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Voltage Surge Protection Circuit for Superconducting Bias Coil

In some circumstances, a superconducting dc magnetizing coil with a magnetic core may be subject to a large induced voltage surge due to coupling with other coils or a sudden break/close of its circuit. For instance, the superconducting bias coil in a saturated iron-core type fault current limiter may experience a huge induced voltage when a short-circuit fault takes place in the power grid. These events may result in damage of the coil or other elements in the circuit. Therefore, it is important to have a measure for protecting the superconducting coil from large voltage surges. We designed a circuit configuration that is capable of suppressing voltage surges on the dc superconducting coil and has no adverse effect on the coil's normal magnetizing capacity. Experiments were carried out to verify the validity of the design. In this paper, we introduce the protective circuitry and its work principle. We also report the experimental results which are in a good agreement with theoretical calculations.