Transformer Winding Resistance Testing

A closer look at the paths carrying current

A transformer’s electrical connections may develop problems long before there is an obvious external sign. A loose connection, deteriorating tap changer contact, or damaged conductor can affect the path current takes through the equipment. Winding resistance testing helps investigate these concerns by measuring resistance through the windings and associated connections.

MVA performs transformer winding resistance testing to support commissioning, scheduled maintenance, troubleshooting, and verification after repairs. The results provide another piece of evidence for evaluating equipment condition and deciding where further investigation is needed.

What can an unexpected reading tell you?

The value of winding resistance testing comes from comparing measurements. Differences between comparable phases, unexpected patterns across tap positions, or changes from previous results can draw attention to a potential problem.

Abnormal readings may indicate:

• Loose, defective, or incorrectly assembled connections.
• High resistance at tap changer contacts.
• Broken conductor strands or partially open winding circuits.
• Other irregularities affecting continuity through the winding circuit.

An unusual result helps direct the investigation, but it does not identify every fault by itself. The location and cause must be evaluated alongside the transformer’s design, test conditions, and other diagnostic findings.

Tap positions help tell the story

A single measurement may leave important questions unanswered. Checking the selected tap positions allows technicians to examine how resistance changes as different winding sections and contacts become part of the circuit.

For example, an unexpected reading at one position may warrant closer examination of the corresponding contact or connection. A consistent irregularity across several positions may suggest a concern elsewhere in the shared current path. Evaluating the pattern helps focus subsequent inspection and testing.

This makes winding resistance testing useful when investigating tap changer concerns or checking electrical continuity following tap changer work.

Reliable comparisons require context

Resistance changes with temperature. A measurement taken on a warm transformer cannot be compared directly with a cold measurement without accounting for that difference. Winding material, connection configuration, tap position, and the stability of the reading also influence how results should be evaluated. Temperature correction helps place measurements on a comparable basis when reviewing factory data or historical records.

For asset owners, this means a useful test report needs more than a list of numbers. Documented conditions and clearly identified measurement locations make the results more valuable for future maintenance and troubleshooting.

Put the test to work at key equipment milestones

Before initial service: Establish reference measurements and check the winding circuit as part of the commissioning program.

During a planned outage: Compare current results with earlier records to investigate changes in connections or tap changer condition.

After repairs: Verify the affected current paths following work involving winding leads, connections, or tap changer components.

When a concern develops: Use resistance measurements alongside other electrical tests to narrow the investigation. Transformer commissioning, maintenance, and troubleshooting are established applications for this test.

The testing scope should reflect the equipment and the reason for the assessment. Routine transformer maintenance, a newly installed transformer, and a unit returning from repair may each require a different combination of tests.

From measurements to a practical next step

MVA brings field testing and transformer service experience together to help customers act on diagnostic findings. Winding resistance results can support decisions about additional testing, connection inspection, tap changer evaluation, or repair planning.

Our capabilities extend to electrical diagnostics, transformer assembly, field repairs, LTC services, and oil processing. When an investigation identifies work that needs to be completed, MVA can help coordinate the next stage with an understanding of the equipment and its service requirements.

Planning a transformer outage or checking equipment after repair? Request a quote to discuss winding resistance testing and the appropriate diagnostic scope for your transformer.