Extruded Cables for High Voltage Direct Current Transmission: Advances in Research and Development

HVDC transmission is not new. Edison, a pioneer of all things DC, constructed a 45 km link connecting Miesbach and Munich in 1882 using rotating DC machines at each end. A few HVDC links were constructed in the next 50 or so years, partly following the advent of mercury arc rectifiers; but they largely fell out of favour when the advantages of AC transmission were realised. AC benefits from using transformers – which are straightforward to manufacture and efficient to use - to step up and step down voltages. The AC induction machine, whilst perhaps not so efficient, is rugged and low in maintenance, and cheap to make. The twentieth century saw the development of electrical distribution systems connecting consumers and generators within cities and of transmission systems using national grids. However, in the infamous words of Bob Dylan, “The times they are a-changin'.” Grids are no longer national – the countries of Europe are connected, the states of the USA are connected, and the provinces of China are connected. We are moving to pan-continental grids covering typically 10 million square kilometres and distances of 4000 km or more. Yet our grids have transmission lines that rarely stretch more than a hundred kilometres at a time. This is partly because of the capacitive and inductive limitations of AC transmission, and partly because they were never designed for anything else. Our networks are getting congested: on occasions more than 90% of the power transmitted in Belgium is not being using by the Belgians, it is simply transmitted across the country connecting the neighbouring countries either side. Our current electrical transmission networks are analogous to road networks with no motorways (or interstate highways.) We need electrical superhighways to transmit huge amounts of power long distances across continents. This is partly to connect consumers with sustainable sources of power. For example, the windy wet northern climate of Europe offers wind and hydroelectric generation, whilst the sunnier climes in the south offer photovoltaic and solar thermal possibilities. Generally, humans shy away from the really wet, windy and hot climates and so long distance transmission is often necessary to connect such regions of generation to centres of population. Nuclear power stations are often distant from cities, perhaps for reasons of safety. Similarly, the Three Gorges Dam situated near Yichang in China, which the guidebook states generates the equivalent of ten nuclear power stations, needs to transmit vast amounts of power to Shanghai, 1000 km to the east. East-west pan-continental electrical superhighways facilitate “power lopping”. The 4000 km width of a typical continent is about 10% of the circumference of the world and therefore corresponds to a two and a half hour time difference. Those in the east will be switching on their kettles for breakfast whilst those in the west are still asleep. Those in the west will be cooking their dinners whilst those in the east are doing the washing up. Transmitting power east and west can therefore reduce a region’s peak power generation requirements. Electrical grids are necessary for the twenty-first century and the lower electrical losses and reduced space demands afforded by HVDC transmission systems when used over longer distances are likely to afford the appropriate solution technologies. As our existing grids have been connected together, we have also seen other problems. Cascading failures, where a failure on one network brings down a connected network leading to a domino effect bringing down successive networks, are difficult to control on massively interconnected AC network systems. Fifty-five million people were left without electricity in the north-eastern and mid-western US states and Ontario in August 2003 after such a cascading failure. HVDC back-to-back links between AC grids are likely to be efficacious firewalls preventing such wide-scale failures. De-regulation and privatisation, smart-grids, and indeed some renewable energy sources such as wind, have led and will lead to faster changing demands and supplies requiring systems that can react more quickly. Without the problems of phase synchronisation, this is more straightforward to achieve with DC than AC systems. Of course the move to HVDC brings with it some technical challenges. Since the 1980s, we have seen the successful wide-scale introduction of extruded coaxial high-voltage AC power cables with polymeric insulation (typically cross-linked polyethylene.) These cables can support voltages of 10’s to 100’s of kilovolts and carry currents of 1000’s of amps. Three phase systems may transport all the energy from a large power station. Early problems with reliability have been overcome through the use of triple extrusion techniques for extruding “semicon” polymeric conductors contemporaneously with the insulation, and through the use of super-clean materials, the exclusion of water, and great care to avoid contaminants and voids in the insulation and protrusions from the electrodes. Due to geometrical effects, the electric field in the insulation is slightly higher nearer the centre conductor, but not greatly so. Although the temperature near the centre conductor can be significantly higher than that at the outer electrode, this makes little difference to the electric field in AC cables, as the field is controlled by the permittivity, which itself is not strongly dependent on temperature. The electric field is therefore defined everywhere by the applied voltage and the cable geometry; this is important since high electric fields are likely to lead to more rapid degradation and possibly to breakdown. Under DC conditions this is not necessarily the case. The electric field may be controlled by the conductivity, which is highly dependent on temperature, and may be enhanced by electrical charge (‘space charge’) that can accumulate in the insulation. An in depth understanding of these phenomena is therefore required to manufacture and operate HVDC systems using extruded power cables. With the resurgence of interest in using HVDC transmission systems, this book is therefore timely and will, I am sure, find itself of great interest to manufacturers and operators of such systems. Although not all the problems are solved, especially with the quest to operate at higher voltage and powers, the book will help to guide the way forward. It will also provide an excellent introduction for researchers in this area and those developing new cable materials and systems. I was therefore very pleased to be asked by Giovanni Mazzanti at the 2012 IEEE CEIDP to write this preface and I would like to congratulate the authors on a job well done! John Fothergill City University London 20 November 2012