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Monday, 15 December 2014

What are Power Transformers?

What are Power Transformers?

Power transformers are those transformers having capacity above 500 kVA and used between the generation and the distribution circuit. A typical power system consists of a variety of transmission and sub-transmission voltages and power transformers are employed at each of these stages where the voltage is to be transformed.

Power transformers are available for step up operation and are mainly used at power plants. These power transformers used at generating stations for the stepping up of voltage level are called Generator Transformer (GT). Power transformers for stepping down the voltage level can be found at all the major and intermediate sub-stations which are the transmission and the sub-transmission part of the power system.

Normal life span of a Power Transformer:

Normal life span of a power transformer is expected to be 30 years when operated under rated conditions of load, temperature etc. Transformers are rated based on the power output they are capable of delivering continuously at a specified voltage and frequency under usual operating conditions without exceeding the internal temperature limitations. Since the ambient temperature is supposed to vary according to the location, , therefore the temperature within a power transformer is usually expressed in terms of the temperature rise above the ambient temperature. Under certain conditions the power transformer may be overloaded and operated beyond its rated capacity with a predictable “loss of life”.

Classification of Power Transformer:

Power transformer may be broadly classified as:
1.  Small power transformers with capacity 500 kVA to 7500 kVA,
2.  Medium power transformers with capacity 7.5 MVA to 100 MVA,
3.  Large power transformers with capacity 100 MVA and above.

The upper range of smaller power transformer may vary throughout the sector.

Dielectric Medium in Power Transformer:

Over the years mineral or silicone oil has been used as insulating and cooling medium in transformers. Recently in 2014, Siemens has successfully produced and commissioned world's first EHV class Power Transformer that uses vegetable oil as dielectric medium.

Power transformers are normally equipped with auxiliary equipment such as fans and pumps. These forced circulation of air or oil increases the cooling and thereby increasing the rating of the transformer. 

Cooling of Power Transformer:

Usually, a power transformer will have multiple ratings corresponding to multiple stages of cooling. An example of multiple ratings would be ONAN/ONAF (i.e. Oil Natural Air Natural/ Oil Natural Air Forced) where the transformer has a base rating where it is cooled by natural convection and an additional rating because of the use of fans to provide additional cooling. A power transformer with a base rating of 16 MVA can have an additional 4 MVA capacity when the fans are ‘ON’, i.e. with ONAF cooling the MVA rating of the power transformer is 20 MVA.

Figure shows a power transformer with fans for additional cooling.

Large power transformers are large in size and cost millions of dollars. They pose unique challenges during their transportation. To know about transportation of power transformers read special arrangements for transportation of Large power Transformers