Implement OFDM For The Smart Grid

Implement OFDM For The Smart Grid

The Smart Grid demands efficient communication between consumers and the generator. To avoid the heavy cost of building a communications network in parallel to the distribution grid, powerline communications (PLC) technology is needed. Orthogonal frequency division multiplexing (OFDM) provides the ideal carrier technology, but the considerations needed to implement it efficiently for the Smart Grid go beyond the choice of carrier.

The traditional electricity grid is evolving into the Smart Grid, and the main criteria for an effective distribution grid lie in the efficient distribution of centrally generated energy to a distributed network of consumers (Fig. 1).

The new energy infrastructure sees the arrival of a bidirectional transfer of energy, both to and from the consumer. Electric vehicles (EVs) plugged into the mains will provide temporary storage of energy to allow peak energy demands to be satisfied without needing to build new and expensive generating capacity.

Such a complex grid demands production and usage data alongside the efficient bidirectional transfer of energy to ensure that consumers are charged and compensated appropriately. There is no budget to support the heavy cost of laying new cable to each home or office or to develop a complex mesh of wireless networks. So, the power cables used to transfer energy will need to be pressed into service to carry data.

Recent Phenomenon

In the medium-voltage (MV) section of the grid, PLC has been used for more than 20 years. The widespread use of PLC on the low-voltage (LV) side of the grid, however, is a more recent phenomenon (Fig. 2).

ENEL in Italy has conducted one major implementation. The utility deployed an automatic meter management (AMM) system based on narrowband PLC using frequency shift keying (FSK) and binary phase-shift keying (BPSK) modulation schemes. This system can perform accurate bimonthly readings of some 35 million meters. Unfortunately, the modulation scheme limits the data rate so the system does not suit real-time information and control.

Utilities often are forced to use slow PLC protocols because of the nature of the low-voltage power grid. The distribution networks are frequently made of a variety of types of conductor, terminating with very different impedances. This kind of network has a response to signal amplitude and phase that varies dramatically with frequency and even as loads on the network change.

Interference is the second major consideration. Electric appliances on the consumer side generate switching noise. Switched-mode power supplies and halogen lamps, which have steadily become more prevalent in users’ homes, also reduce communication reliability.

The low-bit-rate physical-layer (PHY) transmission schemes that have been used so far for PLC can’t support advanced protocols such as IPv6, which will be needed to convey real-time data around the network securely and reliably.

Using FSK or BPSK modulation, the information is transported in a single carrier. The baud rate available is proportional to the bandwidth, but noise and selective attenuation can limit the communication. What is needed is a technology that can cope with the radical changes in frequency-dependent behaviour across the LV network.

Orthogonal frequency-division multiplexing (OFDM) provides the ideal basis for a high-speed PLC system. OFDM divides the available spectrum into a set of narrow parallel frequency channels. The baud rate used in OFDM is proportional to the bandwidth and the complexity of modulation of the subcarriers, which may use a differential form of BPSK or more complex schemes such as eight-ary differential phase shift keying (D8PSK), which allows eight bits to be coded in each transmitted symbol.

If a single OFDM channel suffers excessively high interference, the transmitter can choose to avoid it, favouring clearer channels, or use more intensive error correction. In this way, OFDM can adapt to the changing electrical conditions prevalent on the LV grid.

Two systems that use OFDM have been proposed for PLC. First, EDF proposed G3 in 2009 and plans to test a network of 2000 meters that use it in 2011. Maxim and Sagemcom have developed solutions that support the technology. The alternative, Prime, is the result of the collaboration of a multi-disciplinary consortium including utilities, industrial, and university partners to design a new OFDM-based PLC technology open standard.

Prime offers an open and multi-provider solution supported by more than 30 companies, among them utilities, meter manufacturers, and silicon providers. Founding members of the PRIME Alliance, which was created to promote the Prime solution, include ADD Semiconductor, Current, Iberdrola, Itron, Landis & Gyr, STMicroelctronics, Texas Instruments, and Ziv.

Iberdrola was the initial utility promoter, but now EDP, CEZ Mereni, Fenosa, and Taiwan Power have joined the group. Iberdrola began deploying 100,000 meters in 2010 and intends to complete the full deployment of 10 million meters in Spain within five years. Other utilities are also starting to introduce PLC networks based on Prime.

The Prime consortium used a systematic process of design for the PHY starting from basic requirements. The team fully characterised the grid’s physical media in terms of noise level, noise patterns, attenuation characteristics, and models of impedances.

The work called for the development of automatic equipment able to carry out these tasks, working with utilities to accumulate extensive records. The result of this characterisation process was a large database of noise level, noise patterns, attenuation characteristics, and models of impedances that provided a statistically accurate model of the grid.

In a second step, the media model was used to simulate several different OFDM-based PHY transmission schemes. The approaches tried different header designs and changes to bandwidth allocations, number of subcarriers, subcarrier modulation, and error correction techniques.

For example, some conditions such as switching events can occur on the grid and push noise across a wide spectrum for brief periods. Studies showed that block-level correction did not adequately protect against these events and that other schemes should be used individually or in combination.

In OFDM, the amount of data transmitted per symbol is proportional to the sampling frequency and the number of subcarriers. Longer symbol transmission times increase the robustness against impulsive noise. Coding increases the robustness but also increases the complexity and power consumption. The number of subcarriers provides more robustness, not a higher baud rate.

System Evaluation

The best alternatives were evaluated in the field using the new equipment. After several iterations and a massive field test, the best combination of parameters was selected according the condition of the European grid and the specification of the utility.

The G3 solution uses 36 subcarriers and relatively short symbols of 0.735 ms. The scheme uses repetition and Reed Solomon error correction to increase robustness.

The Prime solution uses 97 subcarriers and longer symbols of 2.24 ms. To avoid the need for repetition and the complexity of Reed Solomon error correction, Prime uses symbols with three times more energy than those of the alternative to increase robustness. This is a more cost-effective solution to increase robustness.

Above the PHY, the media access control (MAC) and upper layers were the result of a collaborative consortium including silicon providers, meter manufacturers, and utilities. The net result of these combined efforts is the finalisation of the PHY, MAC, and convergence layers.

The PHY transmits and receives MAC protocol data units (MPDUs) between neighbouring nodes. The average transmission rate of the PHY layer is around 60 kbits/s and the maximum is 128.6 kbits/s using a bandwidth of 47.363 kHz located on the high frequencies of the A band defined by the European Committee for Electrotechnical Standardisation (CENELEC). 

As the electrical grid evolves into a Smart Grid to improve its robustness in a far more complex energy environment, the communications scheme that underpins this intelligent network has to be fast, dependable, and safe. PLC technology is the most convenient technology to achieve the necessary datarates and robustness.

OFDM solutions such as G3 and Prime have emerged as the main alternatives. However, Prime has demonstrated the most consistently reliable performance thanks to its developers’ use of technical analysis, simulation, and testing.

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