Why measure the DC resistance of transformer windings?

Why measure the DC resistance of transformer windings?

The DC resistance test for transformer windings is a crucial component of both factory acceptance tests and maintenance inspections; it is primarily used to verify the integrity of the winding’s conductive path and the reliability of its connections. Measuring DC resistance allows for the early detection of issues that may arise during manufacturing, transportation, or operation, thereby ensuring the transformer’s safe and reliable performance.
1. Checking winding connection quality
Windings consist of conductors, solder joints, leads, and connectors; any anomaly in these connection points affects the overall resistance.
DC resistance testing can detect:
① Poor winding solder joints;
② Loose lead connections;
③ Excessive contact resistance at joints;
④ Broken conductor strands or abnormal connections.
If the resistance of a specific phase is significantly high, the corresponding connection points usually require a detailed inspection.
2. Assessing tap changer contact status
For transformers equipped with tap changers, different tap positions correspond to different winding turn counts and connection configurations.
Measuring DC resistance at each tap position allows for checking:
① Whether tap changer contacts are making good contact;
② Whether tap switching is accurate;
③ Whether internal connections are reliable.
If resistance is abnormal at one tap position but normal at others, the issue is often related to the contacts or leads associated with that specific position.
3. Checking for three-phase winding balance
In a three-phase transformer, the winding structures for each phase are identical; under normal conditions, the DC resistance should exhibit good symmetry.
Comparing the resistance values ​​across the three phases helps determine:
① Manufacturing consistency among the three-phase windings;
② Any discrepancies in the number of turns;
③ Localized poor contact;
④ Any winding anomalies.
Imbalance in three-phase resistance can affect current distribution and losses during transformer operation.
4. Detecting winding faults
During operation, transformers may be subjected to short-circuit impacts, mechanical vibrations, or overheating, potentially leading to winding deformation or connection anomalies.
DC resistance testing aids in detecting:
① Damage to winding conductors;
② Inter-turn short circuits;
③ Abnormal current distribution in parallel branches;
④ Changes in connections caused by localized winding overheating. A comprehensive assessment typically requires integrating data such as the turns ratio, short-circuit impedance, and no-load test results.
5. Verification of manufacturing processes and product quality
For manufacturers, the DC resistance test is a crucial method for verifying the production process.
Testing allows for the verification of:
① Whether winding fabrication meets design specifications;
② The reliability of welding processes;
③ The correctness of tap lead connections;
④ Whether product consistency meets requirements.
Test data also serves as a factory quality record, facilitating future traceability.
6. Why is data stability required during testing?
Transformer windings typically possess significant inductance, requiring time for the current to stabilize during testing. Reading data prematurely can lead to measurement inaccuracies.
Therefore, the following points must be observed during testing:
① Select an appropriate test current;
② Wait for the resistance value to stabilize;
③ Accurately record the winding temperature;
④ Convert measurement results based on temperature;
⑤ Fully discharge the unit after testing.
Conclusion
The primary objective of measuring transformer winding DC resistance is to inspect the winding’s conductive circuits, welded connections, tap changers, and three-phase consistency. This test enables the early detection of manufacturing defects, connection anomalies, and operational hazards, making it a vital procedure for ensuring factory quality and safe transformer operation.

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