Ripple Current Propagation in Bipole HVDC Cables and Applications to DC Grids

Offshore wind power is attracting increasing levels of research and investment. The use of HVDC transmission and the development of dc grids are topics with similar high levels of interest that go hand in hand with the development of large scale, far from shore wind farms. Technical challenges result from the interaction between power-electronic dc-dc converters and the cables in a dc transmission network. In particular, the propagation of the ripple current in bipole dc transmission cables, constructed with a lead sheath and steel armor, is examined in detail. The finite-element method is used to predict the currents induced in the outer layers of the cable by the ripple current. These results are used along with wave propagation theory to demonstrate that cable design plays a crucial role in the behavior of the dc system. Applications include the prediction of transmission losses, resonance, and high-voltage filter design.