Mechanical stresses in transformer windings

Abstract

The forces and stresses set up in transformer windings and their clamping structures, as the result of external or internal short-circuits or of switching operations, are considered in detail.Various arrangements of windings and tappings in large power transformers are described. Points at which particularly high mechanical stresses occur in concentric windings are discussed in detail. Axial forces between the top and bottom halves of each winding or between different windings are responsible for bending stresses in turns near the ends or adjacent to gaps in the windings, for compressive stresses in the body of the winding, and for tensile and compressive stresses in the clamping gear. Radial forces produce tensile stresses in the outer and buckling stresses in the inner winding. Such stresses are more pronounced in coils adjacent to the main leakage duct. The excessive mechanical stresses which can be caused by internal electrical breakdowns are discussed. The mechanism of cumulative shrinkage, the loss in clamping pressure due to switching stresses and thermal cycles, and the danger of subsequent movement of the windings causing abrasion of the insulation and final electrical breakdown, are considered in detail. A rule is given for the minimum clamping pressure that should be maintained within the windings. Readers interested in the mathematical treatment of the subject-matter are referred to E.R.A. Reports, Refs. Q/T101, 103, 105 and 106, in which quantitative expressions are derived for the maximum values of currents, forces and stresses which can occur in the windings. Certain statements made in this paper on the merits of various forms of design represent the personal opinion of the author. They do not necessarily express the official opinion of the E.R.A.