Qualification of MVAC XLPE Cable Systems for DC Operation – Thermal Stability Test on MVAC Cables

In accordance with climate goals, electrical energy will be produced mainly from renewable energy sources in future. This climate-friendly energy has to be transported to consumer centres. A part of the solution to face this challenge is the medium- and high-voltage direct current transmission (MVDC, HVDC) via cables. There are currently only standards for extra HVDC (EHVDC) and HVDC cable systems available. Therefore, a qualification procedure for MVDC cable systems was presented. This procedure includes tests and simulations for obtaining detailed information about the used insulation material. One of these additional tests is the DC breakdown test, which provides information about the breakdown strength of MV cables under DC stress. During the DC breakdown test a thermal runaway for the investigated 6/10 kV AC XLPE cable at a DC voltage with negative polarity was recorded. The thermal runaway is presented and discussed in detail. To ensure the thermal stability of the used 6/10 kV and 12/20 kV AC XLPE cables at nominal DC voltage, the thermal stability test included in the CIGRE TB 852 was carried out with and without a thermal insulation and for both polarities. These tests have been passed without any objections having been raised and will be discussed in detail. Furthermore, the thermal stability test of the 6/10 kV cable was extended up to a DC voltage with negative polarity of UDC = -140 kV.