A transformer is defined as a passive electrical device that transfers electrical energy from one circuit to another through the process of electromagnetic induction. It is most commonly used to increase (‘step up’) or decrease (‘step down’) voltage levels between circuits.

Power transformers are electric devices that are used to step up or step down the voltage level of its supply source. The stepping up or down depends upon the number of turns of primary and secondary winding. Transformers have two windings, being the primary winding and the secondary winding. The primary winding is the coil that draws power from the source. The secondary winding is the coil that delivers the energy at the transformed or changed voltage to the load. If the number of turns on both the windings is the same, and the losses of the transformer are negligible, we may conclude that the voltage across each of the windings is the same. In this case, the transformer is just utilized in isolating two electrical circuits. Generally, a power transformer is used in stepping up the voltage of the supply to decrease the transmission losses, and then stepping down is done for the distribution purpose at the load centers. Power Transformers are larger size devices that transfer the energy to substations or public electricity supply.

A power transformer is a stationary machine used for transforming power from one circuit to another without changing frequency. This is a very basic definition of transformer. Since, there is no rotating or moving part, so a transformer is a static device. Transformer operates on an ac supply. A transformer works on the principle of mutual induction.


Power Transformers are used in transmission networks so they do not directly connect to the consumers. They are used in transmission networks of higher voltages for step-up and step-down applications (400 kV, 200 kV, 110 kV, 66 kV, 33kV) and are generally rated above 200MVA. They are used in generating stations and transmission substations with high insulation levels. Power transformers are, in fact, a bridge between the power generator and the primary distribution grid.

The main use of power transformers is to convert the low voltage high current to a high voltage low current.

Based on specifications and ratings, a power transformer can be divided into three categories:

Small Power Transformer

Medium Power Transformer

Large Power Transformer


Power transformers are some of the most important pieces of equipment required for operating a power system. It is critical for the installation team to complete several different tests prior to installation. Diligent testing ensures the electrical, thermal, and mechanical suitability of the transformer for the system being served.

There are different tests commonly applied to power transformers. Most testing routines will include most of these tests:

Visual Inspection: This is the simplest of all tests, this test may reveal potential problems that can't be detected by other more sophisticated forms of diagnostic testing. A standard procedure must be established to perform the visual test, identifying the elements to be viewed and criteria for pass or fail judgments. These can vary, depending on the type of transformer and the circumstances of the installation, but most standard visual inspections look for the presence of any sign of physical damage, oil loss or leakage, and integrity of wire connections.

Oil Tests: The oil that provides insulation and cooling properties for a transformer should be tested before the transformer is energized, and periodically as a part of a regular maintenance schedule. It is generally done with a portable testing unit which applies test voltage that increases in intensity until a breakdown point of the oil is detected. An oil sample test can detect several things on a transformer:

Acid number

Moisture content

Power factor

Dielectric breakdown

Interfacial tension

Oil tests are very important for determining the condition of the insulation and the oil.

Phase Relation Testing: This test will detect if two or more transformers have been connected in a correct phase relationship. This test calculates the angular displacement and relative phase sequence of the transformers and can be conducted at the same time as ratio and polarity tests. The voltages of the phase of primary and secondary windings in each transformer can be recorded and comparisons made to get the phase relation between them.

Polarity Testing: Polarity refers simply to the direction of current flow in a transformer, and testing is done to ensure that the windings are all connected the same way, and not in opposing ways that can cause a short circuit. Polarity is a vital concern if several transformers are to be paralleled or bank-connected.

Polarity in a transformer is categorized as either additive or subtractive, and it is tested using a voltmeter. When voltage is applied between the primary bushings and the resultant voltage between the secondary bushings is greater, then it means that the transformer has additive polarity. Three-phase transformers are also checked for polarity by the same means.

Resistance Testing: Resistance testing is conducted several hours after a transformer has stopped conducting current when it reaches the same temperature as its surroundings. The purpose of this test is to check for differences in resistance between windings and opens in the connections. This test ensures that each circuit is properly wired properly and that all connections are tight. Resistance testing is performed using a transformer megohmmeter. Performing this test will allow you to calculate and compensate for load losses as a whole.

Power Factor Testing: The power factor test helps to determine the power loss of the transformer's insulation system by measuring the power angle between an applied AC voltage and the resultant current. Power factor is defined as the cosine of the phase angle between voltage and current. For ideal insulation, the phase angle is 90 degrees, but in practice, no insulation is ideal. The closer the phase angle is to 90 degrees, the better the insulation. The test is performed with a power factor test kit. This test can be repeated during the service life of the transformer and verified against the result obtained during manufacturing, as a check to determine if the insulation is malfunctioning or decaying.

Insulation Resistance Testing: Insulation resistance testing measures the quality of insulation within the transformer. Testing is typically done with a megohmmeter. Some variations in testing results in natural, depending on the moisture, cleanliness and the temperature of the insulation, but to pass, the insulation must demonstrate a higher resistance than prescribed international standards for the type of transformer. The insulation resistance test involves measuring the insulation resistance of a device while the phase and neutral are short-circuited together.

Turns Ratio Testing: This test is used to make sure that the ratio between the windings of the primary and secondary coils follow the proper specifications. This test ensures that the transformer will provide the proper step-up or step down in voltage.


It is very important to test power transformers, as it checks the quality of materials used and the procedure of transformer manufacturing, hence it provides us the details of the life of the power transformer, the behavior of the power transformer on different faults. As a power transformer is highly expensive as well as an important part of the distribution system. Hence it becomes important to test them before using it.

For further information on procurement and testing of Power transformers kindly contact us at GZ Industrial Supplies.

17th Dec 2020 Ernest Orhuebor

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