The isolated power system on the Greek island of Crete – formerly managed by the Power Public Corporation (PPC) and since 2012 by the Hellenic Electricity Distribution Network Operator (HEDNO) – suffers from rapid pollution accumulation during the annual prolonged period without rain. The situation is made worse by persistent northwesterly winds that deposit salt spray onto the network and serve as the primary catalyst for flashover as well as corrosion. A variety of countermeasures have traditionally been employed to cope with these problems, including washing, application of RTV silicone coatings and selective application of silicone rubber insulators on the most affected lines. Based on the experience of the last few years, these solutions have begun to yield tangible benefits in reducing flashover outages at reasonable life-cycle costs. The most visible recent achievement in regard to evaluating the performance of alternative insulation technologies for the Cretan service environment has been a HV insulator test station located alongside a large coastal power generating facility.
INMR Contributor, Raouf Znaidi travelled to Crete in 2014 to meet with HEDNO engineers and reported on their efforts to combat pollution flashover.
The HEDNO power system on Crete is the largest fully autonomous system in Greece, comprised of some 585 km of 150 kV lines as well as 18 substations. Most of this system runs close to the coast due to the geography of the island, which is 260 km long but never more than 60 km wide. The annual dry season starts in April and lasts until the end of October. Over these months, there is steady accumulation of pollution – mostly salt mixed with industrial and agricultural pollutants – that deposit onto insulators and become wetted by high humidity, dew and salt spray from the surrounding Mediterranean. The combination of contamination build-up and rapid onset of wetting often result in pollution flashovers on the most exposed lines. For example, some of the worst years, such as 1985, saw up to 25 outages recorded per 100 km/year – a level that dealt a serious blow to the island’s important tourist industry. Table 1 shows the number of outages on transmission lines and highlights the especially severe pollution affecting northern Crete, where one key line – Linoperamata to Agios Nikolaos to Ierapetra – alone experienced a total of 117 flashover outages since 1994.
HEDNO consultant, Dionisios Pylarinos, investigated this problem for years and reported that the unusually high level of outages on the line running from Linoperamata to Ierapetra is due to severe pollution along the route made worse by the fact that it is also the island’s longest double circuit 150 kV line. Said Pylarinos, “the eastern portion of Crete is more affected by flashover outages due to the impact of salt spray depositing on insulation and this line runs especially close to the sea”.
Another problem related to the continuous salt spray as well as frequent wetting from high summertime humidity is corrosion that affects a range of vulnerable components and structures – from cap & pin insulators to transformer enclosures. These require HEDNO to implement a costly maintenance program where external contractors are brought in to treat HV transformers and other substation apparatus with special anti-corrosion coatings.
Historical Solutions to Reduce Pollution Flashover
Emmanuel Thalassinakis has been Assistant Director at HEDNO’s Islands Network Operation Department and this role has put him in charge of the power system not only on Crete but also on the island of Rhodes to the north. Referring to the long history of flashover problems dating back to the 1980s, he recalled that transmission lines in those days were insulated using standard anti-fog cap & pin glass or porcelain strings. The poor performance of these insulators, however, eventually made Crete one of the first places in Europe to experiment with first generation composite insulators with Teflon housings. Lines in Crete were also among the first to experience the phenomenon of brittle fracture, subsequently traced to poor design of wedge type end fittings on these first generation composite insulators.
Thalassinakis explained that coping with the severe maritime pollution affecting Crete’s lines and substations has been accomplished in a step-by-step process involving applying different solutions as well as predictive countermeasures. For example, costly dead washing of insulators during the 1970s was eventually replaced with live washing by ground crews (circa 1985) and then by more efficient washing using helicopters (1995). Another countermeasure was to increase specific creepage distance of insulators from 32 to 39 mm/kV.
More recently, HEDNO has been implementing an external insulation strategy based on assessing the comparative performance of different RTV silicone coatings as well as silicone composite insulators. For example, three different RTV material formulations have now been evaluated over a decade at the large Linoperamata Substation as well as on problematic lines insulated with traditional cap & pin glass or porcelain insulators. At the same time, HEDNO has come to rely more on composite line insulators to reduce pollution flashovers and also maintenance costs. “We have been working since 2004,” reported Thalassinakis, “on a strategic program to selectively replace glass and porcelain string insulators with silicone composite types”.