Temperature rise test on transformers

In the test, the secondary side of the transformer is short-circuited and a reduced voltage at rated frequency is applied to the primary side. The voltage is set so that rated current flows through the windings (in practice often slightly more, to account for the no-load losses as well). This 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 set in the standard or the specification.

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

• 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

Preparation

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

Test procedure

• Apply a reduced 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
• Extrapolate the winding resistance back to the moment of switch-off (R₀)

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. Start the heat run measurement on the instrument.

During the test, the WinRes-20 measures the winding resistance at fixed intervals, every 30 seconds by default or at an interval you choose, for the number of measurements you enter in the test settings. There is no fixed time limit after switching off.

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 winding resistance at the moment of switch-off, so at the temperature under full load.

The results are displayed as shown in the curve below.

Related: winding resistance measurement.

The test results must meet set limits, often defined in standards or technical documents. For oil-filled transformers, limits may be specified for top-oil rise, average winding rise and hot-spot temperature.

For example, IEC 60076-2 and IS 2026 (Part 2) set temperature rise limits for the average winding and the top oil.

• An excessive temperature may indicate insufficient cooling, internal defects or incorrect sizing.
• The thermal time constant should be within expectations; deviations can point to problems in heat transfer.
• Watch the hot-spot factor: the temperature at the hottest point (hot spot) is often higher than the average winding temperature.

The temperature rise test (heat run test) is essential to ensure that transformers operate within safe temperature limits under full load. Would you like advice on your application, or do you want to know which tester best suits your situation? Contact us or view our instruments.