The alarm temperature of an oil-immersed transformer is a critical safety threshold that triggers protective measures to prevent overheating and potential insulation failure. The exact values depend on design standards, insulation materials, cooling methods, and operating conditions. Here’s a detailed breakdown based on international and regional standards:

　　1. Typical Alarm Temperature Ranges

　　Oil Temperature:

　　Normal Operation: Oil temperature should ideally stay below 85–95°C under rated load.

　　Alarm Threshold:

　　General Distribution Transformers: Alarms often activate at 85–90°C for oil temperature (e.g., S20 series) .

　　High-Power Transformers: Forced-cooled (OFAF/ODAF) units may have higher thresholds, up to 95°C, due to improved heat dissipation .

　　Safety Margin: A 10–15°C buffer exists between the alarm and the maximum allowable oil temperature (e.g., 95°C for A-grade insulation per GB 1094.2-2013) .

　　Winding Temperature:

　　Normal Operation: Winding temperature is typically 10–15°C higher than oil temperature (e.g., 95–110°C under rated load) .

　　Alarm Threshold:

　　General Cases: Alarms trigger at 100–110°C for windings .

　　Short-Term Overloads: During temporary overloads (e.g., 150% load), winding temperatures may briefly exceed 110°C, but sustained operation above 120°C risks insulation degradation .

　　2. Key Standards and Guidelines

　　IEC 60076-2 (International):

　　Oil Temperature Rise: Maximum 60°C above ambient (e.g., 60°C rise + 40°C ambient = 100°C oil temperature) .

　　Winding Hot-Spot Rise: 78°C above ambient, leading to a typical alarm threshold of 118°C (78°C + 40°C) .

　　GB 1094.2-2013 (China):

　　Top Oil Temperature: Limited to 95°C under A-grade insulation (standard for most oil-immersed transformers) .

　　Winding Temperature: Calculated based on oil temperature and thermal gradient, with alarms set 10–15°C above oil temperature .

　　IEEE C57.12.00 (US):

　　Average Winding Rise: 65°C for natural cooling (ONAN), 70°C for forced-directed oil flow (ODAF) .

　　Hot-Spot Rise: 80°C above ambient, aligning with an alarm threshold of 120°C (80°C + 40°C) .

　　3. Factors Influencing Alarm Settings

　　Insulation Material:

　　A-Grade Insulation (Paper): Maximum oil temperature = 95°C, winding alarm at 105–110°C .

　　Nomex Insulation: Used in mining transformers, it allows higher temperatures (e.g., 120–130°C winding alarm) .

　　Cooling Method:

　　ONAN (Natural Cooling): Lower thresholds (oil: 85–90°C, winding: 100–105°C) due to slower heat dissipation .

　　OFAF/ODAF (Forced Cooling): Higher thresholds (oil: 90–95°C, winding: 105–110°C) .

　　Overload Conditions:

　　Short-Term Overloads (e.g., 150% load): Alarms may tolerate 10–15°C above normal thresholds for up to 30 minutes .

　　Emergency Overloads: Prolonged operation beyond 120°C (winding) requires immediate load reduction to avoid irreversible damage .

　　4. Safety Mechanisms Beyond Alarms

　　Trip Thresholds:

　　Oil temperature: 130–150°C (immediate shutdown to prevent fire or explosion) .

　　Winding temperature: 130–140°C (insulation breakdown risk) .

　　Monitoring Systems:

　　IoT Sensors: Real-time tracking of oil and winding temperatures, with remote alerts .

　　Dissolved Gas Analysis (DGA): Detects early signs of insulation degradation due to overheating .

　　5. Practical Recommendations

　　Manufacturer Guidelines: Always refer to the transformer datasheet for exact alarm settings (e.g., CEGB KBSGZY-6300/10/3.45 sets oil alarm at 85°C) .

　　Ambient Adjustments: Reduce load if ambient temperature exceeds 40°C to avoid exceeding thresholds .

　　Mining Applications: Use explosion-proof transformers with lower alarm thresholds (e.g., oil: 80°C, winding: 95°C) due to methane risks .

　　6. Example Alarm Settings

　　Conclusion

　　The alarm temperature of an oil-immersed transformer is a dynamic parameter influenced by insulation, cooling, and operational context. Adhering to international standards (IEC, IEEE, GB) and manufacturer specifications ensures safe and efficient operation. Regular maintenance, including oil analysis and thermal imaging, is essential to prevent unexpected failures and extend equipment lifespan.