How high voltage cable technology is changing cable transmission lines

High-power line between France and Spain

At every stage in its development, the world’s electrical engineering has faced the need to create cable lines with high current-carrying capacity. This task has always been challenging due to a number of difficulties in achieving the required electrical strength of high voltage cable insulation. Such insulation has to withstand high voltage and guarantee the cable reliability over a long period of time, while meeting acceptable parameters of weight per a cable length unit and its outer diameter.

In the 1920s cable manufacturers started covering cables with multilayer oil-paper insulation; at the same time, their mechanical strength was increased by means of a lead sheath. This made it possible to bring the cable line voltage up to 30-40 kV.  The need to increase the voltage of cable lines first lead to the invention of low-pressure oil-filled cables with a lead sheath, after which high-pressure oil-filled cables were designed. The latter are made up of three shielded conductors in a metal pipe filled with mineral oil at a pressure of 1.5 MPa. With time, the cable line operating voltage was raised to 220 kV, and later to 500 kV. However, the maintenance of cable lines with liquid insulation turned out to be rather complicated on account of numerous fast couplings and cable manholes with cable oil charge systems.

The search for more efficient insulating material gave rise to cables with insulation made of cross-linked polyethylene (XPLE-insulated cables). Their high electrical strength and reliability, service life of at least 30 years, simple maintenance and environment-friendly components brought about the world-wide usage of cables with XPLE insulation in the voltage range 35 to 420 kV.

                     Among the CIS cable manufacturers, Ukraine’s cable producer “Yuzhcable” was the first to begin the commercial production of XPLE-insulated cables in the voltage range 6 to 330 kV.

Despite the benefits of XPLE insulation, AC cable lines cannot compete with overhead lines in the area of intersystem communication. Yet, such cable lines are becoming an efficient means of delivering electricity to large urban transmission networks.

Gas-insulated transmission lines

          In Central London, in 2003-2004, a high-capacity cable line with a voltage of 400 kV and current-carrying capacity of 1600 MW was installed. The line uses an XPLE-insulated cable with copper conductors; the conductor cross-section is 2500 square millimeters in phase.
For London, the most acceptable solution was to install cables in a 20-metre underground ventilated tunnel, the diameter of which is 3 metres. The tunnel design allows to lay one more cable line, which is to be gas-insulated (the conductor of each  phase is in a metal pipe filled with SF6 gas).

For instance, a 15-kilometre cable transmission line with a voltage of 220 kV was built in the Moscow region in 2009. It was laid underground, under the highways and industrial facilities. The line capacity is 450 MW.

Another advantage of XPLE-insulated cable lines is the fact that they are used for intersystem communication in DC electricity transmission lines when these need to cross substantial water bodies like sea channels and shallow seas. If it is impossible to install an overhead line in the water area, a seagoing cable line comes as the only reasonable solution. Only one cable is used for unipolar transmission, and two cables – for bipolar transmission (with positive and negative polarity). Should one of the cables be disconnected, the transmission line will operate at half of its capacity.

It is expected that the next step in cable development will be marked by cryogenic cables, which use the effect of superconductivity achieved when metals are subjected to deep freezing.  At the liquid-nitrogen temperature of 77 K the active resistance of copper and aluminum decreases by a dozen times, and at the liquid hydrogen temperature of 20.4 K – by 200 times. Niobium (Nb) and its compounds (Nb 3 Sn, Nb 3 Al) experience the effect of superconductivity at the temperature of liquid helium 4.2 K. The liquefied gas cooling the conductors can be used for the insulation of cryogenic cables.

The capacity of cryogenic transmission lines is evaluated at the range 10 to 100 GW.

Source (Russian language): http://energetika.in.ua

About KEP

KEP (KharkovEnergoPribor Ltd.) produces high voltage testing equipment, including portable testing instruments and cable test vans, for the world’s leading power engineering companies. The KEP’s product range includes solutions for cable tracing, cable diagnostics and fault locating, testing high voltage instruments and safety gear, and oil dielectric testing.

We at KEP believe that our main task is to translate the customer’s requirements into the top-quality product, combining cutting-edge technology with fundamental high voltage testing principles.

Alexei Tiatiushkin
Marketing manager
KharkovEnergoPribor Ltd.
marketing@keppowertesting.uk
http://www.kep.ua