Recent progress in large transformers

Abstract

It is not the intention of the author to present a treatise on transformer design, but rather to describe in general the progress made in recent years and to give examples illustrating the present stage of the art. Separate papers might well be written on several subjects touched upon in this paper, and the author hopes that engineers whose experience enables theni to do so may be willing to prepare such papers.The necessity for transformers of large output and high voltage raised considerable problems in regard to transport, but these have been overcome and the largest units contemplated can be transported by road or rail. The relative advantages and economic uses of 3-phase units and banks of single- phase units are discussed, and it is shown that, while in America the latter arrangement is still popular, in this country and on the Continent 3-phase units are now almost standard. The author also shows that the core type of construction is becoming the most usual for both single-phase and 3-phase units, but for certain special applications the shell design may be preferable. The subject of cooling is discussed in some detail, and it is shown that natural cooling, with or without auxiliary air blast, is now supplanting older water-cooling arrangements. Reference is made to the “Inertaire” transformer, and an interesting protective scheme known as the Buchholz device is described. It is essential that transformers shall withstand transient surge conditions, and the author describes the “non-resonating” type, which is claimed to be surge-proof, but he suggests that transformers built with inter-turn insulation according to a proposed new British Standard Specification will be quite satisfactory in service, and the design will be cheaper and less complicated than for a “non-resonating” transformer. Problems connected with short-circuit conditions in a star/star-connected transformer with tertiary winding are touched upon; examples are also given of transformers having three or even four windings for useful power supply. As regards on-load tap-changing, the author shows that the parallel-winding scheme has become almost obsolete in favour of the midpoint auto-transformer arrangement. A description is given of a booster transformer arranged to give simultaneous control of both voltage and phase angle, as is now necessary for the most efficient operation of large power networks. In view of the growing demand for current-limiting reactors. the author describes a type of oil-immersed reactor which is becoming popular and is displacing the older concrete type of construction.In order to present a picture of the stage now reached in transformer construction, the author has collected technical data concerning a number of outstanding transformers built in recent years by manufacturers throughout the world.