Three Phase Transformer Construction

The most important parts used in the three phase transformer construction are windings and core. Some other parts such as a tank, conservator, bushing, breather etc. are also used along with the core and windings. Simplified construction of a three phase transformer is shown in the figure.
three phase transformer construction

Parts Used in Three Phase Transformer Construction

Laminated Steel Core: The material used for the construction of three phase transformer core is silicon steel. It is used for its high permeability, low magnetic reluctance and low hysteresis loss.
The core is used in form of stacks of laminated thin steel sheets which are electrically insulated from each other by a thin coat of varnish. The core is laminated in order to reduce the eddy current loss. The laminations are typically 0.35 to 0.5 mm thick.
Windings: The cylindrical coils (windings) are placed in a concentric way around the same limb with the low voltage winding placed inside and high voltage winding is placed outside.
The low voltage winding is placed close to the core because it is easy to insulate a low winding from the core. Both the windings are properly insulated from each other and the core. In the construction of three phase transformer, three high voltage and three low voltage windings are used.
Transformer Oil: A mineral oil obtained by fractional distillation of crude petroleum, known as transformer oil is used in the transformers. This oil carries the heat produced in the transformer by convection from the windings and the core to the transformer tank. This oil also improves and maintains the insulation of transformer windings.
A great disadvantage of the mineral oil is its susceptibility to catch and propagate fire. To overcome this disadvantage, a group of synthetic liquids known as Askarales has been developed which is completely fire-proof.
The Askarales offer greater resistance to acid and sludge formation than the mineral oil and, therefore, the life of an Askarel-filled transformer is much longer. But Askarales should never be mixed with mineral oil. Also, the two cannot be interchanged in a transformer.
Conservator: It is a small cylindrical tank mounted on top of oil-cooled transformers and connected to the main tank by a small pipe. It performs the following functions:
It provides space for expansion and contraction of oil which expands on heating and contracts on cooling.
It minimizes the oxidation of oil by limiting the surface area of oil in contact with air.
It confines the oxidized oil to conservator only.
The conservator is equipped with a sight-glass at one end to indicate the oil level in the transformer. The level of cold oil should always be maintained up to the gauge mark on this glass.
Breather: It is a device through which all movement of air from and into the transformer takes place. Air is expelled when the oil expands due to the heating of transformer and is drawn in on cooling of oil. It is installed at one end of the conservator.
The breather is filled with some dehydrator (generally silica gel), which absorbs moisture from the air passing through it so that only dry air can enter the tank. Silica gel is blue in color when dry and turns pink when it gets moisturized. It can be reconditioned by heating in a pan.
Thermometer: A thermometer is installed on all transformers of large rating to indicate the temperature of hot oil. In very large transformers, temperature indicating devices are installed in the winding itself to record the winding temperature. A temperature sensitive alarm system is also installed to sound the alarm whenever there is dangerous temperature rise.
Explosion Vent Pipe: It is a bent pipe with a glass cover at the end. It is installed on top of transformer tank. It provides protection against excessive pressure build up inside a transformer. The transformer can explode in the absence of this device.
Valves: A number of valves are provided on each transformer for filling and draining the transformer oil.
Buchholz Relay: Buchholz relay is a gas relay. It sounds the alarm whenever any fault develops inside the transformer and if the fault is dangerous the relay disconnects the transformer circuit. This relay is fitted in between the main tank and the oil conservator on the transformers of large capacity.
Construction of Three Phase Transformer
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