Common Faults and Solutions for Oil-Immersed Transformers

Oil-immersed transformers are crucial components in the electrical power distribution system. However, like all electrical equipment, they are prone to faults that can affect their performance and reliability. Understanding these faults and knowing how to address them ensures safe operation, longevity, and the continued efficiency of the transformer. Below are some of the most common faults encountered in oil-immersed transformers, along with diagnostic methods and solutions.

1. Overheating Fault

Causes:

· Excessive load: If the transformer is overloaded, it generates more heat than it can dissipate.

· Insufficient oil cooling: Problems with the cooling system, such as a malfunctioning fan or low oil level, can impair the heat dissipation process.

· Internal faults: Issues such as short circuits within the windings or the core can cause localized overheating.

Diagnosis:

· Visual Inspection: Check for discoloration or signs of oil leakage, especially near bushings or connections.

· Temperature Monitoring: Use thermocouples or temperature sensors to monitor the oil temperature. The temperature of the oil should generally not exceed the manufacturer's rated limits.

· IR Thermography: Use infrared thermography to check for hot spots in the transformer body, bushings, and cooling system.

Solution:

· Correct Overloading: If the transformer is overloaded, reduce the load and redistribute it across other transformers if available.

· Oil Level Check: Ensure the oil is at the required level and there are no leaks. Add oil if necessary.

· Cooling System Check: Inspect the cooling fans, radiators, and oil pumps for functionality. Replace any defective components.

· Winding Inspection: For suspected internal faults, perform an insulation resistance test and an oil breakdown voltage test.

2. Insulation Failure

Causes:

· Aging Insulation: Over time, the insulating oil and materials inside the transformer degrade due to electrical stress, heat, and moisture.

· Moisture Ingress: Moisture contamination from external sources or leakage can significantly reduce the insulation resistance, leading to failures.

· Contamination: Oil contamination by particles, carbon, or other foreign matter can impair the insulation's performance.

Diagnosis:

· Dissolved Gas Analysis (DGA): This test helps identify gases dissolved in the transformer oil, which can indicate the condition of the insulation.

· Insulation Resistance Test: Measure the insulation resistance between the windings and between the windings and the ground. A significant drop in resistance is a clear indicator of insulation degradation.

· Tan Delta Test: Measures the dielectric loss in the insulating materials, providing insight into insulation condition.

Solution:

· Oil Filtering and Replacement: If oil contamination is detected, the oil should be filtered and replaced to restore proper insulating properties.

· Moisture Removal: If moisture is identified, drying techniques such as vacuum drying or heating the transformer in a controlled environment can be used to remove moisture from the insulation.

· Partial Discharge Detection: To locate insulation problems, use partial discharge detection equipment to pinpoint weak spots in the transformer.

3. Oil Leaks

Causes:

· Seal Failures: Gaskets, seals, or welds may fail due to aging, poor manufacturing, or external damage, leading to oil leaks.

· Pressure Build-up: If internal pressure exceeds safe limits (due to overheating or load fluctuation), it can cause seals to rupture or leak.

Diagnosis:

· Visual Inspection: Look for signs of oil stains around seals, bushings, and the tank. Leaks may be obvious at the points where components are bolted or welded.

· Pressure Test: Conduct a pressure test on the transformer's tank and seals to detect hidden leaks.

Solution:

· Seal Replacement: Replace damaged gaskets or seals with suitable high-quality materials to ensure a tight seal.

· Pressure Control: Adjust the oil preservation system to prevent excessive internal pressure build-up, ensuring proper ventilation and pressure relief mechanisms.

4. Bushing Faults

Causes:

· Aging and Insulation Breakdown: The bushings are essential for connecting external circuits to the transformer. Over time, aging of bushing insulation or oil contamination can lead to faults.

· Environmental Factors: Moisture, dirt, and physical damage can affect the bushing's performance.

Diagnosis:

· Visual Inspection: Check for visible cracks, oil leakage, or discoloration around the bushings.

· Bushing Monitoring: Conduct tests such as DC voltage testing, power factor testing, and the measurement of leakage currents to determine bushing health.

Solution:

· Bushing Replacement: If a bushing is damaged or its insulation is compromised, replace it with a new one that meets the transformer's specifications.

· Regular Maintenance: Ensure periodic cleaning and inspection of bushings to prevent external damage from dirt or moisture.

5. Partial Discharge

Causes:

· Imperfect Insulation: Partial discharge occurs when there is an insulation breakdown in a localized area, typically at weak points in the winding or bushing.

Diagnosis:

· Ultrasound Testing: Use ultrasound detectors to find sources of partial discharge within the transformer.

· Dissolved Gas Analysis (DGA): A rise in certain gases, such as acetylene or ethylene, indicates partial discharge activity in the transformer.

Solution:

· Insulation Repair or Replacement: If partial discharge is detected, inspect the insulation system, locate the source of the issue, and perform repairs or replace the damaged parts.

6. Abnormal Noise

Causes:

· Loose Parts: Vibration from loose components such as windings, bolts, or core can create abnormal noises.

· Magnetic Saturation: Magnetic core saturation can lead to buzzing or humming sounds.

· Flow of Oil: The movement of oil inside the tank during operation can sometimes produce noise.

Diagnosis:

· Sound Analysis: Listen for any irregular or loud noises that differ from the normal operational hum. Use sound recording equipment to analyze the noise patterns.

· Visual Inspection: Check for any loose parts or components that might be causing the noise.

Solution:

· Tighten Loose Components: Ensure all bolts and connections are securely tightened. Replacing any loose parts may eliminate noise.

· Magnetic Field Adjustments: If magnetic saturation is suspected, adjustments in operational conditions or core maintenance might be necessary.

Conclusion

Effective fault diagnosis and timely resolution of issues are essential for maintaining the performance, safety, and longevity of oil-immersed transformers. Regular inspections, monitoring, and maintenance can help prevent common failures such as overheating, insulation breakdown, oil leaks, and bushing faults. By understanding the potential issues and their solutions, operators can ensure transformers operate efficiently and avoid unexpected failures.