CASE STUDY: MINERAL OILS
This article describes the test methodology and discusses the study results that may be of interest to transformer manufacturers and operators when selecting insulating materials or qualification processes.
A study was conducted to assess the compatibility between each of four leading transformer oil products and each of eight common transformer component materials. The sample of four transformer oil products tested included two primarily naphthenic oils and two isoparaffinic-based transformer oils. Results of the test were twofold. First, both isoparaffinic-based transformer oils demonstrated compatibility across a wider set of transformer component materials than either of the tested naphthenic oils. Second, two commonly used materials in transformer components, neoprene and nitrile, demonstrated poor compatibility across all four of the tested transformer oils. This article describes the test methodology and discusses the study results that may be of interest to manufacturers and operators of transformers.
Introduction and Scope
Transformers include many types of insulation materials in addition to the oil, each material critical to the system performance. Solid insulation materials like rubber seals/gaskets or elastomers are frequently examined for compatibility. Separate from these, resin, laminate, silicon, paper and others may also be included.
Materials compatibility testing is essential for the understanding of long-term performance of transformer oil design. One well known issue in transformer operation is oil leakage from gaskets. In fact, seal gaskets have the potential to become a critical root cause of failure in transformers. CIGRE’s working group conducted a survey and found that 13% of power transformer failures were due to leakage [1]. According to a Renewable and Sustainable Energy Review [2], damaged seals and oil leakages account for 32% of outages. Another study on failures in power system transformers concluded that the most common cause of failure in 20-400 MVA transformers was general ageing and insulation deterioration [3].
Compatibility testing between the transformer oil and other solid insulation materials in the transformer is essential for the understanding of long-term performance of transformer oil design.
Therefore, understanding compatibility is necessary to ensure safe operation. The less influence the oil has on the gasket materials, the less deformation and deterioration of the gasket will occur. Not only is the choice of a high-quality transformer oil important in this equation, ensuring the use of high-quality materials is also necessary for safe and long lasting transformer design. It is known that rubber gaskets can leach particles into the oil due to the solvency nature of the oil. Not only will this degrade the quality of the gasket, but it can also reduce the electrical insulating properties of the oil and impact the overall insulation performance of the system. It is evident from the study outlined here that the quality of the gasket material plays a key role in achieving compatibility with mineral oil.
Significance and Use
The compatibility between the transformer oil and other solid insulation materials in the transformer is important for long term reliability. The magnitude of changes in the electrical, physical, and chemical properties of the oil after immersion of the solid materials are important for determining the presence and extent of contamination and therefore the suitability of the oil and materials in that system. The test method utilized may also help confirm the quality of the solid materials since both high quality and commercial grade options are available for use in transformers. In the case of power transformers where longevity and long-term performance are important, it is important to use high quality grades, especially for any elastomeric components.
Standard and Experiment
The standard method ASTM D3455 Compatibility of Construction Material with Electrical Insulating Oil of Petroleum Origin was used. The method is quite involved in terms of preparation, execution, and it requires accurate measurement of the oil properties in the analysis. In the method, each oil sample receives an oven-treated test specimen (solid components) immersed in it and together each sample is treated in an oven at 100°C for a duration of 164 hours. After this period, the sample is removed, allowed to cool, and the test specimen is removed. The oil is then tested for select physical, electrical, and chemical properties to detect and quantify the resulting contamination that may come from immersion at high temperature. At this point, the oil and solid material can be evaluated for suitability in the transformer design. Table below lists the test specimens chosen for the test.
The quality of the gasket material plays a key role in achieving compatibility with mineral oil.
Table 1. Solid test specimens for compatibility testing
The method itself provides guidance for (1) the types of materials typically analyzed, (2) a specified solid quantity to oil volume ratio, and (3) the analytical methods recommended to assess oil quality after the immersion period. Though specifications for the oil tests are recommended, they do not define the compatibility of a system per se because values for these limits were derived from typical values of most mineral oils available on the market at the time the method was finalized. For this reason, it is often helpful to do comparative testing, e.g. testing multiple oil products or testing alongside a known baseline.
The Transformer Oils Being Compared
Figure 1 shows the composition of the four distinct transformer oil products chosen for this study. Two typical transformer oils containing high naphthenic carbon content were compared with two oils (Univolt™ and Univolt Plus transformer oil) containing higher isoparaffin content and lower naphthenic carbon content. While containing both naphthenes and isoparaffins, these two oils will be referred to as “isoparaffinic oils”.
Results
The 2021 analysis by Doble Engineering shows high variation in the compatibility results for neoprene, nitrile, nylon and less variation for copper, paper, and laminate. In fact, all oils are determined to be compatible with laminate, copper, silicon steel and paper. Therefore, the focus of the discussion is on the more susceptible materials: neoprene, nitrile, nylon and copper with resin. Table 2 shows the final compatibility determination for each oil.
Figure 1. Percentage of aromatic carbon content (Ca), naphthenic carbon content (Cn), and isoparaffinic content (Cp), based on ASTM D2140. Naphthenic Oils 1, 2: Doble TOPS Survey 111. Univolt, Univolt Plus TO: Doble TOPS Survey 112
Table 2. Compatibility results of ASTM D3455. NC = not compatible, C = compatible
Figures 2-9 show the test results of the oil properties compared for a single material. Figures 2-5 focus on the measured Power Factor at 100°C (PF100) of the oil after immersion. Figures 6-9 show the measured Interfacial Tension (IFT) after immersion.
Figures 2-5. Power Factor at 100°C (PF100) of the oil after immersion
Figures 6-9. The measured Interfacial Tension (IFT) after immersion
The isoparaffinic oils exhibited a lower PF100 for all materials and maintained a higher IFT, especially for the most susceptible elastomer materials like neoprene and nitrile. For Figures 2-9 above, the isoparaffinic grades have better demonstrated performance.
The study informed that both grades of neoprene and nitrile were actually incompatible with all the oils. This result was not unexpected considering neoprene is known to show poor compatibility with mineral oils. While this study shows it is not advisable to use neoprene or nitrile in transformer construction, higher quality of grades of these types of materials should be available which may show better compatibility with mineral oil than the commercial grades have. For example, nitrile with a high concentration of acrylonitrile would be a better choice.
OEMs and utilities that are concerned about the longevity of their transformers should consider using an isoparaffinic-based transformer oil to maximize material compatibility.