Features of main power house transformers for Queenston plant

Interest in the main step-up transformers for the New Queenston generating station of the Hydro-Electric Power Commission of Ontario centers largely about their great physical size and the heavy short-circuit currents to be dealt with. Physically they are the largest single-phase transformers in operation today. In spite of the fact that they are for 25-cycle service, to our knowledge even their kv-a. rating is exceeded by only one bank of single-phase transformers now in operation, namely, the 70,000-kv-a., 60-cycle bank at the Colfax Station of the Duquesne Light Company. Figures are given for the weights and dimensions of these transformers and their component parts as well as the performance calculated from test results. The mechanical forces in 25-cycle transformers are inherently high because of the relatively large number of turns necessary to develop the voltage, and because of their relatively low impedance to the flow of short-circuit currents. As the forces depend upon the square of the ampere-turns, it is evident that both the above conditions contribute materially toward increasing the forces. To understand the bracing necessary to withstand the electromagnetic forces developed under short-circuit conditions, it is necessary to understand the nature of these forces as well as their magnitude. The nature and magnitude of the mechanical forces existing between current carrying coils are discussed, (1) for a single turn coil in space, (2) for two single turn coils arranged concentrically and lying in the same plane, (3) for two single turn coils arranged coaxially and lying in parallel planes. The conclusion is reached that as long as primary and secondary coils are adjacent there is no limit on the shape of the coil from the mechanical point of view, as all stresses acting in the plane of the coils are neutralized and there is therefore no force of any magnitude tending to distort the coils. The conclusions reached when single turns are considered hold equally as well for coils or groups of coils, so that in considering the forces in a transformer these fundamentals must always be kept in mind. Thus by interleaving the primary and secondary coils, it is possible to overcome completely any limitations which the mechanical forces with other arrangements of coils may dictate and to choose a coil shape which adapts itself most readily to the solution of the other important problems of the design, namely, insulation and ventilation. The Queenston transformers employ the interleaved type of construction familiarly known as the shell form, with rectangular pancake coils forming the alternating groups of primary and secondary coils. The distribution of the mechanical forces in these transformers is analyzed in detail. The effects of imperfect distribution of turns and of taps are shown to be very undesirable. All of the required voltages could have been obtained with considerably fewer leads and taps but the reduction in the insulation difficulty through the elimination of extra leads would have been accompanied by an increase in the mechanical forces due to unbalancing conditions on tap connections. In these transformers the maximum stress occurs on the first under voltage tap and has a value equal to 136 per cent of the maximum stress with the full winding. Having analyzed the various types of forces to be met with in the design of transformers of this type of construction, it will be interesting to examine the mechanical supports which have been provided in these units. The proper ventilation and insulation of a transformer is equally as important as the adequate mechanical support of the winding. The system of bracing used in these transformers is particularly interesting, in that, in spite of the substantial construction used, the other vital factors of ventilation and insulation have not been impaired in the least.