Mechanical strength of power transformers in service

The mechanical stresses in power transformers have steadily increased with transformer size and supply-system capacity. Short-circuit currents are generally based on the rupturing capacity of circuit-breakers which has increased in 25 years from 1 500 MVA to 25 000 MVA. The position has been accentuated in recent years by the established practice of auto-reclosing, which makes repeated switching on to possible faults a normal practice, and the introduction of fault throwing for inter-tripping purposes, where dead short-circuits are deliberately created under normally controlled conditions. Using only simple mathematics, it is shown that the mechanical strength of a transformer is not a simple single value as is implied in the short-circuit clauses of standard transformer specifications. Some of the strains are progressive, and some of the stresses cumulative, leading to short-term and long-term characteristics. The resulting categories of mechanical strength are defined as initial, critical and ultimate, the last named being usually the crucial one. It is shown how the performance in service can be predetermined and the expectation of life, in terms of number of short-circuits, predicted for any given operating conditions. Service records of failures on short-circuits are analysed and found to support the predicted values. Methods of improving the expectation of life are considered.