Core Function and Structure
The core serves as the main magnetic circuit of the transformer. It is made of 0.35mm to 0.5mm thick silicon steel sheets. Power transformers primarily 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
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 will reveal that the dissolved gas content in the oil exceeds the standard.
Transformer Core Maintenance and Overhaul
Use a clean, lint-free white cloth to wipe off oil stains and impurities from the core surface.
If the silicon steel sheets have curled edges or翘角 (warped corners), carefully repair them using a wooden mallet.
Check that the core oil duct spacers are arranged neatly; tap the oil duct spacers lightly to ensure there is no looseness; check that there are no foreign objects in the core oil ducts.
Check that there is a clear and uniform gap between the pressure plate and the upper yoke; check that the bolts of the grounding piece 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 meter (megohmmeter) to measure the insulation resistance between the core and the through-core bolts/steel tie straps. It should show no significant change compared to previous tests.
Disconnect the connecting piece between the upper clamp and the core, as well as the connecting piece between the steel pressure plate and the upper clamp. Use a 2500V insulation resistance meter 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：原文中的“2V绝缘电阻表”应为笔误，根据工程标准已修正为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 core electrostatic shielding condition. Use a 1000V insulation resistance meter to measure the insulation resistance of the core electrostatic shield to ground, which should be greater than 100 MΩ.
Check the connection and insulation condition of the core grounding piece. The core is only allowed to have one grounding point. The grounding piece is generally made of a copper sheet with a thickness of 0.5mm and a width of not less than 30mm. It is inserted between 3 to 4 levels of the core. For large transformers, the insertion depth should be no less than 80mm. The exposed part should be insulated to avoid short-circuiting the core.
Precautions
(Note: This section repeats the content from the "Multi-point Grounding Faults" section above)
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.
