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CAUTION: Always de-energize a transformer and ground its case before entering or working on the transformer. Connect the ground to all transformer terminals. Remove these connections before re-energizing the transformer. The life of your dry type transformers can be improved with proper maintenance. An inspection and maintenance schedule should be established to maximize the life of this equipment. Evidence of rusting, corrosion, and deterioration of the insulation, varnish or paint should be checked, and corrective measures taken when necessary. Auxiliary devices should be inspected and serviced during these inspections.
The frequency at which ventilated dry type transformers should be inspected depends on operating conditions. For clean dry locations an annual inspection may be sufficient. For more harsh environments where there is dust or chemical fumes, a more frequent inspection may be required. Usually after a few inspections, a schedule can be established based on the existing conditions. Dirt or residues on windings or insulators should be removed to permit free circulation of air and to reduce the possibility of insulation breakdowns. Particular attention should be given to cleaning windings and vents. The windings may be cleaned with a vacuum cleaner, a blower, or with compressed air. The use of a vacuum cleaner is preferred, followed by compressed air or nitrogen. Maintain adequate ventilation during cleaning. The compressed air or nitrogen should be clean and dry and should be applied at a relatively low pressure (not over 25 lb/in2). Lead supports, tap changers and terminal boards, bushings, and other major insulating surfaces should be brushed or wiped with a dry lint-free cloth. The use of liquid cleaners is undesirable because some have a deteriorating effect on insulating materials. The insulation resistance test is used to determining the moisture content of a transformer’s insulation material. The insulation resistance test should be made before conducting an applied voltage test. The test data should be recorded with temperature and humidity at the time of measurement. If a dry type transformer’s insulation resistance readings do not meet the minimum values in the table below, it should be dried before the applied voltage test is performed or before the transformer is placed in service.
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Windings
should be inspected for dirt, especially on insulating surfaces
or where it could restrict air flow. Also check for loose connections,
for condition of tap changers, terminal boards and for the general
condition of the transformer. Check for signs of overheating
and tracking or carbonization marks.