Types of Transformers - 1

Furnace Transformers

Furnace Transformers are Transformers which are specially designed for powering Arc Furnaces. These Transformers are designed to withstand high currents and severe voltage fluctuations. They are also designed to withstand higher than normal temperatures.
The windings are mechanically strengthened to withstand the huge forces generated due to the flow of high currents. Reactors are often used to smoothen the fluctuations. Operation of the Furnace breaker which can trip frequently generates surges and operational over voltages.
Furnace Transformers are designed to provide On load or no-load tap Changer. They also have a built in reactor for long arc Stability. The bushings can be air cooled or water cooled.
Transformers for DC Furnaces have an attached rectifier. DC Furnace Transformers are also designed to withstand the harmonics generated by the rectifier operation.
Furnace Transformers are generally used in the steel industry for smelting iron and refining steel.

Generator Step Up Transformers

Generator Step-up Transformer units are used to increase the voltage of a generator and connect the supply to a bus bar. For instance, if the generator has a voltage of 3300V and the busbar a voltage of 6600V, the Step Up Transformer will have a ratio of 3300/6600.
Generator Transformers are also used to limit the fault level of the generator in case of a fault.
Transformers used for these applications are called Generator Step-up Units. These Transformers are designed to operate at near full load.
Every time a fault occurs in the grid, these transformers are subjected to stress.
As these transformers are connected to the grid, they are subjected to constant voltage fluctuations.    Voltage fluctuations cause stress to the transformer windings. These windings have to be specially designed to withstand these stresses. Localized heating is another issue which is a result of over excitation by the generator during voltage fluctuations. Localized heating can damage the metallic accessories of the transformer.
Hence, the magnetic circuit is specially constructed to have very low leakage flux.
Generator Step Up Transformers are available in both single and three phase units. They can also be designed to withstand wide temperature variations.

Hermetically Sealed Transformers

Hermetically Sealed Transformers are transformers which are airtight i.e. they do not allow the air from the atmosphere to enter the transformer. These transformers are totally filled with oil. Preventing the entry of air protects the oil from oxidation and consequent deterioration.
Since the transformer is totally sealed, water cannot enter the transformer. This protects the insulation from moisture.
The transformer has low maintenance as there is no need for testing the oil or the insulation for a period of 10 years. This reduces the life cycle cost of the Transformer.
Since, there is no ingress of water or air into the transformer, the aging of the transformer insulation and oil are considerably reduced.
Hermetically sealed Transformers are used applications where it is difficult to conduct maintenance such as Offshore Platforms. Hermetically sealed transformer are more expensive to purchase. However, since they have lower maintenance costs and downtime, the life cycle cost of these transformer is cost effective as compared to conventional transformers.

Converter Transformers

Converter Transformers are equipment which convert the voltage and the frequency of the system voltage. Converter Transformers are used in applications such as Variable Frequency Drives, Electric Traction, copper refining and even in HVDC Transmission.

Converter Transformers have a converter unit on the primary or secondary side. The voltage is stepped up or down in the transformer and then converted into the desired frequency in the converter circuit. Alternatively, the frequency can be changed in the converter prior to stepping the voltage up or down.
Converters are used to couple two AC power systems asynchronously.
Converter Transformers are designed to withstand high harmonic content.

Medical Isolation Transformers

Medical Isolation Transformers are transformer used in medical facilities to power medical equipment. These transformers are designed to provide isolated AC power with noise suppression and are designed to withstand AC transients.
Noise can disturb sensitive medical equipment and interfere with measurements. Medical Transformers are designed with withstand short-circuits and are explosion proof.
They are also designed to have very low leakage currents. Excess leakage currents can cause unwanted and sometimes dangerous physiological effects in the patient.
Medical Isolation Transformer come in a box shaped enclosure to which the equipment can be connected.
Transformers are designed to withstand a very high hipot voltage.

Traction Transformers-An Overview

Traction Transformers are used in locomotives to step down the voltage from the overhead power lines. These transformers are similar to power transformers. However, they can be designed to function at different frequencies from 16 2/3 Hz to 50 Hz. The size of the core will vary depending on the frequency the transformer is designed for. At lower frequencies, a higher cross section of the core is required.
Cooling is carried out by different means such as through the use of silicone or mineral oil.
These transformers have a heater winding if they are operated at low temperatures. Traction Transformers will also have multiple traction windings. They may also have auxiliary windings for other circuits.
Traction Transformers are generally roof mounted. Some locomotives have transformers below the chassis as well.