Testing TransformersTesting transformers is more important than many people realize, and whenever CT or PT testing is conducted, it’s important to ensure that technicians use the correct transformer testing equipment to ensure that all electrical, mechanical, and thermal standards are being met.

For each different type of transformer, the inspector should conduct the specific recommended tests to ensure efficient and safe inspection and operation. Today we’ll be covering four of these methods so that you can have a better idea of the most common testing transformers procedures.

1. Turns Ratio Testing

Turns ratio transformer testing is commonly used to ensure that the winding ratio between the primary and secondary coils are aligned to recommended specifications. This type of transformer testing also ensures the transformer will provide either step-up or step-down voltage. A step-down transformer, for instance, comprised of 100 primary turns and 10 secondary turns will work to reduce the voltage by a factor of 10 — corresponding to the secondary coil — while multiplying the current by 10 as well.

2. Insulation Resistance Testing

Insulation resistance transformer testing, also known as the Megger test, is used to determine the quality of insulation within the transformer itself. These tests are conducted with a megohmmeter, one of the necessary transformer test instruments, that operates similar to a multi-meter. In order to pass the test, the insulation of a transformer must be determined to have a greater resistance than defined by international standards for that transformer type. If it measures any lower it could signify an issue with the insulation which may require replacement.

3. Power Factor Testing

Power factor transformer testing is the process wherein the power loss of the insulation system is tested by measuring the angel the resulting current of power that occurs when AC voltage is applied. For the test results to be optimal, the angle of the current should measure 90 degrees; however, more often than not, insulation is never perfect. As a rule, the closer to 90 that the current is, the better the insulation is.

This test is completed with a power factor test kit, and it can be completed regularly throughout the life of the transformer. This can help detect deteriorating or malfunctioning insulation over time and give you an idea of when the transformer will need to be replaced.

4. Resistance Testing

This type of transformer testing once a transformer has been left to settle at the same temperature of the surrounding air. The reason for this is to check for any differences between the opens and windings within the transformer. This helps ensure that all the circuits are still wired and connected correctly. This test is conducted using an ohmmeter.

Overall there are eight types of transformer testing; however, these are four of the most common. All of these tests are critical for maintaining the integrity of the transformer and without them, the operation can be severely hindered.

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  1. Can you recommend a company that can test my condominium building’s 50 year old dry 750 KVA, 13,200 volt transformer? We are trying to decide if we should replace it. We are located in Philadelphia, PA.
    Thank you,
    Tom Klein

    1. So how do u tells that the transformer needs replacement….and for how offen did the transformer service took after the next service…?

      1. Thanks for reaching out! We recommend testing the instrument transformer initially following manufacturing at the manufacturer’s location; annually in the field (after install) using one of our field analyzers; upon redeployment if one is ever removed and then recommissioned to service; and upon deployment if one has been in inventory for over a year before initial installation.

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