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

Figure 2.1 (a) Pongamia seeds, (b) Jatropha seeds.

      2.2.5 Rapeseed Oil (Canola Oil)

The rapeseed oil or canola oil is extracted from the crude rapeseed oil after three refinement procedures, which are alkaline refinement, vacuum distillation, and bleaching [45, 46]. The basic attributes of the refined oil are shown in Table 2.2. The refined form of rapeseed oil exhibits lighter color, lower acid value, and enhanced electrical properties when compared with the crude form of the oil. The flash point of the rapeseed oil is above 300 °C, which is nearly twofold the flash point value of the conventional MO used in transformers. The dipolar characteristic of ester molecules produces higher values of dielectric permittivity (ε_r), and dielectric dissipation factor (tan δ ) of the insulating rapeseed oil than transformer MO.

2.3 Processing and Pretreatment

The potential methyl ester‐rich biological seeds such as Pongamia pinnata, Jatropha Curcas, etc., are the primary feedstock for the preparation of natural ester. Therefore, before processing the seeds for the extraction of the crude oil, pretreatment is required. This process involves a series of steps mentioned in Figure 2.2.

      2.3.1 Extraction of Oil

From Figure 2.2, it is seen that the biological seeds are collected when they mature. Thereafter, the thick cover on the oil nut is removed by peeling. Once the seeds are peeled off, they are allowed to undergo heat treatment for drying. The drying process is of two types; one is the natural method of drying by exposing the seeds to sun light, and another is the laboratory method using the heating ovens. After that, the treated seeds are dried completely, and removal of dead seeds and solid impurities are carried out through separation technique. Once the separation is over, the seeds are now ready for extraction of crude oil.

Table 2.2 Basic properties of NEO [2,6–8, 13, 21, 26].

      Source: Based on Rakesh and Thomas [2]; Wilhelm et al. [6]; Kumar et al. [13]; Maharana et al. [26].

Properties Oils	Kinematic viscosity at 40 °C (cSt)	Flash point (°C)	Acid value (mg KOH/g)	AC BDV (kV)	Pour point (°C)
Jatropha oil	33.8	260	0.06	73	−3
Crude Pongamia oil	32	250	1.3	87.5	−25
Pongamia oil Methyl Ester	12	284	0.9	82	−1
Rapeseed oil	43	325	0.03	73	−18
Biovolt A (mineraltec)	36.1	312	0.05	55	−21
Soya oil	33.1	318	0.08	42	−12
Rice oil	32.4	318	0.04	37	−15
Sunflower oil	38	312	0.06	37	−6
Coconut oil	25.7	298	0.005 – 0.05	35	23
Neem oil	108.79	288	—	43	—
Mustard oil	95.74	289	—	41.6	—
Punna oil	129.66	280	—	42.3	—
Castor oil	155.71	280	—	31	—
Palm oil	5.06	186	0.005	81	−32.5
Mineral oil	8.13	146	0.01	35	−40

Figure 2.2 Processing of natural esters from seeds.

      2.3.1.1 Mechanical Extraction

Mechanical extraction is a conventional yet widely used crude oil extraction process followed for biological oil production. This method of extraction is suitable for industrial and large‐scale crude oil production. The treated seeds are permitted to flow into the mechanical extruder and allowed to crush the seeds through a twin screw extruder as seen in Figure 2.3a. In this process, the crude oil is collected at the bottom of the extruder whereas the by‐products are separated out.

      2.3.1.2 Chemical Extraction

Initially,