According to records from an Iranian Utility, 70% of high voltage line faults are caused by inappropriate insulation. Also, a technical researcher from Virginia pointed out that as often we think of generators as a suspect to power failures, insulation failure would often be the cause and records are increasing. As one of the key factors in our electrical system, different maintenance methods were adapted to preserve the effectivity and functionality of electrical insulators. In order to evaluate which among these methods are highly effective and efficient, data were sourced out from different studies, documents and technical evaluation conducted by different organizations to come up with a comparison mainly focusing on the valuation per method.
Traditional Periodic Washing
This method used distilled water to wash high voltage insulators periodically. This method proved as effective as any method, however, it is discouraged by many for its time and material consumption. Time and interval of washing depends on the degree of contamination, actual site weather condition, and design of insulators.
Water washing from ground zero provides insufficient pressure to drive the cleaning at a typical high voltage tower height. Linemen needed to climb towers and wash using sprays under energized condition. This is easier but the safety considerations are exorbitant and adding up cost for special equipment and a high resistivity or low conductivity water (as per IEEE 957). Periodic washing could also be done under de-energized condition to exempt the clearance requirements of washing. However, cost of shutdown should be considered for the total maintenance cost. Washing period is typically more than one time annually.
Since the 1960’s, utilities have used "silicone grease" with good results to prevent the effects of pollution on the insulators surface. Its water repellent and arc track resistant surface, it encapsulates pollutants, has a good performance and lower lifetime costs than water washing. Greasing can be applied in two ways: manually or by spraying.
Silicone grease application depends on the severity of site pollution, from medium level to very heavy. An increase on the severity of pollution level will also increase the grease thickness and weight requirement as well as the overall maintenance costs. Compared to traditional washing, a correctly applied silicone grease would last for a year. Thus, silicone grease application is required only one time annually.
Room Temperature Vulcanizing Silicone Coating
RTV coating or high voltage insulator coating (HVIC), has been developed and applied rapidly as it suppresses the development of leakage currents due to a dynamic and interactive surface which retards water film formation and contamination solubility. Its hydrophobic surface prevents water filming, then the surface oil coats the contaminant particles and renders them hydrophobic. This reduces the contaminants’ ability to combine with water to form an ionic solution. The RTV coating maintains these features over the long life of the installation. They are the most competitive solution over long term, by providing arc track resistance additives, unparalleled UV resistance, Superior adhesion, wide temperature stability and hydrophobicity. Its long-term effectiveness affects significantly on lowering the total maintenance cost. Same with silicone greasing, insulator surface should be prepared before coating. Spraying is the most efficient method of RTV application. Another advantage to the overall cost is the ability to apply RTV coating under de-energized condition. Thus, power shutdown costs can be excluded on the computation for annual maintenance cost.
Different studies were conducted as to compare the equivalent costs per method introduced for insulator preservation. As to this document, we will be focusing on the comparison between traditional washing and RTV application only, since the cost for silicone greasing lies between the other two as mentioned on the above sections. Breakdown of the total maintenance costs were considered – Material and installation cost, power outage cost and annual leakage cost.
Material and installation cost can be easily be define by itself, this includes the mobilisation, equipment, manpower, man-hours and all factors needed to complete the activity. Power outage cost refers to the equivalent loss of income due to power outages, this include outages caused by flashovers, tripping and maintenance. Annual leakage cost is the total amount loss due to leakage current for uncoated insulators. An average of 2 mA leakage current per string can be obtained for an uncoated insulator.
Comparison table for the equivalent annual costs of traditional washing and HVIC application are shown below.
For ten (10) years, the total annual costs for both methods are presented below. Note that power outage cost for traditional washing starts after Year 4.
Material and installation cost of traditional washing is way much lower than HVIC application on its first year. However, HVIC does not need to be applied every year, collectively, the total cost incurred by the traditional washing considering all aspects will be 5 times larger than the HVIC application. By all means, HVIC application is the most cost-effective mitigation method.