The advantages of cross-linked polyethylene cable with multi-core circular strand structure are as follows:
(1) The electric field is more uniform than that of the sector conductor, which is beneficial to increase the voltage level of the cable.
(2) Increase the flexibility or curvature of conductors. Cores twisted by multiple conductors have good flexibility and larger curvature.
Because when a single metal conductor falls in love along its radius, the outer part of its center line circle will inevitably elongate, while the inner part of its circle will shorten. When multiple conductors are twisted, the inner and outer parts of the core can move and compensate each other. When bending, the plastic deformation of the conductor will not be caused, which greatly improves the softness and stability of the core. The flexibility of cables is approximately proportional to the square root of the number of strands. The more strands, the easier to bend. However, the flexibility of cables is also limited by the outer protective layer. Therefore, in the manufacture of conductors with different nominal cross sections, a certain number of strands is specified. And in order to prevent the phenomenon of twisting, the direction of twisting of each layer is opposite from left to right, so that each layer of wire has a fixed position and is not easy to disperse. When bent, the elongation of each layer of wire is the same.
The size of conductor core is calculated by cross-sectional product in square millimeter. The standard of wire core cut is different in different countries. At present, the specifications for core truncation of medium and low voltage cables are: 2.5, 4, 6, 10, 16, 25, 35, 50, 70, 95, 120, 150, 185, 240, 300, 400, 500, 630, 800, etc., and high voltage XLPE insulated heat-shrinkable tube cables. At present, the commonly used core truncation specifications are 300, 400, 630, 1000 and so on. The cross sections of copper and aluminum conductors are all produced according to the above specifications.
The conductors used in XLPE insulated cables are generally compacted cores. In order to reduce the size of conductors, the conductors must be compacted by rolling wheels after stranding to reduce the gap between the wires. Because of the existence of these voids, the nominal cross section of a conductor is smaller than the area contained by its outer circle, and the ratio of these two areas is the compaction coefficient. Normally, the compaction coefficient of the non-compacted conductor is 0.73 ^ 0.77, while that of the compacted core is 0.88-0.93.
The compaction coefficient of conductor is one of the key factors to determine the quality of XLPE insulated cable. Some countries with fast development of XLPE cable adopt compact cores or solid cores. Compression cores can prevent moisture diffusion. Water in conductors is one of the causes of water tree and breakdown of cable, and will seriously affect the life of cable. Several European companies such as ASEA and Sieverts in Sweden,Germany, The VDE company of China and the NEF company of the Netherlands both adopt higher compaction coefficient and non-compaction core standard to adapt to other kinds of power cables. In order to adapt to the situation of various countries, IEC has not made special provisions on the compacted cores, nor has China made provisions on the compacted cores for cross-linked cables, which is not conducive to our development and use of cross-linked polyethylene cables. Compression and non-compaction structures are stipulated in Japan and the United States. The conductor structures of JISC-3606 in Japan and IPCEA in the United States have compaction coefficients up to 0.93 and 0.94, which greatly prevent water from entering the conductor along the longitudinal direction.