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Heatwaves and underground MV cable joints failures
No full textIn the last years, the summer meteorological conditions became much hotter and more arid as one of the consequences of the climate changes. These conditions have caused and are causing an increasing number of unexpected failures in underground medium voltage cables, mainly in correspondence of cable joints which are weak points of the system. The anomalous hot weather conditions during the summer period may last for several days (heatwaves), worsening appreciably the consequence of this phenomenon; the stress obviously increases as the number of the warmer days and lack of rain prolongs, heating cable and joint insulating materials with their internal defects and voids, wherein partial discharge phenomena may be easily triggered at this higher temperature under the action of the electrical field greatly increased in correspondence of such defects and voids. The present paper reports a study on the hotter day sequences in some Italian main cities with the aim to establish a correct definition for the term heatwave, to be used when dealing with underground cables failure topic
Standardized methods for the determination of breakdown voltages of liquid dielectrics
No full textIEC 60156 and IEC 60897 were standardized with the purpose of evaluating the breakdown voltage of liquid dielectrics both under AC and impulse voltages. AC breakdown voltage determination is a simple method but only able to provide information on the presence of contaminants. The limit of this test is the large scatter in the results and, for this reason, this IEC standard has been recently revised. Impulse breakdown voltage determination allows discrimination of different dielectric liquids, also in terms of their chemical composition. This test has only limited application because of the dimensions of the high voltage generator. The present paper shows a comparison among breakdown voltages of various mineral oils and natural esters under both AC and impulse voltages. The paper presents also an effort to describe how these two tests may reveal the electrical performance of the dielectric liquid under test. © 1994-2012 IEEE
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