Classification And Requirements Of Power Transformer Oil Type
The insulation system of oil-immersed power transformers is a cornerstone of their safe and reliable operation, serving as a critical barrier against electrical breakdown. This system is categorized into two primary types: internal insulation and external insulation, each playing distinct roles in safeguarding the transformer’s core components.
Internal insulation, which protects the winding assembly and internal conductive parts, is further split into main insulation and longitudinal insulation. Main insulation refers to the insulating layers between windings and grounded structures, as well as between individual windings themselves. In modern oil-immersed transformers, the oil-paper barrier insulation configuration is widely adopted for main insulation due to its excellent dielectric properties and thermal stability. Main insulation is additionally classified as graded insulation or full insulation. Graded insulation is designed such that the insulation level near the transformer’s neutral point is lower than that at the winding ends—an optimization tailored to the reduced voltage stress experienced at the neutral side. In contrast, full insulation features uniform insulation strength across both the head and tail sections of the windings, making it suitable for applications where consistent voltage distribution is required.
Longitudinal insulation, on the other hand, addresses insulation needs within the same winding. This includes insulation between different turns of the coil, between winding layers, and between segments of the same winding. Its primary function is to prevent electrical arcing between adjacent conductive parts, ensuring the winding operates as a cohesive, insulated unit even under fluctuating load conditions.
Key Performance Requirements for Insulation Systems
Oil-immersed power transformer insulation must meet rigorous performance standards to withstand the demands of electrical grids. First and foremost, it must reliably endure both normal operating voltages and transient overvoltages—such as those caused by lightning strikes or grid switching—without dielectric breakdown. Equally important, the insulation system must tolerate normal operating currents, overcurrents from load fluctuations, and short-circuit currents, which generate significant thermal and mechanical stress.
Moisture resistance and aging stability are also critical attributes. Exposure to moisture can degrade insulation performance, leading to reduced dielectric strength and increased risk of failure. Similarly, long-term operation at elevated temperatures can cause insulation materials to age, compromising their structural and electrical integrity. A high-quality insulation system must resist these factors to ensure the transformer’s long-term reliability and minimize maintenance requirements.
Core Insulating Materials in Oil-Immersed Transformers
Oil-immersed power transformers rely on a suite of specialized insulating materials, each selected for its unique electrical, thermal, and mechanical properties. These materials work in tandem with transformer oil to form a robust insulation system:
- Transformer Oil: Acts as both an insulating medium and a heat transfer fluid, dissipating heat generated by the windings and core while preventing electrical arcing between components. Its high dielectric strength and low viscosity ensure optimal performance under operating conditions.
- Insulating Cardboard: Manufactured primarily from unbleached sulfate fibers, this material features a porous structure that enables superior air permeability, oil absorption, and water absorption. When impregnated with transformer oil, it forms a high-performance insulation layer. For applications requiring enhanced heat resistance, polyamide fiber-based cardboard is often used, significantly extending the transformer’s service life. Insulating cardboard is utilized in components such as insulation tubes, struts, spacers, partitions, and angle rings.
- Cable Paper: Produced from high-quality kraft pulp, cable paper is available in two common specifications for transformers: DL2-08 (0.08mm thickness) and DL2-12 (0.12mm thickness). Its primary applications include conductor surface coating insulation, coil interlayer insulation, and lead wire insulation—making it one of the most essential insulating materials in oil-immersed transformers.
- Telephone Paper: Crafted from kraft pulp, telephone paper is valued for its thin, uniform structure, making it ideal for coil conductor insulation and coil end insulation. It provides reliable dielectric protection in tight spaces within the transformer.
- Crepe Paper: A specialized insulation material derived from kraft pulp-based cable paper through additional processing, crepe paper exhibits exceptional electrical performance in oil, including high average breakdown voltage and low dielectric loss tangent. Its flexibility and oil compatibility make it perfect for wrapping transformer outlet terminals, ensuring secure insulation in high-voltage areas.
Challenges and R&D Directions for Oil-Paper Insulation
A key characteristic of oil-paper insulation systems is the significant difference in electric field strength between the oil gap and the cardboard. Specifically, the electric field intensity in the oil gap is considerably higher than that in the cardboard, making the oil gap the primary weak point in the insulation structure. This discrepancy arises due to the differing dielectric coefficients of the two materials—when these coefficients are mismatched, the oil gap bears a disproportionate share of the electrical stress, increasing the risk of breakdown.
To address this challenge and optimize insulation design, industry researchers are focusing on minimizing the dielectric coefficient gap between oil and cardboard. When the dielectric properties of these materials are more aligned, the electric field distribution becomes more uniform, reducing the strain on the oil gap. This not only enhances insulation reliability but also enables the miniaturization of the transformer’s insulation structure, contributing to more compact, efficient transformer designs. As a result, the development of advanced low-dielectric-constant cardboard materials has emerged as a key research and development priority, with the potential to revolutionize oil-immersed transformer insulation performance and design.