What is the significance of developing a DC winding resistance tester?
The transformer winding resistance tester is one of the top performance testers specifically made for large motor and power transformer applications, basically, the winding resistance testing is a vital part of the manufacturing quality assurance program and power transformer maintenance. Usually, the large motors and transformers are subjected to overloading, vibrations and environments with high temperature variations, this winding resistance test kit can be used to perform measurements and assure that the connections are made accurately without any shorts or opens. However, winding resistance tester is highly effective to use in finding the tap changer abnormalities, the tap changer of power transformer is especially the most critical element of a power system.
The purpose of the DC winding resistance test is as follows:
1.Inspect the welding quality of internal wires and leads in the winding: Poor welding such as cold solder joints, false solder joints, or oxidized welds will lead to an abnormal increase in contact resistance. Under normal conditions, the unbalance degree of three-phase winding resistance should be ≤2% (for transformers with capacity ≥1000kVA). If the resistance value of a certain phase is more than 5% higher than the other two phases, it is likely that the joint welding of that phase winding is poor. For example, during the maintenance of a 10kV/500kVA transformer, the measured resistance of phase A is 12.5mΩ, phase B is 12.3mΩ, and phase C is 18.7mΩ. After disassembly, it is found that there is an oxide layer at the weld between the lead wire of phase C winding and the terminal. After cleaning and re-welding, the unbalance degree of three-phase resistance drops to 1.2%, which meets the standard requirements.
2.Inspect the contact status of each gear of the transformer tap changer: Wear, oxidation, or poor contact of the tap changer contacts will cause the resistance value of the corresponding gear to be too large or fluctuate. During the test, the switch should be switched step by step and the resistance value recorded. After normal gear switching, the resistance should be stable within the corresponding range, and the resistance variation between adjacent gears should be uniform (deviation ≤3%). For example, when testing a 35kV transformer in "Gear Ⅱ", the resistance value suddenly rises from the normal 8.6mΩ to 15.8mΩ, and returns to normal after switching to other gears. After disassembly, it is found that the "Gear Ⅱ" contacts have severe ablation marks, and the test returns to normal after replacing the contacts.
3.Inspect whether the winding or lead wire is open-circuited: When the winding wire is broken or the lead joint falls off, the resistance value will be infinite (displayed as "OL" on the multimeter) or far exceed the design value. Taking a 110kW motor as an example, the designed resistance of its stator winding is 3.2mΩ. If the resistance of a certain phase shows infinity during the test, it can be determined that there is an open circuit in that phase winding. Further investigation reveals that the wire at the end of the winding is broken due to vibration, which needs to be re-welded and repaired.
4.Inspect the correctness of parallel branch connection and whether there is an open circuit: For windings composed of multiple parallel wires (such as low-voltage windings of large transformers often use 6-12 parallel wires), if one or more wires are open-circuited, the resistance of that branch will increase, leading to three-phase unbalance. For example, the low-voltage winding of a 220kV transformer adopts 8 parallel wires. Under normal conditions, the resistance deviation of each parallel branch should be ≤1%. During the test, it is found that the resistance of one group of branches is 8% higher than that of other branches. After disassembly, it is confirmed that 2 parallel wires are broken in the middle of the winding, resulting in a reduction in the effective conductive cross-sectional area and an increase in resistance.
5.Inspect whether there is a short circuit between winding layers and turns: A short circuit between turns or layers will reduce the effective number of turns of the winding, resulting in a significant drop in resistance value. Under normal conditions, the deviation between the measured resistance value and the design value should be ≤±5%. If the deviation exceeds 10%, a short circuit fault should be alerted. For example, during the maintenance of a 380V motor, the designed winding resistance is 4.5mΩ, but the measured value is only 3.8mΩ, and the three-phase unbalance degree reaches 6.3%. Through turns ratio test and partial withstand voltage test verification, it is confirmed that there are 3 turns short-circuited in the winding. After disassembly and rewinding, the resistance is restored to 4.4mΩ, which meets the design requirements.
How does a DC winding resistance tester work?
The major applications of winding resistance tester are given below:
1.Transformer Primary and Secondary Winding Resistance
Test content: Measure the DC resistance of high-voltage (primary) and low-voltage (secondary) windings across all tap changer positions, including phase-to-phase and phase-to-neutral resistance values.
Purpose: Verify the consistency of winding manufacturing (e.g., wire cross-section uniformity, winding tension) and detect hidden defects such as turn-to-turn short circuits, poor solder joints of internal leads, or loose tap changer contacts. It also provides a baseline for subsequent maintenance comparisons to assess winding aging or damage.
2.Windings of Motors and Generators
Test content: Focus on stator windings (three-phase) and rotor windings (for wound-rotor motors/generators), measuring phase resistance and branch resistance (for parallel-wound windings).
Purpose: Identify issues like open circuits in winding conductors, poor connections at terminal blocks, or inter-turn short circuits caused by insulation degradation. For generators, it also helps evaluate the integrity of rotor winding joints, preventing overheating or breakdown during high-speed operation.
3.PT (Potential Transformer) / CT (Current Transformer) / CVT (Capacitor Voltage Transformer)
Test content: Measure the resistance of PT primary/secondary windings, CT primary (busbar-type or wound-type) and secondary windings, and CVT series winding segments.
Purpose: Ensure accurate voltage/current transformation by checking winding continuity and connection reliability. Poor winding resistance (e.g., open circuits or contact defects) can lead to measurement errors, affecting protective relay operation and power system metering accuracy.
4.Instrument Transformers (General Category)
Test content: Cover specialized instrument transformers (e.g., coupling transformers, zero-sequence current transformers) by measuring winding resistance of both primary and secondary circuits, including auxiliary windings.
Purpose: Validate the electrical integrity of windings to avoid signal distortion during power system monitoring. It also helps detect manufacturing flaws (e.g., uneven winding layers) or in-service damage (e.g., winding deformation due to short-circuit electrodynamics) that could compromise measurement precision.
5.Voltage Regulators
Test content: Measure the winding resistance of the regulator’s series/ shunt windings and tap selector contacts, across all voltage adjustment ranges.
Purpose: Ensure smooth voltage regulation by confirming the reliability of tap switch contacts and the uniformity of winding resistance. Abnormal resistance (e.g., sudden increases in a certain tap position) indicates contact wear or winding damage, preventing voltage instability or regulator overheating.
6.Bus-bars, Cable Lines, and Switch Contact Points
Test content: Measure the contact resistance of bus-bar joints (e.g., bolted or welded connections), cable core conductors (including termination points), and switchgear contacts (e.g., circuit breaker contacts, isolating switch contacts).
Purpose: Detect poor contact caused by oxidation, loosening, or ablation, which can lead to localized overheating, energy loss, or even arcing faults. For cables, it also verifies conductor continuity and identifies hidden breaks or compression defects in terminations.
The transformer winding resistance tester can always perform measurements on very large transformers, thanks to its highly filtered and regulated constant-current output. This stable current supply suppresses interference from the winding’s inherent inductance and power supply ripple, ensuring accurate and repeatable resistance readings even for high-inductance, large-capacity transformers.
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