Oil-Immersed Distribution Transformer

Distribution transformers are used to decrease the medium voltage (3.3 -36 kV ) to low voltage level (≤ 1000 V ) . ELTAŞ A.Ş. manufactures oil immersed transformers from 25 kVA  to  5000 kVA .


  • Distribution transformers can be ordered as single phase or three phase units.
  • ELTAŞ A.Ş. uses inhibited or uninhibited mineral oil in its productions. If requested,non-flammable fluids can be used as well.
  • Distribution transformers are produced with ONAN ( Oil -Natural Air -Natural ) or ONAF ( Oil -Natural Air -Forced ) cooling systems.
  • Distribution transformers are manufactured as open to atmosphere with conservator, sealed to atmosphere or hermetically sealed with gas cushion.
  • Distribution transformers are provided with off-load tap changers or on-load tap changers.
  • Distribution transformers can be manufactured with cable box or protection box.
  • Distribution transformers are manufactured with bushings with ceramic insulators or with resin type plug-in bushings.


The purpose of ELTAŞ A.Ş. is to fulfill customer’s requests completely and to provide highest quality. ELTAŞ A.Ş. manufactures distribution transformers according to customer requirement, contract requirements or technical standards such as TS 267 60076-1, IEC 60076-1, BS 171, DIN, ANSI, ENEL, ENDOSA, CSA.

Windings for distribution transformers

For the low voltage coils, either copper or aluminum can be used as conductor material. The windings are made of foil or a flattened and paper or nomex insulated wire. If the coils are made of foil, the layers are insulated using a thermal upgraded paper as well as a covering made of  DDP resin.

Depending on customer's requests, copper or aluminum conductors are used in high voltage windings. Coils are wound with fully automated computer-controlled machines. The layer insulation looks like a dagger, because of the stripped electrical insulation paper ( kraft or thermal-upgraded, diamond patterned ). The structure of this specially formed layer insulation increases the ability to withstand the impulse voltage. The layer insulation also helps in achieving another important goal: to produce a rigid coil without space. In other words a coil of excellent quality.



Cores of transformers are made of cold rolled grain oriented silicon steel. Cores are produced by using first class quality of steel. The choice of the material is made according to no-load values required by the customer. Cores are cut to length without burr by modern transformer lamination cut to length machine produced by GEORG. Cores are produced as step-lap, over-lap, 90° cut with or without holes.


Active Part and Cover

The assembly of core, coil and top cover are designed according to increase  the resistance against short circuit forces. The top cover, including the active part of standard transformers, is mounted to the tank with bolts. Another application is direct assembly of active part to the tank. The connection of parts of the top cover, high voltage bushings and the arching horns are made of stainless materials or of materials covered by a special coating which is resistant to corrosion. The arrangement of the bushings, the accessories, the protection box or the cable box is done on the cover or the side of the transformer according to customer requests.



Tanks and top covers for transformers are made of mild steel. Cooling surfaces of distribution transformers are constructed with corrugated walls and they also form the lateral surfaces of the tank. Bottom plate, sides and frame are sealed by welding.

For transformers up to 4000 kVA , corrugated walls can be used as cooling surfaces. Also, it is possible to use radiators as cooling surfaces. Every transformer tank needs to pass an oil leakage test after welding. Therefore, each tank is taken under pressure after being produced. Finished tanks and top covers are sandblasted commensurate with DIN specifications. If the customer has specific requests or demands related to sandblasting, ELTAŞ A.Ş. has sufficient machinery to fulfill all these requirements. It is possible to enclose the bushings on the top cover into a protection box made of appropriate steel according to IEC 60144. According to BS 2562, the cable box can be installed to the sides of the tank.



Transformers are in the colour of RAL 7033 as standard. Transformer tanks are painted by the method of dipping or spraying. Method of painting is chosen in accordance with customer requests and working conditions of the transformer. ELTAŞ A.Ş. offers different applications, different thickness of film or transformers with different colours as per customer requests. Also, if requested, hot dip galvanized tanks can be produced.


Dry and Filling

The active parts of transformers such as core and coils are dried under high vacuum in a furnace. The process of drying varies according to transformer’s type, power and voltage. Oil is filled under high vacuum.

The following accessories are mainly used to protect transformers:

1. Oil Level Indicator

Magnetic oil level gauges are used in order to follow and to control the oil level in the conservator of transformers. Oil level indicators give alarm and tripping signals in case of sudden changes of the oil level.


2. Air Dehumidifier

Air dehumidifier are utilized for transformers with oil conservator. It is connected to the oil conservator. Via silicagel, it removes moisture and dust coming into the conservator. The amount of  Silicagel depends on the amount of oil inside the transformer.


3. Oil Thermometer

Oil thermometers, which are located in thermometer pockets and welded on the top of  the cover or on the side of the tank, follow and control the maximum temperature of the oil. It is available with maximum pointer, without electrical contact or with single or double electrical contact. If the temperature exceeds the estimated values, thermometers with contact give alarm or tripping signals.


4. Buchholz ( Gas Collecting and Alarm) Relay

The Buchholz-Relay collects the gasses which are formed by discharges or arcs with low or high intensity inside of the transformer. It used for transformers with conservator. The relay can be with single or double contact. If certain amount of gas is collected on the Buchholz-Relay, both the alarm and tripping signals are activated. Moreover, Buchholz-Relay sends out an alarm signal in low level of oil. The Buchholz-Relay detects failures like overheating and prevents an increase of failure by signalling.


5. Pressure Relief Device

Even though hermetic protection relay is used in transformers, pressure relief device is used additionally. It helps to cover the pressure that expands inside of the transformer.


6.Hermetic Protection Relay

Hermetically sealed transformers equal and greater than 630 kVA are equipped with a Hermetic Protection Relay. It consists the functions of Oil Level Indicator, Temperature Indicator, Buchholz-Relay as well as Pressure Relay.



Besides, upon customer’s request, special accessories such as winding thermometer, digital thermometer, gas collecting unit can be used  for  oil immersed power transformers.