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

In the Hybrid HTS Current Limiter, current limitation is due to the transit ion to the normal resistive state of YBCO bars, which are shorting the seco ndary of a transformer. The HTS bars act as current driven switches allowin g two states, a low impedance state in which the bar is in its superconduct ing state and a higher impedance state arisen after the quench, From our me asurements, we can conclude that the triggering current is essentially rela ted to the contact resistance, which induces a hot spot. The resistance dev eloped 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 ce nter of the bars, in the adiabatic approach. In order to stimulate a multi hot spot quench, we have adjusted the triggering current by Mg doping the Y BCO, thereby homogenizing the quenching current along the bars. We have als o developed switching elements based on a quasi isothermal approach of the quench, which diminishes the high thermal gradients developed in the adiaba tic quench induced by a hot spot, Measurements of the performances of both switching systems are reported.