Conductivity Of Insulators In A Dry State: High Resistance Does Not Mean Absolute Insulation.
In dry environments, the surface conductivity of polymer suspension insulator is typically maintained at an extremely low level. Compared to humid or dirty conditions, the electric field intensity distribution on a dry suspension type insulator surface is more uniform, and the leakage current is almost negligible. In the actual operation of transmission lines, this high-resistivity characteristic is fundamental to ensuring system stability. Once exposed to fog, dew, or light rain, the electrolyte in the contaminants adhering to the surface of composite tension insulator dissolves, causing a sharp increase in surface conductivity, thereby destroying the original insulation advantages.
Determining Factors of Surface Conductivity
The high-resistivity characteristic exhibited by high voltage transmission line insulators in a dry state does not mean that the material itself is absolutely non-conductive, but rather that its surface conductivity is within an extremely low threshold range.
Coupling of Materials and Environment
The bulk conductivity of composite insulators (such as silicone rubber materials) is closely related to the surface state. Under dry conditions, the insulator surface lacks the water film medium necessary to form a conductive path. Even in the presence of neutral particulate contaminants, due to the lack of water dissolution, these particles cannot form continuous ion migration channels, resulting in extremely low leakage current values. Taking a 20kV voltage level experiment as an example, the leakage current measured in the dry state of the silicone rubber insulator is much lower than that in the wet state. This high-resistivity state ensures that the voltage distribution of the insulator string mainly depends on its geometric capacitance and volume resistance, rather than the surface contamination layer resistance.
Critical Transition from High-Resistivity State to Dry-Band Discharge
Although the insulation performance of the insulator is excellent in the dry state, the operating environment is always dynamically changing.
Formation of Dry Bands and Local Arcing
When there is an uneven contamination layer on the overhead power line insulators surface and it encounters localized moisture, areas with high current density (such as near the steel feet) will first form dry bands due to the Joule heating effect. The appearance of dry bands forces a severe distortion in the voltage distribution on the tension insulator surface. The electric field strength is significantly enhanced in the dry band region. Under certain microstructures, the local electric field strength in the dry region can reach several times that of the conventional region due to charge accumulation. When this local field strength exceeds a critical value, a local arc will be ignited.
Partial Discharge Initiation Mechanism
Regarding the initiation of partial arcs, the process of charge capture and release needs to be considered. Before the formation of the dry zone, the charge capture effect on the insulator surface begins to appear. When a water film adheres to the surface of salt particles in the contamination layer (even during the seemingly "dry" transition period), its charge capture ability is significantly enhanced, leading to a distortion of the electric field intensity on the opposite side of the electrode. This enhancement of the electric field caused by the accumulation of micro-area charge is a key physical process in the transition from the dry state to the pre-flashover state. If left unmonitored, this weak discharge may further develop into a penetrating flashover accident.
