What is the mechanism of accelerated insulation aging of power capacitors in high temperature environments?
Release Time : 2025-09-23
The mechanism that accelerates insulation aging in power capacitors under high-temperature conditions involves the synergistic effects of multiple physical and chemical processes. Its essence is the irreversible degradation of the insulating material's molecular structure driven by thermal energy. Insulating materials undergo a complex series of degradation reactions at high temperatures, with thermal oxidation being a key mechanism. High temperatures accelerate the breakage of polymer chains within the insulating material, rupturing previously stable chemical bonds and generating a large number of free radicals. In the presence of oxygen, these free radicals further trigger a chain oxidation reaction, significantly reducing the polymer's molecular weight and generating low-molecular-weight products such as carboxylic acids and aldehydes. These oxidation products not only alter the material's chemical composition but also destroy its original crystalline structure, causing the insulating material to gradually lose mechanical strength and electrical properties.
Thermal decomposition is another important mechanism by which high temperatures accelerate insulation aging. When the temperature exceeds the thermal stability threshold of the insulating material, the polymer backbone undergoes random scission, producing volatile small molecules. This decomposition process directly leads to material quality loss and a gradual reduction in the thickness of the insulating layer. In power capacitors, the thermal decomposition of impregnants is particularly critical. Decomposition products can alter the dielectric properties of the dielectric, while the released gases can generate localized high voltages within the sealed container, further exacerbating mechanical stress on the insulating material. Low-molecular-weight substances produced by decomposition can also migrate to the electrode surface, forming conductive pathways and triggering partial discharge.
Partial discharge exhibits a vicious cycle in high-temperature environments. High temperatures reduce the dielectric strength of the insulating material, making previously tiny air gaps or impurities more susceptible to discharge. Each discharge generates transient temperatures reaching thousands of degrees Celsius, sufficient to carbonize the surrounding material and form conductive pathways. The formation of these pathways, in turn, reduces the local electric field strength, encouraging the discharge to extend deeper into the material. High temperatures also accelerate the diffusion of discharge products, rapidly expanding the carbonized area and ultimately leading to through-hole breakdown of the insulation layer. Particularly in oil-immersed power capacitors, ozone generated by partial discharge can chemically react with the impregnant, generating acidic substances that further corrode the insulating material.
Mechanical damage caused by thermal stress is an indirect but significant factor in high-temperature aging. The difference in thermal expansion coefficients between the insulating material and the metal electrode creates shear stress at the interface under high temperature conditions. Repeated thermal cycling causes this stress to accumulate, forming microcracks within the material. These cracks not only provide pathways for moisture and impurities to enter, but also alter the electric field distribution, creating concentrated electric fields at the crack tips and accelerating the development of local discharge. For metallized film power capacitors, thermal stress can also cause delamination between the metal plating and the base film, rendering the self-healing function ineffective and significantly reducing the reliability of the power capacitors.
High temperatures alter the dielectric properties of the insulating material, creating a coupled thermal-electrical aging mechanism. As temperature increases, the dielectric constant and dissipation factor of the material generally increase, resulting in increased Joule heating under the action of the electric field. This self-heating effect further increases the local temperature, creating a positive feedback loop. High temperatures can also alter the distribution of space charge within the material, distorting the internal electric field. The electric field strength in some areas can reach several times the average value, accelerating insulation degradation in these areas.
Thermomigration of chemical additives is particularly pronounced at high temperatures. Additives such as antioxidants and plasticizers added to insulating materials gradually migrate from the material's interior to the surface or interface regions under high temperatures. This migration reduces the additive concentration within the material, resulting in a loss of its original protective effect. Furthermore, the concentration of additives on the surface can alter local electrical conductivity. For composite insulation systems, differences in additive concentrations at the interfaces between different materials can also lead to compatibility issues, resulting in delamination or voids.
High-temperature-accelerated aging processes exhibit significant synergistic effects. Thermal, electrical, mechanical, and environmental stresses reinforce each other under high temperatures. For example, microcracks generated by thermal aging can reduce the partial discharge inception voltage, accelerating electrical aging. Meanwhile, gases generated by electrical aging exacerbate thermal expansion, increasing mechanical stress. This multi-factor interaction causes insulating materials to age at high temperatures much faster than the combined effects of any single factor, ultimately resulting in a significant reduction in the service life of power capacitors.