Reliability and long time behaviour of MV-distribution switchgear in severe environmental conditions - examination of SF

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SF6 insulated RMUs from Siemens are designed and tested for operation in severe environmental conditions and have proven its reliability on thousands of units in service since 1983. International standards give only limited recommendations of how to test switchgears for special and severe environmental conditions and which risks have to be considered for an expected life time of more than 30 years. To accumulate field experience, tests are carried out on switchgears which were operated up to 20 years in different regions in the world. Such tests include e.g. dielectric tests, mechanical tests or dew point measurements inside the SF6 compartments. One of the examined 24 kV-RMU was in service continuously from 1985 to 2000 on the south west coast of Norway with salty atmosphere and has been used since 2000 also in sandy and traffic atmosphere after moving in a substation for temporary installation. The investigations were performed in July/August 2004 in the Testing Laboratory Medium Voltage Frankfurt am Main, Germany. In detail, the following tests were carried out: (i) checking the condition of the gas; (ii) checking the condition of the electrical contacts; (iii)dielectric tests including partial discharge measurement; (iv) mechanical tests; and (v) visual inspection on the SF6-vessel and mechanical parts, e.g. for corrosion. The examination showed a good condition of the primary circuit, the SF6-tank and the other materials. The insulating-gas itself has got minimal humidity with a dew point of less than -30°C. Out of this examination we can derive a conclusion that the RMU from Siemens will have an expected life time which is much longer than 30 years. It also shows that the chosen design and the intensive aging tests, e.g. according to IEC 60932 are suitable to prove the long time behaviour for switchgear in severe environmental conditions. (7 pages)