Temperature rise test is a critical diagnostic procedure used to assess the performance and durability of oil-immersed transformers. This test evaluates whether the temperature limits of transformer oil and windings adhere to the manufacturer's specifications, ensuring optimal functionality under operational conditions.
The results of this test are essential for verifying the transformer's ability to manage heat dissipation effectively, thereby extending its lifespan and preventing potential failures.
The temperature rise test for transformer oil involves precise procedures to ensure accurate results. The following steps outline the process:
Preparation: The LV winding of the transformer is short-circuited.
Temperature Monitoring: Thermometers are strategically placed in a pocket on the transformer’s top cover, and at the inlet and outlet of the cooler bank.
Voltage Application: A voltage equivalent to no-load losses plus load losses at a reference temperature of 75°C is applied to the HV winding.
Loss Measurement: The total losses are measured using the three-wattmeter method.
Top Oil Temperature Monitoring: Hourly readings of the top oil temperature are recorded.
Mean Oil Temperature Calculation: Hourly readings from the cooler bank inlet and outlet thermometers are used to determine the mean temperature.
Ambient Temperature Measurement: Thermometers are placed around the transformer to measure ambient temperature at specified intervals.
Stability Verification: The test continues until the top oil temperature stabilizes, with a maximum variation of 3°C within an hour.
An alternative method involves extending the test until the top oil temperature variation is less than 1°C per hour for four consecutive hours. The final value of temperature rise is the lowest reading during this period.
During the test, the LV winding is short-circuited, and the HV winding is energized. This setup results in negligible core losses. To achieve realistic temperature rise conditions, additional copper losses are introduced to simulate the thermal impact of core losses.
| Condition | Winding (°C) | Top Oil (°C) |
|---|---|---|
| Natural oil circulation, air-cooled | 55 | 50 |
| Forced oil circulation, air-cooled | 60 | 55 |
| Sealed transformer with conservator | 60 | 50 |
| Unsealed transformer | 45 | 45 |
These limits represent the permissible rise above the cooling medium's temperature, ensuring safe operation under specified conditions.
The winding temperature rise test measures the thermal stability of the transformer windings. The procedure includes:
Stabilization: After completing the oil temperature test, the current is adjusted to the rated value and maintained for one hour.
Shutdown Preparation: The power supply is switched off, and short-circuit connections are opened while maintaining operation of fans and pumps (if applicable).
Resistance Measurement: The resistance of the windings is measured at intervals of 3-4 minutes for up to 15 minutes.
Graph Plotting: A graph of hot resistance versus time is plotted to determine the resistance at the moment of shutdown.
Temperature Calculation: Using the resistance-temperature relation formula:
Here, is the cold resistance at temperature , and is the hot resistance. This calculation provides the winding temperature rise.
Conducting a temperature rise test provides numerous advantages, including:
Enhanced Reliability: Ensures the transformer operates efficiently within thermal limits.
Prolonged Lifespan: Reduces the risk of overheating, which can degrade insulation and other components.
Performance Validation: Confirms compliance with design specifications and regulatory standards.
Risk Mitigation: Identifies potential thermal issues before they lead to failures.
Customer Assurance: Demonstrates the manufacturer’s commitment to quality and safety.
Ambient conditions significantly impact the results of temperature rise tests. Factors such as room temperature, air circulation, and the presence of external heat sources must be controlled to ensure accurate measurements. Standardizing these variables enhances the test's reliability and replicability.
The efficiency of cooling systems directly affects the thermal performance of oil-immersed transformers. Proper maintenance of components such as radiator fins, fans, and pumps ensures optimal heat dissipation. Additionally, using advanced cooling techniques, like forced oil circulation, can improve temperature regulation under high-load conditions.
The temperature rise test is a vital procedure for assessing the thermal performance of oil-immersed transformers. By evaluating the temperature limits of both oil and windings, this test ensures the transformer's reliability, safety, and longevity. Incorporating precise methodologies, monitoring ambient conditions, and leveraging efficient cooling systems are essential to achieving accurate results.
For manufacturers and operators, these tests not only validate product performance but also enhance customer confidence. Regular temperature rise testing is an investment in the transformer's operational efficiency and durability, providing long-term benefits for both the user and the environment.
