Alternative Liquid Dielectrics for High Voltage Transformer Insulation Systems. Группа авторов

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2 Processing and Evaluation of Natural Esters

       Niharika Baruah1, Rohith Sangineni1, Mrutyunjay Maharana2, and Sisir Kumar Nayak1

       1 Indian Institute of Technology, Guwahati, Assam, India

       2 Xi’an Jiaotong University, Shaanxi, China

2.1 Introduction

      Transformers are the critical elements of the electrical grid and their effective operation is vital to the health of the whole network. It is of utmost importance to sustain the continuity of supply and prevent financial losses. The mineral oil (MO) is most generally used in the transformer as the dielectric fluid alongside the solid insulation like the pressboards and kraft papers. The development of environment friendly vegetable oils (VOs) or natural ester oils (NEOs) for transformers in place of MO gives us an alternative insulation system. MO is procured from the petroleum‐based fossil fuels and will deplete in the long run. Also, the biodegradability of MO is poor and hence causes environmental concern if spillage occurs. Alternatively, NEOs are achieving prominence because of their environment friendly nature, better biodegradability, higher fire and flash points. NEOs also have thermal conductivity better than MO; however, the viscosity is higher than MO, which is a disadvantage as it prevents flowability of the oil.

      NEOs are derived from plant sources and there is a variety of them available in the market. Some of them are the linseed oil, castor oil, corn oil, Pongamia pinnata oil (karanji oil), soybean oil, punna oil, neem oil, Jatropha Curcas oil, palm oil, sunflower oil, olive oil, peanut oil, cottonseed oil, rapeseed oil (canola oil), coconut oil, hazelnut oil, and mustard oil [1–8]. The experimentation with NEOs for insulation purposes dates back to the twentieth century as these oils are readily biodegradable, have higher fire points, and are sustainable. The NEO also has higher water absorption capacity than the MO. A little concern over the use of NEOs made from edible oils like peanut and sunflower is that abundant use of NEOs can lead to a food crisis; therefore, these oils should preferably be made from nonedible seeds [9].

      The crude oil, when extracted from the seeds, is highly acidic and viscous in nature. The acidic nature and viscosity can be reduced by following the transesterification process of the NEOs [1, 2]. However, the transesterification process reduces the flash point of the NEO. To overcome the reduction in flash points, the NEO is blended with its transesterification‐processed oil. The blended oil has low viscosity, higher flash point, and higher impulse breakdown voltage (BDV) [2]. The proper blending of NEOs with MO can also be done for lowering the pour point at the same biodegradability [10].

A typical fatty acid composition of few NEOs is tabulated in Table 2.1. The unsaturated fatty acids are at risk of oxidation. Thus, the oxidation stability is lower for NEOs as compared with MO. Though these unsaturated fatty acids help in absorbing the gases released, the oxidation stability needs to be addressed. Another major drawback of the NEOs is the high values of pour point. The high pour points and high viscosity of VO are due to the presence of the fatty acids [14]. The viscosity, however, decreases to a certain extent with increase in temperature in a running transformer. The problem of high pour point and low oxidation stability can be dealt by using additives. Some of the additives used as antioxidants are tert‐butyl hydroxy quinone (TBHQ) and butylated hydroxytoulene (BHT). One of the additives used to lower the pour point is polyalkyl methacrylate [8]. The other disadvantage of the NEO is poor dissipation factor. NEOs are proven to be useful only in high‐voltage transformers and capacitors [1, 15]. The NEOs may be best utilized in power network with transformers in the coastal areas, where the oil spillage may result in serious threat to coral life and the aquatic life.

The different kinds of studies like electrical, chemical, and physical analyses of the oil are performed to estimate the life of the insulation. The aging study of oil can give information on the cellulose aging, low‐level partial discharge (PD), contamination, static electrification, circulating currents, and faults in the transformer. Aging and degradation studies are conducted to assess the gases, furans, phenols, cresols, and metal particles that are formed in the oil during the life of the insulation [11]. The acidity in MO increases with oxidation, whereas the acid generation in NEOs is dominated by the hydrolysis phenomenon. The NEOs are preferred to be used in sealed transformers due to the drawbacks of oxidation and hydrolysis [16]. It is also observed that the paper insulation immersed in NEOs have longer life than the paper immersed in MO [15]. The degree of polymerization (DP) of paper is lower for MO immersed paper insulation with aging compared with NEO immersed paper insulation. This is due to the higher moisture retention capability of the NEO [17]. The BDV is also observed to be higher in NEOs as compared with the MO.

Table 2.1 Typical fatty acid composition of some vegetable oils.

      Source: Based on Beltrán et al. [8]; Oommen [11]; Dung and Huong [12]; Kumar et al. [13].