What is Transformer Testing and Its Types

The electrical device which can transfer electrical energy from one circuit to another circuit (without any connection) by means of mutual induction between the windings, is called a transformer. It works on the principle of electromagnetic induction. The transformer consists of the primary winding and secondary winding. It is used to increase or decrease the voltage levels of a circuit. In the entire electrical distribution system, the transformer is the main part. If the transformer doesn’t work properly, then the entire distribution system may get damaged and there will be no chance of transferring electrical energy. The electrical circuit can get damaged due to mechanical failures, electrical damage, thermal damage, and winding deformity. So, the functioning of the transformer should be checked by using the transformer testing process to avoid failures. The transformer testing can be done to determine the specifications, and performance of an electrical transformer. To meet the specific design and specifications, transformer testing should be done during the manufacturing process.

Types of Transformer Testing

In order to reduce electrical failures, mechanical failures, winding deformation, thermal failures and insulation breakdowns of a transformer, the transformer testing should be done. The transformer testing can be done in different types. They are,

• Routine tests
• Type tests
• Special tests
• Pre-commissioning tests
• Periodic or condition monitoring tests
• Emergency tests.

Transformer Testing Done at the Factory

The transformer tests done at the factory include the following.

Type Tests

Type tests of the transformer are to verify whether the transformer is designed as per the customer’s expectations and design specifications. So, the transformer has to go through this type of test during the manufacturing itself. To confirm the Customer’s design expectations, the transformer has to go through some type of tests. These tests are done in a prototype unit or individual unit in a production lot. The type tests of the transformer verify and confirm the basic design of a transformer in a production lot.

The type tests of the transformer include measurement of various specifications of the transformer. They are,

• Measuring winding resistance of the transformer
• Testing of transformer turns ratio
• Testing of transformer vector group
• Measuring the impedance voltage of a transformer
• Measuring short circuit impedance of a transformer
• Measuring load loss of the transformer ( includes short circuit test)
• Measuring of no-load loss and current of a transformer ( includes open-circuit test)
• Measuring insulation resistance of a transformer
• Dielectric tests
• Testing of temperature rise of the transformer
• No-load tap changer tests
• Vacuum tests on tank and radiators

Routine Tests

To verify and confirm the operational performance of a transformer, routine tests are carried out on a transformer in a production lot. These tests should be done on every unit of manufacturing. These tests include all tests except temperature rise test and vacuum tests. They are,

• Measuring winding resistance of the transformer
• Testing of transformer turns ratio
• Testing of transformer vector group
• Measuring the impedance voltage of a transformer
• Measuring short circuit impedance of a transformer
• Measuring load loss of the transformer ( includes short circuit test)
• Measuring of no-load loss and current of a transformer ( includes open-circuit test)
• Measuring insulation resistance of a transformer
• Dielectric tests
• No-load tap changer tests
• Oil pressure test to check the leakages at joints and gaskets on the transformer

Special Tests

Special tests help to provide useful information to the user. These tests are carried out during the maintenance and operation of the transformer.

Some of the special tests carried out during the maintenance of the transformer are,

• Dielectric tests of transformer
• Short-circuit test in transformer
• Measuring the impedance of three-phase and zero-sequence of transformer
• Measurement of the acoustic noise level of the measurement.
• Measuring harmonics of the no-load current in the transformer
• Measuring the amount of power taking by the oil pumps and fans in the circuit
• Measurement of some components in the circuit such as Buchholz relay, pressure relief devices, temperature indicators, oil preservation systems and many more.

Transformer Testing Done at the Site

The transformer tests done at the site include the following.

Pre-commissioning Tests

The tests carried out before the order or authorization of the transformer at the site are called pre-commissioning tests. These tests help to check the installation process and compare the results of the transformer during the factory tests.

Pre-commissioning tests of the transformer include,

• Operational checking of the protection system
• Insulation resistance (IR) measurement
• Measurement of capacitor bushings
• Measurement of voltage ratio (turns ratio)
• Measurement of vector group or polarity of the transformer
• Measuring of winding resistance
• Measurement of vibration of the transformer
• Test of the magnetic balance of the transformer
• A measure of Frequency Balance Response (FRA) of the transformer
• Measurement of floating neutral point
• The measure of short-circuit impedance and magnetizing current of the transformer
• Operational measurement on OLTC’S
• Measurement of stability of differential, REF of transformer
• Measurement of bushing current transformers (BCT’S)

Periodic/Condition Monitoring Tests

These tests are used to enhance the performance and check the condition of the transformer periodically. The condition of the transformer is checked periodically whether it meets the customer requirements. These tests are carried out on the site after the operation of the transformer on a regular basis like weekly, monthly and annually. The periodic maintenance plans of the transformers depend on the type of transformer used.

These tests help to detect the faults during the early stages by monitoring the performance of the transformer periodically. For example, in the measurement of insulation resistance of the transformer, if the value falls below the normal, it shows that the defect is in the beginning stage.

Emergency Tests

These tests are carried out at the site to check the problems or damages of the transformer during the operation. For example, high-temperature measurement even though the ventilators work efficiently, and also includes measurement of resistance in windings and analysis of oil used in the cooling of the transformer.

FAQs

1). What is the need for transformer testing?

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

2). How to maintain the transformer?

The maintenance of the transformer can be done by checking the leakage of the oil of the transformer.

• By using the filtration method, dust, sludge, and dirt can be removed
• Periodic monitoring of loose connections on both sides of the windings
• Bushings should be cleaned properly to avoid cracks
• Periodic dissolved gas analysis should be done.

3). What is the purpose of insulation resistance measurement in transformers?

It is necessary to measure the insulation resistance between the primary and secondary windings (HV and LV) with respect to earth.

4). What is the purpose of winding resistance measurement?

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

5). What type of relay is used in transformer protection?

Buchholz Relay is used to protect the transformer from the damages occurring inside the transformer like short-circuit, winding faults, insulation breakdowns, etc.

Thus, this is all about the transformer testing types. The purpose of the transformer testing is to check the performance of the transformer during the manufacturing as per the user’s specifications. Here is a question for you “What are the advantages of transformer testing?”