Effects of Steel Tapes on Internal Pressure in Mass Impregnated HVDC Cables

Résumé

Thin steel tapes providing radial mechanical support is a vital component in mass impregnated non-draining (MIND) HVDC cables. When the insulation volume increases with temperature during operation, the tapes work against the expansion and make sure that the cable is not permanently deformed by repeated heat cycles. It is belived that the application of steel tapes also compresses the insulation, increasing the internal pressure building up when the cable is loaded. The oil impregnant has a much higher thermal expansion coefficient than the paper, which can cause both radial flow of oil when heating and cavity formation - and thereby the risk of dielectric breakdown - when cooling. Steel tapes are important in this regard, as they may enhance there effects by reducing the insulation volume. Through measurements of internal pressure and thermal expansion as function of temperature, this work confirms that there is a significant difference in pressure behaviour in cables with the steel tapes in place and when they are removed. Measured pressures ange from some 0.3 bar belom atmospheric at 20 °C, to approximately 8 bar at 46 °C. The experimental results are compared to available literature, and a new mathematical model that calculates the pressure at the insulation/sheat interface.