Epoxy resin cast dry-type transformers are widely used in power systems, and their short-circuit withstand capability directly affects the reliability and safety of equipment operation. The enormous electrodynamic forces generated by short-circuit faults can lead to winding deformation, insulation damage, and even equipment failure. Therefore, improving short-circuit withstand capability through structural improvements is a key direction in dry-type transformer design. The following systematically elaborates on structural improvement measures to enhance short-circuit withstand capability from aspects such as winding structure, end support, pad design, internal positioning, material optimization, and process control.
Winding structure optimization is the core of improving short-circuit withstand capability. Traditional layered windings are prone to axial displacement due to axial forces during short circuits, while foil windings, by winding the conductors into continuous metal foil layers, significantly reduce lateral leakage flux, thereby reducing axial electrodynamic forces. Foil windings have low inter-layer voltage, large capacitance, and more uniform voltage distribution, further enhancing mechanical stability. Furthermore, adopting a segmented cylindrical winding structure, through optimized inter-segment spacing and insulation design, can effectively disperse stress concentration during short circuits and avoid localized deformation. The reactance of the high-voltage and low-voltage windings is highly matched, eliminating helix angle phenomena, ensuring ampere-turn balance, and bringing axial force close to zero, fundamentally improving short-circuit withstand capability.
The support structure at the winding ends is a crucial element in preventing deformation. During a short circuit, the ends are prone to rotational forces due to leakage magnetic fields, leading to winding twisting or displacement. Adding insulating end rings at the ends and using high-strength glass fiber reinforcement enhances the rigidity of the end structure. Simultaneously, placing silicone rubber buffer pads between the ends and the clamps increases friction to limit displacement and absorbs vibration energy, reducing impact damage. The low-voltage foil winding ends are integrally cast, ensuring a flat end face consistent with the high-voltage winding height, further enhancing the stability of the end support.
The spacer, as a key support component between the winding and the clamps, directly affects short-circuit withstand capability. Traditional spacers are prone to breakage due to insufficient strength or poor toughness, leading to winding loosening. Improvements include using high-density epoxy glass cloth sheets for the spacers, which have high strength and good toughness, effectively dispersing stress. A superior solution utilizes glass fiber reinforced molded pads, cast using a unique process, which combine high compressive and shear strength with low partial discharge. The contact surface between the pads and the winding is designed with an arc shape to increase the contact area and avoid stress concentration; elastic positioning devices are installed between the pads to limit displacement while buffering short-circuit impacts.
The winding positioning and axial clamping structure is crucial to preventing axial displacement. Positioning bosses are provided on the contact surface between the pads and the winding to limit radial movement of the winding; a steel pull plate-type elastic buffer structure, with special positioning and elastic devices installed between the pads and clamps, ensures constant clamping force while mitigating axial impacts during short circuits. The winding clamping uses a combination of steel pull strips and disc springs, absorbing vibration through elastic deformation to ensure the winding is always under clamping, preventing aggravated deformation due to loosening.
Material selection and process control are fundamental to structural improvements. The epoxy resin and glass fiber composite material must possess high mechanical strength, a low coefficient of thermal expansion, and excellent electrical properties. By optimizing the resin formulation and adding nanofillers or fiber reinforcement phases, the impact resistance and crack resistance of the material can be significantly improved. Vacuum casting ensures full resin penetration, eliminating bubbles and defects to form a dense insulation layer; the curing process uses a stepped temperature profile to avoid internal stress. Furthermore, the winding conductors are cold-drawn from oxygen-free copper rods, eliminating sharp corners and burrs, reducing electric field concentration, and lowering the risk of partial discharge.
Optimization of internal air channels and heat dissipation structures indirectly improves short-circuit withstand capability. Dry-type transformers increase heat dissipation area and reduce operating temperature rise by incorporating axial ventilation channels within the windings. Improved temperature uniformity reduces structural stress caused by thermal expansion and contraction, preventing insulation layer cracking. The air channel design must balance mechanical strength and heat dissipation efficiency; glass fiber mesh is used to reinforce the air channel walls to ensure no deformation under short-circuit impact.
Structural improvements must be deeply integrated with the manufacturing process. The foil winding employs an automatic winding machine and interlayer DMD prepreg paper to ensure winding accuracy and insulation reliability; the segmented cylindrical winding is cast after vacuum impregnation to eliminate inter-segment gaps; the overall casting mold design must consider resin flowability to ensure simultaneous curing of thick and thin sections. Short-circuit testing is added to the quality inspection process to verify the effectiveness of structural improvements through sudden short-circuit tests, forming a closed-loop optimization from design to manufacturing to testing.
