Choices and considerations for the selection of power cables in tunnel installations

This paper considers cables in the voltage range of 33 kV to 550 kV. A wide choice of cable types exist to suit different tunnel situations. At voltages up to and including 220 kV, extruded XLPE cables offer a combination of adequate electrical performance, low maintenance and good fire performance. At voltages of 275 kV and above the most reliable cable system remains the fluid filled paper/PPL (polypropylene paper) cable, both in terms of the cable dielectric and the accessory dielectric. The risk of fire spread is greater with this type of cable and precautions are required to be taken. XLPE cables and in particular their joints are becoming available at 275, 400 and 500 kV, initially with small conductors. However, these are electrically less reliable and care is required to be taken in the specification of type approval tests, prequalification test, manufacturing routine tests, manufacturing sample tests and in jointer training. Prototype GIL (gas insulated line) systems are being proposed and although offering the advantage of high ratings, pose significant engineering problems with respect to electrical reliability and service life. Wherever possible the cable tunnel construction should be designed to suit the thermomechanical requirements of the cable system to achieve maximum circuit reliability. Flexible cables apply minimal loads to the tunnel wall. GIL cables can impose discrete loads at certain positions and this needs to be taken into account in the tunnel design