Harmonics in MVDC Cable Systems - Study on Thermal and Electric Stress

MVDC can assist in resolving issues with the transition to renewable energy by expanding transmission capacity. Due to the use of converters, MVDC systems may be affected by harmonic currents and voltages. However, harmonics are not currently considered harmful in MVDC systems. Nevertheless, harmonics may have an impact on the electric field distribution and temperature in cable systems. Therefore, multiphysics FEM simulations are carried out to examine the general impact of harmonics in MVDC cables. The MVDC cable system experiences an electric field enhancement because of the influence of harmonics. The field enhancement shows the highest impact closest to the conductor. Furthermore, the average relative field enhancement is equal to the relative amplitudes of the harmonics. To investigate the field enhancement for different insulation material properties, different types of XLPE material (AC and DC) are investigated. Findings show that the influence of harmonic voltages rises with the non-linear conductivity of the insulation material. To examine the impact of harmonic currents on the ampacity of MVDC cable systems, FEM simulations are presented. The dielectric losses of MVDC cables compared to conductor losses are smaller than 0.01% and therefore negligible. Harmonic currents cause a very slight reduction in ampacity.