Seismic Displacement at Interconnection: Points of Substation Equipment

During an earthquake high-voltage substation equipment may experience substantial displacement at interconnection points, resulting in relative displacement between interconnected components. Sufficient slack must therefore be provided in the conductor between the moving components to prevent the impact loading that can occur as a result of a shortage of slack. The first step in estimating the required conductor slack is to determine the individual displacement that each interconnected equipment may experience. Once this is known with sufficient accuracy, methods to estimate the required slack such as prescribed in the IEEE 693 standard may be used. This paper presents a simple and useful method to estimate the expected displacement at the interconnection point of substation equipment under seismic excitation based on knowledge of the fundamental frequency of an equipment, its damping and the use of a response spectrum. A survey and analysis of qualification reports on standalone equipment revealed that for many types of equipment, the displacement is composed almost entirely of the contribution of the first vibration mode. The generalized single-degree-of-freedom method is therefore recommended to evaluate the displacement using the first modal-participation factor and the spectral displacement from the required response spectrum. Mean and bound values of this factor were determined and are presented for most classes of equipment used. Shake table tests were performed to investigate the effect of the flexible connection that is present a the top of equipment on this factor. It was found that, when sufficient slack is provided, its main effect, on average, is to decrease the standalone modal-participation factor. The method presented is thus of practical use, specially when no data is available on the expected displacement of a given type of equipment.