Page 478 - Industrial Power Engineering and Applications Handbook

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tests, to represent the behaviour of all components By exercising these checks, most problems can be
and devices during the test and their repercussions on averted at the installation stage. The remainder can be
the process they are controlling. avoided in the design of mechanical systems/construction
of a machine rather than its electrical design, even in the
14.6.9 Preventive measures case of an electrical machine, to ensure their required
behaviour during an earthquake.
In general, this discussion relates more to primary systems, The following may be a few such design areas that
which include civil foundation and structures, on which may be considered, in improving the performance of a
the whole building rests. Correct civil foundations, struc- machine during an earthquake:
tures, columns, beams and trusses are major components
in mitigating the effects of an earthquake. All these must Bearings and bearing housings in a rotating machine,
be capable of withstanding the shocks and vibrations of in view of the very small gap between the stator and
an earthquake, according to the response spectra the rotor.
constructed for that location. It may be noted that at Rigidity of doors, bus and wiring systems etc. in a
higher floor levels, the building tends to act like a vibrating switchgear or controlgear assembly.
filter and may transmit to the object frequencies close to Limiting the use of brittle metals such as ceramic or
the natural frequency of the secondary structure. In other porcelain. Although, avoiding the use of such materials
words, the multi-frequency band of the ground movements in some cases may not be practical, as in the case of
may reduce to a narrow frequency band, almost lightning arresters, bushings and insulators used in an
dominating the natural frequency of the secondary system HT switchgear, instrument and power transformers and
and may become a potential cause of a likely resonance reactors.
with the structure. Objects on upper floors may thus be It is mandatory to employ only static or microprocessor-
subject to higher accelerations, sometimes many times based relays, components and devices wherever
more than ground accelerations. Hence the necessity to possible.
avoid a resonance at the design stage itself. It is possible Extra care needs be taken on the quality of fasteners
to do this, by keeping the fundamental frequency of the and the method of jointing and bolting.
floor or structure, where the secondary systems are to be Resilient but rigid foundations such as by providing
mounted, away from the predominant frequency that may spring mounts or rubber pads for machines on the
filter out from the ground level. floor or for components and devices mounted on the
More precautions in mounting the electrical or mecha- machine so that they are able to absorb the vibrations,
nical machine, component and device on each floor or caused by resonance and quasi-resonance effects, due
structure wouldcontain themagnification of seismic effects to filtered out narrow band ground movements. The
to a large extent. Critical machines and components then stiffness of the foundation (coefficient of the restoring
need be designed with such a natural frequency that they force, k) may be chosen such that it would make the
do not resonate with the filtered-out predominant frequency natural frequency of the equipment
of the structure or foundation. This is possible by suitably
modifying the method of mounting or making small
alterations in the construction of the object. It may be
noted that a resonance may take time to build up and an
earthquake may not last that long. The occurrence of a much less than the ground (exciting) frequency, W. It
resonance, therefore, may not be as significant as the is recommended to keep > 2, to achieve an effective
quasi-resonance (i.e. initial stages while the resonance is P
still building up) and this is reflected by the jagged peaks vibration isolation.
of the RS. The latest practice in the field of civil and The foundation and machine body/structure must
structural engineering is to build a resilient ground floor prevent internal resonance, which is possible by slightly
rather than arigid one, which can absorb the most vibrations altering their designs.
of a seismic event and filter out to the upper floors only
the frail motions. For more details, refer to the work The critical items may then be checked for the required
available on the subject, as mentioned in the references. response spectra (RRS) as discussed above.

Relevant Standards

IEC Title IS BS
60044- 1/1996 Specifications for current transformers 2705 BS 7626/1993
General requirements Part 111992
Measuring current transformers Part 2/1992
60044-2/1997 Application Guide for voltage transformers 4201/ 199 I BS 7625/1993
Specification for voltage transformers 4 l46/l 99 1 BS 7629/1995
General requirements
60044-411980 Method for measuring partial discharges in instrument 11322/1990 BS 6184/1992
transformers