Modelling of a high-temperature superconductor HVDC cable under transient conditions

The carbon neutrality goal of achieving net-zero by 2050 has sparked significant interest in offshore wind farms. With the offshore wind farms installed > 100 km away from the seashore, underground power transmission infrastructure is a necessity. Considering the power cable length and low net effective losses, a high-voltage direct-current (HVDC) system is chosen for this study. Also being lossless in DC operation, the HVDC cables are considered to be made out of high-temperature superconductor (HTS) material. But, unlike copper/aluminium, HTS material has a sharp transient behaviour as a function of the operating current, temperature and field. Thus, under transient conditions, as the fault current ramps up, this can lead to an increase in the HTS operating temperature, causing either degradation or permanent damage to the HVDC cable. In this paper, an HTS HVDC cable model has been developed in MATLAB/Simscape coupling both electrical and thermal models. For this study, a 100 km long coaxial 100 kV/1 GW DC HTS power cable is considered and modelled both as a lumped element and distributed element model with 100 elements to compare and evaluate the cable parameters along the length. The parameters include temperature distribution, resistance, critical current, and losses at different spots throughout the length of the cable. To simulate the transient condition, a line-to-ground (LG) fault is considered and the current distribution between the copper former and HTS tapes is studied. Using this cable model, the maximum temperature of the HTS and coolant both in the superconducting state and transient state are evaluated and presented. In comparison to the distributed model, the lumped model displayed different thermal and electrical values.