Dump Resistor Research Articles

Energy Extraction System for the Superconductive Solenoid with Current 5100 A and Stored Energy 22 MJ

At the FAIR (Facility for Antiproton and Ion Research, Darmstadt, Germany) accelerator complex under construction, one of the key experiments is the PANDA experiment. The main field of the PANDA spectrometer is provided by a superconducting solenoid. The energy of the magnetic field stored in a superconducting solenoid reaches 22 MJ at an operating current of 5100 A. In the case of the transition of the superconducting cable used for the solenoid winding into a resistive state, the safe extraction of stored energy is required to protect the magnet. The energy extraction system proposed by the authors provides energy dissipation inside the external dump resistor, and not in the section of the superconducting cable that has passed into resistive state, while preventing its damage. The most critical elements of the energy extraction system are the current breaker, with the help of which the dump resistor is introduced into the power circuit of the superconducting solenoid, and the dump resistor itself. A modernized three-phase circuit breaker was chosen as current breaker. A unique design of the energy extraction system including dump resistor with a low parasitic inductance has been developed.

First Validation of Robust REBCO Insert Concept on a Large 20-Pancake Prototype Reaching Up to 25 T

We recently proposed a robust REBCO HTS (RareEarth BaCuO) insert coil concept in the framework of a project to upgrade the 25 T Cryogen-Free Superconducting Magnet at the High Field Laboratory For Superconducting Materials to reach 30 T. In this concept, the conductor consists of a two-tape bundle to mitigate the risks posed by local tape degradation. So-called edge impregnation is used to enhance the mechanical stiffness while reducing delamination risk, providing conduction-cooling capability at the same time. This insert is to be protected against the risk of thermal runaway simply by using dump resistor, thanks to an early detection concept. The different ideas forming the robust REBCO insert coil concept were tested successfully on various small scale prototypes. We report here their first implementation on a large-size insert coil, consisting of 20 stacked pancakes from the 30 T CSM upgrade insert. The mechanical behavior of the coil is modelled and validated with experiments. The thermal runaway detection setup is introduced. Its sensitivity and selectivity is discussed based on previous works on thermal runaway scenarios.

EV’s as energy storage on urban light rail systems — A synergy of requirements

The increasingly urgent need to decarbonize transport is leading to a much greater uptake of electric vehicles (EVs) in countries across the world. Also, the installation and use of urban light rail systems (trams) is seen as a way of breaking the reliance of commuters on the internal combustion engine, and therefore car ownership. Due to the simplicity of design, most conventional tram systems use unidirectional substations to draw power/energy from the utility supply. Due to their very nature, the substations are not able to return excess regenerated energy from the trams back into utility supply, with this energy often being dissipated in dump resistors onboard the trams to prevent over-voltages on the tram system. This paper explores the possibility of using EV’s as temporary trackside energy storage systems on urban light rail systems through the use of bi-directional connection interfaces (chargers), which allow use of the vehicle battery in typical V2X scenarios. The paper uses the city of Sheffield (UK) Supertram network as an example network on which the effect of EV energy storage could be studied.

Active De-Excitation System Using Magnetically Coupled Secondary Coil and Charged Capacitor for Fault Protection of Superconducting Coils

Coils excited for high magnetic fields have large discharging time constants owing to high inductance and low resistance for large operating currents. This type of coil requires a de-excitation system to reduce the current as quickly as possible when a fault occurs and to minimize the damage caused by Ohmic heating. This paper introduces a new de-excitation system that rapidly reduces the current discharged through the dump resistor by using the active current of the secondary coil. To verify the performance of the proposed method, a de-excitation system utilizing the primary superconducting coil and secondary copper coil was constructed and discharging tests were performed. Good agreement was obtained between the circuit simulation and test results. The performance of the proposed system was compared with those of other de-excitation systems, and it was confirmed that the proposed approach provides a feasible and powerful means of reducing the fault energy load and that it reduces this load more than the other systems considered. In addition, the effects of the charging voltage, coupling coefficient, capacitor bank configuration, and diode on the performance of the de-excitation system were analyzed and information about the secondary section that most effectively reduces the current was obtained.

Fast Discharging Behavior of High Temperature Superconducting Coils With Copper Rings and Plates

Fast energy extraction using dump resistances is an effective method for quench protection of high temperature superconducting (HTS) magnets. Copper frames are often applied in HTS magnets for conduction cooling and mechanical support. Previous studies have shown that copper plates can accelerate the energy release of HTS coils during discharging operations with dump resistance, which is favorable for fast quench protection. Copper rings distributed at the inner and outer diameters of HTS coils can provide a strong mechanical support for the possible electromagnetic strain in high magnetic fields. This paper studies the effect of copper rings and plates on fast discharging process of an insulated HTS double pancake coil by finite element method. An optimized structure composed of copper rings and plates is proposed, and its effect on fast discharge is analyzed. The results show that copper plates show better performance than copper rings in accelerating energy release and reducing hot spot temperature of the coil. During discharging process, the optimized structure can absorb considerable energy from the HTS coil and significantly reduce the hot spot temperature. The optimized structure is a promising alternative for fast quench protection of HTS magnets.

The Effect of Different Copper Discs on the Discharge of Superconducting Coils

High temperature superconducting (HTS) magnets often work at high energy density and have slow quench propagation speed, so a quench will present a serious risk to the safety of magnets. The quench protection method based on the dump resistance can effectively reduce the current and release the energy in the HTS magnets. However, too large dump resistance may cause excessive voltage across the magnets. A quench protection system consisting of dump resistances and metal discs has been proposed. Copper discs are often embedded in HTS magnets for conducting cooling and mechanical support. In the discharging process of HTS magnets, the copper discs can absorb energy from the magnets through magnetic coupling, thus accelerating the current decay of the magnets. This quench protection method is more effective than using dump resistance alone. In this paper, the effect of different copper discs on the discharging process of HTS coils is discussed. Eight types of copper with different residual resistivity ratios (RRR) are applied. The results show that with the increase of the RRR of the copper disc, the current decay rate of the coil increases, and the energy absorbed by the copper disc from the coil increases. The role of different copper discs in the fast quench protection of the coil can be sorted as: RRR = 300 > RRR = 100 > RRR = 80 > RRR = 60 > RRR = 40 > RRR = 30 > RRR = 20 > RRR = 10. The copper disc with RRR of 300 shows the best performance in quench protection of HTS coils.

Design optimization for the quench protection of DTT's superconducting toroidal field magnets

The paper is focused on the optimal design of Fast Discharge Unit (FDU) for the quench protection of the Toroidal Field (TF) magnets of the Divertor Tokamak Test facility (DTT), an experimental facility under construction in ENEA Frascati Research Centre (Rome, Italy). The FDU is a safety key component that protects the superconducting magnets when a quench is detected through the fast extraction of the energy stored in the magnets by adding a discharge dump resistor (DR) in the TF magnets circuit. A comparison between a fixed DR and a switched variable DR has been implemented by changing resistor parameters and by using multiple current control of the power electronics components (IGCTs).The new configuration allows to reduce the maximum voltage and the thermal stresses both for superconducting magnets and for FDUs (Fast Discharge Units), so reducing the insulation level of all TF (Toroidal Field) coil circuits, including also the power supply, reducing the hotspot temperature on the TF coils and the specific energy through them. Another advantage of the proposed configuration is the reduction of the size of all electrical devices of TF coil circuits so achieving a more effective and reliable design, also decreasing the overall costs.

Design and test results of a novel quench protection circuit for a HTS ship propulsion motor

This paper describes a novel configuration of a quench protection circuit for a high-temperature superconducting (HTS) ship propulsion motor that can protect the HTS coils from an unexpected outage in its cryogenic cooling system. The quench protection circuit consists of conductor parts placed adjacent to the HTS coils and dump resistors and diodes placed in the cryogenic-temperature and normal-temperature areas, respectively. The conductor parts are shaped and arranged such that the magnetic coupling with the HTS coil is extremely large. Thereby, the current of the HTS coils can be instantaneously attenuated immediately after the quench protection is activated. With this configuration, the electromagnetic energy of the HTS coils can be absorbed in the dispersed parts. This can make the dump resistors smaller and make it easier to install them on a HTS ship propulsion motor. By analysing the equivalent electric circuit model of the protection circuit, we derived the conditions under which instantaneous current attenuation of the HTS coil occurs. In accordance with the conditions, we have fabricated a quench protection circuit that protects one HTS field pole of the 3 MW HTS motor developed by Kawasaki Heavy Industries. The fabricated circuit was empirically evaluated and proved to be highly effective in quench protection. The present quench protection circuit technology contributes to the practical application of HTS ship propulsion motors.

Detection and Protection Against Quench/Local Thermal Runaway for a 30 T Cryogen-Free Magnet

The upgrade to 30 T of the existing 25 T cryogen-free magnet (25 T CSM) at the High Field Laboratory for Superconducting Materials, Tohoku Univ., will consist in replacing the existing High Temperature Superconductor insert made of Bi-2223 by one wound using REBCO tape. The new insert is designed to produce 16 T instead of 11 T. In order to have good dynamics and allow sweeping mode experiment, the insert will be conventionally isolated, as it was the case for the previous Bi-2223 version. In previous work we modelled thermal runaway phenomenon and showed that a sensitive dissipative voltage detection technique can give sufficient warning to protect an HTS coil using external dump resistor even in the case of sharp local critical current drop. For the 30 T insert, the conductor will consist of a two tape bundle so as to reduce the chance of such sharp Ic drop due to tape performances inhomogeneities or local damage. We simulate the quench dynamics in the LTS outsert coupled with the HTS insert to demonstrate that the existing detection/protection system of the 25 T CSM can be re-used and determine the current dumping dynamics. We then use our thermal runaway modelling tool to study the behavior of the future 16 T REBCO insert in case of local sharp defect and determine realistic detection threshold for its safe protection.

Parametric study of the TF coil design for the European DEMO

In addition to a reduced number of feeders, which would simplify their integration in the cryostat of the tokamak, the present study considers the possibility to reduce the voltage during safety discharge of the toroidal field (TF) coils. A lowered discharge voltage would ease the manufacture of the TF coils and reduce the operational risks. Both aspects closely related to the discharge time constant, determined by the ratio LTF/Rd, where Rd is the resistance of the dump resistor and LTF the inductance of the TF coil. The supply of current to two or more TF coils in series requires a reduced discharge voltage per TF coil. The present study considers a fixed total TF current of 14.9 MA-turn provided by the DEMO baseline 2018. The discharge voltage can be reduced by increasing the TF conductor current, and hence lowering the inductance, or by an enlarged discharge time constant. For an increased discharge time constant, the copper cross-section in the TF winding pack needs to be increased to limit the hot spot temperature to acceptable values. A parametric study of the impact of increased TF conductor currents and discharge time constants on the radial build of the TF winding pack has been performed. The advantages of a possible reduction of the TF discharge voltage need to be compared with the disadvantages of an increased radial build.

Test Results of Fast Decaying Current-Induced AC Losses in SHMS Superconducting Magnets at Jefferson Lab

The 11 GeV Super High Momentum Spectrometer (SHMS) of Hall C, part of the recent 12 GeV accelerator Upgrade at Jefferson Lab, was successfully commissioned in 2017. Fast current discharges of the SHMS Q2, Q3 quadrupoles and dipole superconducting magnets experienced some level of operational difficulty, during commissioning and normal operation. Measurements and analyses demonstrate that the fast current discharge leads to substantial AC losses in the conductor and subsequently triggers a quench-back effect. The details of the measurements and analyses have been reported previously. This paper focuses on the test results of the magnets using a reduced value of the protection dump resistor for each magnet to lower the current decay rate. The test results confirmed that the reduced dump resistances eliminated the quench-back effect for the SHMS Q2, Q3 and Dipole magnets, thus improving their cryogenic operability.

Optimal Design and Performance Evaluation of Dual-Capacitor Switching (DCS) Quench Protection System for Superconducting Magnet

In recent years, a new quench protection system using capacitor and switches has been announced to rapidly extract energy from high temperature superconducting (HTS) magnets. When a quench occurs, the quench protection system activates four metal oxide-semiconductor field-effect transistor switches in sequence, and the energy stored in the magnet is extracted by a capacitor to an external resistor. To verify the feasibility of the proposed dual-capacitor switching (DCS) quench protection system, DCS system was tested using a small-scale coil with a 3.3 mH self-inductance. The experimental results show that the energy extraction of the magnet is faster than the quench protection system using a conventional dump resistor. However, as the system is sensitive to the capacitance, inductance, and four resistances, it is necessary to analyze and optimize the design parameters for an application to various magnet systems. In this study, a modified DCS system is introduced to remove the constraints of capacitance, and the effect of all the design parameters on DCS protection performance is analyzed. The optimal design of modified DCS protection system for the best performance is determined. The performance of the system is also evaluated by applying a design that is optimized for the scale-up HTS coil with a 30 mH self-inductance.

Quench Phenomena in a Conduction-Cooled Fast-Ramping High Temperature Superconducting Magnet

A conduction-cooled HTS magnet was tested to produce an AC magnetic field up to 4 T for an adiabatic demagnetization refrigerator (ADR). Quench detection was conducted by measuring the voltages in the conductor, and the protection system was constructed with an external dump resistor. The magnet, which is equipped with extensive copper thermal drains, was conduction-cooled by a two-stage Gifford-McMahon cryocooler to approximately 6 K. The magnet succeeded to stably generate an alternating magnetic field between 0 and 3.5 T at a ramp rate of 229 mT/s without any pre-quench signs; however, in the case of a maximum 4 T experiment, a quench occurred, and the magnet was permanently damaged. The quench location was identified as the place where the highest perpendicular field from the magnet was generated. It is speculated that the already-formed normal zone before quench grows very fast during the dump process, and part of the magnetic energy is dissipated in this normal zone, causing the excessive temperature rise of the conductor. The detailed experimental results are presented and discussed.

Quench Experiments of Conduction-Cooled Coated Conductors With Various Copper-Stabilizer Thicknesses

We experimentally studied the quench properties of REBa2Cu3Oy (RE-123) coated conductors with various plated-copper thicknesses (20 and 40 μm). A short sample of coated conductors was conduction-cooled to 45 K, a magnetic field (μ0H, up to 2 T) was applied perpendicular to its wide face to control the critical current, an operating current was supplied, and subsequently quench was initiated using a small heater. Normal zone propagation velocities (NZPVs) were measured at various operating currents, and the NZPVs of coated conductors with various copper thicknesses were compared with each other. To understand the impact of the copper stabilizer on quench protection, hot-spot temperatures were measured during the processes that simulate quench detection using voltage taps and protection using dump resistor. The maximum hot-spot temperatures were plotted against the operating current as well as the overall current density, and the impact of the thickness of the copper stabilizer on hot-spot temperature was examined. The impact of the initial temperature on hot-spot temperature was also studied.

Experimental Study on Quench Detection and Protection Conditions of Copper-Stabilized Coated Conductors Using Short Samples

In order to clarify the conditions for successful quench detection and protection of magnets wound with REBa2Cu3Oy (RE-123) coated conductors, experiments using short samples of copper-stabilized coated conductors were performed. A short sample of coated conductor was conduction-cooled to 30 K, a magnetic field was applied (µ0 H up to 2 T), a current was supplied, and subsequently quench was initiated using a small heater. When a normal voltage (over a preset threshold) was detected, the supplied current would be decreased exponentially. This procedure simulated the conventional quench detection (detecting voltage) and protection (using a dump resistor; current decaying exponentially with a time constant = coil inductance/resistance of dump resistor) for superconducting magnets. We examined how the critical current, time constant of current decrease, and voltage threshold for quench detection affected the hot-spot temperature. By comparing the critical currents and n values of a sample before and after quench, we identified the maximum current at which the sample exhibited no degradation (not-degrading current). The effect of transverse thermal diffusion and longitudinal thermal conduction was evaluated using one-dimensional/zero-dimensional quench simulation. The conditions for successful quench detection and protection were evaluated using not-degrading currents as well as the current at which the maximum hot-spot temperature reached 300 K.

Spark Detection and Search for High-Voltage Paschen Leaks in a Large Superconducting Coil System

The plasma fusion experiment Wendelstein 7-X (W7-X) uses a system of 50 nonplanar and 20 planar superconducting coils. These coils produce the magnetic field that is required to confine the plasma. Magnetic flux densities up to 3 tesla can be reached in the center of the plasma. Supercritical helium is used to cool down the coils to operating temperatures below 4 K. The coils are specified for currents up to 18 kA. A critical issue in all superconductors is the occurrence of quenches. These are unwanted local transitions from superconductivity to normal conductivity. If that happens, the coil current has to be discharged as fast as possible into a dump resistor. However, the strong current change will produce a self-induced high voltage (HV) with values up to several kilovolts. Therefore, the electrical coil insulation versus ground has to be HV proof to avoid a high-current arc in any circumstance. Worst case scenario is a quench that is induced by a loss of thermal insulation after a leak in the helium supply lines, because Paschen-minimum conditions could be given. This article describes some HV test procedures and techniques employed to test and qualify the coil system against that scenario. Some of these techniques are used today for the routine coil tests. Some other turned out as inefficient for daily use. Essential for a safe machine operation is the detection of insulation defects, but also on their localization along the superconductor, for the sake of a later repair.

Quench protection system for an HTS coil that uses Cu tape co-wound with an HTS tape

A quench protection system for a high-temperature superconducting (HTS) coil using Cu tape co-wound with an HTS tape was studied by numerical analysis using a thermal model of a coil winding pack. In normal operation, the voltage across the resistive zone Vs in the HTS coil was monitored by measuring the voltage difference between the HTS coil and the co-wound Cu coil. When Vs exceeded the quench detection voltage, the HTS coil was disconnected from the power supply and the energy stored in the HTS coil was dumped into a dump resistor. At the same time, the terminals of the co-wound coil were connected to another resistor, and some of the current in the HTS coil was quickly transferred to the co-wound Cu tape coil due to the tight magnetic coupling of both coils. This is expected to decrease the hot-spot temperature due to the quick decrease of HTS coil current and to protect the coil from damage caused by overheating at the hot-spot. The analysis investigated the effectiveness of this method.

Electromagnetic Design, Fabrication, and Test of LPF1: A 10.2-T Common-Coil Dipole Magnet With Graded Coil Configuration

R&D of high-field accelerator magnets is ongoing at the Institute of High Energy Physics for the prestudy of high-energy accelerators in the future. As the first step, a 12-T subscale common-coil dipole magnet named Let the Proton Fly (LPF1) with two 7-mm apertures and graded coil configuration was designed, fabricated, and tested. With 4 NbTi double-pancake racetrack coils outside and 2 Nb3Sn double-pancake racetrack coils inside, this hybrid dipole magnet was designed to provide a 12-T main field in two apertures operating at 6100 A. The load line ratio is 83% @ 4.2 K for the designed current and field. To reduce the field enhancement at the ends of the coils, the coils were designed with different lengths. All of the six coils were wound with superconducting Rutherford cables and impregnated with epoxy CTD-101k. Bladders and keys technology was used to prestress coils during the assembly of LPF1. A 0.1 Ω dump resistor and a voltage-dependent varistor were used for the quench protection during the test of LPF1. LPF1 was tested at 4.2 K under the self-field and a field plateau was shown around 10.2 T after the 13th quench. The parameters of the design, process of the fabrication, and the test performance of LPF1 are presented in this paper.

Quench-Back Management for Fast Decaying Currents in SHMS Superconducting Magnets at Jefferson Lab

The Super High Momentum Spectrometer (SHMS) of Hall C, part of the 12-GeV upgrade at Jefferson Lab, was successfully commissioned in 2017. Early operation showed that fast current dumps decays of the SHMS Q2/Q3 and dipole superconducting magnets triggered quenches, causing some level of operational difficulty. Tests and detailed analyses indicate that nonquench induced fast current decay could result in substantial ac loss in the conductor and subsequently trigger a quench-back effect. The OPERA/ELECTRA software package was used to calculate the amount of heat deposited in the copper stabilizer from a fast current decay. The resistances of external energy dump resistors were lowered to slow the fast dumping of the magnet's current to reduce or eliminate the quench-back effect. A worst-case adiabatic quench scenario was analyzed, assuming no external dump resistor and no liquid helium surrounding the coil, to ensure the safety of the magnets. The stress levels in the coil imposed by winding, collaring preload, Lorentz force, and temperature gradient during a quench were also examined. The Tsai-Wu material failure criterion was used to determine the acceptable combined stress level. Linear orthotropic analysis of the coil indicates that the magnets can be operated safely with appropriately sized dump resistors. Fast dump tests with the modified dump resistors have been planned to verify the performance and suitability.

A method for interplay analysis between quench protection and quench characteristics of close-wound superconducting magnet

The quench protection of the close-wound superconducting magnet is a key factor during the design process, especially when used on the particle therapy gantry system. Therefore, the interplay between quench protection and coil quench needs to be considered from the earliest stage of the design. For this purpose, a method using Cryosoft code is presented in this paper. Based on the quench protection electric circuit, the current and voltage evolution of the quench protection circuit and the temperature and normal zone evolution of the superconducting coil all can be obtained with this method. By studying the allowable action time for the quench protection associated with the operating current, the quench voltage threshold and the resistance of the dump resistor are determined, which is 0.4 V and 0.3 Ω. The differences of the voltage evolutions obtained with different kinds of dump resistor connections are also studied. The results show that it is better to have the dump resistor directly connected to the coil than with a switch. The advantage of this method is its integration and comprehensiveness and can be used in the design procedure of different kinds of magnets.