Abnormal Conditions of Transformers and Handling Measures

• Transformer Blog
• Tue, 17 Mar 2026 02:44:52 +0000

1. Abnormal Temperature Rise

   • Phenomenon: During operation, the top oil temperature or winding temperature of the transformer rises abnormally, exceeding allowable values or normal levels under historical similar operating conditions.

   • Handling Measures:

     • Verify Parameters: Immediately check the ambient temperature and load current, compare with historical data, and confirm whether it is a false alarm.

     • Inspect Cooling System: Check if all coolers (fans, oil pumps, water pumps) are operating correctly, checking for stoppage, reverse rotation, or blockage. If the cooling system fails, put standby coolers into operation according to regulations or attempt to restore the faulty equipment.

     • Check Oil Level and Color: Check if the conservator oil level is normal and if the oil color has darkened or turned black.

     • Check Load: If the cooling system is normal, check if the temperature rise is caused by overload. If long-term overload exists, reduce the load according to regulations.

     • Check External Environment: Check if the ventilation in the transformer room is good and if the cooling vents are closed or blocked.

     • Internal Inspection and Sampling: If the above checks are normal, suspect internal faults (such as core multiple grounding, partial discharge, inter-turn short circuit, etc.). Arrange to take an oil sample for chromatographic analysis, and listen for any abnormal internal sounds from the transformer.

     • Shutdown for Handling: If the temperature rise exceeds the limit and continues to increase, or if chromatographic analysis indicates a serious internal fault, immediately apply to shut down the transformer for handling to prevent the fault from expanding.

2. Abnormally High Oil Level: If the oil level may exceed the upper limit due to a temperature rise, and it is confirmed not to be a false oil level, oil should be drained to lower the level to the normal height corresponding to the current oil temperature to prevent overflow.

3. Abnormally Low Oil Level: If the transformer oil level is found to be significantly lower than the normal level corresponding to the current oil temperature, the cause must be investigated. If oil needs to be topped up while the transformer is energized, the Buchholz relay protection should be temporarily switched to the signal position, and adding oil from the bottom of the transformer is strictly prohibited.

4. Low-Temperature Startup: When starting a transformer in a low-temperature environment, if the oil flow is hindered due to excessively low oil temperature, it should be started under no-load conditions without turning on the coolers. Simultaneously, closely monitor the top oil temperature. As the oil temperature rises, gradually increase the load and engage the appropriate number of coolers based on the situation, allowing the transformer to transition smoothly to normal operation.

5. Core Multiple Grounding: If multiple grounding of the core is found with a high grounding current, analyze the specific cause. If suspected to be caused by metal burr bridging, an attempt can be made to burn off the burrs by discharging a charged capacitor at the grounding point. If it cannot be resolved without lifting the core, schedule a core lift for repair. Before the defect is completely eliminated, current-limiting measures should be taken to control the grounding current to around 100mA, and operational monitoring should be intensified.

6. Abnormal Operating Sound: The sound of an operating transformer is related to its capacity, voltage, and load. If the sound is significantly louder compared to similar equipment, it may be related to structural design, manufacturing process, or installation stability, sometimes also caused by core over-excitation. If faint, intermittent discharge sounds are heard internally, it could be due to metallic foreign objects at the bottom of the tank or suspended burrs in the oil. If the abnormal sound persists and cannot be eliminated, the transformer should be immediately shut down for inspection and repair.

7. Bushing Discharge:

   • Discharge at Base Flange: If static discharge sounds occur at the base flange of a high-voltage bushing after a new transformer is commissioned, it is usually due to poor contact between the flange and the tank (poor grounding). This can be resolved by applying silver powder or semiconductor paint to the base of the flange after de-energizing the transformer.

   • Discharge at Top: If slight discharge sounds occur at the top of the bushing, it should be considered that air might be trapped at the top, which can be solved by de-energizing and venting.

8. Severely Damaged or Leaking Bushing: If the transformer bushing is severely damaged or leaking oil, causing the oil level to continuously drop, or if there are obvious internal discharge sounds, the transformer should be immediately shut down for inspection and repair.

9. Tank Leakage: If the transformer tank is leaking oil, causing the oil level to continuously drop below the lower limit of the oil level indicator, the transformer should be immediately shut down for handling.

10. Serious Fault (Fire, Protection Failure, etc.): If the transformer emits smoke, catches fire, or experiences other faults endangering safety, and the corresponding protection devices fail to trip, the operator on duty should immediately and manually perform an emergency shutdown of the transformer.

11. Buchholz Protection Signal Operates: When the Buchholz protection signal operates, the transformer should be inspected immediately. For newly commissioned transformers or those after oil top-up, if the gas accumulated in the gas relay is colorless, odorless, non-flammable, and identified as air by chromatographic analysis, the transformer can continue operating. If the gas is flammable and appears repeatedly, serious internal defects should be highly suspected, requiring a comprehensive judgment to decide whether to shut down for handling.

12. Buchholz Protection Trips: After the Buchholz protection trips, the transformer must not be re-energized until the cause is found and the fault is eliminated. During inspection, focus on analyzing the characteristics of the gas inside the relay. During an internal fault, the arc decomposes insulating materials, producing a large amount of gas that drives the insulating oil to impact the gas relay, causing the trip. The severity can be judged by the gas volume, and the nature of the fault (distinguishing between oil decomposition and solid insulation decomposition) can be judged by the gas composition. For example, a high content of CO and CO₂ in the gas mostly originates from solid insulation materials. Preliminary judgment of the fault state can be made by observing the gas color and flammability: Grayish-white gas often involves insulation paper or pressboard; yellow gas often involves wooden insulation; dark blue or black gas is often caused by oil gap breakdown; flammable gas indicates an arcing fault inside. Flammability testing must be conducted in a safe area after collecting the gas, and igniting the gas directly at the relay vent valve is strictly prohibited to avoid exploding the conservator. If there is no gas in the relay and the transformer oil color is normal, consider the possibility of protection maloperation, focusing on checking the secondary circuit. If a secondary circuit problem is confirmed with a clear cause, the transformer can be restored to operation after taking preventive measures, without the need for a core lift inspection.

13. Overcurrent Protection Trip: The transformer overcurrent protection trips. Possible causes include:

    • A short circuit occurs on the secondary side outgoing line, and the protection for that line fails to trip.

    • The transformer itself has a fault, but the differential and Buchholz protections did not operate for some reason.

    • The overcurrent protection setting is incorrect, or the protection device itself malfunctions, causing a false trip.

    • A short circuit occurs on the secondary side busbar.

    • A short circuit fault occurs in the circuit breaker or disconnector on the transformer’s secondary side outgoing line.

14. Differential Protection Trip: The transformer differential protection trips. Possible causes include:

    • A short circuit fault occurs within the transformer itself (oil-immersed transformers usually accompanied by Buchholz protection operation).

    • A short circuit fault occurs on the primary or secondary side leads or bushings.

    • Differential protection maloperation. Causes of maloperation typically include incorrect secondary circuit wiring, faults in the current transformer or its secondary circuit, or unreasonable protection settings. Such maloperations often occur during external faults (e.g., short circuit on the secondary side outgoing line) due to unbalanced currents in the differential circuit caused by high through-fault current. If the trip coincides with an external fault or a large impact load, and the transformer itself shows no abnormalities like oil spewing or Buchholz operation, differential protection maloperation should be highly suspected. Once obvious defects are found in the protection device or its secondary circuit, it can be concluded that the transformer is not faulty, and the trip was due to protection maloperation.