8 Common Testing Procedures for Electrical Transformers

Types of testing required before putting a transformer in service

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Transformers that increase (step-up) or reduce (step-down) electrical voltage are used in many industrial and public utility applications. Wherever they are used, 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. Most of the tests performed on power transformers are defined in national standards created by IEEE, NEMA, and ANSI. Each type of transformer and each contractor or utility provider will have a specified regimen of recommended tests, but it is critical that these be conducted diligently by the installation team to ensure safe and efficient operation of the system.

There are 8 different tests commonly applied to power transformers. Most testing routines will include most of these tests.

Turns Ratio Testing

The transformer turns ratio 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.

The turns ratio is calculated by dividing the number of turns in the primary winding by the number of turns in the secondary coil. This calculation defines the expected output of the transformer and gives the corresponding voltage required on the secondary winding. In a step-down transformer designed to reduce the voltage, the number of turns in the secondary coil must be lower than in the first, while in step-up transformer, the secondary coil must have more turns than the first coil.

The ratio is calculated under no-load conditions, using a tool known as a turns ratio tester. Done correctly, the test can identify tap changer performance, shorted turns, open windings, incorrect winding connections and other faults inside transformers.

Simultaneous readings of voltage are taken to the low-voltage and high-voltage windings area after the voltage is applied to one winding. The ratio is the division between the high reading and low reading. If it is a three-phase transformer, each phase is tested individually.

Insulation Resistance Testing

Commonly known as the Megger test, insulation resistance testing measures the quality of insulation within the transformer. Testing is typically done with a megohmmeter, a tool similar to a multi-meter but with a much higher capacity. 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. It is recommended that tank and core should always be grounded when this test is performed and that each winding is short-circuited at the bushing terminals. Resistances are then measured between each winding and between all other windings and ground.

Power Factor Testing

The power factor test determines 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, and the connections are the same as for the Megger test (the insulation resistance test). 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. 

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 ohmmeter.

Winding resistance is calculated by measuring the voltage and current simultaneously—ideally, the measured current will be as close to the rated current as possible. Performing this test will allow you to calculate and compensate for load losses as a whole.

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.

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.

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
  • Dielectric breakdown
  • Power factor
  • Moisture content
  • Interfacial tension

Oil tests are very useful for determining the condition of the insulation and the oil. Based on these results, a maintenance program for the transformer can be established.

Visual Inspection

Although this is the simplest of all tests, a visual inspection 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/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 manufacturer's labels, signs of physical damage, the condition of welds, oil loss or leakage, integrity of wire connections, and the condition of valves and gauges (if present).