Ricardo Taborda

A test series carried out in the shaking table of the Institute of Engineering requested by the Mexican Federal Commission of Electricity (FCE) to evaluate the dynamic behavior of an antiseismic device of base isolation used in electric interrupters of energy substations, is detailed.

After a brief introduction, in sections two and three, characteristics of the interrupter and the isolation system are detailed as well as the measurement, recording and analysis equipment used. In section four a brief description of the tests is made, which consisted on subjecting the interrupter to a white noise signal, a sweep of frequencies test, to the reproduction of October, 1995 and January, 2003 Manzanillo earthquakes, and to a M=8.3 synthetic recording, corresponding to an earthquake originated in the cost of Guerrero for soils with up to 0.5 s of dominant period.

The report’s fifth section thoroughly describes attachment and instrumentation of the equipment. The interrupter bases on a steel frame connected in its four corners to pendulum-type seismic isolators, which are welded to metallic plates firmly settled to the shaking table. Acceleration, deformation and displacement transducers were used in the instrumentation at the base and at strategic points of the interrupter.

Results obtained during the different tests are described in section number six. Through transfer functions between induced and recorded signals on the shaking table in the white noise test, the alterations caused by the table interaction are evaluated. The sweep of frequencies test, consisting on applying sinusoidal functions, allowed obtaining a first estimation of the system’s vibration frequencies.

Besides letting know the dynamic properties of the structure, tests with selected earthquakes also allowed evaluating the performance of the antiseismic device utilized. The records obtained were analyzed by means of non-parametric identification techniques in the frequency-domain with a conventional spectral process in which, by means of transfer functions between the power spectra at different points of the interrupter, the system vibration frequencies were identified as well as their variation during the events, through a time-frequency analysis with 5 s windows. Additionally, frequencies were also identified by means of a modal parametric technique. System damping was calculated by applying the logarithmic decrement expression in the later stage to the excitement in which the system was in free vibration.

Additionally, acceleration and displacements animations of the instrumented points were done, which made it possible to visualize the contribution of the antiseismic devices.

The above described analysis and identification process indicated a non-linear behavior of the system. During the tests with earthquake recordings, changes of up to 50 percent in the fundamental frequencies of the system were observed. In spite of it, a satisfactory behavior of the isolation system utilized was observed.

Finally, in the report, the following conclusions stand out:

• The largest dissipation of energy concentrates on the antiseismic devices.
• The devices showed to be efficient for mitigating the effects of the intense earthquakes tested to the interrupter.
• The dynamic properties of the interrupter show variations depending on the excitation.

These conclusions and other observations made during the tests suggest the following recommendations:

• To make similar studies with simulated earthquakes corresponding to the intensities expected in the regions of higher seismic risk.
• To seismically instrument FCE substations in high seismic risk areas to evaluate the behavior of their different components.