Orientation of Power Equipment in Seismic Qualification Testing

Abstract eng:
IEEE 693, “IEEE Recommended Practice for Seismic Design of Substations,” is used to seismically qualify electrical power equipment in the United States. It requires equipment operating at or above 161 kV to be seismically qualified by shake-table testing. Input test motions are specified by the Required Response Spectrum (RRS). The response spectra of theoretical shake-table input records are typically within 5% of their target values. The Test Response Spectra (TRS), which are derived from table-mounted accelerometers, are typically within 25% of the RRS. Some power equipment is structurally simple and modes of vibration can be estimated, however, small variations in the method of installation can cause a large change in the mode shape orientation. When equipment is tested it is generally assumed that the modes of vibration are aligned in the directions that the table motions have been optimized. If the actual mode shapes are aligned along different axes, the equipment can be over or under test by as much as 60% when using theoretical analysis and deviations in actual table motions may be larger. Thus, minor changes in the method of installing the equipment on the shake table can cause large variations in the degree of over or under testing. For complex systems the modes of vibration can have different orientation for each mode so that it is not possible to orient the equipment to conform to the directions for which the TRS has been optimized. This demonstrates that even when testing is done correctly qualification results may not meet qualification objectives.

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