Bamji_DEIS - F E A T U R E A R T I C L E Electrolurnin ceA...

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FEATURE ARTICLE Electrolurnin ce- A Technique to Detect the Initiation of Degradation in Polymeric Insulation Key Words: Diagnostic technique, degradation mechanisms, electroluminescence, power cables, polymeric insulation he demand for polymeric insulating materials in power apparatus such as cables, transformers, mo- T tors, and capacitors is growing. Polymers have excel- lent electrical properties such as high breakdown strength, -900 kV/mm, low dielectric loss, 6 < lo”, and high dc resis- tivity, > 10l6 L2.m. In addition, good mechanical stiffness, high corrosion resistance, ease of formation, and the low cost of manufacturing and maintenance often makes them the best choice of insulating materials for many applications. As we approach the 2lSt century, a 500 kV underground power transmission line using crosslinked polyethylene (XLPE) will be put into service between Tokyo and Chiba in Japan [l]. In North America, underground high voltage ca- bles operate at lower voltages, but the maximum operating stress even in the highest voltage rated cable does not exceed 20 kV/mm. This value is significantly lower than the break- down strength of the polymeric insulation; yet the power ca- ble is susceptible to long-term electrical degradation. Even with the present-day technology, it is almost impos- sible to completely eliminate defects that can be accidentally introduced into polymeric insulation during material pro- cessing and cable manufacturing. When the cable is ener- gized, the defects act as points of electrical stress enhancement where the degradation can initiate. In cables operating in a dry environment, the degradation takes the form of a tree-like growth called electrical trees [2], while ca- bles operating in a humid environment develop water trees [3]. Even when the water tree completely bridges the insula- tion between the central and outer conductors, XLPE insula- tion can withstand the full operating voltage applied to the cable. However, a water tree will ultimately convert to an electrical tree [4], especially when subjected to lightning and/or switching surges. Once an electrical tree starts, cable Soli S. Bamji National Research Council Canada breakdown is imminent and can occur in less than a year at service voltage. Thus, for power cables operating in dry as well as in wet environments, it is the electrical tree that is usually responsible for cable breakdowns. Electrical treeing has two distinct time periods. The first is the incubation period, or the initiation phase, when hardly anything appears to occur in the insulation, and the second is the propagation period, during which partial discharges oc- cur and the tree grows. The initiation phase of electrical tree- ing is not well understood, but it has been established that prior to electrical tree inception, light is emitted at points of electrical stress enhancement in the polymer. This light is not due to partial discharges (PD) but due to the phenomenon of electroluminescence (EL). The intensity of EL is at least two
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Bamji_DEIS - F E A T U R E A R T I C L E Electrolurnin ceA...

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