Core Function: Serves as the main magnetic circuit of the transformer.
Core Material: Silicon steel sheets with a thickness of 0.35 to 0.5 mm.
Core Structure: Power transformers mainly adopt a core-type structure.
The core forms a closed magnetic circuit within the transformer and also acts as the skeleton for mounting the windings (coils). It is a crucial component for both the electromagnetic performance and mechanical strength of the transformer.
The core is the magnetic circuit part of the transformer, consisting of core limbs (where the windings are mounted) and core yokes (which connect the core limbs to form a closed magnetic circuit). To reduce eddy current and hysteresis losses and improve the magnetic permeability of the circuit, the core is built by interleaving and stacking silicon steel sheets (0.35mm to 0.5mm thick) coated with insulating varnish. The cross-section of small transformer cores is rectangular or square, while that of large transformers is stepped to make full use of the space.
1) Multi-point Grounding Faults of the Core
The insulating paperboard between the bottom clamp feet and the iron rail falls off or gets damaged, causing the laminations at the yoke to touch the feet and create a ground.
Due to wear on the submerged oil pump shaft, metal powder enters the oil tank and accumulates at the bottom. Under the influence of electromagnetic forces, it forms a bridge, connecting the lower yoke to the feet or the tank bottom, resulting in multi-point grounding.
The thermometer socket on the tank cover is too long and touches the upper clamp, yoke, or the edge of the side limb, creating a new grounding point.
The wooden spacer between the lower clamp and the yoke step gets damp or dirty (covered with oil sludge), causing its insulation resistance to drop to zero and forming multi-point grounding.
Metal foreign objects like iron nails or welding rods fall into the tank, causing the core laminations to contact the tank and form a ground.
After transformer installation, the positioning pins on the tank cover used for transportation are not flipped over or removed, causing multi-point grounding.
2) Core Overheating Faults
There are many causes for transformer core overheating, such as winding short circuits, overload operation, poor or abnormal core grounding, short circuits between core laminations or local core short circuits, yoke bolt grounding, core flux leakage, high power supply voltage, and blocked core cooling oil ducts. Besides the above, poor oil circulation, low oil level, oil deterioration, large burrs around the core laminations, and uneven gaps during core stacking can also cause overheating faults.
Core partial overheating faults basically occur on the core and clamps. If a transformer in operation experiences core overheating, especially partial overheating, it will generate characteristic gases such as H₂, CH₄, C₂H₂, and C₂H₆. Chromatographic analysis (DGA) will reveal that the dissolved gas content in the oil exceeds the standard.
Transformer Core Maintenance & Overhaul
Wipe the oil stains and impurities off the core surface using a clean, lint-free white cloth.
If there are curled edges or翘角 (warped corners) on the silicon steel sheets, carefully repair them using a wooden mallet.
Inspect the spacers in the core oil ducts; they should be neatly arranged. Lightly tap the spacers to ensure there is no looseness; check that there are no foreign objects inside the core oil ducts.
Check that there is a distinct and uniform gap between the pressure plate and the upper yoke; ensure the grounding tab bolts on the steel pressure plate are not loose. The insulating pressure plate should remain intact without damage or cracks, and have appropriate tightness.
Use a 1000V insulation resistance tester (megger) to measure the insulation resistance between the core and the through-core bolts/steel straps. There should be no significant change compared with previous tests.
Disconnect the connecting pieces between the upper clamp and the core, as well as between the steel pressure plate and the upper clamp. Use a 2500V insulation resistance tester to measure the insulation resistance of the core to the clamp and ground, which should be no less than 100 MΩ. After measurement, reliably reset the connecting pieces. (注：原文的“2V绝缘电阻表”应为笔误，已修正为电力行业标准的2500V)
Use wrenches and torque wrenches to tighten the fasteners on the upper and lower clamps, upper beams, side beams, feet, pressure nails, and through-core bolts one by one.
Check the condition of the core electrostatic shield. Use a 1000V insulation resistance tester to measure the insulation resistance of the core electrostatic shield to the ground, which should be greater than 100 MΩ.
Check the connection and insulation condition of the core grounding tab. The core is only allowed to be grounded at a single point. The grounding tab is generally made of a copper sheet with a thickness of 0.5 mm and a width of not less than 30 mm. It should be inserted between 3 to 4 levels of the core laminations. For large transformers, the insertion depth should be no less than 80 mm. The exposed part should be insulated to prevent short-circuiting the core.
