Seismic Response of High Voltage Transformers

The IEEE 693-1997 standard is a guideline for the seismic qualification of electrical substation equipment. High voltage bushings, typically attached to the top of voltage transformers, are one electrical component highly susceptible to damage during strong shaking. For seismic qualification of bushings, it is assumed that the dynamic amplification that occurs from the ground to the base of the bushing is a constant factor of 2.0. Despite the use of this standard, recent seismic events have resulted in a large number of high-voltage bushing failures, thereby raising doubts on the accuracy of the assumed amplification factor of 2.0. The main objectives of this analytical study are to identify the critical parameters of supporting transformer structures that affect the seismic response of bushings and to quantify and compare the dynamic response of voltage transformers with the predictions of the IEEE-693 1997 standard. Three-dimensional finite element models were developed for five different voltage transformers that varied in voltage rating and manufacturer. Time history analyses were then performed on each of the models in order to determine the dynamic response characteristics as well as the amplification that occurs at the base of the bushing. The results of the analyses show that the transformer structures were much more flexible than currently assumed. In addition, the local flexibility of the transformer top upon which bushings are attached significantly reduces the natural frequency of the bushings. The dynamic amplifications occur predominately at two frequencies: the natural frequency of the transformer frame and the natural frequency of the bushing. The highest amplifications occurred when these two natural frequencies were close to each other. For all transformers, the lower frequency transverse direction consistently resulted in larger amplifications than the higher frequency longitudinal direction. Only one transformer had an amplification value greater than 2.0 at the bushing frequency.