95_Laurent_spacechg_tree - IEEE Transactions on Dielectrics...

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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 2 No. 1, February 1995 27 Influence of Space Charge Buildup on the Transition to Electrical Treeing in PE under ac Voltage M. Mammeri, C. Laurent Laboratoire de Genie Electrique, UniversitC Paul Sabatier, Toulouse, France and J. Salon Center de Physique Atomique, Universitk Paul Sabatier, Toulouse, France ABSTRACT Our investigation is focused on the understanding of electrical aging in polymeric materials under 50 Ha ac voltage. In this report, a needle electrode molded into the insulant to simu- late defects producing local field enhancement is used. Spe- cial emphasis is given to low density polyethylene. The tran- sition between the discharge-free electroluminescent state to micropartial discharge (PD) state (early electrical tree prop- agation phase) is studied with a sensitivity reaching 0.01 pC. Optical diagnosis is carried out simultaneously. At the moment of tree initiation, electrical discharges from 0.04 to 0.1 pC oc- cur in the positive half cycle. A very small electrical tree of 5 pm was observed. Using the phase angle of the first measur- able PD with respect to the applied voltage offers additional information helping to understand tree initiation. Then a cor- relation between the magnitude of electrical discharges and the characteristic traces of local breakdown (electrical tree) is re- ported. We proposed an interpretation based on the similarity with grounding tree experiments in which the initiation of a lo- cal breakdown is mainly due to a strong Poisson field radiated by a space charge region in the vicinity of the needle tip. 1. INTRODUCTION LECTRICAL aging in solid dielectrics is still a poor- E ly understood phenomenon. For this reason much work has been actively performed to clarify this mecha- nism. The technical aim is to improve the reliability of the insulated system. This report deals with polymeric materials as those used for the insulating of power cables. Degradation due to electrical discharges at the surface of the insulant does not fall within the scope of this pai per. Also excluded are water treeing related phenomena, which concern water impregnated polymers. Under nor- mal operating conditions, polyethylene (PE) insulation of power cables are submitted to voltage gradient up to 15 kV/mm. Basically, two situations can lead to dielectric aging: the presence of gaseous cavities in which PD can occur, or the existence of field-enhancing defects lead- ing to some high field effects. In both situations, aging ends up with the generation of electrical treeing leading to rapid breakdown [l]. Our aim is to understand aging in discharge-free situation under ac divergent field. Charge injection from field-enhancing tips was shown 1070-9878/95/ $3.00 @ 1995 IEEE
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28 Mammeri et al.: Influence of Space Charge Buildup on Electrical l'keeing in PE to be the first step in a series of pretreeing events [2,3].
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95_Laurent_spacechg_tree - IEEE Transactions on Dielectrics...

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