Numerical analysis of current-limiting cooperation of a 20 mva superconducting transformer and cable

In a previous study, we developed a 66/6.9 kV-2 MVA REBa₂Cu₃O_y (REBCO) superconducting transformer with a current-limiting function that was cooled with subcooled liquid nitrogen at 65 K. By considering the flux-flow state, it is possible to quantitatively explain the behavior of REBCO superconducting transformers and cables subjected to a sudden short circuit. Our goal was to demonstrate that limiting an excess fault current to triple the rated current after 0.2 s (while maintaining a superconducting state) was possible with the cooperation of superconducting transformers and cables. For that purpose, a numerical analysis was performed by coupling thermal equations and circuit equations. Consequently, the numerical analysis demonstrated that limiting the excess fault current to triple the rated current after 0.2 s can be achieved by adjusting the thickness of the stabilizing copper layer of REBCO superconducting tapes and the length and critical current, I_c, of the superconducting cable. Through these simulations, the balance of I_c and thickness of copper layers was shown to be important in limiting overcurrent and normal transition rates of the superconducting winding of the transformer and cable.