Temperature rise test is the type test of the transformer . The purpose of temperature rise tests are to check the temperature rise of transformer winding, core and insulating oil under the specified conditions. Judge whether there is local overheating at the oil tank surface, structural parts, lead and bushing, and connections of on-load tap-changer of the transformer. Determine the transformer's state and relevant parameters under normal operation state.
The temperature rise test for distribution transformer involves precise procedures to ensure accurate results. The following steps outline the process:
Preparation
Temperature Monitoring
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
Ambient Temperature Measurement
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 |
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:
Conducting a temperature rise test provides numerous advantages, including:
Enhanced Reliability
Prolonged Lifespan
Performance Validation
Risk Mitigation
Customer Assurance
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.
