DC Resistance Test of Dry-Type Transformers: Methods, Steps & Safety Tips

Dry-type transformers are sealed systems that rely on insulating materials such as laminated paperboard. These materials are vulnerable to moisture and degradation from long-term electrical and thermal stress. Regular DC resistance testing is essential to ensure insulation integrity, detect hidden faults, and maintain safe, efficient operation.

Purpose of DC Resistance Testing

The DC resistance test verifies the quality of winding connections and welding, identifies issues such as inter-turn short circuits or poor tap changer contact, and provides baseline data for further tests like turns ratio and temperature rise analysis.

Common Testing Methods

Bridge Method

Single-arm bridge (Wheatstone): Suitable for resistances above 1Ω. Balance is achieved by adjusting bridge arms and calculating resistance accordingly. Often used for high-resistance transformer windings.

Double-arm bridge (Kelvin): Ideal for low resistance below 1Ω. This method uses a four-terminal configuration to eliminate lead/contact resistance and improve measurement accuracy—especially suitable for precision components.

Voltage Drop Method

This method applies DC current through the winding and measures the voltage drop. Resistance is calculated using Ohm’s Law (R = U/I). It is quick and practical for field testing but requires careful wiring to avoid errors.

Test Procedure

Preparation

Record ambient temperature, humidity, and winding temperature. Ensure the test equipment is properly grounded. Use a tester with ≥0.5 accuracy and select test current according to the target resistance.

Wiring & Execution

Connect clamps firmly to the tested terminals. Isolate non-tested windings. Confirm correct wiring and ensure no personnel are near the equipment during testing. Start the test and monitor closely for anomalies.

Data Recording & Evaluation

Document current and voltage readings. Convert values to standardized temperature (usually 20°C) for accurate comparison. Analyze against historical data. If results are abnormal, recheck contacts and repeat the test as needed.

Critical Notes for Safe & Accurate Testing

Maintain a dry, temperature-controlled test environment around 20°C. Ensure all instrument functions are normal before testing. Follow procedures precisely to control current levels and prevent data distortion. Clean and store instruments properly after testing to extend their service life and prepare for future use.

DC resistance testing is vital for dry-type transformer maintenance. It ensures electrical continuity, reveals potential faults, and supports broader performance evaluation. By applying correct methods, following test protocols, and observing precautions, technicians can ensure the reliable operation and extended service life of transformer assets.