G3-PLC Power-Line Technology Makes the Smart Grid Smarter

By Lee H. Goldberg

Contributed By Hearst Electronic Products


The recently approved ITU G3-PLC networking standard supports high-speed, highly reliable IP-based communications across existing power lines, allowing data and control messages to flow across the generation, transmission, and distribution systems that comprise a regional Smart Grid (Figure 1). It was developed to provide robust connections between Smart Grid elements to allow the application of advanced billing and demand management techniques to customer loads and to efficiently integrate conventional and renewable-based distributed energy resources, including solar or wind farms.

Although initially developed as a joint effort between Maxim Integrated Products and Electricite Reseau Distribution France (ERDF), G3-PLC is a non-proprietary specification that has been selected to serve as a base technology for international standards development efforts such as IEEE P1901.2, ITU Ghnem/G.9955, and IEC/CENELEC T13.

Power-line communication technologies

Figure 1: Power-line communication technologies like G3-PLC are essential for providing reliable data links between the many elements that comprise a Smart Grid (Courtesy of the G3-PLC Alliance).

As with earlier PLC technologies, G3-PLC communicates across existing power lines without the need for additional wire or fibers. Offering higher data rates, further reach, and better error correction than previous narrowband power-line communications (NB-PLC) standards, this open, IPv6-based standard will accelerate widespread development of internet-based energy management systems. This brief introduction to the G3-PLC standard is intended to provide an overview of its capabilities, the technologies that it uses, and the first wave of semiconductor products that support it.

Inside G3-PLC

Unlike the Home Plug PLC standard, which is intended to support relatively high data rates across short distances within a building, G3-PLC is a so-called “narrowband” power-line communication technology that uses the lower carrier frequencies and slower data rates to provide longer reach and more reliable service. As we shall see shortly, it uses a combination of adaptive orthogonal frequency-division modulation (OFDM), forward error correction (FEC) technology, and repetition coding, to support highly reliable data links of 3 to 5 miles in length across the noisy, challenging environments usually found on power lines.

Much like the ADSL/VDSL technologies commonly used to push broadband data across phone lines, G3-PLC uses OFDM modulation, which creates multiple channels (or “tones”) across whatever chunk of spectrum it is assigned. Each tone occupies a 25 kHz channel and, depending on line conditions, can be modulated with either differential quaternary phase-shift keying (DQPSK) or differential 8-phase-shift keying (D8PSK). If maximum range or interference resistance is needed, the tones can be modulated with simpler dual binary phase shift key (DBSK) modulation. In cases where line conditions are especially poor, the DBPSK mode can be further enhanced using an optional “Robust Operation” (ROBO) mode which provides stronger error correction and repetition coding (at the cost of reducing the channel’s effective data rate).

G3-PLC’s media access control protocol (MAC) is designed to support mesh network topologies. Since there are usually many PLC devices communicating across a single power line, it uses an improved CSMA/CA (carrier-sense, multiple access/collision avoidance) MAC that listens for both the carrier signal and the length of the packet being transmitted before assigning itself a random back-off period.

The convolutional algorithm used to generate OFDM tones can be adjusted to work within any portion of the 10 to 490 kHz spectrum that a particular country allocates for PLC networks. It supports the full 145.3 to 478.1 MHz band defined by the FCC for U.S. power-line communications. PLC-3’s tones can also be adjusted to cover any or all of the four bands within the 35.9 to 90.6 kHz “CENLEC A” Band used in most of Europe, as well as the CENELC B and C Bands which occupy 98.4 to 121.8 kHz and 128 to 137 kHz, respectively. G3-PLC’s ability to perform “tone masking” (i.e., turn individual OFDM tones on or off) allows it to avoid parts of the spectrum occupied by external noise sources. Tone masking also allows it to co-exist with any incumbent communication technologies (such as IEC 61334, IEEE P1901 and ITU G.hn) that might be sharing the channel.

The same ability to select tones that offer the best channel characteristics helps G3 reach across the transformers that sit at the boundaries between high-, medium-, and low-voltage portions of the power distribution network. By selecting frequencies that pass more easily through the transformer’s inductive reactance and applying the error correction capabilities available in its optional ROBO mode, G3-PLC signals can emerge with enough signal strength to reach up to an additional mile. This enables multiple meters and gateways located on low-voltage neighborhood loops to communicate with a single data concentrator located on the medium-voltage side of a distribution network. Recent tests demonstrated its ability to support a 3.1-mile-long connection which included a transition through an MV/LV transformer (Figure 2). This capability is especially important in North American distribution networks where a single medium-voltage leg often serves many homes. It can also dramatically cut deployment costs in rural areas by allowing fewer concentrators to support isolated meters.

G3-PLC’s robust characteristics

Figure 2: G3-PLC’s robust characteristics allow it to support data links across transformers (Courtesy of Texas Instruments).

G3-PLC’s frame structure (Figure 3) is designed to carry encapsulated compressed IPv6 (6LoWPAN) packets. Each frame starts with a preamble, composed of SYNCP and SYNCHM symbols, which is used for synchronization and detection and provides a known sequence to facilitate AGC adaptation. This is followed by the frame control header (FCH), which contains the control information required to demodulate the data frame, with data symbols transmitted next. The PHY also supports a shorter ACK/NACK frame, which only consists of preamble and the FCH and is used to verify that a message was successfully received. Since G3-PLC networks are IPv6-based, other protocols (such as AMI/AMR DLMS-COSEM over IEC4-32) must be encapsulated as they enter the network.

Elements of a typical power-line modem (click to enlarge)

Figure 3: A typical G3-PLC data frame (Courtesy of Maxim IC and the G3PLC Alliance).

Implementation issues

The power-line modems that will link the Smart Grid’s G3-PLC-enabled system elements usually consist of a coupling transformer, an analog front end (AFE), a signal processor/controller and interfaces to the modem’s host processor and other resources (Figure 4). The AFE includes an active input band pass filter that provides low noise, low harmonic distortion, and low input bias. The AFE’s programmable gain amplifier (PGA) scales the incoming signal’s amplitude to match the range of the analog to digital converter (typically 10 to 12 bits) that feeds the modem’s digital signal processing element where the data stream is extracted.

Elements of a typical power-line modem (click to enlarge)

Figure 4: Elements of a typical power-line modem (Courtesy of Texas Instruments).

Outgoing packets are injected back into the power grid by the PLC transmitter stage which is capable of a high output current. The digital portion of the transmitter stage converts the outgoing data stream into series of xPSK-modulated tones and is then converted to an analog signal. This conversion is accomplished using either pulse-width modulation (PWM, again see Figure 4) or a fast ADC. The resulting analog signal is filtered for out-of-band emissions and passed to a high-power driver/transformer stage that matches the low impedance of the AC power line.

Open standards promote multiple silicon solutions

Thanks to its status as a non-proprietary specification, its high performance, and advanced capabilities, G3-PLC technology is currently supported by several major IC makers and equipment manufacturers in the Smart Grid sector, including Enexis, ERDF, Maxim Integrated Products, STMicroelectronics, Texas Instruments, Cisco, Itron, Landis & Gyr, Nexans, Sagemcom, and Trialog. At the time of this writing, silicon solutions are already available from Maxim and Texas Instruments, and STMicroelectronics is expected to introduce its own G3-PLC-capable products sometime this year.

Maxim introduced the first fully compliant G3-PLC modem chipset, which pairs the MAX2992 processor (an enhanced version of the MAX2990 power-line modem) with the MAX2991 AFE. Both modems are based on the 32-bit MAXQ30 microcontroller core which implements both the physical (PHY) and MAC layers. The 2990 uses a combination of software and hardware accelerator cores to perform channel estimation, adaptive tone mapping, and routing protocols. An on-chip authentication coprocessor with AES-128 encryption/decryption provides security and authentication.

Operating in the 10 to 490 kHz band, the MAX2991 is the first AFE specifically designed for OFDM signal transmission over power lines (Figure 5). The device boasts an integrated band-select filter, AGC, and a 10-bit Rx ADC. The transmit path features an integrated wave-shaping filter, programmable predriver gain, and a 10-bit DAC. The AFE’s programmable filters enable it to be configured to comply with CENELEC, FCC, and ARIB standards. An evaluation kit based on the chipset is also available.

Maxim’s MAX2991 analog front end

Figure 5: Maxim’s MAX2991 analog front end features a 10-bit ADC/DAC paired with adaptive equalization and digitally-programmable filters (Courtesy of Maxim Integrated Products).

Texas Instruments’ G3-PLC modem solution is based on the F28069 (Dev Kit for F28069, Product Training Module for F2806x), an application-specific variant of their C2000 Piccolo family of DSP-oriented MCUs and the AFE032 analog front end. The processor includes hardware accelerator blocks for FFT, Viterbi and CRC functions that are designed to work as extensions of the CPU’s instruction set. Eliminating separate reads, writes and interrupt service routines that are usually used to work with accelerator cores can help compress the code required to perform a particular function from hundreds to dozens of lines.

As a result, the part can support G3-PLC’s MAC and PHY functions as well as the convergence layer APIs. It also has enough reserve memory and processing power to support standards-transparent performance enhancements at the receiver (including preamble detection and adaptive equalization) that enable operation under extremely challenging channel impairments, many of which are common in areas with aging grid infrastructures. TI also offers the TMS320F28027 kit, a complete PLC modem development kit (Figure 6). The kit includes two modems, two control cards, cables, a power supply, and all the necessary software needed for design and development of PLC systems.

Texas Instruments’ PLC modem development kit

Figure 6: Texas Instruments’ PLC modem development kit includes all the hardware and software elements necessary to explore G3-PLC and other power-line communication technologies (Courtesy of Texas Instruments).

Summary

Fast, secure, reliable, and cost-effective communications are critical for the "Smart Grid." G3-PLC is a new OFDM-based power-line communications (PLC) technology. Two-way communications networks based on G3-PLC will provide electricity network operators with intelligent monitoring and control capabilities so they can optimally use existing resources and seamlessly integrate new, energy-harvested renewable resources. This article has examined the fundamentals of G3-PLC and discussed chipset solutions meeting the standard. Further product information can be obtained by using the links provided to the Digi-Key website. In-depth G3-PLC specifications can be obtained from the References and Resources listing below.

References

  1. A paper on G3-PLC testing is available from the IEEE
    http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=5764382&tag=1
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