Transformer is composed of two windings (or three windings for three-winding transformer or a single core three phase transformer) whose main purpose is to step-up or step down an AC voltage and AC current. Transformers can be called differently depending on its function, capacity, and construction;
A transformer that is of high MVA capacity and used in high voltage application like in large power plants and high voltage transmission system is commonly called Power Transformer. In most power plants, they called this kind of transformer, the Main Transformer or Step-up Transformer.
In most distribution systems where the primary voltage is around 34.5kV or less, the transformer that is being used to step-down the voltage is called a distribution transformer. Usually, distribution transformers are single phase, have a secondary voltage of 220V/480V, and have a MVA rating of less than 1 MVA. More information here: http://www.jcmiras.net/jcm/item/48/
Depending on the insulation and cooling medium, transformers can be oil-filled or dry-type transformers. Most dry type transformers are used for indoor application since oil-filled transformers can be fire hazard.
There are various kind of transformers depending on its function;
Potential or Voltage Transformer (PT) – This is commonly used for metering or relaying applications. It steps down a very high primary voltage to around 110 volt or lower. Though, transformers are always thought as composed of sets of windings, there are some potential transformers that are made of series of capacitors (no winding at all) that works like a voltage divider to step down the voltage. This is called a Coupling Capacitor Voltage Transformers (CCVT). PT’s has a low MVA rating.
Current transformer (CT) – If PT is used to step down the voltage, CT works with the same purpose - that is, to step-down the high primary current for metering or relaying purposes. CT’s can be bushing type or core type. Like PT, CT has a low MVA rating.
Both Current and Potential transformers are also called instrument transformers.
Voltage Regulator - Voltage regulator is actually a transformer that has adjustable taps to regulate or maintain the voltage level of the regulated system at an acceptable level.
Non-Instrument Transformers in the Power Plant
Transformers in a Power Plant have various name designation depending on its assigned function;
Main or Step-up Transformers – This is used to step-up the voltage of the generated power from generator voltage (usually 13.8kV) to the transmission line voltage (like 138kV or 230kV).
Auxiliary or Station Service Transformers - This kind of transformer is used to step-down the generator or grid voltage for plant’s auxiliary and station load.
Excitation transformer – Excitation transformer is used for the excitation system of the generator.
Unit Transformer – Aside from auxiliary transformer, some power plants, like those plants with large auxiliary loads (usually thermal plants), have a dedicated transformer for the per units auxiliary loads’ electricity requirements. This is called Unit Transformer.