Photo: Siddharth Kankaria / Research Matters.
Researchers at the Indian Institute of Science, Bangalore have studied the behaviour of insulators that protect overhead power transmission conductors, in the presence of fog and mist. Polymeric insulators, which are made of various kinds of rubber, have been used in recent times because they are light weight, require lesser maintenance, and resist wetting of surface. However, they are also more sensitive to certain environmental and physical conditions. This study, led by Prof. B. Subba Reddy from the High Voltage Laboratory, Department of Electrical Engineering, has shown some light on the effect of environmental factors like fog and mist on these insulators.
Corona discharge is a common phenomenon that occurs when the electric field around a conductor is intense enough to cause ionisation of the surrounding air and form a localised conductive region. Often manifesting as a purple glow, it not only leads to power loss but also degrades the insulator in contact. Also, the effect of this corona discharge on the insulator varies depending upon weather conditions. While previous studies have shown the effect of corona discharge in dry air conditions, this is the first study that highlights the effects under foul weather.
“In the service conditions, the corona discharge activities will be intense under mist or fog condition”, explains Prof. Reddy. “The present study aims to understand this cumulative effect of mist or fog with corona particularly on polymerictransmission line insulators”, he adds. The study was also extended to acidic fog conditions that are seen due to increased acidity of rain in polluted or industrialised areas.
The researchers had to set up a suitable experimental facility for the study. A corona chamber was fabricated with separate inlets for injecting natural air and normal/acidic fog. Electrical activity was generated using electrodes and the effects of corona, thus generated, on insulator samples were recorded. The samples were further analysed for changes in surface properties.
The researchers observed that the effects of corona discharge under fog conditions were rather different from normal air conditions. “We found that the corona activity was intensely concentrated in fog conditions, which may have led to the local surface damages that were observed on the polymeric insulators”, Prof. Reddy remarks. “Study with acidic component in the fog has also shown a positive influence onthe degradation of the polymeric insulator”, he adds. The surface resistivity of the fog-treated samples was found to be drastically reduced, that could lead to surface conduction and flashover. Formation of nitric acid was also observed on these samples, which may cause fracture and failure of the insulator in the long run.
Resistance to corona discharge is a parameter that is yet to be standardised for polymeric insulators. This study gives valuable insights into the reasons for degradation and expected long-term performance of polymeric insulators in various environmental conditions. More importantly, the study has brought forth a viable method to judge a material’s performance in non-ideal conditions. “This method will depict the real service condition unlike the other methods proposed in the literature,” signs off Prof. Reddy.
About the authors and paper:
Dr. B. Subba Reddy is the Principal Research Scientist at the High Voltage Laboratory, Dept. of Electrical Engineering at IISc, Bangalore. D. Shakthi Prasad is a Ph.D. student in Dr. Reddy’s group. Web: http://hve.iisc.ernet.in/~reddy/index.html Contact: reddy@hve.iisc.ernet.in
The paper was published in IEEE Transactions on Dielectrics and Electrical Insulation.