Transformer theory Maintenance Resources（变压器理论维护资源）.pdf

1 Transformer theory 1.1 INTRODUCTION The invention of the power transformer towards the end of the nineteenth cen- tury made possible the development of the modern constant voltage AC supply system, with power stations often located many miles from centres of elec- trical load. Before that, in the early days of public electricity supplies, these were DC systems with the source of generation, of necessity, close to the point of loading. Pioneers of the electricity supply industry were quick to recognise the ben- efi ts of a device which could take the high current relatively low voltage out- put of an electrical generator and transform this to a voltage level which would enable it to be transmitted in a cable of practical dimensions to consumers who, at that time, might be a mile or more away and could do this with an effi - ciency which, by the standards of the time, was nothing less than phenomenal. Todays transmission and distribution systems are, of course, vastly more extensive and greatly dependent on transformers which themselves are very much more effi cient than those of a century ago; from the enormous gener- ator transformers such as the one illustrated in Fig. 7.5, stepping up the output of up to 19 000 A at 23.5 kV, of a large generating unit in the UK, to 400 kV, thereby reducing the current to a more manageable 1200 A or so, to the thou- sands of small distribution units which operate almost continuously day in day out, with little or no atten