The power system in Israel operates in a sub-tropical as well as desert environment. A long dry season lasts some eight months during which all types of contaminants – from marine to dust to industrial and agricultural – accumulate on insulators. This pollution layer tends to be adhesive and is therefore only partially removed during the four months of rain that follow.
Responding to unacceptably high levels of outages triggered by pollution flashovers during the early to mid 1990s, Israel Electric (IECo) became among the first utilities in the Middle East to research and eventually embrace widespread application of composite line insulators. Today, these represent a large and still growing proportion of the insulators on the overhead network, from 22 kV to 400 kV.
Until now, however, there has been hardly any use of composite insulator housings for apparatus such as bushings and transformers. Rather, the preferred countermeasures at substations affected by pollution have included washing and, more recently, application of RTV silicone coatings.
In 2014, INMR visited the 170 kV Alon Tabor Substation, in the northern interior, where localized pollution from near-by cooling towers presents a constant challenge. In 2000, this became site of IECo’s first application of silicone-housed GIS bushings. But what makes the installation even more noteworthy is the fact that, as with all porcelain at the substation, these composite insulators have recently also been coated with RTV silicone.
As Manager of Substations for Israel’s Northern Region, Amir Rozenstein is responsible for ensuring the reliable operation of a broad range of HV apparatus. “We have quite a mix of equipment at our substations and from many different suppliers,” he says. “That makes it all the more challenging for us to be able to identify and respond to any developing problem before it becomes critical and leads to costly failure.”
In order to assist maintenance staff to monitor the condition of equipment, IECo relies on teams of predicative maintenance specialists (SPDMs) who conduct periodic substation inspections using a variety of diagnostics, including thermal, acoustic, ultraviolet and visual. Findings of any incipient problem are then communicated to Rozenstein who must schedule the appropriate remedial action in a timely manner.
Rozenstein explains that like most power utilities these days, IECo is focused on reducing expenses such as washing in spite of operating in a warm coastal environment marked by plenty of dust. For example, he says that much of the country’s 400 kV system needs to be washed twice a year and, apart from the direct cost, lines and stations typically have to be taken out of service in the process since the water used is not demineralized
One of the strategies to optimize maintenance costs has been to prioritize which lines are in greatest need of washing and at what specific intervals. This task has been accomplished in co-operation with the Transmission Team, based largely on helicopter inspections using infrared and ultraviolet cameras. At the same time, starting from the early 2000s, IECo made a strategic decision to adopt composite designs as the standard for most new insulator purchases since these were expected to require little to no washing.
At substations, dealing with pollution has also shifted progressively away from scheduled washing. However, unlike the case for overhead lines, the transition has been almost entirely toward coatings applied to porcelain insulators rather than use of composite housings. Rozenstein explains that this process first began more than 20 years ago with silicone grease but has since focused on RTV material due to problems linked to having to change the grease about every 3 years.
The decision to rely heavily on RTV silicone, notes Rozenstein, was based largely on tests conducted at one of the network’s most heavily polluted sites – an open air substation located less than 200 meters from a factory for alkali salts on the Dead Sea (at -427 m also the lowest point on earth). Here, RTV coatings were found to remain effective in suppressing leakage current even after 6 years of operation in one of the most severe service environments imaginable.
Arie Avner, from IECo’s Maintenance Dept., reports that coating porcelain insulators at substations affected by pollution has been ongoing now for over 10 years and indeed is still underway. “Experience overall has been positive,” he says, “and we have found that coatings solved a lot of the problems we had 15 to 20 years ago.”
Still, the performance of coatings is being monitored closely. In the northern region, for example, Avner explains that inspectors are sent to coated substations at least twice a year – usually before summer and again before winter – at a time of day when humidity levels are high. Their task is to verify coating performance by monitoring partial discharge activity both acoustically and visually. In cases where the coatings are found insufficient to deal with the level of accumulated pollution, washing is conducted at low pressure so as to remove surface dust but not risk causing permanent damage to the coatings.