Electrical Power Transformer: Definition & Types of Transformers

Power Transformer Definition

A power transformer is a static machine used for transforming power from one circuit to another without changing the frequency. As there is no rotating or moving parts, a transformer is classified as a static device. Transformer operates on an AC supply. Transformers operate based on the principle of mutual induction.

Use of Power Transformers

Generation of electrical power in low voltage level is very much cost effective. Theoretically, this low voltage level power can be transmitted to the receiving end. This low voltage power if transmitted results in greater line current which indeed causes more line losses.

But if the voltage level of a power is increased, the current of the power is reduced which causes reduction in ohmic or I2R losses in the system, reduction in cross-sectional area of the conductor i.e. reduction in capital cost of the system and it also improves the voltage regulation of the system. Because of these, low level power must be stepped up for efficient electrical power transmission.

This is done by step up transformer at the sending side of the power system network. As this high voltage power may not be distributed to the consumers directly, this must be stepped down to the desired level at the receiving end with the help of step down transformer. Electrical power transformer thus plays a vital role in power transmission.

Two winding transformers are generally used where ratio of high voltage and low voltage is greater than 2. It is cost effective to use auto transformer where the ratio between high voltage and low voltage is less than 2.

Again a single unit three phase transformer is more cost effective than a bank of three single phase transformers unit in a three phase system. But a single three phase transformer unit is a bit difficult to transport and have to be removed from service entirely if one of the phase winding breaks down.
power system

Types of Transformers

Transformers can be categorized in different ways, depending upon their purpose, use, construction etc. Note that sometimes these classifications overlap – e.g. a transformer can be both a three phase transformer and a step up transformer at the same time. For more information, some of the best electrical engineering books explain the workings of a transformer in more detail.

The types of transformers are as follows:

Step Up Transformer and Step Down Transformer

Step up transformers converts the low voltage (LV) and high current from the primary side of the transformer to the high voltage (HV) and low current value on the secondary side of the transformer.

Step down transformers converts the high voltage (HV) and low current from the primary side of the transformer to the low voltage (LV) and high current value on the secondary side of the transformer.

Three Phase Transformer and Single Phase Transformer

A three phase transformer is generally used in three phase power system as it is more cost effective than single phase transformers. But when size matters, it is preferable to use a bank of three single phase transformer vs a three phase transformer, as it is easier to transport than one single three phase transformer unit.

electrical transformer

Electrical Power Transformer, Distribution Transformer and Instrument Transformer

Power transformers are generally used in transmission network for stepping up or down the voltage level. It operates mainly during high or peak loads and has maximum efficiency at or near full load.

Distribution transformer steps down the voltage for distribution purpose to domestic or commercial users. It has good voltage regulation and operates 24 hrs a day with maximum efficiency at 50% of full load.

Instrument transformers include C.T and P.T which are used to reduce high voltages and current to lesser values which can be measured by conventional instruments.

Two Winding Transformer and Autotransformer

A two winding transformer is generally used where the ratio between the high voltage and low voltage side is greater than 2.

It is more cost effective to an autotransformer in situations where the ratio between the high voltage and low voltage sides is less than 2.

Outdoor Transformer and Indoor Transformers

As the name suggets: outdoor transformers are designed for installing outdoors.

Whereas indoor transformers are designed for installing at indoors (who would have guessed!).

Oil Cooled and Dry Type Transformer

This classification pertains to the transformer cooling system used within the transformer.

In oil cooled transformers, the cooling medium is transformer oil. Whereas in the dry type transformer, air cooling is used instead.

Core type Transformer

There are two main types of transformer windings – core type, and shell-type. There are also berry type transformers.

A core type transformer has two vertical legs or limbs with two horizontal sections named yoke. The core is rectangular in shape with a common magnetic circuit. Cylindrical coils (HV and LV) are placed on both the limbs.

Shell Type Transformer

A shell type transformer has a central limb and two outer limbs. Both HV, LV coils are placed on the central limb. The double magnetic circuit is present.

Berry Type Transformer

In a berry type transformer, the core looks like the spokes of a wheel. Tightly fitted metal sheet tanks are used for housing this type of transformer, with transformer oil filled inside.

History of Power Transformers

If we want to know the history of transformer we have to go back long in the 1880s. Around 50 years before that in 1830 the property of induction was discovered, and this is the working principle of transformer.

Later the transformer design was improved resulting in more efficiency and lesser size. Gradually the large capacity of transformers in the range of several KVA, MVA came into existence.

In the year 1950, 400KV electrical power transformer was introduced in high voltage electrical power system. In the early 1970s, unit rating as large as 1100 MVA was produced. Various manufacturers manufactured 800KV and even higher KV class transformers in the year 1980.

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