OVER CURRENTS

In the electrical grid there is a possibility that a fault can happen. This fault can be a short-circuit between individual phases or between phases and earth. The current during these short-circuits is several times larger than the nominal load current. They are interrupted by the use of circuit breakers that disconnect the cable, thus interrupting the over-current or short-circuit current.

Heating up

In the time from the occurrence of the short-circuit to its removal the high temperature superconducting (HTS) cable has to sustain the short-circuit current. Because the short-circuit current can be several times larger than the nominal current the critical current is exceeded and the consequence can be that the HTS cable conductor heats up.

Short-circuit experiments

To investigate the short-circuit currents effect on the critical current of the HTS cable conductor a number of short-circuit experiments have been made on a HTS cable conductor.

Measured critical current (Ic) as a function of short-circuit currents [1].

As shown in the figure, short-circuit currents can have an effect on the critical current of the HTS cable conductor. To protect the HTS cable conductors from these short-circuit currents a protection with high-speed relays could be an option. Without protection the conductor can handle over-currents of order 10 times I_c, thus around 20 kA for a 2 kA cable.

 

[1] K. H. Jensen, C. Træholt, E. Veje, M. Däumling, C. N. Rasmussen, D. W. A. Willén, and O. Tønnesen, "Overcurrent Experiments on HTS Tape and Cable Conductor", Applied Superconductivity Conference 2000.

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