Transformer Winding Resistance Tester: DC Resistance Measurement with Demagnetization for Power Transformers

The Importance of Transformer DC Resistance Testing

DC resistance measurement of transformer windings is one of the most fundamental and revealing diagnostic tests in transformer maintenance. By passing a known DC current through each winding and measuring the resulting voltage drop, the winding resistance can be calculated with high precision. Changes in winding resistance from baseline values indicate problems such as shorted turns, broken strands, loose internal connections, high-resistance tap changer contacts, and poor bushing connections.

Standards such as IEEE C57.152 and IEC 60076-1 specify DC resistance testing as part of routine transformer maintenance and acceptance testing. The test is performed on each phase winding individually, and results are compared between phases and against factory test reports to identify abnormalities.

AC-Powered Transformer Tester: EK-C367A

The EK-C367A is an AC-powered transformer DC resistance tester designed for workshop and substation testing where AC power is available. It provides a resistance measurement range of 0.1 milliohm to 50.00 kilohm with 1 micro-ohm resolution, test current up to 10A, and 24V output voltage. These specifications cover the full range of power transformer winding resistances from large distribution transformers to medium-power units.

The 5-inch touchscreen color display presents measurement results clearly and allows intuitive navigation through test functions and settings. Built-in printer capability generates immediate hardcopy test reports in the field, while Bluetooth and USB connectivity support digital data management. A standard reference resistor is included for calibration verification.

The built-in demagnetization function is a critical feature that sets professional transformer testers apart from general-purpose resistance meters. After DC resistance testing, the transformer core retains residual magnetism that can affect subsequent tests and normal operation. The automatic demagnetization cycle safely removes this residual magnetism, eliminating the need for a separate demagnetization procedure.

Battery-Powered Portable Tester: EK-C367B

The EK-C367B provides the same measurement capability as the EK-C367A (0.100 milliohm to 50.00 kilohm, 10A, 1 micro-ohm resolution) in a battery-powered portable format. This model is designed for field testing at remote locations where AC power may not be available, such as pole-mounted transformers, pad-mounted units, and outdoor substations.

The lithium battery provides sufficient capacity for multiple transformer tests on a single charge, making the EK-C367B a practical choice for mobile transformer maintenance crews. All other features match the AC model, including the 5-inch touchscreen, printer, Bluetooth, USB, standard reference resistor, and automatic demagnetization.

Testing Procedures and Best Practices

Before performing DC resistance testing, ensure the transformer is properly isolated and all windings are discharged. The instrument should be allowed to reach thermal equilibrium with the ambient environment, and the temperature at the time of testing should be recorded for temperature correction of results.

Connect the test leads using the four-wire Kelvin method to eliminate lead resistance. Apply the test current and wait for the reading to stabilize, which may take several minutes for large transformers due to the high inductance of the windings. Record readings for all phases and tap positions as specified in the test procedure.

Compare results between phases, noting that phase-to-phase variation should typically not exceed 2 percent for similar windings. Compare against previous test results and factory data, investigating any significant changes. After testing is complete, run the automatic demagnetization function before disconnecting the instrument.