Dry-type transformers are widely used in power distribution and supply systems, and their safe and stable operation is crucial to the reliability of the entire grid. To ensure that the winding and terminal connections meet operational standards, the DC resistance test is an essential diagnostic method. This test helps identify poor connections, loose welding points, short circuits between turns, or excessive contact resistance. This article provides a systematic explanation of the principles, methods, precautions, and common troubleshooting points of the DC resistance test, enabling electrical engineers and maintenance personnel to fully master this important procedure.
1. Purpose and Importance of DC Resistance Test
The primary purpose of the DC resistance test is to apply a stable DC current to the transformer winding, measure the voltage drop, and calculate the resistance. By comparing measured resistance values with design data, factory test values, or between tap positions, the test can reveal:
1. Whether winding joints are welded firmly or loose;
2. Possible short circuits between turns or incorrect winding turns;
3. Reliability of tap changer switching and contact condition;
4. Balance of resistance values among windings, ensuring even current distribution and avoiding local overheating.
Therefore, the DC resistance test is a key procedure required by electrical testing regulations and an important step to ensure operational reliability of dry-type transformers.
2. Test Principle
The principle is based on Ohm’s Law, applying a DC current to the winding and calculating resistance from measured voltage and current:
R = U / I
where R is the winding resistance, U is the voltage across the winding, and I is the current. Since copper or aluminum conductors are temperature-sensitive, results must be corrected to the reference temperature of 20℃ for accurate comparison.
3. Test Equipment and Preparations
Before testing, the following equipment and tools are required:
1. DC resistance tester with high current output and fast measurement capability;
2. Thermometer for winding temperature measurement and correction;
3. Insulated gloves, goggles, and other personal protective equipment;
4. Recording forms for documenting resistance values at different tap positions.
Before the test, ensure the transformer is completely de-energized and properly grounded for safety.
4. Step-by-Step DC Resistance Test Procedure
Step 1: Verify Connections
Connect the tester’s current and voltage leads according to the transformer wiring diagram. Use the four-terminal method to eliminate the influence of lead resistance.
Step 2: Select Tap Positions
Test resistance at all available tap changer positions to ensure complete data coverage.
Step 3: Apply DC Current
Start the tester and gradually increase the current to the rated test value. Wait for the reading to stabilize before recording voltage and current values.
Step 4: Record Data and Temperature
Record the resistance values and winding temperature, converting them to the 20℃ reference value.
Step 5: Compare and Analyze
Compare results with factory or standard values. A resistance difference greater than ±2% between phases may indicate connection or tap changer issues.
5. Safety Precautions
1. Ensure the transformer is fully discharged before testing to prevent residual charge hazards;
2. Avoid touching exposed conductors and always wear protective gear;
3. Disconnect the current circuit before the voltage circuit after testing to prevent arcing;
4. Perform temperature correction if ambient temperature differs significantly from the reference;
5. Recheck abnormal readings to rule out errors caused by poor contact.
6. Common Problems and Solutions
High resistance values: Usually caused by loose connections or poor welding. Retighten or recheck joints.
Low resistance values: May indicate short circuits between turns. Verify with additional tests such as induced voltage withstand test.
Unbalanced resistance among phases: Suggests tap changer issues or winding defects. Inspect mechanical contacts.
Unstable readings: Likely due to loose tester connections. Recheck all connections.
7. Applications in Transformer Maintenance
DC resistance testing is not only required for acceptance testing of new transformers but is also essential during routine maintenance and overhaul. Regular testing helps:
1. Detect potential winding faults early;
2. Monitor tap changer operating conditions;
3. Provide data for preventive maintenance, reducing downtime risks;
4. Support transformer lifetime assessment and reliability analysis.
The DC resistance test of dry-type transformers is a crucial diagnostic method for ensuring operational safety and reliability. By following standardized procedures, accurate data analysis, and strict safety precautions, engineers can identify hidden issues early and take corrective measures. For power system operators and maintenance staff, mastering this test is not only a technical requirement but also an important safeguard for reliable power supply.
Leave A Comment