Xiaoze Pei

Experimental Tests on a Superconducting Fault Current Limiter Using Three-Strand

Magnesium diboride (MgB2) in simple round wire form has shown significant potential to be a low-cost resistive superconducting fault current limiter (SFCL). The commercial exploitation of MgB2 SFCLs for typical distribution network voltages requires a considerable scale-up of the current-carrying capability of MgB2 wire. The two obvious options to achieve this are to increase the wire size and use multifilament wire and/or use multiple wire strands. This paper assesses the current capacity for multi-strand wire and presents results of initial tests on a three-strand MgB2 demonstrator SFCL coil. Three strands of monofilament MgB2 wire were braided together to form a single multi- strand wire. The wire was fabricated, wound on a ceramic former and heat treated by Hyper Tech Research, Inc. One of the key performance aspects of this type of wire arrangement is ensuring each strand of wire carries the same current. This paper demonstrates the potential for increasing the current capacity of SFCL using multi-strand MgB2 wire to that required for distribution network levels. The paper includes a detailed analysis of the results and the implications for the practical design of commercial SFCLs.

\hbox{MgB}_{2}

A prototype resistive superconducting fault current limiter (SFCL) was developed using single-strand round magnesium diboride (MgB2) wire. The MgB2 wire was wound with an interleaved arrangement to minimize coil inductance and provide adequate inter-turn voltage withstand capability. The temperature profile from 30 to 40 K and frequency profile from 10 to 100 Hz at 25 K were tested and reported. The quench properties of the prototype coil were tested using a high current test circuit. The fault current was limited by the prototype coil within the first quarter-cycle. The prototype coil demonstrated reliable and repeatable current limiting properties and was able to withstand a potential peak current of 372 A for one second without any degradation of performance. A three-strand SFCL coil was investigated and demonstrated scaled-up current capacity. An analytical model to predict the behaviour of the prototype single-strand SFCL coil was developed using an adiabatic boundary condition on the outer surface of the wire. The predicted fault current using the analytical model showed very good correlation with the experimental test results. The analytical model and a finite element thermal model were used to predict the temperature rise of the wire during a fault.

Wire

A resistive superconducting fault-current limiter (SFCL) has been developed using round magnesium diboride (MgB2) wire. The SFCL coil was wound using an interleaved coil arrangement to minimize the total coil inductance. The SFCL coil demonstrated reliable and repeatable current-limiting properties during testing. However, the wire temperature of the SFCL coil increases quickly during quench tests, and several minutes are required for temperature recovery after the fault is cleared. The SFCL coil therefore was fully integrated with a vacuum interrupter to quickly remove the SFCL coil from the circuit once a fault occurred. This allowed the SFCL coil to recover quickly while a bypass resistor acted as the current limiting resistance. A fast-acting actuator and its control circuit were designed and built to provide automatic control for the operation of the vacuum interrupter. The SFCL with the prototype vacuum interrupter was successfully tested. The energy dissipated in the SFCL coil was significantly reduced by integrating the vacuum interrupter. The fault tests with different potential fault currents also proved that the operation of the vacuum interrupter is independent of the fault current level. This prototype demonstrated the potential of a cost-effective and compact integrated SFCL and vacuum interrupter for power system applications.

Experimental testing and modelling of a resistive type superconducting fault current limiter using MgB2 wire

Resistive superconducting fault current limiters (SFCLs) offer the advantages of low weight and compact structure. Magnesium diboride (MgB2) in simple round wire form has been previously tested and shown to be suitable as a low-cost resistive SFCL. The primary objective of this work was to design a resistive SFCL for an 11-kV substation using multiple MgB2 wire strands. This paper will look into the options for the coil design. Two types of low-inductance solenoidal coils, namely, the series-connected coil and the parallel-connected coil, were theoretically examined and compared. This paper also reports the experimental results of two multistrand MgB2 prototype coils used as a resistive SFCL. This paper demonstrates the potential of SFCL coils using multistrand MgB2 wire for distribution network levels.

Experimental Tests of a Resistive SFCL Integrated With a Vacuum Interrupter

Superconducting machines offer the significant advantage of smaller volume, lighter weight, and increased operating efficiencies compared with traditional electrical machines. To date, superconducting machines have utilized a superconducting dc field winding on the rotor of a synchronous machine. This increases the system complexity because it requires cryogenic cooling on the rotating part of the machine. The stator in these machines is generally composed of a set of conventional ac copper wire coils. Round magnesium diboride (MgB2) wire however has the potential to form low-cost ac stator coils for a superconducting machine. This could enable a stationary superconducting ac stator winding to be fabricated, reducing the complexity associated with the cryogenic cooling. This paper presents initial test results on a prototype ac solenoidal coil to represent a typical ac stator coil for a superconducting machine using MgB2 wire. The diameter of the wire with insulation was 1 mm and the coil was wound in a double-layer solenoidal arrangement. The magnetic flux density distribution, quench current level, long duration operating current level, and the ac losses of the coil were measured and discussed. This paper demonstrates the potential of MgB2 wire to develop a superconducting ac stator winding for a superconducting machine.

Resistive Superconducting Fault Current Limiter Coil Design Using Multistrand MgB2 Wire

Medium voltage direct current (MVDC) distribution networks have been considered for various applications, such as offshore wind farm collector systems, all-electric naval vessels, and aircraft. MVDC circuit breakers are a critical technology to directly manage faults in multi-terminal DC (MTDC) networks. However, DC current breaking is much more challenging than in AC systems because there is no natural zero-crossing of the current waveform to aid fault isolation. This paper reviews existing MVDC circuit breaker technologies and also discusses their advantages and disadvantages. This paper also introduces new topologies that can be applied in MVDC applications. The operation of several hybrid DC circuit breaker topologies with aided commutation is included. The paper illustrates that a hybrid DC circuit breaker with aided commutation can clear a fault within 2-5 msecs with low losses, this shows great potential for future MVDC applications. The implications for the practical design of commercial MVDC circuit breakers are also discussed.

Design, Build and Test of an AC Coil Using

Magnesium diboride (MgB2) in a simple round wire form has been tested and shown to be suitable as a low-cost resistive superconducting fault current limiter (SFCL). The commercial exploitation of MgB2 SFCLs requires a considerable scale-up of the current-carrying capability of the MgB2 wire. A multistrand MgB2 wire was developed for an SFCL coil to increase the current capacity. This paper will briefly report on the experimental results on a three-strand MgB2 coil used as a resistive SFCL. An improved analytical model that predicts the behavior of the three-strand SFCL coil was developed, taking the temperature and critical current variation along the wire into consideration. Variations in the critical current along the wire are to be expected as a consequence of normal manufacturing tolerances. The predicted current using the improved analytical model showed good correlation with the experimental test results at different fault current levels. The improved analytical model is a useful tool for the practical design of commercial SFCLs.

\hbox{MgB}_{2}

AC losses in high-temperature superconductors are an important design parameter for large-scale power applications. It is often necessary to have a metallic containment vessel to house the superconducting element in a closed-loop cryogenic system. This, however, produces additional ac losses due to induced eddy currents in the walls of the vessel. This paper focuses on the investigation and understanding of the induced eddy current loss in the containment vessel. The total ac losses of a 1-m yttrium barium copper oxide coil were experimentally measured in three different containment vessels. The predicted eddy current loss from finite-element modeling combined with the measured superconductor hysteresis loss demonstrated good agreement with the experimental results. This paper also includes the implications for practical design of commercial cryostat containment vessel for superconductor power applications to minimize ac losses

Wire for Use in a Superconducting Machine

Direct current (DC) circuit breakers are a key enabling technology for fault management in multiterminal high-voltage DC (HVDC) systems. DC fault isolation is challenging due to the high rate of rise of the fault current and the lack of natural current zero-crossings found in ac systems. In this paper, we present a novel superconducting hybrid dc circuit breaker that utilizes the intrinsic characteristics of the superconductor material. The automatic quench of the superconductor coil as a result of a high fault current transfers the current from the mechanical switch to the semiconductor switch. The isolating mechanical switch is able therefore to open at low current and recover its dielectric capability rapidly. A low voltage DC circuit breaker prototype has been built using a multistrand magnesium diboride (MgB2) coil, a vacuum interrupter, and an insulated-gate bipolar transistor module. This prototype successfully demonstrated interruption of 500 A DC within 4.4 ms. This paper presents the design of the superconducting hybrid breaker prototype and a detailed analysis of the experimental results. This superconducting hybrid dc circuit breaker has significant potential for scaling up the high-voltage and high-current applications.

A review of technologies for MVDC circuit breakers

The aerospace industry has ambitious environmental emissions and noise reduction targets that have led to some radical proposals for future aerospace transportation technologies. One of the disruptive technologies identified is hybrid electric propulsion. The use of electrical machines for aerospace propulsion is not a new concept. Fully superconducting machines have the potential to deliver the step-change in specific torque, power and efficiency capabilities required for large civil transport aircraft applications. However fully superconducting machines are still in their infancy. This paper looks at the electromagnetic design of two different stator design concepts for an AC fully superconducting machine for an aerospace distributed fan motor using a benchmark aerospace specification. A benchmark aerospace specification of 1 MW was chosen and the design of a conventional permanent-magnet machine was used to assess the performance of the two equivalent fully superconducting AC motor designs. The AC fully superconducting machine includes superconducting bulk magnets mounted on a conventional rotor core and an MgB2 superconducting wire wound stator in a non-magnetic core. The paper looks at a fully superconducting air-cored stator design to reduce weight and a new yokeless stator design is proposed to reduce the armature losses. The paper will look at the key design issues of the different motor designs in relation to the current aerospace targets for efficiency and power densities.