Temperature rise test on transformers

In the test, nominal voltage and frequency are applied to the primary side of the transformer, while the secondary side is short-circuited. This causes the nominal current to circulate through the windings, which gradually raises the temperature. The test runs until both the oil and the windings reach a stable temperature.

This final temperature must lie within the limits specified by the designer.

The time it takes to reach that stable level gives the thermal time constant of the transformer; the final temperature is the temperature rise at full load.

Why is this test important?

• It shows whether heat dissipation is adequate under load
• It confirms the transformer stays within its specified temperature limits
• It bears on service life, performance and safety
• It is often required by industry and technical specifications

How do you perform a heat run / temperature rise test?

Preparation

• Measure the cold winding resistance (R_cold)
• Record the initial winding and oil temperatures

Test procedure

• Apply voltage to the primary side
• Short-circuit the secondary side so nominal current circulates
• Maintain the load until the temperature change per unit of time becomes negligible
• Then measure the winding resistance to calculate the temperature

Analysis and calculation

• Determine the thermal time constant
• Calculate the final temperature rise
• Derive the winding resistance at the actual operating temperature R0​

Equipment: WinRes-20

The WinRes-20 measures the winding resistance of transformers, including the full-load resistance R₀ during a heat test, by applying up to 20 A DC.

Its software reports on the measurements automatically.

Once the heating phase is complete and the transformer is ready to be tested, the operator follows these steps:

1. Configure the test and equipment settings.

2. Disconnect the transformer.

3. Connect the test device (WinRes-20).

4. Perform the temperature rise test within 10 minutes.

During the test, the WinRes-20 measures the winding resistance continuously for the test duration entered in the test settings.

The results are not shown immediately when the test completes. To view the final result, first save it and then go to the test reports. Select the relevant result and press calculator. Then enter R Cold and T Cold, the material temperature, and the heat test calculation to obtain R₀: the measured winding resistance at the actual operating temperature of the tested transformer.

The results are displayed as shown in the curve below.

Related: winding resistance measurement.