Numerical Analysis of Thermo-Electric Field for AC XLPE Cable in DC Operation Based on Conduction Current Measurement

With rapid development of renewable energies and dc loads in distribution grids, converting ac distribution grids into dc operation has become a promising method to utmost utilize the present ac distribution grid in large cities. This paper reports a numerical investigation of thermo-electric fields of a three-core 10-kV ac cross-linked polyethylene (XLPE) cable and cable joint in the dc operation. DC conduction current for the XLPE and silicon rubber (SiR) was evaluated at different temperatures and electric fields. The thermo-electric field simulation was performed by COMSOL Multiphysics according to the measured data. The obtained result showed that the threshold of XLPE at 65 °C is 3.1 MV/m, whereas the threshold of SiR at 60 °C is 2.2 MV/m. It was found that the maximum electric field of the cable operated in dc voltage up to 12.5 kV was below the threshold of 3.1 MV/m. The maximum electric field in the cable joint appeared at the high voltage screen tube, whereas the maximum electric field at the high voltage screen tube was less than the threshold of SiR at a temperature difference of 25 °C. Moreover, the maximum transmission power of the three-core 10-kV AC XLPE cables running at the ±10-kV bipolar dc operation was 1.36 times of the power in the ac operation. It was elucidated that the three-core ac 10-kV XLPE cable could be operated reliably at ±10 kV in bipolar dc topology with sufficient safety margin. It is beneficial to upgrade the ac distribution grid to the dc distribution grid.