Routine test you should perform before commissioning a Dry-type transformer?

What are Dry-type transformers?


The dry-type transformers are air-cooled, and the normal ambient air cools their windings. They are exclusively designed with proper ventilation to let the circulating air flowing in its fins to maintain it’s working temperature, and make it efficiently operate without any external aid. Like other transformers, they are also static devices.

These transformers are economical, safer, and efficient. They are designed for indoor as well as outdoor applications and are for lower loads.

The dry transformers are easy to install as they don’t require any cooling oil or some other medium. They don’t need any fire-proof vault as well. 

What are types of transformers?

1) Cast Resin Dry Type Transformer – These transformers come with the windings that are encapsulated with epoxy resin to make them moisture-proof.

2) Vacuum pressure Impregnated Transformer (VPI) – These transformers are exclusively designed for commercial and industrial applications to withstand extra high temperatures, with moisture-resistant polyester sealant. They are manufactured up to 30 MVA rating using vacuum pressure impregnation.

For lower voltages, the windings are of foil or strip in continuous layers. Whereas for higher voltages the windings are made up of disks that are connected in series or parallel depending upon power rating and the voltage.

3)  Vacuum Pressure Encapsulated (VPE) – These transformers use silicon-based resin instead of polyester for insulation purpose. As it results in a thicker coating, has higher resistance against moisture, salt, and high humidity. They are excellent for harsher environments.

What are various routine tests conducted on a dry-type of transformer?

Various types of different tests are conducted on these transformers to confirm their adherence to the specifications, and the claimed performance., which are classified into two classes-

A) Transformer Tests done by the Manufacturer

  1. Type Tests – Type tests are conducted to ensure that the transformers are complying to the specifications, technical standard and regulation they are designed to abide. These tests are conducted by the accredited laboratories or testing centres.
  2. Routine Tests – The routine tests are conducted to confirm the operational performance of a sample unit from a production lot. These tests are conducted in the premises where the transformers are manufactured.
  3. Special Tests – These tests are other than Type Tests and Routine Tests, that are agreed between manufacturer and purchaser.

B) Transformer Tests done at Site

  1. Pre-Commissioning Tests – These tests are conducted before commissioning of the transformer at the site of erection. They are conducted to assess the proper installation and to compare the low voltage tests with factory test reports.
  2. Periodic Monitoring Tests – These tests are conducted periodically to monitor the condition of the transformer to let it perform continuously without fail.
  3. Emergency Tests – These tests are conducted during emergency conditions to have a better understanding of the transformer status during emergency periods. They are generally performed to check the overall insulation strength of the transformer.

All dry-type transformers are subjected to following routine tests –

  1. Insulation Resistance Tests
  2. Winding Resistance Tests
  3. Voltage ratio Test
  4. Polarity Test
  5. No-load losses Test
  6. Short Circuit or Full Load Test
  7. High Voltage Test
  8. Dielectric Test
  9. Applied Voltage Test
  10. Induced Potential Test
  11. Partial Discharge Test
  12. Magnetizing Current Test
  13. Power Factor Test
  14. Phase Relation Test
  15. Sound Level Measurement

Insulation Resistance (IR) Test

The Insulation resistance test ensures the dryness of the insulation. It is measured in Mega-ohms and varies from the transformer to transformer. The insulation resistance is dependent on the temperature and is typically measured on the ambient temperature. In case, it is measured at a higher temperature; then the correction factor has to be considered before finalizing the value for insulation resistance.

The insulation resistance value measured at the factory is considered as the standard insulation resistance of the transformer, which is later compared with the field measurement values to consider the health of the transformer. This test is also known as the Meggar Test.

The insulation resistance is measured between High Voltage windings to Low Voltage windings, High Voltage windings to ground and Low Voltage windings to ground. While testing the remaining all other parts of the transformer not under test are always grounded.

Winding Resistance Test

This test is conducted as a routine test and field test as well.It is measured by the current-voltage method. Here the test current is injected in the winding, and the corresponding voltage drop across the winding is measured to know the winding resistance by Ohm’s law. 

This test is conducted to ensure that all circuit is wired correctly with tight connections.

Voltage Ratio Test

The voltage ratio is defined as the ratio of the primary voltage to the secondary voltage (VR). The voltage of the windings is directly proportional to the number of turns on the coils. It is conducted with the Transformer Turns Ratio Tester (TTR), and is done to ensure the correct turns ratio of the primary turns to the secondary turns. It let us know the tap changer performance, open windings, incorrect winding connections, shorted turns, and other faults inside the transformer.

The phase voltage is applied at one winding by a bridge circuit, and the ratio of the induced voltage is measured at the bridge.

Polarity Test

The polarity test is conducted to ensure that similar windings are connected. The transformers are required to be connected in parallel to increase the rating of the entire system and to feed more connected load than the specified rating of the transformer. The windings of similar polarity are connected. There are two types of polarity

1) Additive Polarity – If the windings of the two or more transformers are connected, then the voltages of the connected transformers are added together.

2) Subtractive Polarity – In case the opposite windings are connected, it results in subtractive polarity.

If the transformers are connected in reverse polarity in parallel operation than it leads to an accident resulting in damaging the system.

No-Load losses Test

The no-load losses tests are conducted to check the operational economy of the transformer. These tests are conducted with open HV windings, and feeding another winding with the rated voltage and the frequency. These losses occur as long as the transformer is operated.

They are also termed as core-loss., which got generated when the rated flux density is induced in the core. These losses occur even when the transformer is not feeding any load or also when the secondary winding is open.

In case the voltage supply is insufficient to provide the pre-requisite voltage to get the desired current then capacitors are used to meet that gap. The transformer winding temperature has to be stabilized before conducting the test. Both the winding temperature, as well as the resistance, is noted.

The applied current is kept between 25%-100% of the rated current, and the measurements are taken quickly to avoid the increasing winding temperature, which could otherwise lead to errors.

In case the temperature goes above standard temperature, that is 75 degrees C then the necessary corrections are to be made to the readings.

Short Circuit or Full Load Test

This test is conducted to determine the copper losses, which are also known as winding losses.

This test is conducted by connecting the test voltage at the rated frequency to the HV side through a variac, while the LV side is kept short-circuited. A low voltage is applied on the HV side and is slowly and slowly increased until the rated current is obtained on the HV side. At this reading, the wattmeter represents the total full-load Copper losses of the transformer that comprises of both primary Cu loss and the secondary Cu loss. As the applied voltage is just 5 to 10% of the normal voltage in comparison to the rated primary voltage, therefore the core losses are considered as negligible and the result represents the full load copper losses.

High Voltage Test

This test is conducted to ensure the transformer’s performance at the rated electrical conditions, that ensures the effectiveness of the insulation of the transformer.

Dielectric Tests

Dielectric Tests are conducted to ensure the claimed overall insulation strength of the transformer. The full test voltage is applied for 60 seconds between the windings under test to all other windings, magnetic core of the transformer and the enclosure connected to the earth. These tests verify the dielectric strength of the insulation and confirm transformers capability to withstand the test levels defined in the standards.

There are three different types of dielectric test conducted on the transformers-

  1. Applied potential test
  2. Induced potential test
  3. Impulse test (Type Test)

Applied Voltage Test

In it, the terminals of all the three phases of the HV winding are shorted and brought to a point (say point-1). All other terminals of other windings are tied to another point (say point-2), and are first connected to the tank and then to the ground. The test voltage is applied between these two points depending upon the specifications of the transformer usually for a minute at the respective frequency.

Similarly, other windings are also tested as per test voltage generally referred to as Hipot level.

Induced Potential Test

It ensures the insulation level within the windings like in inner-layers, turns to turn and section to section. This test is conducted between all the windings simultaneously. The Class I Transformers are tested at twice the rated voltage while the Class II Transformers are tested in another way. Here the enhancement level voltage of the transformer is first induced for a shorter duration of typically 1 minute, which is followed by the long duration induction of the voltage, usually one hour, during which the voltage is measured. It is also known as ‘Separate Source Voltage withstand test’.

Partial Discharge Test 

The partial discharges are the minor discharges occurring in the insulation of the transformer, which doesn’t lead to the breakdown of the insulation but deteriorates the system by damaging the insulation of the transformer slowly and slowly. Besides, these discharges also produce radio-frequency voltages that interfere with radio communications.

The partial discharges are measured either in Radio Influence Voltage in micro-volts or can be measured in charge in Pico-Coulomb. It is typically conducted in combination with the induced voltage test.     

The partial discharge voltage is measured by giving an elevated level of electrical stress between the transformer windings and the other insulation.

Magnetizing Current Test

The magnetizing current test ensures the sufficient reluctance of the magnetic circuit that is required to establish the flux in the core. It indicates and locates the defects in the magnetic core structure, problem in tap changers, failure in between turn insulation, shifting of windings, and much more. The DC resistance is measured before conducting the test.

As long as the measured exciting current is within the 30% range of the earlier conducted test, the transformer is considered fit to use. Otherwise, in case the measured exciting current goes 50 times higher than the calculated value during factory inspection, then there’s a fault in the winding, and this needs further analysis.

Power Factor Test

This test is conducted to determine the power loss of the transformer’s insulation system. It indicates the malfunctioning or decaying of the transformer’s insulation and is as the power angle between the applied AC voltage and the resultant current.

The power angle in ideal conditions is 90 degree, but as no insulation is perfect; therefore, it is never precisely 90 degree but something closer to it. Closer the phase angle is to 90 degrees, better is the transformer.

Power factor is the cosine of the phase angle between the current and the voltage. The test is conducted with a power factor testing kit, and its value is considered in comparison with that achieved during the manufacturing of the transformer.

Phase Relation Test

The phase relation test is conducted to ensure the correct phase relationship between two or more transformers. It describes the angular displacement and relative phase sequence of the transformers. It is in continuation of the turn ratio and the polarity test. The phase voltage of the primary and secondary windings of the transformers is compared to know the phase relation between them.

Sound level Measurement:

Sound level measurement checks the sound generated by the transformer during regular operation. The sound comes from the core of the transformer due to Magnetostriction phenomena. It is because of the very-small mechanical vibrations, which occurs at 120 Hz frequency or it’s higher harmonics.

The test is conducted by energizing the transformer at the rated voltage and frequency, and the sound generated is measured at four-five different places around the transformer in decibels(dB). The average of these figures is the sound of the transformer.


Transformers are a vital part of any installation, whether residential, industrial, or commercial. Therefore, it is advisable to conduct the routine test before installation rather than troubleshooting the failures later or replacing the transformer itself. It costs negligible to perform the tests and inspect it beforehand rather than wasting productivity, comfort, energy, money, and time later on. We, Servostar servo Voltage Stabilisers are an established renowned manufacturer based out of India and are catering to entire pan India in power equipment like Servo Stabilizers and all type of transformers including isolation transformers for domestic households and other industries. We have a proven track record of supplying quality transformers and voltage stabilizers to various industries, commercial setups, residences, and the utility. We do provide consultancies also.