Water Treeing in XLPE MV Cables under AC and DC Stress

Abstract:
To reach the intended climate goals, the number of renewable energy power plants will continue to grow in future. In this context, offshore and onshore wind power plays a significant role. However, this climate-friendly energy has to be transported to consumer centres in an economically and ecologically efficient way. To address this challenge, medium and high voltage direct current (MVDC, HVDC) transmission via cables will play a dominant role in future. Currently, individual wind power plants are often connected by MVAC cable systems to a wind park. Afterwards, the energy can be transported by HVAC or HVDC cable systems to consumer centres. In this regard, it should be mentioned that the energy from such wind power plants could also be collected by MVDC systems. These MVDC cable systems could be placed in wet conditions where water can enter the XLPE insulation. The water that enters the XLPE could cause water trees in the cable insulation. This process can lead to premature failure of the cable insulation. This contribution analyses the occurrence of water trees and the growth behaviour of water trees in standard MVAC XLPE cables under different voltage stresses such as AC (50 Hz) and DC. To allow the water to enter the insulation, the XLPE cable test objects were built up without shield, water barrier layers, and sheath and the insulation was prepared with artificial defects to facilitate water ingress.