Power Transmission by Way of Submarine Cables (1975)

The paper reviews design, manufacture and laying of submarine power cables, according to the experience of one of the largest manufacturers in the world. Some of the more important installations are shortly described. Emphasis is given to the modern cable projects for: a) linking off-shore nuclear generating stations to the mainland; b) transmitting to the mainland the electric power generated off-shore as an alternative to oil or gas pipeline; c) interconnecting nearby oil platforms.1. INTRODUCTION. The importance of power transmission y way of submarine cables was steadily rowing during the last 20 years. As a matter of fact, on one hand we witnessed an important increase in the number of islands to be electrically linked to the mainland as well as in the number of high voltage networks to be interconnected; on the other hand new and more advanced techniques were made available by the submarine cable makers.For the next decade one can easily foresee a further increase in the number and importance of submarine cables. It will be due not only to the growing demand for connections of the previously mentioned kind but also to the solution of new types of problems, such as underwater crossings between off-shore nuclear generating plants and the mainland, interconnections of nearby oil platform, exploitation of oil or gas off-shore fields by way of power stations placed on site and connected to the mainland by submarine cables.It is easy to anticipate that in future submarine cables at voltages up to 700+750 kV (either d.c. or a.c.) to transmit power in the range of 1000+2000 MVA per circuit will be needed. Pirelli have manufactured and laid many submarine cables (Sardinia Corsica - Tuscany at 200 kV d.c., Long Island Sound at 138 kV a.c., Majorca - Minorca at 132 kV a.c. and in future 200 kV d.c., to quotethe more important ones only), and are at present manufacturing 300 kV d.c. cables to connect the Vancouver Island to the mainland. Their experience in this field is therefore of major importance and is here briefly summarized.2. TYPES OF SUBMARINE CABLES. The types of submarine cables so far used by Pirelli are as follows:The impregnated-paper cable, which is mainly suitable for d.c. connections up to 250+300 kV. The present experience is limited to water depths in the range of 500 m, but deeper waters may be reached by adding a special reinforcement to limit mechanical stresses.The oil-filled cable, which is suitable for voltages up to 700+ 750 kV, both d.c. and a.c. Being liquid filled this cable can be laid at depths of 1000 m without special problems. The cable design is dependent on the feeding length, i.e. larger oil ducts are necessary for longer crossings.The extruded insulation cable (EPM type), which is suitable for voltages up to 60 kV a.c.; its use at d.c. is still under study. Having no lead sheath, this cable is lighter and easier to handle than the previously mentioned ones. In comparison with polyethylene, EPM offers a better resistance to partial discharges: this is an important feature for extruded insulation where, in spite of strict manufacturing and delivery tests, the presence of a few, small air bubbles can never be surely avoided.