This book addresses the very latest research and development issues in high voltage technology and is intended as a reference source for researchers and students in the field, specifically covering developments throughout the last decade.
Inspec keywords: power cables; earthing; high-voltage engineering; zinc compounds; SF6 insulation; partial discharges; pulsed power supplies; electric breakdown; polymer insulators; arresters
Other keywords: ZnO; SF6; earthing; pulsed power; optical methods; high voltage engineering; polymeric insulators; surge arresters; insulation systems; power cables; partial discharge; air breakdown
Subjects: Power system protection; Gaseous insulation, breakdown and discharges; Power line supports, insulators and connectors; Power convertors and power supplies to apparatus; Power cables; Dielectric breakdown and discharges
Studies of air breakdown began in the eighteenth century. Two names are pre-eminent: Franklin and Lichtenberg, although contemporaries were active. Franklin's work grew out of his interest in lightning a long spark while Lichtenberg drew tree-like discharges, now called corona, across the surface of a large cake of resin. These two men defined two broad approaches to the study of breakdown which are perpetuated to this day in experimental and theoretical work. In the late 19th century, the emergence of modern physics, exemplified by the work of Townsend and his successors, permitted knowledge of the process of ionisation to be applied to these phenomena. The two approaches were thus linked and another concept from the 18th century, the electric field, became established as paramount in all discussions of the subject.
This chapter reviews the basic ionisation processes which occur in SF6, the streamer mechanism which controls breakdown under relatively uniform field conditions, and the influence of electrode surface roughness on breakdown at high pressure. The characteristics of the partial discharges (corona discharges) which occur under the non-uniform field conditions associated with certain types of defect are then discussed. Following a discussion of the various PD diagnostic techniques that have been proposed for use in GIS, an account is given of the principles of the UHF technique for detection of PD in metal-clad equipment. Finally, the design and calibration ofthe sensors used in UHF monitoring are discussed and an explanation given of the interpretation of the PD patterns recorded in practical UHF monitoring systems.
Benjamin Franklin was born in 1706, and was fascinated in his youth by electrical phenomena. In 1732 he founded with his collaborators the Library Company of Philadelphia with the support of the Perm family, and achieved a new insight into electrical science with his definition of a 'single electric fire'. This is equivalent to the free-electron concept, and implies the principle of the conservation of charge.
The study of partial discharges is not an academic pursuit. It is driven by a very practical desire, a desire to understand a phenomenon which can be utilised to infer the level of integrity within the insulation systems of high voltage plant and which, in itself, constitutes a serious stress degradation mechanism. The role of partial discharge studies remains twofold: to enable the development of new insulating systems which are resistant to partial discharge stressing and to predict remnant life for existing plant systems. To this end, studies in partial discharges involve a balance a balance between understanding the phenomenon and being able to measure it. On one side lies the understanding of how partial discharges cause degradation, and which are the key parameters of activity to be measured. On the other side lies the ability to make the measurement, often under very adverse conditions.Partial discharges are localised gaseous breakdowns which can occur within any plant system provided the electric stress conditions are appropriate. Because the breakdown is only local, failing to result in a following current flow, it is described as partial.
High voltage systems are often subject to transient overvoltages of internal or external origin. The resultant surges travel along the transmission line and can cause damage to unprotected terminal equipment. Corona losses and the earth return path can attenuate and distort the surges, but the magnitude of the surge may still exceed the insulation level of the equipment. Surge arresters provide a limitation of the overvoltage to a chosen protective level. The superiority of the recently developed zinc oxide (ZnO) material over earlier silicon carbide (SiC) renewed interest and boosted the use of surge arrester protection.
The product developments that have taken place since John Looms published, in 1988, his excellent book that covered much of this subject have mainly concerned polymeric materials especially silicone rubber. Although the principal modifications made to porcelain and glass insulators over this period are those required for high voltage DC applications, much more has nonetheless been learned about the performance of such insulators under AC energisation. The discussed proposals of some years ago for ultra high voltage transmission systems resulted in the production of very high mechanical strength insulators of the cap and pin design. However, as such proposals generally failed to come to fruition, at least one insulator manufacturer now has a product that is ready, should a market become available in the future.
This chapter reviews some aspects of insulation coordination and overvoltages on transmission networks. The purpose of insulation coordination is to ensure that the probability of insulation breakdown is limited to an acceptable value and that any breakdown is restricted to self-restoring insulation. It is based on computing the most severe overvoltages occurring on the network and relating these to the breakdown characteristics of the insulation through appropriate margins to obtain withstand voltages for the network components together with the statistical risk of insulation failure.
Three-phase power systems are earthed by connecting one or more selected neutral points to buried earth electrode systems. Such earths are referred to as system earths. At electrical installations, all non-live conductive metallic parts are interconnected and also earthed to protect people against electric shock, and in this role, the earth is referred to as a protective earth. Under normal conditions, there is only a residual current or no current at all in the earth path. However, very high magnitudes of current return to source via the earth path under fault conditions. The earth also conducts lightning currents and the current path may involve part of a power system either directly or by induction. The earthing system, or part of it, may therefore also be specifically designed to act as a lightning protective earth.
This chapter provides a general introduction to the vast subject of circuit breakers, interruption strategies, characterisation and performance evaluation of gaseous/solid insulation systems, the identification of strategic online condition monitoring (CM) methods and diagnostic techniques (DT) as applied to modern switchgear systems, before and post deregulation. Although it has only been possible to touch very briefly on these aspects together with certain related strategic issues including sub station control systems (SCS), also working networks harder and environmental matters, important strategic issues have been considered briefly and a generous listof references, including CIGRE electronic sources is included, which will enable the reader to explore this developing topic in greater depth.
The name cables is given to long current-carrying devices that carry their own insulation and present an earthed outer surface. In this context, overhead lines for example, are not considered as cables. Power cables have a coaxial structure: essentially, they comprise a central current-carrying conductor at line voltage, an insulation surrounding the conductor and an outer conductor at earth potential. AC cables are generally installed as a three-phase system and hence the outer conductor should only carry fault and loss currents.
This chapter discusses the different numerical analysis method for the determination of electrical field distribution in high voltage equipment. It discusses and compares the finite-difference method and the boundary element method.
Optical techniques are attractive for measurements and monitoring under high voltage conditions for several reasons. They enable measurements to be made remotely via free space so providing a high degree of geometric isolation of the measuring equipment from the high voltage environment. They also provide a means via optical fibre transmission for penetrating into high voltage enclosures for monitoring purposes with a high degree of inherent electrical insulation.
Pulsed power originated with the invention of the Marx bank in the early 1920s, enabling the simulation of lightning strikes and switching operations on power system components. Between the 1960s and the beginning of the twenty-first century, the demands of high energy pulsed power applications have stretched Marx bank techniques enormously. Refinements of triggering and switching techniques, and control and exploitation of the stray capacitances in the systems, enable 100-200 ns rise times to MV levels and above with MJ of energy being stored and delivered. The requirements of exotic radiographic, e-beam and plasma pinch loads have demanded the use of pulse forming networks or lines interposed between the Marx and the load to provide the requisite pulse shaping and power levels. Meanwhile the rise time capability of low energy Marxes has been reduced to nanosecond levels as a result of developments in capacitor and switching technologies. The latest direction is now for the development of ultrafast high energy Marxes with adequately low inductance for direct feed of compact e-beam loads.Pulsed power has principally developed in response to high energy physics and weapons programme requirements; plasma drivers, x-ray drivers, magnetron drivers, laser drivers, ion beam steering and acceleration. However, the industrial application of pulsed power is increasing.
