The balance of power: Developing a smart meter

Khurram Waheed, Freescale Semiconductor -October 15, 2013

Electric utilities face the challenging task of matching highly variable demand to fixed supply. Peak demand can be a multiple of average demand, making it hard to get full value from highly capital intensive power generation plants. Offering consumers a time-based incentivized tariff, or having the capability to remotely manage the schedules of high consumption equipment, is a more efficient and controllable way of balancing power supply and demand. To offer a time-based tariff, the utility must know when the user is consuming power, which means there must be a communication path from the utility to the consumer’s meter.

Obviously, wireless standards are candidates for utility-meter communication, but they have problems. Zigbee-based 2.4 GHz technologies have been proposed, but in practice they suffer from a short range of 20-50m which doesn’t provide a cost-efficient connectivity path back to the utility. Adding mesh networking can extend the range of a Zigbee network, however large intra-meter distances in rural and suburban settings, and intervening building structures, present a challenging environment for wireless communication in the ISM 2.4GHz band. As an example, a crowded multi-story apartment complex presents intricate coverage and connectivity issues that can quickly make a wireless network expensive to install and maintain. The new IEEE 802.15.4g Smart Utility Networks (SUN) standard is designed to address these challenges using various frequency bands, but it’s not ready for mass deployment. The biggest drawback with wireless is the need to use scarce and valuable spectrum, which might not be cheaply and/or readily available.

The obvious communications path is the powerline itself. Why not piggyback a signal along the electricity supply?

For a very long time, utilities have been using powerlines to communicate with generation equipment and provide voice communication between sub-stations. This involves adding a signal to high-tension lines. These signals typically use amplitude modulation or Frequency Shift Keying and work at a modest data rate of 1 kb/s or less, the low data rate allowing the signal to propagate over long distances. This technology is used to switch in and out distribution units, to check the integrity of the grid, and to provide basic voice communication to remote generation and sub-stations. However, there are thousands more electricity meters than distribution units and this approach doesn’t scale well for utility-meter links.

To communicate with meters, there is need for a higher data rate and a technology that supports multiple users in a semi-autonomous and agile communication network. The G3-PLC narrowband orthogonal frequency-division multiplex (OFDM) powerline communication standard has been designed for this purpose and provides the ability to overcome unique powerline communication challenges. The standard borrows many technologies from wireless, for example OFDM communication, forward error correction mechanisms such as Reed-Solomon, Viterbi convolutional decoding, and time and frequency domain interleaving to name just a few.

The powerline itself is a very challenging environment, which in many ways is noisier than wireless channels. The noise on a powerline is highly non-stationary and has a Gaussian component (similar to wireless) and a non-Gaussian, impulsive noise component (very different from wireless) that can be either periodic or aperiodic (Figure 1).

Figure 1: Time domain capture of typical powerline noise in an industrial building (a) raw power line noise samples, (b) Cenelec-A band powerline noise that will be experienced by a G3-PLC modem.

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