Just as the general society becomes more environmentally alert, the same goes for the fields of engineering and power distribution. This is notable when looking at the transformer oils studied in this report. Mineral oil is the oldest, most commonly used transformer oil. It is not good for the environment in the event of leakage.
However, it has good insulating capabilities when it comes to high resistivity and low tangent delta. Ester oil is an up-and-coming transformer oil as it originates from various types of vegetable oil it is both readily biodegradable and organic. In terms of resistivity this oil is lower compared to the mineral oil by around a factor 100. Isoparaffin oils, similarly to mineral oil, originates from crude oil. They are however refined to the point of reducing the negative environmental aspects while keeping the high insulating capabilities.
These insulating capabilities are shown in table9.2. Isoparaffin A is not classified as readily biodegradable.
However, as mentioned in "Types of transformer oil", chapter4, isoparaffin A is close to or has achieved the criteria required for the classification according to the description of the refining process. Isoparaffin A has a resistivity that is slightly lower compared to the mineral oil. A step in the right direction when trying to create a transformer oil that both has the environmental aspects of ester oil and the insulating properties of mineral oil. The newest oil, isoparaffin B, which became commercially available late 2019, is both classified as readily biodegradable and has a resistivity on par with the mineral oil. Indicating a positive trend in the field of transformer oil.
10. Conclusion
Four different transformer oils, mineral, ester, isoparaffin A and isoparaffin B are investigated in this thesis. It is found that relative permittivity is relatively constant for all transformer oils. Bringing up the argument to reduce the required parameters for the ion drift model down from four to three. Low voltage is recommended when doing experiments, as the dielectric parameters should be measured at thermodynamic equilibrium.
The experiments in this thesis are all performed at a voltage under 5 V. A voltage of this magnitude should not affect the ohmic behaviour of the transformer oil.
The influence of moisture on the resistivity of mineral and ester oil are investigated. Ester oil has a lot higher moisture tolerance than mineral oil. The resistivity decreases almost linearly in ester oil, resistivity in mineral oil has a more complex relationship with moisture. To the extent where some small increment in moisture might be a good thing. Something that is not commonly recommended. This of course requires more research, especially when it comes to the affect said moisture would have on the other insulating material (pressboard) in the transformer.
This thesis compares two measuring techniques, IDA Diagnostic System and The Triangular Method. IDA is easy to use and can calculate various dielectric parameters while the triangular method is solely for resistivity measurements. While IDA is industrially used, it is also a complex system. The triangular method only requires a function generator, an electrometer and a computer containing Labview. It is found that by tweaking the triangular method algorithm slightly, the technique should yield resistivity measurements similar to IDA.
A new measuring cell is designed and built. This new cell removes some durability issues the original cell had. When doing a measurement that considers the resistance of the transformer oil in its calculation. It is found that the contact resistance between the transformer oil and the measuring cell significantly affects the overall result. It is therefore the recommendation to always consider the contact resistance before making any official statements regarding said measurements. A rough estimate can be achieved by placing the electrodes far apart, thus minimizing the total effect the contact resistance has on the measurement. This could require higher applied voltage as the measured current will decrease with higher resistance. Making it more noise sensitive.
It is found that the loss tangent (tan δ) and concentration of ions decreases with increasing resistivity for all four transformer oils. The relaxation time increases with increasing resistivity. Experiments done with mixtures of isoparaffin A & B and Mineral & Ester confirms that a linear relationship between resistivity and mixture percentage does not exists.
There is a positive trend in the field of insulating transformer oils as newer oils show both good dielectric properties while being classified as readily biodegradable. Table 10.1shows how the newly commercially available transformer oil isoparaffin B has resistivity on par with mineral oil while being biodegradable just as the organic ester oil. The resistivity values presented have not been adjusted with contact resistance as the contact resistance is only known for the mineral oil and isoparaffin oil A.
Transformer oil Resistivity [Ωm] Biodegradability
(Oldest)Mineral 9.2×1012 No
Ester 6.6×1010 Readily biodegradable
Isoparaffin A 1.2×1012 No*
(Newest)Isoparaffin B 8.6×1012 Readily biodegradable Table 10.1: Resistivity and biodegradability for the investigated oils.
*While not classified as readily biodegradable, according to the refining process, it has met the criteria. See chapter4.
11. Future work
In this chapter some interesting observations throughout the thesis that the author feels could require more research and some more practical, clearly defined, work are suggested. These ideas and suggestions can act as a basis for future master thesis works.
• Testing other properties when adding small amount of moisture (5-10 ppm) to mineral oil such as dielectric breakdown and time of deterioration of cellulose in pressboard.
• Implementing the new algorithm to the Labview script. That is, extracting the capacitive & resistive currents from the existing program and correcting the measured resistivity according to equation 9.9.
• Since both Isoparaffin B and ester oil are readily biodegradable. Mixing these because of the big difference in resistivity, could yield interesting, cost-effective transformer oil.
• More investigation into what causes the different electrode material to yield various resistivity measure-ments. Can be done by using different oils with many small increments in distance between electrodes.
• Investigating the electrochemistry of the various transformer oils and the electrode material.
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