several interoperable standards, notably Lon and DLMS.
Lon is standardized under the ANSI/EIA framework as EIA-709.1 and EIA-702.2 for the media access control (MAC) and physical layers, respectively. DLMS is standardized under the International Electrotechnical Commission (IEC) framework in the IEC 62056 and IEC 61334 standards.
To ensure operation in noisy environments, FSK and BPSK devices must measure the in-band noise level and received signal strength on each frequency. Then the software chooses the best frequencies for communication.
The Semitech SM6401 PLC transceiver, for example, estimates in-band noise level and received signal strength for each carrier frequency. Subsequently, the software chooses the optimal frequencies for data transmission.
The strides made in communication technology have led to the development and deployment of more advanced modulation techniques. For instance, OFDM has proven particularly effective because it can adjust to the noise environment, resulting in a more robust, more capable communication network over the CENELEC bands of operation. It has paved the way to new standard efforts, such as the PRIME Alliance and G3-PLC.
The SM2200 developed by Semitech represents one of the advanced OFDM solutions specifically developed to support applications on the LV (< 100 V) and MV (1 kV to 33 kV) power-distribution networks (Fig. 1). Operating at data rates reaching 175 kbits/s, the SM2200 utilizes 54 carrier groups into 18 independent channels.
OFDM, OFDMA Boosts Smart-Grid Communications
OFDM transmits large amounts of digital data over a noisy channel, such as the power grid. It splits the signal into multiple smaller sub-signals, which are transmitted simultaneously at different (orthogonal) frequencies. Each smaller data stream is then mapped to an individual data sub-carrier and modulated using some form of phase-shift keying (PSK) or quadrature amplitude modulation (QAM), such as BPSK or quadrature phase-shift keying (QPSK).
In addition to its high spectral efficiency, an OFDM system reduces the amount of crosstalk in signal transmissions. Furthermore, it can efficiently overcome interference and frequency-selective fading caused by multipath.
While OFDM addresses communications in noisy Smart Grid environments, it still falls short when trying to achieve reliable communications in these very harsh conditions. Thus, to boost reliability, the OFDM method can be combined with a multiple access scheme to create orthogonal frequency-division multiple access (OFDMA).
As seen with the SM2200, multiple access is achieved in OFDMA by assigning subsets of subcarriers to individual data streams (Fig. 2). This allows simultaneous transmission of several individual data streams.
OFDMA further improves OFDM robustness to fading and interference. More importantly, though, the individual data streams can be used either to communicate with multiple nodes (power meters) simultaneously or for redundancy, significantly enhancing system reliability.
Conclusion
Harsh noise and variations in equipment and differing standards make for difficult communications over the power grid. Reliable communications boosts throughput by increasing the bandwidth and reducing data-packet retries. This is particularly important for Smart Grid implementations, because concentrators can communicate with a larger number of meters. Such throughput enables multiple daily readings, which leads to better control over the grid.
Traditional PLC techniques like BPSK and FSK are insufficient in these noisy environments. That has created lots of interest in OFDM-based modems, as they greatly improve communications bandwidth and reliability. The addition of multiple access channels, otherwise known as OFDMA, can create more frequency flexibility by assigning subsets of subcarriers to individual data streams.