[0001] The present invention is related to a bipolar integrated concentric cable according
to the preamble of the claims.
[0002] HVDC cables have been in two configurations; either in mono-polar or in bipolar circuits.
The mono-polar circuit consists of one high voltage cable and earth, sea or metallic
return conductor is employed to complete the circuit. The bipolar circuit consists
of two high voltage cables usually with equal rating, but with opposite polarity.
Another limitation of prior art cables is the fact that to reduce the magnetic fields
around the cables to a minimum the two cables must be laid very near each other or
must be touching each other. This increases the complexity of laying the cables also.
[0003] A cable of the described type is disclosed in EP 0 938 102, comprising a central
conductor having a negative polarity, a first insulation layer, a second conductor,
a second insulation layer and an outer sheath.
[0004] The bipolar integrated concentric cable according to the present invention, can be
used for bipolar as well as monopolar transmissions and avoids the disadvantages of
prior art cables, such as environmental problems. This is achieved with the cable
according to the invention as defined with the features stated in the claims.
[0005] The only figure discloses a cross section of the cable according to the present invention.
[0006] A center conductor 1 is surrounded concentrically by a number of layers, the major
of which is an insulation 2, surrounded by a second conductor 3. A second insulation
4 is surrounding a second conductor 3 and is surrounded by a lead sheath 5.
[0007] Inside the outer armour layer 7 is a plastic sheath 6, comprising reinforcement.
[0008] According to the invention, the first and second insulations 2 and 4 are made of
the same materials, such as thin PP-film. Up to 50 kV to earth the thickness of both
insulations may be the same, however from 50 to 100 kV the thickness of the insulation
4 increases twice as fast as for the insulation 2, e.g. at 100 kV and higher to earth,
insulation 4 is twice as thick as insulation 2. The losses through insulation 2 are
approximately twice the losses through insulation 4, which influences the thickness
of insulation 2.
[0009] For the cable according to the invention negative polarity is connected to the central
conductor 1 and the positive polarity is connected to the second conductor 3. The
cable according to the present invention furthermore has both conductors inside the
metallic sheath, both of which are at high voltage and insulated from the ground.
[0010] The voltage between the center conductor and the outer conductor is twice the sum
of the nominal voltages to ground. The thickness of the first insulation 2 may be
up to twice the thickness of the insulation 4 between the second conductor 3 and the
outer metallic sheath. Because of the fast increase in the cable size the present
invention is mainly aimed for low to medium transmission capacities, such as up to
350 to 400 MW and voltages up to approximately 200 kV.
[0011] The present invention results in a cable with rotational symmetric cross-section
that allows both phases to be laid and eventually buried in one campaign with conventional
technology. It also allows the present invention to be laid to large depths.
1. A bipolar integrated concentric cable, comprising a central conductor (1) connected
to the negative polarity, concentric layers of insulation (2, 4) and a second conductor
(3) connected to positive polarity, characterized by the inner insulation (2) and the outer insulation (4) being made of the same material,
the thickness of the outer insulation (4) increasing twice as much as the thickness
of the inner insulation (2) by increased voltage.
