Temperature Rise Test of Transformer

The temperature rise test is an essential procedure for comprehensively assessing the performance and safety of electrical transformers. This examination focuses on how efficiently a transformer dissipates heat during its operational phase, crucial for maintaining winding and oil temperatures within prescribed safe limits. Effective temperature management is pivotal for ensuring the extended longevity and reliability of transformers, making this test a critical aspect of quality assurance in the manufacturing and maintenance of electrical systems.

Test Setup and Procedure:

1. Short-Circuiting the LV Winding:

   • The low-voltage (LV) winding is deliberately short-circuited to replicate realistic load conditions. This ensures the transformer operates at its rated current, providing valuable insights into its performance under typical load scenarios.

2. Thermometer Placement:

   • Thermometers are strategically placed at specific locations to capture crucial temperature data. This includes installing one in the transformer top cover pocket and two at the inlet and outlet of the cooler bank. These thermometers meticulously monitor oil and winding temperatures, offering a detailed understanding of thermal behavior.

3. Applying Voltage to the HV Winding:

   • The high-voltage (HV) winding is subjected to the application of rated voltage. Simultaneously, total losses are measured using three wattmeters, providing a comprehensive assessment of the transformer's energy dissipation characteristics.

4. Hourly Readings:

   • Continuous monitoring of the top oil temperature involves recording hourly readings. These readings contribute to the calculation of the mean oil temperature during the test, offering a nuanced view of temperature fluctuations over time.

5. Ambient Temperature Measurement:

   • Close monitoring of the ambient temperature around the transformer is conducted, recognizing its influence on the overall thermal performance during the test.

Determining Final Temperature Rise:

Two approaches are commonly used:

1. Steady Value Approach:

   • The test continues until the top oil temperature increment is consistently less than 3°C per hour. This approach ensures a robust evaluation, and the final temperature rise is determined based on this steady value.

2. Four-Hour Stability Approach:

   • Monitoring the variation in top oil temperature rise over a four-hour period, with the stipulation that it does not exceed 1°C per hour. The lowest reading within this period is considered the final temperature rise, providing a nuanced assessment of stability.

Compensating for Core Losses:

• Core losses, which are voltage-dependent, are compensated for during the test. Additionally, there is an acknowledgment and compensation for additional copper losses arising due to the absence of core losses.

Temperature Rise Limits:

• For oil-immersed transformers, temperature rise limits are well-defined:
  • Natural oil circulation: Winding 55°C, Top oil 60°C.
  • Forced oil circulation: Winding 60°C, Top oil 65°C. Adhering to these limits ensures the safety and reliability of transformers in various operational scenarios.