cells are series and parallel connected with gaps of typically less than 1 mm to achieve a high packing density on the blanket. The operation voltage of the solar array is built up by the series connected solar cells with each cell contributing about 0.5 V to the array operation voltage. The array wiring is bonded to the rear side of the kapton blanket which provides excellent electrical insulation from the cells on the front side. There is presently no practical experience with arrays operating in the multi-kV range. Even the range between 100 and some 1000 V for earlier application needs basic research on the impact of elevated voltages on solar array design and operation. First experimental results were obtained in a study which was recently performed under ESA contract (2). It is a well known fact that electric discharge phenomena will occur when electrodes are in contact with dielectric materials. With increasing electrode voltage a critical field strength £1)d will be reached at which partial discharges are sporadically initiated in the dielectric. With higher voltages the intrinsic breakdown strength Eb will be exceeded which results in a breakdown of the electrode voltage. This breakdown usually happens far beyond the critical field strength Epd. Solar arrays operating at voltages above this so-called partial discharge inception voltage will have a limited lifetime, depending on several parameters (material properties, geometry of defects, voltage frequency, etc.). The empirical stress lifetime law is given by t-E" = const. and can be used to examine the long-time stability of insulating materials. The experimental data found for glass fibre reinforced kapton are shown in Fig. 3. The test samples represent a small section of a lightweight flexible solar array with solar cells on the front side of a glass-fibre reinforced kapton substrate. The solar array wiring is bounded opposite to the cells on the rear side of the substrate.
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