Metallic Core - The core provides a low reluctance path for electromagnetic flux and supports the primary and secondary windings. It is made by stacking thin sheets of high-grade grain-oriented steel which are separated by thin insulating material. In order to minimize the hysteresis and eddy currents, the carbon content of the core steel is maintained below 0.1%. When it is alloyed with silicon, eddy currents can be reduced. A typical three-phase transformer core is shown in the picture above. Each limb carries the primary and secondary winding of each phase. The limbs are magnetically coupled by the yokes. There are two types of core constructions: core type and shell type.
Holding Frame - Holding Frame used to hold all the laminations of the core so that during working of transformer the laminations remain braced against heavy electromagnetic forces produced due to varying flux in the core.
Winding - Transformer carries two sets of winding per phase – Primary winding and secondary winding. These winding consists of several turns of copper or aluminium conductors, insulated from each other and the transformer core. The type and arrangement of winding used for transformers depend upon the current rating, short circuit strength, temperature rise, impedance and surge voltages. Out of the primary winding and secondary winding, the one which is rated for higher voltage is known as High voltage (HV) winding and the other is known as the Low voltage (LV) winding. The high voltage winding conductors are thinner than the low voltage conductors and surround the LV winding from outside. The LV winding is placed close to the core. In shell-type transformers, the winding is split into several coils (few turns of a conductor). The HV coils are sandwiched between the LV coils. Whereas in core type transformers, windings are classified into four types: Multi-layer windings, Helical windings, Disc winding and foil winding. The choice of winding type is determined by the number of turns and its current carrying capacity.
Tank - The main tank is a part of a transformer serves two purposes:
ü Protects the core and the windings from the external environment.
ü Serves as a container for oil and support for all other transformer accessories.
Tank bodies are made by fabricating rolled steel plates to containers. They are provided with lifting hooks and cooling tubes. In order to reduce weight and stray losses, aluminium sheets are also used instead of steel plates. However, aluminium tanks are costlier than steel ones.
On Load Tap Changer - Tap changers are used to adjust the secondary voltage of transformers. They are designed to change the turns ratio of the transformer as required. There are two types of tap changers: On-load tap changers and Off-load tap changers. Off-load tap changers are designed to operate only when the transformer is not supplying any loads whereas on-load tap changers are capable of operating without interrupting the current flow to the load. Automatic tap changers are also available.
Bushings and Terminals - For connecting incoming and outgoing cables, terminals are present in transformers. They are mounted upon the bushings and is connected to the ends of the windings. Bushings are insulators that forms a barrier between the terminals and the tank. They are mounted over the transformer tanks. They serve as a safe passage for the conductors connecting terminals to the windings. They are made from porcelain or epoxy resins.
Radiator Wings/Cooling Tubs - The power lost in the transformer is dissipated in the form of heat. Dry transformer are mostly natural air cooled. But when it comes to oil immersed transformers, a variety of cooling methods are followed. Depending on the kVA rating, power losses and level of cooling requirements, radiators and cooling fans are mounted on the transformer tank. The heat generated in the core and winding is passed to the surrounding transformer oil. This heat is dissipated at the radiator. In larger transformer forced cooling is achieved with the help of cooling fans fitted to the radiators.
Breather - Breather is present in all oil-immersed transformers that have a conservator tank. It is necessary to keep the oil-free from moisture. As the temperature variations cause the transformer oil to expand and contact, air flows in and out of the conservator tank. This air should be free from moisture. Breather serves this purpose. A breather attached to the end of air pipe such that the air enters and exits the conservator through it. The silica gel present in the breathers removes moisture for the air and delivers moisture free air to the conservator.
Buchholz relay - Buchholz relay is one of the most important parts of oil immersed transformers rated over 500kVA. It is a oil and gas actuated relay which is used to sense faults occurring in the parts immersed in the oil. Short circuits occurring under the transformer oil generates enough heat to decompose the oil into hydrogen, carbon monoxide, methane etc. These gases gradually move towards the conservator tank through the connecting pipe. Buchholz relay, which is mounted on the pipe connecting the conservator tank and the main tank, senses these gases and activate the trip and alarm circuits. The trip circuit opens the circuit breaker supplying current to the primary winding and interrupts the current flow.
Explosion Valve - An explosion vent acts as an emergency exit for oil and air gases inside a transformer. It is a metallic pipe with a diaphragm at one end, held slightly above the conservator tank. Faults occurring under oil elevates the pressure inside the tank to dangerous levels. Under such circumstances, the diaphragm ruptures at a relatively low pressure to release the forces from within the transformer to the atmosphere.
Control Panel - Control panel is a metal box which contains important electrical devices that control and monitor a mechanical process electrically.
