Water trees in polyethylene insulated power cables: Approach to water trees initiation mechanism

• Published in: Electric power systems research Vol. 180; p. 106158
• Main Authors: El-Zein, A., Mohamed, Kh, Talaat, M.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01.03.2020; Elsevier Science Ltd
• Subjects: Bouquet size; Breakdown; Cables; Cracks; Cross-linked polyethylene; Diameters; Electric fields; High density polyethylenes; Insulation; Internal pressure; Low density polyethylenes; Polyethylene; Power cables; Solid dielectric; Tree inception field; Trees; Water tree length; Water trees; Tree inception field; Water trees; Solid dielectric; Bouquet size; Water tree length
• ISSN: 0378-7796; 1873-2046
• DOI: 10.1016/j.epsr.2019.106158

•The minimum electric field required to initiate the electric tree has been investigated.•A new breakdown equation for water tree with breakdown index has been introduced.•A new introduced mechanical ratio index LcrPm has been presented. Water trees occurring in polyethylene (PE) insulated power cables at defects exist on the inner or outer semiconductor layers or in PE insulation medium. Water trees start with the existence of water source and penetration under the action of high electric fields. The inevitable micro air voids (1 μm radius) in PE medium with an internal pressure 4450 Torr, and 2.3 dielectric constant, will lead to an electric field (E) of 1.08 MV/cm which can be called tree inception field (Ei). For a protrusion defect of 200 μm tip radius and 2.5 mm insulation thickness, the minimum required breakdown field in PE is 0.417 MV/cm at 2.5 per unit (pu) over voltages. In this paper a new breakdown equation has been introduced related to the injected energy (U), electric field (E) and air void radius (r) with the so called breakdown index (n) under two cases: First case, for n<25, the air void radius increases till 5 μm diameter (water bouquet value). But for the second case, n≥ 25, the air void will crack to many branches in free space and these assume 25 cracks at n=25. The crack length (Lcr) differs according to the mechanical strength (Pm) of PE insulation. According to the new introduced equations Lcr equal 197 μm, 248 μm, and 332 μm for Pm12 N/mm2 low density polyethylene (LDPE), 15 N/mm2 high density polyethylene (HDPE), and 20 N/mm2 cross-linked polyethylene (XLPE) respectively. The water will penetrate these cracks and the progress of water trees starts.