Oil-filled cables

Abstract

The ever-growing demand for electrical power throughout the world has frequently led to electrical generation being concentrated in large fossil, hydro or nuclear power stations built in carefully selected sites. The sites chosen are, of necessity in some instances, remote from the areas where the power is required and this in turn has led to the need for bulk transmission of power from the power stations to these areas. In order to transmit large quantities of power economically over great distances, two possibilities present themselves, namely increasing the a.c. transmission voltage and using a high voltage d.c. transmission system. While the larger proportion of the transmission system is normally by overhead lines, most systems require to have one or more sections undergrounded for practical, environmental and other reasons, and where the transmission voltage is of the order of 150kV or above, the majority of installations throughout the world use the well proven oil-filled cable system for underground transmission. The basic concept of the oil-filled cable is that there shall be full impregnation of all parts of the cable insulation at all times and that all space under the metallic sheath shall be completely filled with oil. The advantage of this is that it totally eliminates the creation of voids which is a common problem with ordinary paper-insulated cables. Voids are unintentional spaces in the insulation of an impregnated cable, filled with air or other gases usually at low pressure. They are caused by the bending of the cable during manufacture and installation, imperfect impregnation, or expansion and contraction phenomena associated with varying loads or overheating of the cable. At the stresses required for the higher voltages, these voids are subjected to ionisation, and the breakdown stress is considerably less than that of the dielectric intended to replace it. The voids nearest the conductor are first to break down, the chemical and thermal effects of ionisation causing permanent damage to the paper insulation. Breakdown of the cable eventually becomes inevitable. Another advantage is that the current carrying capacity of the oil-filled cable is increased as compared to that of a solid one. […] This is seen from the fact that the amount of current that can be passed through a cable is limited by the rise in temperature of the conductors and insulation. One way of increasing the current carrying capacity is by cooling the conductors, and in this way, the size of conductors may be decreased while the power transmitted is increased as compared with the ordinary paper-insulated cable. The thermal resistivity of the ground in which the cable is laid is also an important factor in the current carrying capacity of the cable. In fact, special back-fills of low thermal resistivity are used so that the heat from the cable is dissipated even faster. […]