While epoxy resin insulator bursting is uncommon in the long-term operation of gas-insulated metal-enclosed switchgear (GIS) and gas-insulated transmission lines (GIL), it is often catastrophic. Fault tracing analysis shows that the origin of these accidents often points to tiny air gaps introduced inside high voltage standoff insulators during production or operation. This microscopic structural defect is considered a significant contributing factor to insulation failure. A deep understanding of the physical process from air gap formation to failure is fundamental to improving insulation reliability.

Partial Discharge and Electric Field Distortion Induced by Air Gap

When an air gap exists at the interface between the metal insert of high voltage epoxy and the epoxy casting material, the electric field strength borne by the air gap will increase exponentially because the dielectric constant of air (≈1) is much lower than that of epoxy composite material (usually 4~6). The smaller the air gap width, the larger the corresponding air gap electric field, making partial discharge highly likely. Although this microscopic discharge has limited energy, its long-term effects cause electro-corrosion of the air gap wall, gradually carbonizing the epoxy resin and generating conductive channels. We divide this process into clear evolutionary stages: crack initiation and propagation, fracture, and final explosion. In the initial stage, electrical potential energy dominates crack initiation and propagation along the interface.

Evolution Mechanism from Microcrack to Insulation Explosion

As the degradation process progresses, the dominant mechanism of the accident shifts. After the crack propagates to a critical size, mechanical potential energy begins to dominate. At this point, the structural integrity of epoxy resin insulator is severely compromised, and its mechanical strength drops sharply. Once the system encounters a short-circuit current surge or additional mechanical stress from operational overvoltage, the mechanical potential energy will instantly reach the material's stress critical point. This brittle fracture under electromechanical coupling manifests as high voltage epoxy resin exploding without warning in a very short time, posing a serious threat to surrounding equipment. Data shows that the probability of explosion due to air gap defects is significantly higher under AC voltage than under DC conditions.

Interface Process Optimization and Full-Process Non-Destructive Testing

To address the core pain point of air gaps, the industry is building defenses from two dimensions: manufacturing processes and testing methods. On the manufacturing side, by adding a buffer layer with appropriate elasticity to the surface of the insert, the thermomechanical stress caused by the difference in thermal expansion coefficients between the metal and epoxy resin can be effectively absorbed, preventing the formation of interfacial gaps at the source. Simultaneously, optimizing the vacuum casting process and strictly degassing the mixture are key steps in eliminating air bubbles within the casting.

On the quality control side, the limitations of single detection methods are becoming increasingly apparent, and the synergistic use of multiple methods is becoming a development trend. X-ray imaging can directly display internal macroscopic defects, but it is not sensitive to micro-cracks or debonding. Ultrasonic testing has high sensitivity to internal defects, but it requires a coupling agent and is difficult to scan complex curved surfaces. In recent years, new technologies such as nonlinear ultrasonic methods have shown the potential to detect early-closing cracks and material performance degradation, providing the possibility for early warning of latent defects. Only by combining precise manufacturing processes with highly sensitive, end-to-end non-destructive testing can the invisible killer of air gaps be truly eliminated.