Medium Voltage Transformer Selection Guide

A medium voltage distribution transformer is a transformer that provides the final voltage transformation in the power distribution system, stepping down or up to the voltage used in the network lines to the level used by the end-users such as industrial and commercial properties like offices, homes, and large factories. There are two types of distribution transformers according to cooling type: dry-type transformers and oil-filled transformers (also known as liquid-filled transformers). A cooling method is important to stop the transformer from overheating and becoming a potential fault. Overheating is inevitable when transformers are under load, but it must be solved because temperature rises are not acceptable when transformers are being used.

Oil type distribution transformers are the most used transformer type in the world. Windings are cooled by a special insulated oil which has excellent electrical insulating properties and is stable at high temperatures. The construction of transformer can be hermetically sealed or with conservator tank. Also, oil type transformers can be used in outdoor application because its protection class is IP55. There are some specific codes for cooling type of oil immersed transformers such as ONAN (Oil Natural Air Natural), ONAF (Oil Natural Air Forced) and OFAF (Oil Forced Air Forced). Oil type distribution transformers normally have ratings from 50 kVA to 5000 kVA or more according to application.


Dry-type transformers use air as the cooling method, while oil-filled transformers use oil instead of air. Some public places do not use oil-filled transformers because they are considered to be a big fire hazard and use dry-type transformers. This is because the liquid oil has potential to be flammable. So that, without fire or danger of explosion, it is possible to place dry type transformers near centers where population is high because of fire risk. Its primary and secondary windings are encapsulated with epoxy resin. This encapsulation helps to prevent moisture to penetrate to affect the winding material. Complete protection is achieved by this cast resin encapsulation so that the transformer can work without disruption in highly moisture prone area.  Dry type transformers can be cooled by both natural air-cooling method (AN) and fan cooling method (AF). Six pieces fan groups are mounted at the bottom of the transformer. There is a control panel that enables to run these fans according to the temperature of the windings with temperature sensors. In addition, fans can be used to increase the power of transformer up to 40% when it is necessary. Generally, F and H class of insulation is used to insulate the primary and secondary winding. It is because these classes have high temperature withstand property which enables up to 155 C for F and 180 C for H class of insulation. Power ratings of dry type transformers starts from 250 kVA to 8000 kVA according to application. Because windings are inside the cast resin construction, dry type transformer are used for indoor application. Protection class is IP00 but this can be increased by using a special cabinet.

Selection of distribution transformer are very important. We have prepared a selection guide to make your transformers selection easy. This is a guide to understanding how to select three phase distribution transformers. Oil type or dry type? Specify your capacity and let us know. Our expert engineering team will immediately provide you technical and commercial support.

VEKMAR is the official distributor of HITACHI Energy, Legrand, Eaton, Astor, Eltaş and SEM transformer. We are ready to help you on selecting transformer with technical and commercial support.

Click below link to get the transformer selection guide.

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