Under specified cooling conditions, transformers may operate according to their nameplate ratings. For oil-immersed transformers, the allowable temperature is monitored by the top oil temperature. The top oil temperature must comply with manufacturer specifications and shall never exceed 95°C. To prevent accelerated oil degradation, the top oil temperature should not regularly exceed 85°C.
The applied voltage shall generally not exceed 105% of the rated value. Under this condition, the secondary side may carry rated current.
Transformers may operate under normal overload or emergency overload conditions:
Normal overload may be used regularly, with permissible values determined by the load curve, cooling conditions, and the pre-overload load level.
Emergency overload is permitted only in emergency situations (e.g., when a transformer remains operable after a fault).
If any abnormal phenomenon is observed during operation (e.g., oil leakage, low oil level in the conservator, abnormal heating, unusual noise), immediate corrective action shall be taken. The transformer must be taken out of service immediately if any of the following occurs:
Unusually loud, uneven internal sounds, or explosive noises.
Under normal cooling conditions, the temperature is abnormally high and continues to rise.
Oil or explosion vent expels oil.
Oil level drops below the minimum mark on the indicator due to leakage.
Significant oil discoloration or carbon deposits appear in the oil.
Severe bushing damage or discharge.
If the transformer overload exceeds permissible limits, the load shall be adjusted promptly. When the oil temperature exceeds allowable limits, the cause must be identified and corrective measures taken. The following actions are required:
Check the transformer load and cooling medium temperature, then compare with the expected temperature under those conditions.
Verify the accuracy of the thermometer.
Inspect the mechanical cooling system or transformer room ventilation.
If the oil temperature is 10°C higher than normal under the same load and cooling conditions, or if the temperature continues to rise despite constant load — and the cooling system, ventilation, and thermometer are all normal — internal failure (e.g., core overheating, inter-turn short circuit) is likely. In such cases, the transformer shall be immediately de-energized for repair.
If the transformer oil has solidified, the transformer may be placed into service with load, but careful attention must be paid to top oil temperature and oil circulation.
If the oil level is significantly lower than the level corresponding to the current oil temperature, oil shall be added immediately. If the oil level drops rapidly due to a large leak, the Buchholz relay must not be set to alarm-only mode; instead, leak-stopping measures shall be taken quickly, followed by immediate oil filling.
When the Buchholz relay alarm (signal) operates:
Inspect the transformer to determine the cause — air ingress, low oil level, or secondary circuit fault.
If no external cause is found, analyze the gas accumulated in the relay.
Gas characteristics:
Colorless, odorless, non-flammable gas → air separated from oil → transformer may continue operation.
Flammable gas → transformer must be de-energized for thorough investigation.
⚠️ Safety note: When testing gas flammability, do not bring flame directly to the relay top; keep flame 5–6 cm above it.
If the relay operation is not caused by air ingress, check the oil flash point. A drop of 5°C or more from previous records indicates internal failure.
If the Buchholz relay trips the circuit breaker and flammable gas is confirmed, the transformer shall not be re-energized without special inspection and testing.
Common causes of signal-only operation (alarm without trip):
Air ingress due to leakage, oil filling, or poor sealing in the cooling system.
Gradual oil level drop from temperature reduction or leakage.
Small gas generation from minor internal faults.
Through-fault (external short circuit).
Causes of combined alarm and trip (or trip alone):
Severe internal fault.
Rapid oil level drop.
Protection system secondary circuit failure.
In rare cases, such as after repair, rapid air release from oil may also cause tripping.
Two types of leaks: weld seam leaks and seal leaks.
Weld seam leaks: Repair by welding. The core and windings must be lifted out, and the oil completely drained before welding.
Seal leaks: Identify the cause — improper installation (misaligned gasket, uneven or insufficient compression) — correct accordingly. If the gasket is aged or damaged (e.g., rubber becomes sticky, loses elasticity, or cracks), replace the sealing material.
Regular inspections and monitoring help detect anomalies or faults early, preventing serious accidents. The following items shall be checked periodically:
Unusual sounds — uneven noises, discharge sounds, etc.
Oil level — normal? Any oil leakage or seepage?
Oil temperature — top oil temperature generally ≤85°C.
Bushings — clean? Any cracks, damage, or discharge?
Connections/terminals — overheating signs?
Explosion vent diaphragm — intact?
Buchholz relay — any oil leakage? Is it completely filled with oil?
Breathing system — clear? Oil seal breather oil level normal? Silica gel not saturated with moisture?
Cooling system — operating properly?
Grounding connection — external ground wire intact?
Regular adherence to these operation and maintenance guidelines will significantly enhance transformer reliability and service life. Xinhong Electrical is committed to providing high-quality three-phase oil-immersed transformers and professional technical support. For more information or specific application advice, please contact our team.
