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It has been found that the loss of superconductivity in HTSC wires under ac current overload is accompanied by the change in sign of the phase difference between current and voltage. In a stabilized second-generation HTSC wire under an overload the oscillations of ac current and voltage amplitudes have been found due to the oscillations of the normal zone size in the superconducting layer. The change in sign of the phase difference can be applied for a timely registration of a normal zone nucleation in current-carrying conductors of superconducting equipment.

The application of high-temperature superconductor resistors (HTSC resistors) in ac electrical equipment and networks has been considered to protect them from fault currents and single-phase to earth faults. To improve the response speed and thermal withstand capability of HTSC fault current limiters the use of a stable overloaded regime in composite HTSC wires has been proposed. Basic circuits has been developed for the application of stabilized HTSC wires of low-ohmic value in protective resistors for ac networks that enables to increase the apparent resistance in the circuit by few orders of magnitude. The results are given of measurements of characteristics of first generation HTSC wires with high critical parameters in resistive state in a wide range of overcurrent. Prototype instantaneous current limiters with high thermal capability using the HTSC protective resistors have been manufactured and tested. The estimated values are given for the design parameters of the HTSC protective resistors applicable in electrical networks.

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The possibility of using high-temperature superconductors of the second generation in protective resistors (HTS resistors) for electrical equipment is considered. It is proposed to use mesh fiberglass electrical insulation in the HTS resistors to ensure the efficiency of the heat sink and maintain a stable overloaded operation. Physical modeling of such resistors was performed, which showed the preservation of their overload capability and the suppression of thermal instabilities due to the presence of mesh insulation.