Quench behavior of the switching elements of a hybrid HTS current limiter

In the hybrid HTS current limiter, current limitation is due to the transition to the normal resistive state of YBCO bars, which are shorting the secondary of a transformer. The HTS bars act as current driven switches allowing two states, a low impedance state in which the bar is in its superconducting state and a higher impedance state arisen after the quench. From the authors' measurements, one can conclude that the triggering current is essentially related to the contact resistance, which induces a hot spot. The resistance developed during the switching time enables the limiter to work. However, the maximum resistance which can be arisen during the quench is limited by the low propagation speed of the transition front, from the contacts to the center of the bars, in the adiabatic approach. In order to stimulate a multi hot spot quench, the authors have adjusted the triggering current by Mg doping the YBCO, thereby homogenizing the quenching current along the bars. They have also developed switching elements based on a quasi isothermal approach of the quench, which diminishes the high thermal gradients developed in the adiabatic quench induced by a hot spot. Measurements of the performances of both switching systems are reported.