In distribution and power transformers, windings are designed so as to provide maximum strength to axial and radial forces occurring during a short circuit. LV (low voltage) windings are generally from a foil or paper insulated flat conductor. HV (high voltage) windings are however an enamel insulated round wire conductor or a paper insulated round or flat conductor. Electrolytic copper or aluminium conductors are used. In between the layers of a coil, a special insulating material with increased insulation and high oil absorption is used, such as a craft paper, shellac paper (DDP, DPP) roll pressboards, etc.




In transformers, the core consisting the magnetic circuit is made up from cold-rolled, grains-oriented (CRGO) and low-loss electrical silican steel. Core steel can be in varies thicknesses and qualities and are made up from high-quality silican steel such as M3, M4, M5, MOH, ZDKH85, ZDKH95 and Amorphous steel, depending on the client’s requests, warranted iron loses (no load losses) and applicable specifications. Being highly sensitive, core sheets are trenched, cut to required lengths and arrayed. With hub steel, zero blurring is very important for iron losses. Additionally, step-lap, overlap, 45°, 90°, punctured or un-punctured hub sheets can be made by using different cutting and arraying techniques. Arrayed core steels are compressed by means of yoke compressing irons and studs to minimize iron losses and vibrations.




The active part of a transformer consists of the core, coils, upper cover, tap changer, bushings, and protection and connection members on the upper cover. When assembling the active part, the transformer must have a structure to provide the best strength against short-circuit mechanical forces. Therefore, strength stability is increased by additional measures during the construction of the active part. The active part is mounted on the upper cover of the boiler, using screws. Connections of the active part can be made on the upper cover or directly on side walls of the tank as well.




In tank manufacturing, the tank lower pan, side walls and the upper cover are made by using black sheet metal to quality St 37. Side surfaces that carry out the cooling function in transformers are made up from cooling members manufactured of 1,2 mm or 1,5 mm thick sheet metal, depending on the transformer’s power, called corrugated wall panels. For bigger transformers (>2500 kVA), radiators are used instead of corrugated wall panels. Every tank through with welding operations is subjected to oil-leakage test (pressure test). All surfaces of the tank to get in contact with oil are cleaned by sanding and special chemical cleaning methods for surface preparation prior to painting.





The active part through with the assembly operation is dried in a special high-vacuum drying furnace for a period of time set forth within a predetermined program depending on power and voltage values, and placed in the tank. Then oil filling is done under vacuum. Since the drying operation takes away the moisture in the materials making up the active part, it is a very important process for the insulation of the transformer.





Tanks complete with manufacturing are prepared by sanding and chemical cleaning methods prior to painting to ensure a longer lifetime for the applied primer and two final coats of paint. Tanks are painted with one coat of primer and two final coats of paint with a total thickness not less than 105 µ (microns). Although the RAL 7033 cement gray is used as a standard, non-standards are also used depending on the client’s request. The primer and the paint of final coats are epoxy-based, double-component, acrylic substances and are selected from first-class materials that can endure working and environmental conditions for a long time. For painting, pouring and spraying methods are used.




Routine tests are applied to all transformers through with manufacturing. Type tests and special tests are however conducted upon the client’s request and depending on agreements.


  • Conversion ratio measurement and connection group determination,
  • Measurement of winding resistances,
  • Applied-voltage test,
  • Induced-voltage test,
  • Measurement of no load loss and current,
  • Measurement of load loss and impedance voltage,
  • Insulation resistance measurement,
  • Core and body insulation checking test,
  • Leakage test.



  • Temperature rise test,
  • Lightning impulse voltage test,
  • Measurement of audible noise level,
  • Mechanical strength test against short circuits.



  • Capacity and insulation loss factor measurement (Double test),
  • Insulation resistance measurement (Megger test),
  • Transformer oil dielectric puncture strength measurement,
  • Measurement of harmonics (No load current),
  • Partial discharge test,
  • Zero component impedance measurement.