Transformer facilitates the delivery of power electric energy at minimum power loss. The basic parts of a transformer are core, the primary winding and secondary winding. Apart from these, there are various other components such as insulation, transformer oil, cooling arrangements, protection relays, enclosure etc present in larger transformers. Let’s discuss the working principle of a transformer before diving into the topic.
Transformer – Principle of operation
A transformer is a static device that works on the principle of electromagnetic induction. When an alternating current flows in the primary winding of a transformer, a varying electromagnetic field is generated which induces EMF in the secondary winding. The magnitude of the induced EMF is proportional to the turns ratio.
Parts of a transformer
The following are the various transformer parts:
- Terminals and bushings
- Transformer oil
- Oil Conservator
- Radiators and fans
- Explosion vent
- Tap Changers
- Buchholz relay
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. In shell-type construction, the windings are surrounded by the core as shown below:
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.
Insulation is the most important part of transformers. Insulation failures can cause the most severe damage to transformers. Insulation is required between the windings and the core, between windings, between each turn of the winding and between all current-carrying parts and the tank. The insulators should have high dielectric strength, good mechanical properties and high-temperature withstand ability. Synthetic materials, paper, cotton etc are used as insulation in transformers.
The core, winding and insulation are the most basic parts of a transformer and are present in all types.
The main tank is a part of a transformer serves two purposes:
- Protects the core and the windings from the external environment.
- 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.
5. Terminal and bushings
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.
6. Transformer oil
In all oil-immersed transformers, transformer oil provides added insulation between the conducting parts, better heat dissipation and fault detection features. Hydro-carbon mineral oil is used as transformer oil. It is composed of aromatics, paraffin, naphthenes and olefins. Transformer oil has a flashpoint of 310 degree Celsius, the relative permeability of 2.7 and a density of 0.96 kg/cm3.
7. Oil Conservators
Oil conservator is moved on the top of the transformers and is located well above the tank and bushings. Normally a rubber bladder is present in some oil conservators. The transformer oil expands and contracts with increase and decrease in temperature. The oil conservator provides adequate space for oil expansion. It is connected to the main tank through a pipe. A level indicator is fitted to the conservator to indicate the oil level inside.
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.
9. Radiators and fans
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.
10. Explosion vent
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.
11. Tap changers
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.
12. 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.
Apart from all the parts of a transformer discussed above, there many other sensing instruments (temperature sensors, pressure sensors etc.), indicators, protection relays, heat exchangers (for efficient cooling) and valves present in huge transformers. They are application-specific and are present in huge transformers.