Causes of Winding Cracking in Epoxy Resin Cast Dry-Type Transformers and the Influence Mechanism of Thermal Cycling

1. Multidimensional Triggering Factors of Winding Cracking

Winding cracking is not the result of a single failure but rather the interplay of multiple factors including materials, processes, structural design, and operating environment.

1.1 Evolution of Insulation Structure and Mismatch of Material Properties

1.2 Improper Control of Casting and Curing Processes

During casting, if high-standard vacuum casting is not adopted or degassing is insufficient, air bubbles are likely to remain inside the resin. During the curing stage, if the heating, holding, and cooling profiles are improperly controlled, the resin cannot cure slowly and uniformly, resulting in excessive internal residual stress that creates inherent defects predisposing the material to later cracking.

1.3 Mechanical Stress and Electromagnetic Force Impact

During operation, transformers are subjected to external mechanical vibration as well as enormous electromagnetic forces during short-circuit events. If the coil winding is not tight or the support and fixing methods are inadequate, repeated electromagnetic forces and vibration can cause coil deformation, insulation layer damage, and even cracking.

1.4 Harsh Operating Environment

Prolonged exposure to high temperatures accelerates thermal aging and thermal decomposition of the resin, reducing its mechanical strength. In hot and humid environments, moisture ingress causes ester hydrolysis in the resin, forming micro-pores on the surface. Additionally, corrosive gases in environments such as chemical plants (e.g., sulfur dioxide, chlorine) can attack the insulation material and degrade its molecular structure.

2. Influence Mechanism of Thermal Cycling on the Resin Insulation Layer

Among the triggers described above, thermal cycling (temperature cycling) is the core physical mechanism that runs throughout the equipment's entire life cycle, acting as a "catalyst" that accelerates crack propagation. Research indicates that "thermo-mechanical fatigue" caused by thermal cycling is far more likely to induce micro-cracks in the insulation layer than continuous high-temperature exposure. Its impact on the resin insulation layer is primarily reflected in the following four aspects.

2.1 Accumulation and Fatigue of Alternating Thermal Stress

Due to the CTE mismatch between the resin and the conductor, and between the resin and the glass fiber reinforcement, thermal cycling generates alternating tensile and shear stresses at the material interfaces. As the number of cycles increases, these stresses weaken the interfacial bonding strength, micro-cracks initiate and gradually propagate, ultimately developing into through-cracking.

2.2 Deterioration of Material Microstructure

Under the alternating temperature stress of thermal cycling, the cross-linked network of epoxy resin gradually degrades, and the free volume within the material increases. At the microscopic level, this manifests as voids and micro-cracks on fracture surfaces; at the macroscopic level, it appears as a decrease in the AC breakdown strength of the insulating material.

2.3 Critical Effect of Glass Transition Temperature (Tg)

When the operating temperature approaches or exceeds the glass transition temperature (Tg) of the epoxy resin, the coefficient of thermal expansion rises sharply and the modulus undergoes significant change. At this point, the material transitions from a rigid glassy state to a highly elastic state, its deformation resistance drops dramatically, and the internal stress concentration caused by thermal cycling more readily leads to interfacial debonding and cracking.

2.4 Vicious Cycle Triggering Partial Discharge (PD)

Cracking caused by thermal cycling creates micro-air gaps within the insulation layer. Since the dielectric constant of resin is far higher than that of air, under AC voltage the electric field strength in the air gap is distributed inversely proportional to the dielectric constant, resulting in extremely high field strength within the gap. This readily triggers partial discharge. The high-energy particles generated by PD further erode and degrade the epoxy resin, forming a vicious cycle of "cracking → PD → electrical erosion → aggravated cracking" that ultimately leads to insulation breakdown.

3. Summary and Systematic Defense Strategy

In summary, winding cracking in epoxy resin cast dry-type transformers is essentially a material fatigue and structural failure problem caused by multi-factor coupling. To fundamentally resolve this issue, a systematic defense strategy must be established across both manufacturing and operational phases.

Only by combining the enhancement of intrinsic material properties with life-cycle operation and maintenance management can the erosive effects of thermal cycling be effectively resisted, ensuring the long-term safe and stable operation of dry-type transformers.