A core of a transformer is similar to the heart to a living being. Considering the importance of coil, many issues are kept in mind while their designing, like manufacturing functional reliability, longer service life, economy etc. Due weight age is given for the careful selection of proper material & optimization of the shape of this unit as the efficiency of the Transformer depends entirely on this crucial component. The transformers are built with fully annealed, Cold Rolled Grain Oriented (CRGO) laminations. Fully mitred designs are used to ensure low losses/low magnetising current & low noise levels.

Pre-loss of each core is measured at initial stage after annealing process to ensure the quality assurance.

The core geometry is so formed accordingly to the LV & HV coils (i.e. rectangular & round coils). Rectangular strip conductors are used in LV windings and round conductors are used for HV windings. The LV conductor (i.e. copper or aluminium) is usually insulated with either high-grade multi-paper covering or with polyesterimide enamel (Class B) insulation. Whereas enameled copper or aluminium conductor is used for the HV winding. The insulation is designed to withstand the specified impulse level. The windings are such designed so as to ensure that the coils are rigid, compact, have high mechanical & di-electric strength & have reduced axial stresses in Short Circuit conditions and also to withstand impulse and over-voltages which prevents movement of conductors of the coil in the event of any short–circuits on the transformer during its operation. The windings are progressively wound (HV over LV) in single one piece rectangular / circular construction for better voltage regulation and mechanical strength. The inter layer insulation is of Epoxy Dotted Paper. Winding are done in clean atmosphere to prevent possible accumulation of dust particles. The coils are further processed for dimensional control through automatic roller pressing winding machine, which improves bonding and short circuit withstanding capability. The provision of ducts in windings is made in order to control temperature gradient and rise for both winding & oil. All windings are checked and tested for all quality parameters and quality reports.

The annealed wound core is assembled with coils along with required amount of insulation & core clamps. The whole assembly is strapped with high tensile steel strap or clamped with the help of tie-rods & core bolts, ensuring that the core-coil assembly is mechanically strong to resist the short-circuit forces, for higher over-load capacity & longer life due to additional cooling area of the coil. It is always ensured that all the HT & LT coils are preshrunk, varnished and dried before assembly. Special care is taken to make the core coil assembly mechanically and electrically strong. For this, vertical and horizontal supports of PC Board of required strength & size, suitably interlocked, are provided. Each HT coil section is separated by press board rings. Yoke insulations are made of PC Board, permalli wood or Bakelite sheets, depending upon the size and voltage class of the transformer. LT leads are taken to the bushing terminals through proper bus bars/flexible jumpers. HT tapping leads & line leads are taken to the respective terminals through SRBP tubes-all suitability braced. And finally the core-coil assembly is dried in thermostatically controlled oven at 95 °C for removal of moisture for about 10-12 hours. At the end of the drying cycle, the core assembly is further tightened to take care of the shrinkage, winding, clamping and connection are checked and tightened before lowering into tank.

Before tanking, all core-coil assembly is subjected to pre-tanking tests which includes insulation resistance, no-load loss measurement & turn ratio. After removing the Core-Coil Assembly from the heating oven, it is thoroughly cleaned by pressurised air and then it is placed into the tank and bolted up. . The tanks are made of mild steel and are of welded type. Suitable stiffeners are provided for robust construction. Enough cooling radiators made from elliptical tubes/pressed steel fins are provided to maintain the temperature rise within specified limits. All necessary accessories / fittings such as bushings, valves, oil level indicator, pressure relief device, MCCB, signal light, relay, etc. are fitted on the tank body after being placed in the tank & later is impregnated with processed (filtered) high quality transformer oil.

Purity of oil used in transformers is of paramount importance. The contamination, inclusion of foreign particles, even of microscope level or water particles, causes harmful effects on proper functioning of the transformer. These further affect the desired functional characteristics. Thus, these harmful elements are kept at bay by filtering the oil. All insulating systems are carefully selected with due consideration given to electrical, mechanical & thermal properties of insulation. The oil is used as a coolant and insulating medium. It is tested for di-electric strength as per IS: 335, before topping up.

After tanking process the complete unit ready for testing. Each unit is tested as per agreed standards of acceptance. Purchaser may depute his representative to witness these tests. These tests includes: Turn-ratio, polarity, exciting current & no-load loss measurement at 100% & 112.5% of rated voltage & normal frequency, impedance & load loss measurement at 50 & 100% loading, induced over voltage test, separate source voltage withstand test, insulation resistance, winding resistance, oil leakage test, pressure test, etc. Special & type tests such as short-circuit test for dynamic & thermal ability, impulse voltage withstand test, temperature rise test, etc. are done at National Approved Labs/ National Accreditation Board/ Govt. Approved Labs (NABL) as per customers requirement.