Transformer insulation structure for dielectric liquids with higher permittivity

Insulation structures based on cellulosic materials and mineral oil have been the standard solution for transformers for the last hundred years, both in distribution and power transformers. These structures typically consist of a solid-liquid arrangement, aimed to divide larger voltage drops in smaller steps, creating small volumes of liquid with high dielectric insulating performance and reaching the most compact design. The total potential difference is borne by sequential voltage drops defined according to the geometry and the permittivity of the materials. Standard arrangements and configurations have been developed by transformer manufacturers according to the voltage levels, as the analytical solution of the distribution of the electrical field may be extremely complex. Even nowadays, where numerical calculation models, mostly based on the Finite Elements Method, are available, the use of standardized solutions is not uncommon, as the detailed calculation may be highly time consuming. When the dielectric liquid is changed, replacing the traditional mineral oil by natural ester liquid, the distribution of stress in the insulation system is no longer the same. The permittivity of natural ester liquid is around 40% higher than that of mineral oil, while the permittivity of pressboard and paper made from kraft pulp, impregnated with natural ester liquids is just slightly higher. The result is a different distribution of voltage drops in the insulation system, which must be taken into consideration when designing a natural ester filled transformer. At first glance, the effect is positive, since the critical parameter is typically the stress, the field strength, in the liquid insulation and the higher permittivity shifts the stress from the liquid into the solid insulation. This modification of electrical field distribution may allow, in some cases, increasing the average field strength and thus a more compact design, as there is a better balance between the stress in the solid and liquid portions. However, the modification of the electrical field distribution may lead also to new critical regions. Corners and labyrinth constructions may have higher superficial dielectric field gradients. Additional edge protection may be required in regions where they were not required with mineral oil, as the stress in the paper will be increased. Less specialized engineers tend to take the breakdown voltage as the only relevant parameter for the insulation design. A large quantity of test data has indicated that the breakdown voltage of natural ester liquid, excluding the undesirable highly divergent field (needle to sphere), is similar to mineral oil. But this does not mean that adjustments in the insulation system design would not be required. This paper will present some common configurations, potential optimizations and areas requiring attention for designing insulation systems using cellulose and natural ester liquids.