The Earthquake Engineering Online Archive

Interaction in interconnected electrical substation equipment subjected to earthquake ground motions

Der Kiureghian, Armen; Sackman, Jerome L.; Hong, Kee-Jeung

PEER-1999/01, Pacific Earthquake Engineering Research Center, University of California, Berkeley, 1999-02, 94 pages (400/P33/1999-01)

The research presented in this paper is part of a larger program aimed at assessing the seismic fragility of electrical substations that form critical nodes in power distribution systems. A typical electrical substation consists of a complex set of interconnected equipment items, such as transformers, circuit breakers, surge arresters, capacitor banks, disconnect switches, etc., many of which support fragile elements such as ceramic bushings. These equipment items are usually connected to each other through rigid conductor buses or flexible cables. Due to dissimilar characteristics of these equipment items, significant dynamic interaction between them may occur during seismic disturbances if the connection is not sufficiently flexible. This investigation aims at assessing the effect of interaction in interconnected equipment items and developing design guidelines for reducing the adverse nature of this effect. The study focuses on two equipment items with a single connecting element. Equipment items are modeled as linear, distributed-mass systems with one degree-of-freedom. Two distinct models of the connecting element are used: a linear spring-dashpot-mass element to represent a rigid bus with linear or linearized properties, and an extensible cable with negligible flexural rigidity to represent a flexible cable conductor. A set of dimensionless response ratios are defined that quantify the interaction effect on each equipment item relative to its response in a stand-alone configuration. A response ratio to quantify the force in the connecting element is also defined. Extensive analytical and numerical analyses are carried out to evaluate the influences of various system parameters on the interaction effect. For the linear connecting element, a simple method of analysis utilizing a response spectrum specification of the ground motion is developed. The response of the cable-connected equipment system is found to be highly nonlinear and asymmetric when the cable is taut. As a result, analysis by the response spectrum method is not possible. Instead, an algorithm for time history analysis is developed and numerical calculations are carried out for typical systems subjected to a selection of recorded ground motions. Based on the experience gained in this study, a set of topics for further study are recommended that, in the view of the authors, would lead to a better understanding of the behavior of interacting equipment items and would result in more reliable and safer designs.

Available online: http://peer.berkeley.edu/publications/peer_reports/reports_1999/9901.pdf