Types and Functions of Transformer Insulating Oil

A power transformer is an electrical device used for energy conversion, capable of transforming alternating current (AC) electricity of one voltage and current into another voltage and current of the same frequency. Transformer oil is a specialized liquid medium designed for insulating and cooling transformers. It possesses excellent insulating properties, thermal stability, arc-quenching capability, and oxidation resistance. It can withstand the effects of high-voltage electric fields over extended periods, effectively preventing internal transformer faults such as arcing, discharge, and explosions, thereby ensuring the normal operation of transformers. Based on the source of the base oil, transformer oil can be categorized into four main types: mineral oil transformer oil, vegetable oil transformer oil, silicone oil transformer oil, and synthetic ester transformer oil.

Mineral oil transformer oil has been used in oil-Immersed Transformers for nearly a century due to its excellent electrical and thermal properties. To this day, it remains the ideal transformer oil for ultra-high-voltage and extra-high-voltage power transformers. Mineral transformer oil is primarily derived from naphthenic-base oils and paraffinic-base oils. From a molecular structure perspective, it consists of straight-chain alkanes, isoalkanes, naphthenes, and aromatic hydrocarbons, each playing distinct roles in transformer oil. Straight-chain alkanes have high freezing points and tend to generate hydrogen under high-voltage fields. Considering both economic and performance factors, straight-chain alkanes are generally not directly used in transformer oil production. Isoalkanes have flash points above 170°C, low freezing points, low acid values, good oxidation stability, and moderate gassing properties, making them suitable for transformer oil. Naphthenes exhibit low freezing points and relatively low flash points, with moderate gassing properties, meeting the electrical performance requirements of transformers. Monoaromatic hydrocarbons among aromatic hydrocarbons offer good gassing properties and can be used in ultra-high-voltage transformer oil. Polycyclic and condensed aromatic hydrocarbons have poor oxidation stability and pose health risks, so they should be removed from transformer oil. Therefore, the ideal components of mineral oil transformer oil are isoalkanes, naphthenes, and small amounts of monoaromatic hydrocarbons.

Vegetable oil transformer oil is produced from natural oil crops through pressing, refining, and modification. It offers advantages such as high flash points, good electrical performance, abundant raw material sources, renewability, and biodegradability. The main components of vegetable oil are triglycerides, and the types and proportions of fatty acids vary significantly among different vegetable oils. Balancing the oxidation resistance and low-temperature fluidity of transformer oil, vegetable oils with higher monounsaturated fatty acid content are the optimal choice for producing vegetable oil transformer oil. Although vegetable oil transformer oil has certain advantages over mineral oil transformer oil, it still faces challenges. These include higher costs—approximately three to four times that of mineral oil—higher viscosity, susceptibility to hydrolysis producing acidic substances, poor low-temperature performance, oxidation and gelation issues, inconsistent oil performance standards, limited maintenance experience, and incomplete processes. These factors constrain the promotion and application of vegetable oil transformer oil.

Silicone oil transformer oil offers not only excellent electrical insulation properties but also advantages such as high flash points, low pour points, and minimal viscosity variation with temperature. It is often used as a substitute for traditional transformer oil in small-capacity transformers below 35 kV. However, silicone oil transformer oil is sensitive to decomposition products from partial discharge or breakdown, has relatively low stability against electrical sparks, and experiences declining insulation stability under repeated electrical stress. Therefore, silicone oil transformer oil is typically used in transformers below 35 kV, and operating temperatures should not be excessively high. It is worth noting that silicone oil gained popularity in the last century due to its superior fire safety performance. However, due to its environmental and cost disadvantages, it has not been widely adopted. Currently, the use of silicone oil is declining globally and is only continued in certain countries or regions (such as the United States) and specific industries (such as tunneling machine transformers).

Synthetic ester transformer oil primarily uses polyol ester base oil. Its most prominent feature is excellent biodegradability, allowing it to be broken down by active microorganisms (bacteria) into carbon dioxide and water within a short period, posing no harm to the environment. This makes it particularly suitable for applications with high environmental requirements, such as offshore wind farms. Synthetic ester transformer oil offers comprehensive performance characteristics, including good fire safety performance, electrical insulation properties, oxidation resistance, and aging resistance, in addition to its biodegradability. Moreover, synthetic ester transformer oil exhibits strong polarity and high affinity for water molecules, maintaining high breakdown voltage even under high moisture content. Its moisture resistance surpasses that of both mineral oil transformer oil and silicone oil transformer oil.