Dry type transformer is a static device which doesn’t use oil as a coolant medium rather windings are kept in normal air in the enclosure. Openings are given in the enclosure which allows air to pass though it which cools the transformer.

According to the design of winding, there are two types of Dry Type Transformer – a) Cast resin type (CRT) dry transformer, b) Vacuum pressure impregnated (VPI) type dry transformer.

A dry type transformer consist of following parts –Core, winding, HT terminals, LT terminals, RTDs, marshaling box, Lightning Arresters etc.

  • Core –The main functions of core is a) to provide flux linkage between primary & secondary windings & b) provide support to the windings. Windings are mounted on the core. Primary & secondary windings are not electrically connected but they are connected magnetically. Core is made up CRGOS (Cold Rolled Grain Oriented Silicon) steel which provides low magnetic path for efficient flux linkage. The max flux density of CRGOS steel is about 1.9 Tesla.
  • Windings – Two windings are mounted on the core, the winding which is connected with supply/source is called primary winding & the other winding which is connected with load is called secondary winding. High or low voltage in secondary winding depends on the number of turns of winding on the core. Number of turns more in secondary winding means high voltage in secondary winding&vice versa.

The function of primary winding is to magnetize the core and the function of secondary winding is to increase or decrease the voltage according to primary voltage.

TAP CHANGING – Tap changing is a process by which voltage regulation can be achieved. Example – let us say there is a distribution transformer 11KV/433V. Output voltage 433V depends on the input voltage 11KV. But input voltage can never be a fixed voltage, it may be either slightly higher or lower. So due to variation in input voltage, outputvoltage also varies. As variation in output voltage is not acceptable for the safety of the equipment as equipments are designed for a particular voltage. So, in order to achieve constant output voltage, tap changer plays as important role. Following settings are there of tap changing to get constant output. Tap changing arrangement is given in HT winding. Tap changing should be done when transformer is completely off & discharged.

(Note– Tap changing arrangement varies with the manufacturer, so always refer transformer’s manufacturers tap link chart before changing it. Chart given below is one of the manufacturer’s  tap link diagram.)

HT & LT TERMINALS –These are the locations where HT (incoming) cable & LT (outgoing) cables are connected with transformer (in case of step down transformer).

RTDs (Resistance Temperature Detectors) – RTDs are the resistance which are used to detect the temperature of the windings. RTDs work on the principle that resistance of wire varies with variation in temperature. RTDs are inserted in the groove of each winding. RTDs are connected with marshaling box /relay box. This relay box displays the temperature of the windings. The material used for RTDs are Nickel, Copper,  platinum etc.


MARSHALING BOX/RELAY BOX/CONTROLLER– It’s a control panel which is installed on the body of the transformer. The functions of controller is to –

  1. Show the winding temperature of R,Y, B phase,
  2. Change display speed of winding temperatures,
  3. It can be programmed to give fan control, alarm & trip signal,
  4. Remote indication of temperature can be provided. etc

(Note – Design of controller depends of manufacturing company so before doing any work kindly refer respective manufacture’s guidelines.)