Power System Feedback from High-Tc Superconductor Shielded Core Fault Current Limiter
The shielded core fault current limiter is attractive with high critical temperature superconductors. The device acts during normal operation as an ideally shorted current transformer; the secondary superconducting loop, preferably a superconducting ring or cylinder, prevents from any flux entering. In case of a fault, the large current in the secondary current quenches the superconductor, and the secondary current is limited due to a considerable voltage drop; flux can now enter the core. The flux induces a primary voltage which counteracts the driving system voltage; this limits the fault current efficiently.
Very fast action is an essential requirement. However, this can give rise to dangerous transient oververvoltages both in the system to be protected which incorporates various inductances, and across the fault current limiter. Transient voltage occurence is discussed by means of equivalent circuits. Some means to control these voltages are highlighted in the view of different characteristics of the superconductor: ideal critical state performance versus slow or fast quench, respectively. The findings are illustrated by two numerical examples. It is shown that the shielded core fault current limiter gives a good chance to limit transient overvoltages down to a tolerable amount by introducing an additional tertiary winding. At the same time, heat generation in the superconductor can be kept very low.
KeywordsCritical Current Density Circuit Breaker Secondary Current Transient Voltage Superconducting Fault Current Limiter
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