EoW November 2009

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EuroWire – November 2009 48
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.
Arkema – USA Email: Website:
Diagram of an HV cable showing strippable insulation ▲ ▲