Enhancing reliability in medium-voltage power line communications

Power line communication (PLC) semiconductor devices transform the electricity grid into a smart grid by turning it into a communications network connecting utilities to their customers and 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 and variations in equipment and standards, communications over the power grid is difficult. To both reliably operate in this challenging environment and to successfully interoperate with previously installed equipment requires new approaches to power line communications.

In most countries, the medium voltage (MV) power distribution network (1–35 kV) reaches far and wide into almost every inhabited area. In China particularly, it extends into remote areas that are often inhospitable and far beyond the reach of wireless communication coverage. This wide geographical spread makes the MV power line a useful communications medium. However, the current MV distribution network presents a number of road blocks for data transfer, arising from the fact that it was never intended as a communication channel. Signals propagating along the power line are subjected to very large amounts of noise, frequency selective attenuations and distortions, many of which vary with time.

Although communication over power lines is extremely challenging, many recent advances in communications algorithms enable the data transfer to become more reliable and feasible across greater communication distances.

Communications over the MV network have many applications, but perhaps the most important is remote monitoring. Remote monitoring includes fault detection, control and most notably, automatic meter reading (AMR). In theory, fast data transfer is not needed for simple monitoring activities, but it is often the case that data repetition is needed to increase communications’ reliability over larger distances. Repetition slows down the effective data rate, implying that extra headroom may be required to achieve the necessary throughput.

In North America, Japan and China, the frequency range of up to 500kHz (i.e. below the AM radio band) is available for power line communications and offers a reasonably wide communications bandwidth. This brief tutorial focuses on the characteristics of power-line communications within this frequency range - narrowband power line communication (NB PLC). It presents the communication techniques currently used in this frequency range and examines their shortcomings. Finally, applications of a novel OFDMA communications modem are presented.

The article will examine smart grid characteristics observed from field trials of medium voltage power-line communications and suggest solutions that enable reliable communications. In particular it suggests using the OFDMA scheme as a means to enhance communications reliability. It is shown by simulation and field trials that the OFDMA scheme significantly enhances both the throughput of data and the reliability of communication, when part of the carrier frequency spectrum is blocked by either noise or attenuation.

Further, we illustrate practical implementation approaches to OFDMA PLC systems -using a reference-design example.

Current power line communication techniques
The main three narrowband communication techniques currently deployed on medium voltage (MV) networks are:

• 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)

Single-carrier narrowband power line communication systems appear to be well suited to implement AMR networks, due to their inherently low costs. However, their potential cannot be realized until problems with noise and signal propagation are solved.

Reliability issues are mostly caused by large attenuation and narrowband interference sources present in the communications channel. The characteristics of the power line channel are notoriously complicated - and present a number of communication difficulties. Unfortunately, the inherently simplistic nature of the common narrowband techniques mean that they are often plagued with reliability problems when applied to this complicated channel.

More recent techniques, such as orthogonal frequency division multiplexing (OFDM) and direct-sequence spread spectrum (DSSS) have proven to be resistant to narrowband interference and to many multi-path effects, making them ideal for high data rate communications [5]. OFDM, despite being used in broadband communications for quite some time, has only recently been explored as a narrowband modulation technique for PLC [6]. Its inherent robustness also makes it an ideal candidate for AMR networks.

Smart-grid network characterization
The communication characteristics of a particular smart-grid network are produced by many
variables, with perhaps the two most critical being the network topology and the loads connected to the  network. This variability means that no two MV networks have identical transmission properties.

Figures 1 and 2 respectively present typical and worst-case channel characteristics, as measured in tests carried out on the MV power line in China’s Hebei province.




In order to enhance the reliability of data transfer though the MV communications channel, an
advanced communications scheme is required to cope with the noise and the fact that many
frequencies are temporarily or permanently blocked to communication.