Cold Rolled Grain Oriented (CRGO) magnetic silicon steel sheets of high quality are used to make the iron core. The most practical transfer of magnetic flux from the limbs to the yoke is achieved by inter-leaving the alternate joints,thus reducing no load losses.
HV and LV Windings
Round enamel -insulated copper conductors are wound to form layer type,disc type high voltage coils. The coils are wound with constant tension directly onto a pressboard cylinder.The layer insulation papers adapted according to electrical requirements. Paper -insulated rectangular copper conductors are used to form wound layer type low-voltage coils ,wound with constant tension directly onto a pressboard cylinder. Ample measures are taken to ensure the mechanical rigidity and the electrical soundness of both the HV and LV coils.
The tank and cover are made of high quality sheet steel ,which has to pass the stringent quality test. The tank is fabricated on a rigid bottom with an oil drainage outlet,corrugated sidewalls and a upper frame.After the components are welded together the first leakage test is performed on the tank .Internal surfaces are prepared for painting by sand blasting. The standard paint finish is a two-coast process of rust inhibiting primer followed by a high specification finish coating. The tank provides structural strength to the complete product and the design has the main influence on heat dissipation.
HT bushings shall have full wave impulse withstand voltage or basic impulse insulation level (BIL) as per the IEC Standard /DIN Standard/IS Standard.
In order to provide a constant output voltage despite of the voltage fluctuation on the input system the high voltage windings are provided with the tapings connected to an off-circuit tap-changer mounted on the tank cover and operated externally.The tapping positions are fixed by means of locking pins.On -load tap changer can be upon request and also the tapping position for the off- circuit tap changercan be adjusted upon customers requirement.
The core-cost assembly and tanks are placed in a vaccum oven chamber to the extract the accumulated moisture.Immediately after drying, the transformers are tanked and filled with pre-dried,degassed transformer oil.The required quality of the oil is confirmed by the laboratory tests.
It protects the tramsformer from damage due to internal short circuit and faults .The alarm contact in the double float design signals oil sinkage and formation of gas,while a trip contact is activated in the event of server malfunctions.This can be available upon customer request for a lower rating Distribution Transformers BELOW 1500kva.
It shows the actual oil temperature .Alarm and Signals are provided as protection against overloading.
Each Transformer shall be supplied with high quality,clean,dry and tested oil which complies to the BS148/84,iec 296 standards.
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