The causes of thermal faults in the core area of oil-immersed transformers

In the core area of the Oil-Immersed Transformer (mainly referring to key components such as windings, cores, and lead connection joints), a thermal failure occurs. The main cause is abnormal increase in local energy loss or obstruction of the heat dissipation channels, resulting in excessive heat accumulation exceeding the designed heat dissipation capacity.

I.Causes of winding-related thermal faults: The winding is the core conducting component of The Transformer, and thermal faults occur frequently. They can be mainly classified into two types: excessive copper loss and poor heat dissipation.  
Winding inter-turn/inter-layer short circuit: 1. Insulation aging: The transformer oil deteriorates, gets damp, or the insulation paper becomes brittle and damaged due to long-term exposure to high temperature and electric field, resulting in the failure of inter-turn and inter-layer insulation and the formation of short-circuit circulating currents. 2. Manufacturing defect: The insulation paper during winding does not overlap tightly, the windings are loose, and during operation, the insulation layer is worn out due to electromagnetic vibration. Or the welding of the leads may not be secure, and the burrs could puncture the insulation. 3. External force impact: Intense shaking during transportation and installation caused the winding to deform and the insulation layer to crack. Short-circuit circulation will cause a sharp increase in local power loss, rapid temperature rise, and even lead to winding burnout.
Overload operation of windings: The actual load exceeds the rated capacity of the transformer, and this condition persists for a long time. The current in the windings increases, and the copper loss rises proportionally to the square of the current. The heat cannot be dissipated in time. The three-phase load is severely unbalanced, and the current of one phase winding exceeds the design value, resulting in local overheating of a single phase.
Poor contact of winding wire joints: There is incomplete welding, oxidation or loose bolts at the welding points and exposed wire ends of the winding wires. The contact resistance increases, resulting in contact resistance loss during power supply, and forming local hotspots.
II. Causes of thermal faults related to the core: The function of the core is to conduct electricity. Thermal faults arise from abnormal iron losses.
Insulation damage of iron core silicon steel sheets: The insulation coating on the surface of the silicon steel sheets has peeled off due to excessive clamping force of the iron core, vibration wear, or moisture exposure, resulting in inter-sheet short circuits and forming eddy current circulation. The eddy current loss increases significantly, causing the local temperature of the iron core to rise.
Multiple grounding of the iron core: Normally, the iron core is only allowed to be grounded at one point. If there are two or more points of grounding, a closed circuit will be formed between these grounding points, resulting in circulating current and causing local overheating of the iron core. Common causes: The iron core clamps or straps may come off, or a metal foreign object may fall into the iron core, resulting in multiple ground points. Excessive gap between iron core joints: The gap at the joint of the iron core laminations exceeds the standard, resulting in an increase in excitation current, an increase in iron loss, and overheating in the joint area.
III. Failure of the cooling system leads to overheating faults: Even if the power loss is normal, poor heat dissipation can cause the core area to become congested.
The reasons are as follows: 1. Transformer oil has deteriorated or there is insufficient oil. Oil deterioration: Long-term operation leads to an increase in oil viscosity, a decline in insulation performance, and a reduction in heat dissipation capacity. Impurities in the oil, increased moisture, and blockage of the oil channels. Insufficient oil: Oil leakage or failure to replenish oil in a timely manner, the oil level being lower than the specified height, parts of the windings and the core exposed to the air, losing the oil-cooling effect. 2. Oil channel blockage: Oil channels between windings and between the core and windings are blocked by insulation paper debris and oil sludge, preventing the oil from circulating and flowing, resulting in heat accumulation in the core area. 3. Cooling device failure: For oil-immersed air-cooled transformers, the fans stop operating or the fan blades are damaged; for oil-immersed water-cooled transformers, the cooling water pipes are clogged or the water pumps fail, causing a sudden drop in cooling efficiency.
 Other Causes ：1. Metal Foreign Matter Intrusion: Metal shavings, tools, etc. remaining from the manufacturing and testing processes fall into the transformer interior. Under the action of electromagnetic force, they may move and puncture the insulation, causing a short circuit, or form a local circulation and generate heat. 2. Overvoltage impact: Lightning strikes and operational overvoltage cause insulation of the windings to break down, leading to local short-circuit thermal failures.