DISTRIBUTION TRANSFORMER INSTRUCTION MANUAL ACCORDING TO SUNBELT TRANSFORMERSWhat are the procedures followed in dealing with the operation of distribution transformers?
SHIPPINGDistribution transformers are shipped completely sealed. Core and coils are assembled in a tank with the insulating liquid covering the coils. This method of construction preserves the quality of insulation, the cooling and insulating liquid by preventing contamination from external sources.
INSPECTION ON RECEIPTWhen a transformer is received, a thorough external inspection should be made before the unit is removed from the truck. If there is evidence of damage and/or indication of rough handling in transit, an inspector representing the carrier should be requested and the manufacturer immediately notified.
HANDLINGThe transformer should always be handled in the normal upright position unless information from the manufacturer indicates that it can be handled otherwise. Where a transformer cannot be handled by a crane, it may be skidded or moved on rollers into place, depending upon compatibility of transformer base design and the type of surface over which it is to be moved. During the handling operation, care must be taken to prevent overturning.
INTERNAL INSPECTIONBefore opening a transformer, take samples of the insulating liquid from the top and bottom of tank and test the dielectric strength. The dielectric strength should be 27.5 kv or higher. If it is lower, the transformer should not be placed in service until the dielectric strength has been restored by filtration.
STORAGEIt is advisable to locate a transformer, complete with liquids in its permanent location even if it will not be placed in service for some time. It is well to check the paint finish and to repair all damaged painted surfaces. If the transformer is shipped and stored in dry inert gas, the gas pressure should be maintained and periodically tested. If an oil-filled, indoor-type transformer is stored outdoors it should be thoroughly covered to keep out rain. A transformer should not be stored or operated in the presence of corrosive vapors or gases, such as chlorine.
LOCATIONAccessibility, ventilation and ease of inspection should be given careful consideration in the location of transformers. Self-cooled transformers depend entirely upon the surrounding air for carrying away their heat. For this reason, care must be taken to provide adequate ventilation.
PREPARING FOR SERVICEBefore any work is done on a transformer in preparation for service, a careful inspection of all external parts is needed to disclose any evidence of mistreatment or damage. This inspection should include a check of all parts required to complete the erection, making certain that all parts have arrived and are in first-class condition. Accessible bolted parts should be checked for tightness. Pressure tests should be taken and the liquid checked to determine both its physical level and dielectric strength. Any indication of leaks, which may have resulted in moisture entering the transformer should be noted and appropriate action taken.
MAINTENANCE DURING PERIODS OF SHUTDOWNClean any contamination from bushings Rotate the tap changer handle back and forth a few times. This will clean the contacts. Be sure to return the handle to its original position if no change in voltage ratio is desired. CHECKING FOR LEAKS: Check pressure vacuum gauge daily the first week of transformer operation. If pressure vacuum gauge stays at zero reading, it indicates a faulty seal. If transformer cannot be de-energized, be careful to not come into contact with live parts such as bushing terminals and leads.
METHOD OF DRYING TRANSFORMERSThe most common method is known as the “short-circuit’ method. Before applying voltage, the transformer’s cooling surfaces must be blanketed with heavy paper, cloth or builder’s felt. The amount of surface to be blanketed can only be determined by trial. The cover must be blanketed to prevent condensation on inside of cover. Ventilate interior by raising manhole cover. If transformer is inside a building provide good ventilation to remove vapors from room. If outdoors, protect the opening from the weather.
OIL (filling transformer)A. CHECKING OIL
Check the dielectric strength of oil while it is still in containers. If free water is present, drain off the water before putting the oil through the filter press. Continue passing oil through the filter press until the prescribed dielectric strength is met.
B. NON-VACUUM FILLING
In cases where vacuum filling is not required, the tank should be filled through the main drain valve. A second opening at the top should be provided to relieve the air being displaced. Full voltage may not be applied to the transformer for a period of 24 hours.
C. VACUUM FILLING
Entrapped air is a potential source of trouble in all transformers. In general, therefore, it is desirable to fill transformers with oil under as high a vacuum as conditions permit. Particularly is it essential to vacuum-fill high voltage transformers shipped in nitrogen or dry gas in order to develop their full insulation strength before they are energized.
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