How to Test Transformer Winding Resistance and Turns Ratio in the Field

Posted by Billy 10/07/2026 0 Comment(s)

Field testing a transformer is not just about confirming that it is energized correctly. It is about finding hidden electrical and mechanical issues before they become failures. Two of the most useful field tests are transformer winding resistance testing and transformer turns ratio testing.

Winding resistance testing helps verify the current path through windings, leads, bushings, connections, and tap changer contacts. Turns ratio testing confirms whether the transformer is producing the correct voltage relationship between windings and can also help identify winding or core-related issues when combined with excitation current and phase angle measurements.

Why Winding Resistance Testing Matters

A transformer winding resistance test is used to detect problems such as loose connections, open circuits, poor contact integrity, and tap changer issues. In the field, abnormal or unstable resistance readings can point to increased heating, phase imbalance, or mechanical deterioration inside the transformer.

For three-phase power transformers and autotransformers, instruments such as the DV Power TWA Standard Series and DV Power TWA Advanced Series are designed for winding resistance measurement and tap changer analysis. we note that these instruments can test six transformer windings with a one-time cable connection and support automatic transformer demagnetization. The TWA Advanced Series also supports automated tap changer operation and DVtest analysis for checking moving contacts during OLTC transition.

How to Perform a Field Winding Resistance Test

Before testing, the transformer should be de-energized, isolated, grounded, and verified safe according to site procedures. Review the nameplate, winding configuration, tap positions, and prior test records if available.

A typical field process includes:

Connect the winding resistance tester to the transformer terminals according to the winding configuration.
Select the appropriate test current for the winding under test.
Allow the reading to stabilize before recording results.
Test each phase and each required tap position.
Compare phase-to-phase results and historical records.
Demagnetize the transformer core after DC resistance testing when required.

The key is consistency. Field teams should use the same connection method, tap sequence, temperature correction approach, and reporting format whenever possible. This makes trend analysis much more reliable.

What to Watch for in Winding Resistance Results

A good winding resistance result is stable and consistent with the other phases, the tap position, and previous test history. A problem may be indicated by:

One phase reading significantly higher or lower than the others
Readings that do not stabilize
Unexpected changes across tap positions
Abnormal OLTC transition behavior
Evidence of poor contact, loose leads, or possible winding damage

For transformers with on-load tap changers, automated testing can save significant field time. RCCE’s listing for the TWA Advanced Series notes that automated test mode can operate the tap changer and record winding resistance results based on selected stabilization criteria.

Why Turns Ratio Testing Matters

A transformer turns ratio test verifies that the ratio between the primary and secondary windings matches the transformer design. It is commonly used during commissioning, maintenance, troubleshooting, and after transport or repair.

Turns ratio testing can help identify:

Incorrect tap position
Shorted turns
Open turns
Incorrect winding connections
Phase displacement or vector group issues
Problems that require further diagnostic testing

For field turns ratio work, the DV Power TRT Standard Series and DV Power TRT Advanced Series. The TRT Advanced Series measures turns ratio, excitation current, and phase shift, and it supports automatic vector group detection and magnetic balance on three-phase transformers and autotransformers. It also includes a built-in true three-phase power source for testing special transformer configurations. (rcce.com)

How to Perform a Field Turns Ratio Test

A typical field turns ratio test includes:

Confirm transformer nameplate ratio, vector group, winding configuration, and tap position.
Connect the turns ratio tester to the high-voltage and low-voltage terminals.
Select the correct transformer type and test voltage.
Run the ratio test on each phase and tap position required.
Record ratio, ratio deviation, excitation current, phase angle, and polarity where applicable.
Compare results against nameplate values, acceptance criteria, and historical data.

Modern transformer ratiometers can reduce manual work in the field. RCCE notes that TRT Standard Series models can output test voltages from 1 V AC to 250 V AC, depending on the model, and support excitation current, phase shift, automatic vector group detection, magnetic balance, and OLTC control through the tester or software.

Combining Winding Resistance and Turns Ratio Testing

The two tests are stronger together. Winding resistance focuses on the conductive path and tap changer contact condition. Turns ratio testing confirms the winding relationship, phase shift, and voltage transformation behavior.

For distribution transformers or field crews that need a portable combined solution, we have the DV Power TWR-H Handheld Turns Ratio & Winding Resistance Tester. The TWR-H is a single-phase handheld instrument designed for both transformer winding resistance and turns ratio measurement. It includes two DC output sources for winding resistance testing, supports demagnetization, measures excitation current and phase shift, and is powered by an internal Li-Ion battery.