Electrical stress control

Claim 2

An arrangement for limiting the electrical stress in a region of high electric field strength associated with high voltage equipment, the arrangement comprising two layers of material that overlap each other and that enclose at least part of said region of the equipment, wherein each of said layers is electrically semiconductive, and wherein the a.c. electrical impedance characteristics of said layers are different from each other.

Claim 3

An arrangement according to Claim 1, wherein one of said layers has a linear electrical impedance characteristic and the other has a non-linear electrical impedance characteristic.

Claim 4

An arrangement according to Claim 1 or 2, wherein the layer having the more non-linear electrical impedance characteristic overlaps the other layer.

Claim 5

An arrangement according to any preceding claim, wherein the equipment comprises a high voltage cable and said region comprises a termination or joint thereof, and wherein the arrangement encloses at least part of the insulation of the cable that is exposed to effect said termination or joint.

Claim 6

An arrangement according to Claim 4, wherein at least one of the layers of the arrangement further encloses at least one of the screen and conductor of the cable.

Claim 7

An arrangement according to Claim 4 or 5, wherein one of said layers has a linear electrical impedance characteristic, said one layer being electrically connected to a ground screen of the cable and extending therefrom along the cable insulation towards the cut-back end of said insulation, and wherein the other of said layers is electrically connected to said screen and extends therefrom, overlapping said one layer, towards the cut-back end of the insulation for a shorter distance than said one layer.

Claim 8

An arrangement according to any preceding claim, wherein said layers are integral with each other.

Claim 9

An arrangement according to any preceding claim, wherein one of said layers is provided as a tubular member and the other of said layers is provided as a coating on the inside of said tubular member or of another tubular member.

Claim 10

An arrangement according to Claim 8, wherein said another tubular member comprises electrically insulating material.

Claim 11

An arrangement according to any of claims 1 to 7, wherein each of said layers is provided as a tubular member.

Claim 12

An arrangement according to any of Claims 8 to 10, wherein at least one of said tubular members is recoverable, preferably heat-recoverable.

Claim 13

An arrangement according to any preceding claim, wherein one of said semiconducting layers comprises an electrically insulating polymeric material that contains carbon black.

Claim 14

An arrangement according to any preceding claim, wherein one of said semiconducting layers comprises iron oxide; or zinc oxide; or silicon carbide; or a polymeric material having dispersed therein at least one particulate filler selected from: compounds having a perovskite type crystal structure; compounds having a spinel crystal structure, other than γ -Fe 2 O 3 and spinel itself; compounds having an inverse spinel crystal structure; compounds having a mixed spinel crystal structure; dichalcogenides of transition metals and of tin; AgI, Prussian blue, Rochelle salt and other alkali metal tartrates, compounds of the formular HX 2 YO 4 , wherein X is K, Rb or Cs and Y is P or As, ammonium fluoroberyllate, thiourea, ammonium sulphate and triglycine sulphate; Si 3 N 4 ; the total weight of said compound(s) being at least 10% based on the weight of the polymeric material and the material having a γ -value of at least 1.5 at least at some direct current (DC) electrical stress between 0.01 kV/mm and 10kV/mm.

Claim 15

An arrangement according to any of Claims 1 to 12, wherein one of said semiconducting layers comprises a polymeric material having dispersed therein a particulate filler selected from: compounds having a perovskite-type crystal structure other than titanates of strontium, magnesium, nickel and barium; compounds having a spinel crystal structure selected from compounds of the general formula A 11 B 111 2 0 4 wherein A is Mg,Co,Cu,Zn or Cd and B is Al,Cr,Fe,Mn,Co or V, provided that when A is Mg, B is not Al, when A is Cu B is not Cr and when A is Zn, B is not Fe, and compounds of the general formula A 1V B2 11 0 4 wherein A is Ti or Sn and B is Zn,Co,Ni,Mn, Cr or Cd; compounds having an inverse spinel structure, other than stoichiometric Fe 3 0 4 but including non-stoichiometric variants thereof; compounds having a mixed spinel crystal structure; MoSe 2 , MoTe 2 , MnO 2 and Sn0 2 ; AgI, Prussian blue, Rochelle salt and other alkali metal tartrates, ammonium fluoroberyllate, thiourea, triglycine sulphate (CH 2 NH 2 COOH) 3 H 2 SO 4 ), ammonium sulphate and compounds of the general formula XH 2 Y0 4 wherein X is K,Rb or Cs and Y is P or As;Si3N4; a mixture of Mos 2 and non-stoichiometric or stoichiometric Fe 3 0 4 ; a mixture of stoichiometric or non-stoichiometric Fe 3 0 4 and barium titanate; a mixture of stoichiometric or non-stoichiometric Fe 3 0 4 and at least one metallic powder selected from Fe,Al,Cu,Mn, Cr,Pb,Ni,Zn and Ag; and a mixture of barium titanate and a carbon black; the total weight of the compound(s) in the filler being at least 10% based on the weight of the polymeric material and the material having a y-value of at least 1.5 at least at some direct currrent (DC) electrical stress between 0.01 kV/mm and 10 kV/mm.

Claim 18

A method according to Claim 15, wherein at least one of said layers comprises a recoverable, for example heat-recoverable, member and is recovered into position on said equipment.

Claim 20

A high voltage cable joint or termination having applied thereto an electrical stress limiting arrangement in accordance with any of Claims 1 to 14.

Claim 21

A kit of parts comprising two layers of semiconductive material, wherein the a.c. electrical impedance characteristics of said layers are different from each other, for use in an arrangement in accordance with any of claims 1 to 14.