The detection of the DC resistance of transformers is a core item in the handover, major overhaul, and preventive tests of transformers. It is mainly used to check the welding quality of windings, the contact condition of the tap changers, and whether there are any short circuits between windings, etc. The current mainstream testing methods have evolved from the traditional bridge method to DC resistance testers. Their core advantages lie in their speed, accuracy, and safety, and they can effectively address the testing challenges posed by the large inductance characteristics of transformer windings.
1. Detection Principle
The basic principle of DC resistance measurement is to apply a constant DC current to the winding, based on Ohm’s Law R=U/I
1. Calculate the resistance value by measuring the voltage. Currently, the main methods used are the four-wire Kelvin bridge method or the developed DC resistance tester method. The latter has built-in advanced power supply and control technologies, which can effectively shorten the charging time caused by the winding inductance.
2. Criteria for result determination
The test results must follow relevant national standards, in accordance with GB6451-2008 and other specifications. The requirements for the three-phase imbalance rate of the transformer’s DC resistance are as follows:
For transformers of 1600kVA and below: phase resistance imbalance rate ≤ 4%, line resistance imbalance rate ≤ 2%
For transformers of 1600kVA and above: when the neutral point is led out, the phase resistance imbalance rate ≤ 2%, the line resistance imbalance rate ≤ 1%
III. Transformer DC Resistance Tester
1. Equipment selection dimensions
(1) Output current
The larger the output current, the faster the charging speed, more suitable for low-resistance winding tests of large transformers; common settings include <20mA, 1A, 5A, 10A, 20A, 40A, 50A.
(2) Measurement range
The equipment’s range needs to cover all the resistance values of the transformer to be tested. For example, the 20A setting can measure 1mΩ to 1Ω, and the 1A setting can measure 10mΩ to 20Ω.
(3) Number of channels
Single channel: measurement by phase, lower cost
Three channels: can simultaneously test the three phases of YN connected transformers, significantly improving the testing efficiency
(4) Accuracy
Mainstream equipment accuracy is ±0.2%, with a resolution as high as 0.1μΩ. Reference accuracy is ±(0.2% + 2 digits), capable of meeting most testing requirements.
(5) Core functions
Pay attention to automatic / rapid discharge (essential for safety), data storage and export (USB / Bluetooth / APP), large-screen touch control, etc., which can improve testing efficiency.
2. Selection suggestions for different scenarios
Distribution transformers (small capacity): Select a single-channel or three-channel instrument with a larger current of 10A-20A
Power transformers (large capacity): Recommend an instrument with a larger current of 40A-50A to ensure rapid testing of low-current large-capacity windings
Pursuing efficient testing: Prioritize a three-channel instrument, while paying attention to wireless transmission and other data management functions to reduce on-site workload
3. Precautions for on-site measurement
Full discharge: After the test, the instrument automatically discharges. Wait until the sound and light alarms completely stop before removing the test line to avoid electric shock or instrument damage
Sufficient charging: Large-capacity transformers have larger inductance, wait until the current stabilizes and the reading does not change before recording the data
Stable temperature: The resistance of the winding is significantly affected by temperature. For every 1℃ change in temperature, the resistance changes approximately 0.4%. Record the current temperature and compare historical data or convert to a standard temperature of 20℃
Good contact: Ensure that the test terminals and connection terminals are tightly connected to reduce contact resistance error; for transformers with operated tap changers, you can operate them several times in advance to remove the oxide film
Do not switch tap positions: During the test, do not switch the no-excited tap changer to prevent arc generation, damage to the switch or contamination of the transformer oil
Post time: Mar-16-2026