As power demand rises over the years, power systems, too, have to keep pace.
An important part of the power system is the transformer. Heat is generated by the energy losses in the transformer core, as well as in the current-carrying windings. The insulating oil used in the transformer acts as a coolant. If the oil does not provide proper insulation, it can lead to the failure of the transformer and a power shutdown. A temperature rise to 200 degrees C in the transformer could lead to a catastrophic fire breakout. Traditional transformer oils are also not biodegradable.
Ester oils have been gradually replacing traditional oils in transformers as they exhibit better insulation characteristics and are also biodegradable. Ester oils have a high fire point — that is, they can handle temperatures as high as 300 degrees C. However, as the size of the transformer increases, the amount of oil required also increases. Keeping large volumes of oil pure becomes a challenge. Even a thin fibre or conducting material contaminating the oil can lead to major problems over time, says Prof R Sarathi, who is with the High Voltage Division of IIT-Madras’s Dept of Electrical Engineering.
He and his team of researchers, including members from the KTH Royal Institute of Technology, Sweden, explored the potential application of nanofillers to improve the ‘dielectric performance’ of synthetic ester oils. (Dielectric materials, while not exactly insulators, can be made to function as insulation. A material is said to have a high dielectric strength if it does not allow current to pass through — in other words, it acts as good as an insulator.)
The project was financially supported through the ‘Nano Mission’ of the Department of Science and Technology, Government of India.
The team introduced nanofillers into ester oil. As desired, the introduction did not change the basic electrical or thermal properties of the oil; and the fillers were also able to enhance the required insulation.
Explains Prof Sarathi: “Nanoparticles in a liquid can provide higher withstanding capability in terms of electrical, thermal and mechanical properties. So, when a local electrical discharge occurs inside the transformer, nanoparticles act as a barrier. They will not allow the discharge to propagate further, even at higher voltages.” Nanoparticles also have a high surface area, as a result of which heat too gets dissipated quickly.
The team chose silica as the material for the nanofiller. Silica not only has good dielectric properties but is also cost-effective.
However, nanofillers need to be dispersed uniformly to avoid clustering in the fluid to achieve the desired insulation . For this, the team experimented with multiple surfactants (which are chemicals like detergents that increase the ‘wetting’ property of a liquid) to ensure uniform dispersion. Between CTAB, oleic acid and SPAN 80, the last was found most suitable.
The team also found that the ‘rheological’ properties were ideal for their objective.
“The idea was to use material that enhances the efficiency of the system without compromising on the electrical and thermal properties,” says Prof Ramesh Gardas of the Department of Chemistry, IIT-M.
The combination was evaluated on three fronts: Did their use lead to other problems, such as increase in viscosity or tendency to evaporate at the operating temperature? Did nanofillers have a large surface area, which makes it easier to exercise control over the system’s heat absorption and to stop further discharge propagation? With the choice of surfactants, did the nanofillers cluster together?
Rheological studies (on the flow and deformation of material under stress or strain) were conducted to check how the electrical and thermal properties of the system changed on the application of stresses such as temperature and voltage. The objective was to choose material that did not undergo permanent changes. The combination of silica and SPAN 80 showed resilience.
“Dispersion of the nanofiller is important. The surfactant helps to disperse the silica evenly and acts as a pillar, lending stability to the nanofiller, so that it stays in position in the base liquid,” says Prof Sarathi, adding that the oil the team designed passed muster.
How exactly does this combination lead to savings for manufacturers, in terms of design? The selected nano fluid could potentially help reduce the volume of oil required for insulation. As a result, the size of the transformer could also reduce by nearly 25 per cent.
The transformer includes optical fibre as part of the design. The researchers have received sponsorship from Easun MR Tap Changers, Chennai.