Development of High Current Impulse High Voltage Power Supply for Superconducting Fault Current Limiter Testing
Superconducting fault current limiters represent an advanced technology for protecting electrical power systems from fault currents. These devices exploit the rapid transition from superconducting to resistive state when the current exceeds a critical threshold, providing automatic current limiting without external control. Testing superconducting fault current limiters requires specialized power supplies capable of delivering high current impulses at high voltage to simulate fault conditions. The development of such power supplies presents unique technical challenges.
The operating principle of resistive superconducting fault current limiters involves the transition from zero resistance in the superconducting state to substantial resistance in the normal state. During normal operation, the superconductor carries current with essentially no losses. When a fault occurs, the current rises rapidly, and when it exceeds the critical current of the superconductor, the material transitions to the normal resistive state. This transition occurs within milliseconds, inserting resistance into the circuit and limiting the fault current.
Testing requirements for superconducting fault current limiters are demanding. The test must demonstrate that the limiter can successfully transition and limit current under realistic fault conditions. The test current must reach the prospective fault current level, which can be tens of kiloamperes for transmission system applications. The voltage across the limiter during the resistive state can reach kilovolts. The test duration is typically tens to hundreds of milliseconds, simulating the fault clearing time of protective relays and circuit breakers.
The power supply for fault current limiter testing must deliver high current impulses with controlled amplitude and duration. The peak current capability must match or exceed the maximum fault current the limiter is designed to handle. The voltage capability must be sufficient to drive the required current through the limiter in its resistive state. The energy storage capacity must support the required impulse duration. The switching capability must enable precise control of the impulse timing and duration.
Energy storage is a fundamental consideration for impulse power supplies. The stored energy must be sufficient to deliver the required current impulse without excessive voltage droop. Capacitor banks are commonly used for energy storage, with the capacitance value determined by the required energy and the acceptable voltage change during discharge. For very high energy requirements, inductive storage or rotating machines may be considered, though these approaches introduce additional complexity.
The charging system replenishes the energy storage between test impulses. The charging rate must be sufficient to support the required test repetition rate. For laboratory testing, where the interval between tests may be minutes or longer, relatively slow charging is acceptable. For production testing or type testing with high throughput, faster charging may be required. The charging system must include appropriate safety interlocks to prevent inadvertent discharge during charging.
The discharge switch controls the delivery of the stored energy to the device under test. For high current impulses, the switch must handle peak currents of tens of kiloamperes with rise times of milliseconds or faster. Triggered spark gaps, vacuum switches, or power semiconductor devices are commonly used for this application. The switch must provide reliable triggering with precise timing and must withstand the voltage stress during the charging phase.
Current measurement during the impulse is essential for characterizing the limiter performance. The current may be measured using shunt resistors, Rogowski coils, or Hall effect sensors. The measurement bandwidth must be sufficient to capture the current waveform during the rapid transition. The measurement accuracy must support the required test tolerances. The measurement system must be isolated from the high voltage present during the test.
Voltage measurement across the limiter provides additional characterization data. The voltage waveform shows the transition from the superconducting state to the resistive state and the subsequent current limiting behavior. High voltage probes with appropriate bandwidth and voltage rating measure the voltage waveform. The voltage measurement, combined with the current measurement, enables calculation of the resistance during the fault and the energy dissipated in the limiter.
Protection systems prevent damage to the test equipment and the device under test in case of unexpected behavior. Overcurrent protection interrupts the impulse if the current exceeds safe limits. Overvoltage protection prevents excessive voltage across the limiter or test equipment. Emergency shutdown systems enable rapid termination of the test if hazardous conditions are detected. These protection systems must be designed for the high power levels involved and must respond quickly enough to prevent damage.
Safety considerations are paramount for high power impulse testing. The energy levels involved can cause severe injury or death if proper precautions are not observed. Interlock systems prevent access to the test area during charging and discharging. Warning systems alert personnel to the hazardous conditions. Personal protective equipment and safety training are essential for all personnel involved in the testing. Comprehensive safety procedures and regular safety audits ensure safe operation of the test facility.
