title: An experimental investigation into the electromagnetic compatibility aspects of high frequency power line communications
Creators: name: Fenton, David Richard
Publisher: name: The Open University
Year: 2006

description: Power line communications technology, long established for low data rate applications, is now charting new territory with respect to data rates and provided services. This can only be achieved by increasing PLC operating frequencies from the low frequency band (below 148.5 kHz) to the high frequency band (1 MHz and upwards). There is now only one technical barrier to widespread deployment - Electromagnetic Compatibility. Existing low voltage power networks are optimised for the safe supply of electrical energy. Low voltage cables are often pseudo co-axial in their cross section, but when high frequency signals are coupled onto the network, part of the signal will be radiated. There is therefore a potential for interference to be caused to legitimate users of the radio spectrum. This thesis, and the experimental program underlying it, seeks to quantify potential problems and to propose mechanisms by which they could be mitigated to the extent that wide scale deployment of PLC networks becomes possible. The first part of the thesis offers a detailed introduction to the topics of electricity supply networks, power line communications, modulation techniques and electromagnetic compatibility. Existing EMC standards are examined and although some do not directly cover power line communications networks, key principals are drawn for later use in standards development. The thesis then seeks to examine the mechanisms by which high frequency interference might be caused. Radio propagation modes are discussed and a clear technical distinction is drawn between localised interference from a single PLC network to an individual radio user, and cumulative interference from wide spread deployment of PLC systems. Both such scenarios are examined in detail. The experimental program IS described quantifying radiated signal strength regression from a number of power networks and at a number of operating frequencies within the high frequency band. In this context, signal strength regression is the rate at which electrical field strength reduces with increasing measurement distance. The experimental setup uses a conventional signal generator to supply single test frequencies of known power spectral density, which are coupled onto a power network. The subsequent radiated signal is received via a conventional antenna and radio receiver at a number of locations surrounding the power network at known distances, and signal regression is derived. The experiment was repeated for a number of different frequencies and at representative urban, suburban and rural locations. Indeed, the experimental technique was evolved over a number of months to allow increased portability of the signal receiving equipment, and hence the number of measurements that could be taken. From the experimental results, presented both In tabular and graphical format, a number of conclusions can be drawn. Firstly, based on these results, antenna factors in the order of 85 dB/m can be expected of power line communication networks. It can be concluded that the field strength regression to be anticipated from PLC networks is likely to be significantly below the -20 dB per decade 'free space' regression figure that has often been used in interference models. In fact a regression figure of -35 dB/decade IS more representative of ground wave propagated interference from PLC networks. It is also possible to conclude that the adoption of orthogonal frequency division multiplexing as a multi-carrier spectral technique offers specific advantages in EMC terms. Due to its nature, it is possible to apply a frequency 'mask' to an OFDM based PLC system. Such a mask might be static, applied on a national or regional basis in order to guarantee non-interference with specific frequencies, for example those used for emergency radio channels. It would also be possible to add a dynamic frequency mask, controllable on each PLC system, to mitigate interference with radio services operating within the PLC operating band.