Parts of the Transformer

The parts of the transformer are shown in Figure 1.1
The main parts of the transformer are discussed here.

Steel Tank

The steel tank is one of the main parts of the transformer. This steel tank is a well-shaped box. Inside it, the winding core and the necessary equipment required for the transformer are installed. This tank is filled with mineral oil or insulating oil. To make the T/F completely safe, the inner and outer surfaces of the tank are cleaned of rust and coated with a paint.

Core

The core is included in the main parts of the transformer.The transformer core is made of laminated steel sheet. Laminated steel sheet provides minimum air gap with continuous magnetic path. By laminating the core, the eddy current loss is reduced. The core of a single-phase transformer consists of two legs and the core of a three-phase transformer consists of three legs. Each layer of core is insulated from the other layer by applying a light layer of varnish. The core is round or rectangular depending on the capacity of the T/F. These cores are made in layers to fit the core perfectly inside the coil. These laminated covers are fastened with bolts, etc. to prevent vibration and noise.

Winding

The winding is also included in the main parts of the transformer. A single-phase T/F has one primary and one secondary winding while a three-phase T/F has three primary and three secondary windings.

The low voltage winding is after the core and the high voltage winding is wrapped over the low voltage winding. Adequate distance is also kept between the two windings for circulation of cooling medium oil. Windings are also made in the form of spring coils which are used in the secondary phase of low voltage and high current transformers.

Conservator Tank

This tank is cylindrical, which is mounted on top of the main tank of the T/F. The main tank and the conservator tank are connected through a pipe.
A Buchholz relay is installed between the main tank and the conservator in a transformer of more than 1MVA capacity.
As the oil heats up and expands, the conservator tank provides space for it, and when the oil cools, it returns to the main tank.
Reduces oxidation by reducing the surface area of ​​the oil in contact with air. Oxidized oil is confined to the conservator tank. A glass tube is attached to one side of the conservator tank, which indicates the level of oil in the T/F. The coolant level must be up to the mark on the gauge on this glass tube. The conservator tank is one of the main parts of the transformer.

Breather

Air is drawn into the T/F through the breather and returned.
When the oil expands due to heat, air escapes, and when the oil cools and shrinks, air enters. A glass vessel is placed inside the breather, in which calcium chloride or silica gel is filled. When air enters the T/F tank, it also contains moisture vapor. The silica gel absorbs moisture and allows only dry air to enter the tank. The color of silica gel is changed after some time when it turns from blue to pink due to continuous absorption of moisture.  This silica gel can be made reusable by heating.

Thermometer

The thermometer indicates the temperature of the T/F oil.  It is applied to transformers of rating above 50 kVA. In large transformers, a temperature indicator is placed inside the winding to record the temperature of the winding. When the temperature rises to a dangerous level, the device warns by giving an alarm signal.

Pressure relief Pipe or Pressure relief vent

This device protects against over pressure build up inside the T/F due to evaporation of oil or other insulating material. It is a curved pipe with a glass shell over the T/F tank. Absence of this device may lead to explosion of T/F tank. During T/F operation, excessive gas pressure is generated inside the tank due to temperature build-up or any other reason, which is reduced by this device. The device is constructed in such a way that it senses the pressure rise in the relief vent with the help of an adjusting spring and bellows and mechanically breaks the bursting plate through a spring-driven shaft and the pressure is released.

Valves

Valves are installed in each transformer for oil extraction, oil filling, filtering, and sampling, etc.

Buchholz Relay

The Buchholz relay is installed in the middle of the pipe leading from the conservator tank to the main tank. This relay is gas operated and has two floats, one float is connected to the trip circuit in case of emergency while the other only gives an alarm signal. When a fault occurs in a T/F, gases are produced due to the decomposition of the insulating material. These gases move upward due to light and reverse the float through the Buchholz relay which completes the alarm or trip circuit and automatically disconnects the T/F from the mains supply.

Bushings

Bushings are used to insulate and bring out the winding terminals from the main tank of the T/F. Various types of HT and LT bushings are used in transformers. Arcing cords are placed above and below the bushing to protect against lightning. A condenser type bushing is wound like a power capacitor and uses good quality craft paper. Its vacuum is filled with high quality condenser oil to greatly increase its dielectric strength. Its main element is enclosed in porcelain from top and bottom and then filled with oil and sealed tightly.

Tap Changing Switch

Off Load Switch

This switch is used to change the voltage ratio of the winding. The high voltage winding taps out and is fed to the tap-changing switch. If the primary voltage is high, more turns are added and if the voltage is low, a few turns are shorted. This switch is off-loading, i.e. it is operated after switching off the T/F.

On Load Switch

On-load tap-changing switches are used with T/F’s where the voltage is to be changed without switching off the power supply.
There are two types of on-load switches, resistor type and reactor type. Resistor type selector switches are used in power distribution transformers and consist of star or delta connected windings. Reactor type in on load tap changers are used in power transmission and distribution transformers.

Oil Gauge

The oil gauge indicates the level of oil inside the T/F. Oil gauges are used with transformers of medium capacity and higher ratings.
The gauge is also linked to an alarm in the control room. Whenever the oil level is low, the oil gauge’s alarm contact is closed, triggering a low oil alarm in the control room.

Radiator

A radiator is attached to the main tank for cooling the T/F. Radiators are used in most of the 50kVA transformers of higher capacity. Radiators are in the form of pipes or tubes, they increase the surface area of ​​the T/F tank, thus providing better cooling. This type of cooling of T/F is called oil natural air natural. In this, the oil of the T/F is heated and rises up and then cools down from the top. The hot oil transfers its heat to the outer surface of the main tank and radiator pipes.    As the air passes through the main tank and radiator, it cools them. The radiator is also included in the main parts of the transformer.

Cooling Fans

On large T/F (Above 1MVA), cooling fans are mounted above the radiator. When the transformer temperature exceeds 70°C, the contacts of the oil temperature gauge are closed, which turns “ON” the cooling fan. Thus, the cooling fan cools the oil passing through the radiator through the air. Such cooling is called the oil natural air force.

Oil Pumps

Oil pumps serve to rapidly circulate the T/F oil in the main tank. Transformers rated above 26MVA are also fitted with oil pumps to achieve cooling. Oil pumps also operate at a certain temperature.

Transformer Insulation

Mineral insulating oil or mineral oil is used in the main tank of the T/F. This oil provides insulation as well as cooling.