Distribution transformers plays an important role in the power system. In order to supply power uninterruptedly, they should work without any failure. To ensure this distribution transformers are tested periodically.

Two important testing methods of 100 kVA distribution transformer done at manufacturing industries are discussed in this article.

1. Insulation Resistance Test
2. Transformer Oil Testing

Before discussing them you should know some basics about testing of transformers.

Transformer Testing Basics

Transformer tests can be broadly divided into two types based on when the tests are conducted.

1. Type Test
2. Routine Test

Type Test

Type tests are done in manufacturing premises to prove that the transformer meets customer’s specifications and design expectations. Type tests are carried out for confirming the basic design expectation of that transformer.

These tests are done mainly in a prototype unit not in all manufactured units in a lot. Type test of transformer confirms main and basic design criteria of a production lot.

Type tests of transformer include:

1. Open circuit and short circuit test.
2. Measurement of insulation resistance
3. Transformer ratio test
4. Transformer oil testing
5. Transformer winding resistance measurement
6. Magnetic balance test

Routine Test

Routine tests of transformer are mainly for confirming operational performance of individual unit in a production lot.

Routine tests are carried out on every unit manufactured.

Routine tests of transformer include:

1. Open circuit and short circuit test.
2. Measurement of insulation resistance.
3. Transformer ratio test
4. Transformer oil testing
5. Transformer winding resistance measurement
6. Magnetic balance test

Routine tests of transformer include all the type tests.

Here, in this article we will broadly discuss about two types of test:

1. Insulation Resistance Test or Megger Test of Transformer
2. Transformer oil testing

Insulation Resistance Test of Distribution Transformer

Insulation resistance is the ratio of applied voltage to the resulting current at a specified time after the voltage is applied. Direct voltage are used for measuring insulation resistance values.

Insulation resistance test of transformer is essential type test. This test is carried out to ensure the healthiness of overall insulation system of an electrical power transformer.

The insulation resistance (IR) test (also commonly known as a Megger test) is a spot insulation test which uses an applied DC voltage (typically 250V dc, 500V dc or 1,000V dc for low voltage equipment <600V and 2,500V dc and 5,000V dc for high voltage equipment) to measure insulation resistance in either kΩ, MΩ or GΩ.

The measured resistance is intended to indicate the condition of the insulation or dielectric between two conductive parts, where the higher the resistance, the better the condition of the insulation.

Ideally, the insulation resistance would be infinite, but as no insulators are perfect, leakage currents through the dielectric will ensure that a finite (though high) resistance value is measured.

Because IR testers are portable, the IR test is often used in the field as the final check of equipment insulation and also to confirm the reliability of the circuit and that there are no leakage currents from unintended faults in the wiring (e.g. a shorted connection would be obvious from the test results).

Procedure of Insulation Resistance Test of Transformer:

The procedure of testing insulation resistance of transformer is given below.

1. Firstly the positive terminal of the megger is connected to primary side and negative terminal to ground.
2. Then readings are recorded after application of voltage.
3. Then again positive terminal of megger is connected to secondary side and negative terminal side to ground.
4. Then we again apply the voltage and readings are recorded.
5. And at last again the positive terminal of mrgger is connected to primary side and negative terminal to secondary side.
6. Then we record the readings after application of voltage.

Sample IR Test and Result

Transformer Oil Testing

Several tests can be performed on transformer oil to determine its condition and many are particularly useful in determining the condition oil that has been in service for a number of years. The most common test performed on oil is the dielectric test. These specifications define the method of preparing a sample, the equipment to be used and the test method.

During factory tests it is useful to take oil samples before testing and again after testing in order to perform a dissolved gas in oil analysis. This is particularly useful for high voltage transformers and in cases when a temperature rise test be performed.

Oil tests are a useful indicator of the condition of the insulation system and the oil and form important elements of any transformer preventative maintenance program. Results taken after periods of service can be compared to baseline measurements taken on the new transformer.

The insulation oil of voltage- and current-transformers fulfils the purpose of insulating as well as cooling. Thus, the dielectric quality of transformer oil is a matter of secure operation of a transformer.

As transformer oil deteriorates through aging and moisture ingress, transformer oil should, depending on economics, transformer duty and other factors, be tested periodically. A good transformer oil must follow some Physical, chemical and electrical properties.