Transformer Testing | Types of Transformer Testing

An electrical device that transfers electrical energy from one circuit to another through mutual induction between windings is known as a transformer. The main working principle of a transformer is electromagnetic induction. There are two types of windings in a transformer, one is primary winding and the other is the secondary winding. This winding is used to increase or decrease the voltage.

Transformers play a very important role in the entire electrical distribution system. If the transformer in this system does not work properly then the entire distribution system depending on it may be damaged. And there will be no possibility of transferring electrical energy.

Electrical circuits can also be damaged due to mechanical failure, thermal loss, electrical loss, and winding distortion. Therefore the operation of the transformer should be checked using the transformer testing procedure to avoid failure.

It is necessary to test the transformer to determine the specificity and performance of the electrical transformer. The transformer should be tested during the manufacturing process to meet the special design and specificity. In today’s article, we will see what is transformer testing and what are its types and today we will get an overview of transformer testing.

Types of Transformer Testing:

Transformers have many types of problems such as mechanical failure, winding deformation, thermal failure and insulation breakage of the transformer, electrical failure, etc. It is necessary to test it to prevent such problems.

The various types of tests performed in a transformer are as follows:

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Transformer Testing Done at the Factory:

The tests performed in the factory are as follows:

• Routine Tests.
• Type Tests.
• Special Tests.

#1. Routine Tests:

Regular testing is done in each production lot to verify and verify the operation performance of the transformer. This test should be performed on each unit of the product. This test includes all tests except the temperature increase and vacuum test. These are as follows :

• Measurement of winding resistance of transformer.
• Testing of the transformer vector group.
• Test of transformer turn ratio.
• To measure the impedance voltage of a transformer.
• Measurement of short circuit impedance of the transformer.
• Measuring the insulation resistance of a transformer.
• Dielectric tests.
• No-load tap changer tests.
• Oil pressure test to check for leakage on joints and gasket on the transformer.

#2. Type Tests:

This type of test is necessary because it provides information on whether the transformer is built to meet the customer’s expectations. Therefore the transformer has to undergo this type of test only during manufacturing.

The transformer has to go through some tests to ensure the design and expectations of the customer. Such tests are performed in a prototype unit or unit of production. The transformer-type test product verifies and confirms the basic structure of the transformer in flour.

Tests of the transformer type include measurements of its various specifications. They are,

• Measurement of winding resistance of transformer.
• Testing of the transformer vector group.
• Measurement of short circuit impedance of the transformer.
• To measure the impedance voltage of a transformer.
• Measuring the insulation resistance of a transformer.
• Dielectric tests.
• Testing the temperature rise of the transformer.
• No-load tap changer tests.
• Vacuum tests on tanks and radiators.
• Test of transformer turn ratio.

#3. Special Tests:

Special tests in the transformer help to provide useful information to the user. Such tests are performed during the maintenance and operation of the transformer.

Some of the tests performed while maintaining the transformer are as follows:

• In short circuit test transformer.
• Dielectric tests of the transformer.
• Measurement of the acoustic noise level.
• To measure the harmonics of the no-load current in the transformer.
• Measurement of the three-phase and zero-sequence impedance of the transformer.
• To measure the amount of power taken by the oil pump and fans in the circuit.

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Transformer Testing Done at the Site:

The tests performed on the site of the transformer are as follows:

• Pre-commissioning Tests.
• Periodic/Condition Monitoring Tests.
• Emergency Tests.

#1. Pre-commissioning Tests:

The test performed on-site before ordering or authorizing a transformer is known as a pre-commissioning test. This test helps to check the installation process and compare the results of the transformer during factory tests.

The pre-commissioning tests of the transformer are as follows:

• Measurement of winding resistance.
• Measurement of vibration of the transformer.
• Test the magnetic balance of the transformer.
• Measurement of the floating neutral point.
• Measurement of capacitor bushings.
• Measurement of insulation resistance.
• Operation checking of the defense system.
• Measurement of voltage ratio (curve ratio).
• Measurement of differential stability, REF of the transformer.
• Measurement of vector group or polarity of the transformer.
• Measurement of bushing current transformers (BCTs).

#2. Periodic/Condition Monitoring Tests:

This test is used to increase the performance and periodically check the position of the transformer. Checking the transformer periodically to make sure it can satisfy the customer’s demand. This type of test is done on a weekly, monthly, and yearly basis after operating the transformer on a regular basis. Periodic maintenance plans for transformers depend on what type of transformer is used.

This test periodically inspects the transformer and helps to detect defects occurring in the early stages. For example in the measurement of the resistance of the insulation of the transformer, if the value falls below normal, it shows that the defect is in the initial stage.

#3. Emergency Tests:

This test is performed on-site to check for problems or damage to the transformer during the operation. The ventilator, for example, works efficiently, although it includes high-temperature measurements and measurements of resistance in the winding and analysis of the oil used in the cooling of the transformer.

Transformer Ratio Test:

The performance of a transformer largely depends on the specific winding completeness of the transformer or the voltage ratio of the transformer. That is why the transformer ratio test is an essential type of transformer test. This test was also done as a routine test of the transformer. Therefore electrical power transformer, the voltage of the transformer, and the turn ratio test is an important test to ensure proper operation.

The process of this test is simple. Keeping the low-voltage winding open we only apply a three-phase 415 V supply for high-voltage winding. To find the actual ratio of the transformer we measure the induced voltage at the HV and LV terminals of the transformer. We repeat different tests for all tap conditions.

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Temperature Rise Test of  Transformer:

An increase in the temperature of the transformer is included in the test of the type of test transformer. In this test, we check whether the winding of the transformer and the increase in temperature of the transformer are as per the specification limit. In this type of test of the transformer, we have to check the limit of increase in oil temperature as well as an increase in winding temperature of the electrical transformer.

Routine Tests of  Transformer:

The routine test of the transformer is specifically to verify the operational performance of the individual unit in the production lot. Regular tests are performed on each unit manufactured.

Special Tests of  Transformer:

Special tests of the transformer are carried out as per the requirement of the customer to get useful information to the user during the operation or maintenance of the transformer.

Induced Voltage Test of  Transformer:

The main purpose of the transformer-induced voltage test is to check the inter-turn and line-end insulation as well as the main insulation between the earth and the windings.

• Keep the primary winding of the transformer open
• Apply three-phase voltage to the secondary winding. The applied voltage should be twice the rated voltage of the secondary winding in dimension and frequency.
• The duration of this test will be 60 seconds.
• The test will start at a voltage less than 1/3 of the full test voltage, and the speed will be increased to the desired value.

The test is successful if there is no breakdown in the full test voltage during this test.

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Most Commonly Asked Questions:

What is the Purpose of transformer testing?

To check if the parts of the electrical transformer are working properly and to check the operation of the whole transformer. To confirm the specifications and performance of the transformer to reduce the risk of failure, testing is required.

How do you test a good transformer?

To test your transformer just touch the red and black pins of your ohmmeter to the opposite end of the transformer wiring. Note the reading on the display. Compare this resistance with the resistance shown on the datasheet of the transformer. These are sometimes listed on the enclosure of the transformer

What is the need for transformer testing?

To check or confirm the specifications and performance of the transformer as per the consumer’s requirements.

What type of relay is used in transformer protection?

Buchholz Relay is used to prevent damage to the inside of the transformer such as short-circuits, winding faults, insulation breakdowns, etc.

What is the purpose of winding resistance measurement?

This test is used to ensure that the resistance of all phases matches the design values. It measures the resistance of primary and secondary windings.

Megger Mto210:

The MTO210 Transformer OhmMeter is a line-operated field-portable device specifically designed to safely and accurately measure the DC resistance of all types of magnetic windings. It can test windings such as motors such as transformers and rotating machines and measure low-current resistance on connections, contacts, and control circuits.

Sfra Test:

Sweep Frequency Response Analysis (SFRA) provides an understanding of the mechanical and electrical integrity of transformers, reactors, and other equipment with test windings. An SFRA instrument sends a signal to a number of different frequencies in the transformer winding and measures the returning signals.

Dissolved Gas Analysis:

Dissolved Gas Analysis (DGA) is an investigation of oil transformer oil contaminants. Insulating materials in electrical equipment release gases as they break down slowly over time. The formation of these dissolved gases is indicative of the effects of decay such as pyrolysis or partial dissolution, and the rate of gas production indicates the intensity. DGA is beneficial for a preventive maintenance program.