Transformer Testing

Transformer Testing2021-03-29T09:57:26+00:00

Transformer Ratio Testing (TTR)

This test is used to confirm that the transformer has the correct ratio of primary turns to secondary turns. Utilizing this test correctly can help one to identify tap changer performance, shorted turns, open windings, incorrect winding connections or other faults inside transformers.

Winding Resistance Testing

Winding resistance measurements are a powerful diagnostic tool for assessing numerous types of faults in transformers. By performing the test correctly the following can be assessed; malfunctioning tap changer mechanisms, partial or dead short-circuited turns, loose connections, poor crimping, broken strands, poor efficiency (Reactive component of conductor losses), measurement of winding temperature, calculate the winding temperature at the end of a heat run.

Insulation Resistance Testing

An insulation resistance (IR) test measures the total resistance between any two points separated by electrical insulation. The test, therefore, determines how effective the dielectric (insulation) is in resisting the flow of electrical current. Such tests are useful for checking the quality of insulation, not only when a product is first manufactured but also over time as the product is used.

Polarisation Index

A Polarization Index (PI) test is generally performed at the same voltage as the Insulation Resistance (IR) test. Where the IR test is performed for a period of one minute, the PI test is performed over a period of ten minutes. This gives the absorption (polarization) current ample time to decay and reveals a more detailed indication of the total leakage and conduction current. As such, PI is a good indication of winding contamination, moisture ingress (leakage currents), and/or bulk insulation damage (conduction currents).

Polarization Index testing is generally performed with an Insulation Resistance (IR) test set, commonly known as a Megger.


The condition of the insulation is essential for secure and reliable operation of your transformer. Measuring the capacitance and dissipation/power factor helps you to determine the condition of insulation in bushings or between windings.

Changes in capacitance can, for example, indicate mechanical displacements of windings or partial breakdown in bushings. Aging and degradation of the insulation, coupled with the ingress of water, increase the amount of energy that is converted to heat in the insulation. The rate of these losses is measured as dissipation factor.

Vector Grouping

What is Vector Grouping? A vector group is the International Electrotechnical Commission (IEC) method of categorizing the high voltage (HV) windings and low voltage (LV) winding configurations of three-phase transformers.

The vector group designation indicates the windings configurations and the difference in phase angle between them.

Why is it important?
As different combinations of winding connections will result in different phase angles between the voltages on the windings, transformers connected in parallel must have the same vector group because the mismatching of phase angles will result in circulating current and other system disturbances.

Frequency Response Analysis

Frequency response analysis (often referred to as FRA or SFRA) is a powerful and sensitive method for testing the mechanical integrity of transformer cores, windings, and press frames, in power transformers.

Every electrical network has a unique frequency response – its so-called “fingerprint”. Network faults or vibrations can cause changes in this frequency response.

Comparing the measurements taken at various phases or on multiple identical transformers, as well as comparing measurements with the former fingerprint of the same transformer, provide indications of any mechanical or electrical changes.

Partial Discharge Testing

Partial discharges (PD), as defined by IEC 60270, are localized dielectric discharges in a partial area of a solid or liquid electrical dielectric insulation system under high-voltage field stress. Partial discharges in a transformer deteriorate its insulation and can lead to failure of the transformer.

Diagnostic PD measurements are recommended after conspicuous measured values, such as increased gas-in-oil values or after a failed PD measurement during acceptance tests.