EoW November 2009

Power cables

Semiconductor shields: EVA or EBA? Conductor and insulation shields homogenise the electrical field to prevent electrical breakdowns. In order to incorporate enough carbon black to make the composition semiconducting ethylene vinyl acetate or ethylene acrylates are often used as the carbon black carrier in MV/HV formulations. These copolymers can accept high filler loading and yet maintain suitable flexibility. They also reduce shrink-back and minimise vented treeing. Specific grades give an extremely smooth surface that is suitable for very high voltages.

EVA is widely used for its excellent cost performance ratio and is the material of choice for insulation shields. For conductor shields, particularly for higher voltage, ethylene acrylates are often preferred for their excellent aging characteristics and thermal resistance for safe performance over many years of service. Lotryl ® 17BA07, an ethylene butyl acrylate copolymer from Arkema, is particularly suited to the manufacturing of MV/HV bonded conductor and insulation screen. The final viscosity of the compound can be fine-tuned with Lotryl 28BA175 or Lotryl 35BA320. It is typically crosslinked with dicumyl peroxide (Luperox ® DC range) for fast curing or with Luperox F or Luperox 101 for longer scorch time. For strippable insulation, shields are often made of a mixture of EVA and nitrile rubber; because it must be in intimate contact with the insulation, the strip force regularity is a critical issue. Arkema EVAs are random copolymers of ethylene and vinyl acetate made by high-pressure radical polymerisation in a tubular reactor. They are designed to ensure steady and uniform peel strength. Evatane ® 33-15, 33-25, 33-45 are particularly well suited to this application. Depending upon the strip force required, EVA with 28% to 40% of vinyl acetate is also commonly used. As for the crosslinking, Luperox F40 range is used with internal mixers, whereas Luperox 801 is preferred for the soaking process.

Diagram of an HV cable showing strippable insulation ▲ ▲

Arkema – USA Email : info.cable@arkema.com

Website : www.arkema.com

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EuroWire – November 2009