A 0.5MVA/1MJ superconducting fault current limiter-magnetic energy storage system (SFCL-MES) is under development. The SFCL-MES is used to enhance the low voltage ride through capability and smooth the output power of the wind farm. The SFCL-MES is composed of four major components: a superconducting coil, a power conversion system, a cryogenic refrigeration system, and a monitoring system. In this paper, the design and development progress of the four major components are introduced in detail.

SigmaPhi and the CNRS develop a HTS SMES to demonstrate the feasibility to storage energy in magnet at very high density. In addition, the same team with the aid of ISL develop a HTS dipole to demonstrate the feasibility of a new concept (called S3EL) of electromagnetic railgun.
For the SMES, the aim is to design and to construct a magnet which stores 1 MJ with a density of 20 kJ/kg of conductor. As the energy stored in the magnet is relatively low, a high critical current density, > 400 A/mm², and a high hoop stress, > 300 MPa, are required to reach the energy density criteria. The only type of conductor capable to achieve these requirements is 2G YBCO coated conductor. Due to the sensibility of YBCO conductor to the transverse field, a D shape torus has been chosen to increase the available critical current. The protection of a HTS magnet is always a difficult issue but also an unavoidable source of weight. To limit the weight due to the stabilizer, the magnet will be protected by an aluminum tape co-welded on the conductor.
The second magnet is a HTS racetrack dipole with an electromagnetic railgun inserted inside. The function of the dipole is double. The first is to decrease significantly the current required by the railgun. Indeed, in a classical railgun the magnetic field is only generate by the rails which require a very high current to be significant. The second is to serve as a SMES powering for the projectile. To maintain the magnetic field of the dipole, only few percent of the energy stored in the dipole are transferred in the projectile at each shot. Naturally the current available for the railgun is limited by the current flowing in the dipole. However by subdividing the magnet in several part and in connecting them in parallel with the railgun, the current can be multiplied.
This paper presents the state of the development for these two HTS magnets on more specifically the electromechanical design.

This work is supported by the DGA (French General Directorate for Armament), in cooperation with ISL (Saint Louis Institute) and CNRS (French National Centre for Scientific Research) within the framework of the BOSSE project.

A hybrid device, which integrates three different applications in energy infrastructures, is presented, offering a significant savings in energy, device cost and space. In the heart of this integrated device is a Fault Current Limiter (FCL) based on saturated magnetic cores. Magnetic saturation is achieved by using a current carrying superconducting coil. The core goes out of saturation only in the rare case of a serious electric short, when current limiting is essential. In the rest of the time (>99.99%), the FCL sits idle. The idea is to use the device during this idle time as a Superconducting Magnetic Energy Storage (SMES). Namely, to use electromagnetic energy stored in this current carrying superconductor for injecting/receiving current into/from the utility in order to improve energy quality. The device is designed in a way that even in its deepest discharge state, the magnetic field generated in the coil is sufficient to maintain the FCL cores in saturation state.
Moreover, it is possible to use also the superconducting coil as a source for high magnetic field that can be used, for example, for magnetic separation of paramagnetic waste, purifying used water during idle time of the FCL.
In this work we describe the concept of the hybrid device as well as its realization in a bench model utilizing liquid nitrogen cooled copper coils instead of superconducting coils. The operation of the complementary FCL - SMES functions is studied.
The suggested hybrid device carries the potential for a breakthrough in integrating green technologies, smart grids and superconductivity.

This work was supported in part by the Israeli Ministry of Infrastructures, Energy and Water Resources

Superconducting magnetic energy storage (SMES) system can be utilized in power grid to improve operation stability and power quality. To better analyze the operation characteristic of SMES, the detailed modeling of SMES is quite necessary. This paper proposes a novel detailed model of SMES considering its AC loss of dynamic operation. The voltage distribution of the HTS magnet can be achieved which can help to the design of magnet insulation and protection method. A voltage source type power converter and the corresponding control strategy are also designed to interface the superconducting magnet and power grid. The co-simulation of MATLAB and COMSOL is conducted to build the detailed model of SMES and verify the effectiveness of power converter and its control strategy.

Abstract: As the manufacture of Superconducting magnetic energy storage (SMES) is mature nowadays, the SMES is widely applied in the power system. In this paper, the application of distributed hybrid energy storage system (DHESS) consist of SMES in micro-grid is studied. At first, transient thermal characteristics and power characteristic of SMES is discussed. Then, the application pattern of the DHESS in the micro-grid is studied. With characteristic of quick power response, SMES is allocated in the DC Sub-micro-grid and utilized to maintain the dc bus steady, while the battery is connected to the AC bus of micro-grid for energy management. At last, control strategies of DHESS which have considering the characteristic of SMES is put forward. The control strategies and the performance of SMES is evaluated in a complex micro-grid model based on the MATLAB/SIMULINK simulation.

Superconducting Magnetic Energy Storage (SMES) is thought to be one of the most effective devices for a large scale electric energy system because it has the advantage of high efficiency and the fastest charge and discharge rate of energy with a high performance. Two kinds of coils with High Temperature Superconducting (HTS) conductors, solenoid and toroid types, are generally adopted for HTS SMES system and each types has different electromagnetic properties and stress characteristics. Recently, the toroid type HTS coil has been considered for a major coil shape for a large capacity SMES system because of the smaller perpendicular component of the magnetic flux density on the HTS conductor. However, 3-dimensional calculations are required to analyze or design the toroid type HTS coil which would be complicated and necessarily take very long calculation time. In order to suggest a better design solution for an HTS SMES coil, we need an accurate, fast, and effective calculation of electromagnetic and mechanical characteristics. In this paper, we suggested an intuitive and effective way to decide the maximum magnetic field intensity in the HTS coil and mechanical stress analysis instead of FEM. We could achieve a remarkable reduction of the calculation time and effective design by using this method. After the successful substitution of this calculation method for the design program we are making, we will also try to make a fast optimum solution for a large scale HTS SMES coil using calculating results, as a future work.

This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education (2014R1A1A2058788)

The interest of energy storage system (ESS) has been increased because of the connection of distribution generators (DGs) through renewable energies. The superconducting magnetic energy storage (SMES) magnet is one of the ESS technology. It stores magnetic energy in superconducting magnets and the number of magnets is required. As the number of HTS magnets is increased, finding proper operation mode is important for HTS power converting system.
This paper dealt with various timing sequential controls of four heater triggered switches and two energy storage magnets (ESM) which consist of two energy storage magnets, 29.1 mH and 29.5 mH, through experiments. The experiments mainly separated two mode. The first one is that the timing sequential controls of two ESMs operate simultaneously. And the other one is that the timing sequential controls operate alternately for charging and discharging the EMSs. Each 5 modes are tested at simultaneous and alternating operating mode for charging the ESMs. And Each 2 modes are tested for discharging the ESMs.

This work was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education. (Grant 2011-0009232).

The Massachusetts Institute of Technology has been performing a preliminary study of Superconducting Magnetic Energy Storage (SMES) magnet configurations under a Pôle MecaTech Cluster collaboration sponsored by the government of the Walloon Region in Belgium. Consortium members include: Ion Beam Applications, S.A. (IBA), CE+T Power (CE+T), Euro-Diesel S.A. (Euro-Diesel), Jema S.A. (JEMA), University of Liege (ULG), Catholic University of Louvain-la-Neuve (UCL), and Liege Space Center (CSL). Various SMES magnet configurations designed and built in the past were considered and compared by their specific energy and magnetic field shielding properties.
Among the others, the Constant Field Toroidal (CFT) configuration attracted our attention and it is the subject of this presentation. It is well known that the toroidal magnet configuration presents superior field containment resulting in very low stray fields. This gives toroids a significant advantage over other, mostly solenoidal, SMES magnet configurations. The main drawback of the conventional toroidal magnets is that the distribution of the magnetic field peaks at the outer side of the inner coil leg and then drops as 1/r, resulting in rather inefficient space utilization and consequently relatively low specific energy. To mitigate this disadvantage a toroidal magnet comprised of graded coils creating an almost constant B-field in a large volume inside the magnet is considered in this study. This significantly enhances the specific energy of the magnet while retaining the advantage of the low stray fields.
The present study presents the description of the magnetic design algorithm and an example of the first level optimization of the stored and specific energy. Numerical simulations are performed using Vector Fields Opera and formulae, and a procedure of a simplified analytical optimization of a magnet system are derived and used to maximize the stored energy or the specific energy (per unit weight of the conductor) of a CFT magnet.

Conceptual Design of High Tc Superconducting Magnetic Energy Storage System for Transient Stability of Power System
Poulomi Brahmachary, Student Member, IEEE, V.V. Rao, Member, IEEE
Applied Superconductivity Laboratory, Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur- 721302, India
Superconducting Magnetic Energy Storage System (SMES) based on High Tc materials is ideally suitable for solving the power system problems as it has the ability to dynamically exchange required amount of energy with the grid. The high coil cost, low voltage rating and lack of optimized online control are resisting it to be available commercially for high voltage grid applications. In this paper effort is given to make an application based conceptual design of a High Tc based SMES system to mitigate the transient stability problem of a power system. The capacity of the required SMES is first decided by simulation results of transient stability by creating a three phase fault in a Single Machine Infinite Bus (SMIB) system. Based on this rating of SMES, a cost effective superconducting coil is designed by optimizing the operating current (decided by Bc – Ic charecteristic) and self inductance (decided by coil geometry) with a minimum length of the High Tc Superconducting (HTS) tape.

The HVDC transmission system needs high-capacity (large inductance, high transport current) DC reactors to decrease the DC current ripple and harmonics caused by power conversion systems. However, high-capacity DC reactors experience a lot of electrical loss due to the resistance of their copper winding. Furthermore, an enormous reactor size, heavy weight, and wide open installation space are required.
In this sense, a HTS DC reactor can be an alternative to diminish disadvantages associated with general DC reactors. The HTS DC reactor was made by thin-walled HTS wire. It can reduced the size and weight of the reactor magnet compared to the conventional reactor magnet. In addition, HTS has zero electric resistance under DC current conditions. It is possible to high level current density operation and electrical losses decrease. From the flux leakage point of view, the toroid-type magnet is an ideal structure. The toroid-type HTS magnet has much less flux leakage because most magnetic flux line patterns are formed by the shape of the toroid-type magnet. Therefore, the installation space can be decreased with the toroid-type HTS DC reactor.
This paper presents an economical study of the toroid-type HTS DC reactor compared with conventional DC reactor. In the economical analysis of the HTS DC reactor, the life cycle cost was calculated including the investment and operation (included maintenance) costs of the toroid-type HTS DC reactor, the construction costs and the costs of secondary equipment (protection, converter, cooling system). To compare the approach with the conventional DC reactor, discussion and calculation is carried out in terms of factors that affect the interconnection: investment cost, reliability, energy loss, capacity. The analysis shows that the HTS DC reactor can be interesting, whereas the toroid-type HTS DC reactor can hardly compete economically with existing conventional DC reactor.

This work was supported by the Power Generation & Electricity Delivery Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea. (No. 20142020103560)

Embedded electrical grids are constantly subject to problems of stability. Some responses can be done with the help of resistive superconducting fault current limiters (RSFCL). In this paper , we propose a new system to stabilize DC grid named superconducting stabilizer (SS).
The SS uses the AC current component to increase the stability margin in DC grid. In fact, if the load increases beyond a threshold limit, the DC electrical grid becomes instable and the current presents DC + AC components. When DC grid works at stable point, only DC current is present in the system. From the point of view of the SS, the fact that it is in its superconducting state induces very low losses in this case that can be negligible in comparison with other losses in DC grid. In the other case (instability case), the AC current component induces losses in the SS. In fact, the value of the AC current exceeds the Ic critical value of the superconducting element. It will be show that the increase of losses in the SS can stabilize the DC grid with efficiency.
To obtain optimal results, the SS has to be dimensioned with great accuracy. The measurements of Ic and AC loss of a superconducting tape are necessary. These measurements have been done with DI-BiSSCO tape and presented in this paper. From these measurements, the useful length of the tape is calculated from a specification of a typical embedded system. Simulations of DC grid instability are presented without and with SS to prove the effectiveness of the proposed superconducting element. Future work is to make real SS in real DC grid.

This work is supported by the National Centre for Research and Development under grant PBS1/A4/1/2012
 

To assess the physical nature of vector potentials, we created a complicated coiled coil to generate a vector potential. The device comprises a long flexible solenoid wound around a hollow cylinder, which gives the entire coil a coiled-coil structure. We call such a coiled coil a ‘vector-potential coil’ (VPC), and when the VPC is connected to a current source, a curl-free vector potential oriented parallel to the cylinder axis is created in the hollow core of the cylinder.
We also implemented a vector-potential transformer (VPT) by adding a secondary coil (a straight wire) in the hollow core of the VPC. Driving the primary coil of VPT with alternating current (AC) caused a voltage difference across the secondary coil, even though the secondary coil was not exposed to any magnetic fields.
The secondary voltage appeared even when the secondary coil was enclosed by a normal conducting material or a magnetic material. A vector potential transmit through the both the electric and magnetic shielding.
We also replaced the secondary coil with a high-temperature superconductor (BSCCO) wire maintained at 77 K. For protection, the superconductor wire was covered by a silver alloy. The cryostat was constructed from polystyrene, which is nonconducting and nonmagnetic, and copper wire was used as the lead wire to connect the voltmeter.
The secondary coil comprising a superconducting cable resulted in the same secondary voltage as a secondary coil made of normal metal (cupper). Thus, regardless of whether the secondary coil was normal or superconducting, a curl-free vector potential acts the same, at least for a macroscopic system.
A disadvantage of the VPT is that it requires a very long solenoid as primary coil, which is not always appropriate for high-frequency applications or applications that require low power consumption. However, these drawbacks may be mitigated by using a superconducting coil for primary coil.

This research was partially supported by the Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Scientific Research (C), 2013-2015 (25420398).

The 1 MVA 11/0.4 kV HTS transformer designed and built by a New Zealand - Australian team recently completed electrical testing. Points of difference for this transformer included: the use of coated conductor throughout with Roebel cable used in the low voltage (LV) winding, the highest LV transformer winding current demonstrated to date at 1.4 kA, and a versatile thermo-siphon cooling system for supplying sub-cooled liquid nitrogen at 70 K. What particularly distinguished this HTS transformer project is the combination of AC loss measurement and modelling of the windings using the minimum magnetic energy variation (MMEV) method. Modelling and measurement agreed well particularly at high current, providing a validation of the application of the MMEV modelling to HTS transformers. Extensions of the modelling to a transformer winding with significantly higher rating, a 40 MVA 110/11kV transformer, have involved the modelling of windings of some complexity - the transformer contains around 1400 turns in the high voltage (HV) winding and typically 80 turns of 16-strand Roebel cable in the LV winding, totalling more than 1200 strands. The modelling predicts that, when coupled with a high efficiency cryocooler, the AC loss of an HTS transformer can be a fraction of the load loss of a conventional transformer.
We report here extensions of the AC loss modelling to 40 MVA HTS transformer windings with 10 % reactive impedance with varying winding parameters: volts per turn, and conductor spacing in the HV winding, both inter-disc and inter-turn spacing. These provide an important check on the dependence of the calculated loss on such parameter changes.
Using the modelled loss we calculate the total cost of ownership (TCO) of an HTS transformer of 40 MVA rating compared to a conventional oil-immersed copper transformer. To derive the maximum value from the low load loss of the HTS transformer it is necessary to target applications where the transformer operates at rated load continuously such as generator step-up (GSU) transformers for base load generation. The results show that that an HTS base load GSU transformer can be competitive with conventional transformers assuming an HTS conductor price of 50 USD/kA.m, consistent with manufacturer’s price projections, and realistic values for the lifetime cost of losses.

When SFCL(Superconducting Fault Current Limiter)s are applied to an actual power system, it is very important to carry out power system studies on the impact of SFCLs to protective relay systems. In general, SFCLs have a negative impact on the protective coordination in power transmission system because of the variable impedance of SFCLs. In Korea, distance relays are applied to 154 kV power transmission systems as the backup protection system. This paper proposes the structure of an adaptive distance relay that be based on a novel protection algorithm and suggest the implementation hardware model using a real-time controller. In addition, we present the performance evaluation and results of the test under various fault simulation using the real-time playback or the real-time simulator.

This work was supported by the Power Generation & Electricity Delivery program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Ministry of Trade, Industry and Energy Republic of Korea.

The ability of the high temperature superconductor (HTS) transformer to deliver energy without loss brings tremendous benefits—compared to conventional ones—to power system planning and operation in terms of secure supply, energy efficiency, cost reduction, safety, and sustainability. Accordingly, practical research on the development of the HTS transformers has been conducted by industries and academia, and its prototypes have been tested in various demonstration projects, resulting in almost being ready for commercialization. In accordance with these efforts for actual implementation of the HTS transformer, an intelligent monitoring system on health status of the HTS transformer should be well-prepared for the purpose of reliable and robust operation at system level. The paper proposes an intelligent monitoring system that can diagnose the health conditions of the HTS transformer based on its physical characteristics, which include magnetic saturation of the iron core as well as thermally dependent resistance, using dynamic state estimation.
In details, the monitoring system is continuously measuring operational quantities such as voltages and currents with high sampling rates of more than 1 kHz, simultaneously observing whether the transformer is operating normally according to its dynamic behavior. The criterion of the observation is determined based on dynamic state estimation applied on two or three consecutive samples followed by statistical evaluation on the goodness of fit of the measurements to dynamic models of the HTS transformer; the evaluation finally quantifies the goodness of fit into the confidence level, which indicates the health index of the HTS transformer, i.e., the low confidence level means existence of any internal faults. The merits of the proposed monitoring system are that it can keep tracking of the dynamic, physical characteristics of the HTS transformer and that the criterion of existence of faults is adaptive to the operating situations; for example, the dynamic model can reflect the change of resistance from the superconducting state to normal one due to inrush currents generated when the HTS transformer is energizing.
The paper presents results of numerical simulation with various fault situations to validate the feasibility.

A transformer is one of the most important elements in the electric power system. The conventional transformers have a very high volume/weight ratio and some accidents occur because of the oil. The superconducting transformers can reduce this ratio and avoid accidents, because they need liquid nitrogen and not oil. Besides that, a new function can be observed in this type of prototype: the limitation of fault current, which provides protection to the power system. Two types of single phase transformer were designed and built in laboratory: 1) A superconducting transformer, with both windings made with second generation (2G) high temperature superconducting wires from Superpower (SCS4050); 2) A conventional transformer with both windings made with copper wire, having exactly the same parameters of the superconducting one. The transformation ratio was designed to be 2:1, with 64 and 32 turns. The coil transformer was constructed in a double pancake configuration for all of the windings. In order to estimate the self-field and help in the coil design, simple FEM simulations were made supplying a current below the critical value. The core applied in this prototype has almost 20cm high. The VxI curves of the coils allowed to estimate the operation region of the transformer. Short and open circuit measurements were made in each prototype. Results comparing both topologies under operational and short circuit conditions will be presented in the final version of this work. Those experimental results will help us to model the superconducting transformers in future works, in order to upscale new prototypes.

In another work, we developed a single-phase and a three-phase current-limiting transformers using magnetic flux transfer between the independent primary and secondary iron cores. This arrangement has been set-up as a switch-gear with DC current in the grid. The DC current is used to trigger the transition between the normal state to superconducting state to stop the energy transfer. This method is found to be useful to increase the life-time of circuit breakers. The designed arrangement is able to limit the fault current in the grid for the usual protection. In our experiments, the coupling superconducting wire had DC and AC current simultaneously, in fault and no-fault conditions. We show that the results are found both experimentally and simulations for the realistic cases. With decoupling AC energy transfer, it is shown that conventional switching off may be replaced with this proposed application and it is likely that it may be used in the real potential applications of the grid.

This research has been financially supported by Republic of Turkey Ministry of Development (Grant No. 2010K120520).

With the growing demand of quality power for modern industries, India is facing difficulty to maintain the grid against frequent instabilities occurring in various subsystems of power grid (generator, transmission line and load). The most crucial among them is Transient Stability problem of alternator during large disturbance. Superconducting Magnetic Energy Storage Systems (SMES) based on High Tc materials are ideally suitable and cost effective for solving this. It can instantly exchange required amount of magnetic energy with the grid. This paper describes a conceptual design of a High Tc based SMES system of 33MJ/ 150 MW capacity.

Liquid nitrogen (LN2) based insulation systems for superconducting components are state of the art. As the dielectric strength of LN2 based insulation systems can be significantly reduced if bubbles occur due to heat losses of the current carrying conductor, a solid insulation system using LN2 only for cooling but not as electrical insulation could be an alternative.
Due to the fact that polymeric materials feature comparable high volume shrinkages when cooled to LN2 temperatures they are ineligible in pure form. This paper deals with syntactic foam as a solid substitution of LN2 (LNT) based insulation systems. Syntactic foam consists of a polymer matrix with embedded hollow microspheres (HMS) with diameters of several 10 µm. Compared to the pure material a polymer filled with HMS features a lower density, a lower relative permittivity and a reduced thermal contraction. Former investigations of syntactic foam also show good dielectric properties of syntactic foam at LNT. Within some superconducting components the superconductor needs to be cooled through the insulation system, thus, the knowledge of the thermal conductivity at LNT is necessary.
For the investigation of the thermal conductivity of syntactic foam, epoxy resin (ER) and unsaturated polyester resin (UPR) serve as matrix materials. The HMS used are made of silanized glass and ceramic. A silanization of the HMS results in a better adhesion between matrix and HMS. By comparing the measurement results determined at cryogenic temperature with those at ambient temperature the influence of the temperature on the thermal conductivity is estimated.
The results show that syntactic foams feature reduced thermal conductivities at cryogenic temperature (∼60 % less). Furthermore, syntactic foams based on ER are thermally more conductive than syntactic foams based on UPR. Silanized glass HMS cause a decreasing thermal conductivity due to an increasing filling degree. The thermal conductivity of syntactic foam with ceramic HMS increases with increasing filling degree.

The authors would like to thank Mr. Daniel Schürmann, Mr. David Elsner and Mr. Guido Seelbach for their assistance in preparing and carrying out the experiments.
This project is funded by Deutsche Forschungsgemeinschaft (DFG) under the reference number SCHN 728/10-1.

Resistive high temperature superconducting fault current limiters (R-SCFCL’s) based on second generation superconducting tapes (2G tapes) are successfully used in the medium voltage level. To develop R-SCFCL’s for high voltage applications exceeding 100 kV the dielectric properties of liquid nitrogen as insulation medium in different arrangements have to be investigated more in detail.
Considering R-SCFCL’s the 2G tapes are arranged in bifilar coils. Adjacent tapes are insulated by a combination of solids and liquid nitrogen resulting in a so-called surface or gliding discharge arrangement. Surface discharges, well-known in gasses (Toepler's law), occur on the surface of the solid insulation and are able to grow from one electrode to the other leading to a flashover at applied voltages remarkably smaller than the breakdown voltage of the solid material. The behavior in liquid nitrogen is not known in detail up to now.
In this context surface discharge arrangements formed by thin polymer foils have been investigated. Specimens of foil material were inserted between two electrodes and immersed in liquid nitrogen within an open isolated vessel. The applied AC voltage was raised until flashover occurred and the shortest overhang of the foil material was recognized as flashover length. The investigations were performed for the polymer materials PI, PTFE and PET and thicknesses of the foils from 75 µm to 250 µm. The results reveal that the maximum flashover length is rather small in comparison to arrangements in gasses. At a creepage distance above 10 mm breakdown of the polymer foils occurs before flashover in most cases.
Preliminary measurements applying surge voltages have been performed. But significant results require far more measurements, as surge voltage discharges occur statistically distributed. Further tests applying surge voltages and additionally measurements within a closed cryostat shall be object to future research.

This work is funded by the German Federal Ministry of Education and Research (BMBF) and cared by Projektträger Jülich (PtJ). The authors thank Mathias Noe and Stefan Fink from ITEP/KIT and Judith Schramm and Achim Hobl from Nexans for fruitful discussions and their advice to this work and the company DuPont for providing foil samples.

Liquid nitrogen is applicable as coolant and insulation for high temperature superconductors and offers a more economical alternative to liquid helium because of its higher liquid temperature. For failure-prevention knowledge of the electrical withstand capability is required. To gain this knowledge many experiments have been done, which showed that there are a lot of influences on the breakdown voltage. These influences are for example, the pressure, temperature, size and form of gaseous nitrogen bubbles, applied voltage form, electrode geometry and many more.
Experiments conducted at this institute covering some of these influences have already been presented in several other works. These experiments combined with the available scientific literature led to the evolution of practically-oriented formulas to calculate the breakdown voltage in liquid nitrogen with and without gas bubbles.
In this work, the evolution of the mentioned formulas is presented in addition to an explanation of their respective background. Following, possible influences on the breakdown voltage found in the formulas are emphasized. Then the degree of effect these influences have on the calculated breakdown voltages is calculated and presented. Finally a ranking of which influence has the greatest influence on the breakdown voltage is given.

Resistive high temperature superconducting fault current limiters (R-SCFCL’s) based on second generation superconducting tapes (2G tapes) are successfully used in the medium voltage level. To develop R-SCFCL’s for the high voltage application exceeding 100 kV it is necessary to know the dielectric properties of liquid nitrogen more in detail, especially if sharp edges, occurring for example at 2G tapes or at connection conductors, cannot be avoided.
Consequently in this work the breakdown characteristics of so-called strongly inhomogeneous electrode configurations in liquid nitrogen were investigated. An electrode configuration of a plate electrode and a needle electrode with a rounding radius of roughly 0.1 mm or ball electrodes with radii from 0.5 mm to 4 mm was used. The distance between the electrodes was adjusted in the range of up to 12.5 mm. The electrode configuration was immersed in liquid nitrogen within an open isolated vessel and the AC breakdown voltage was measured in dependence of the rounding radius r of the high voltage electrode and the electrode distance d to the grounded plate electrode. For each combination of r and d the mean breakdown voltage was calculated from 15 single measurements.
The measuring results using the ball electrodes reveal that the mean breakdown voltage U follows approximately the empiric law U = c·d^b with b = 0.4 and c directly proportional to r. The mean values of the breakdown field strength are comparable high. The lowest value of 60 kV/cm is reached for r = 0.5 mm and d = 12.5 mm. For the needle electrode with a radius of roughly 0.1 mm the mean breakdown voltages are remarkably smaller and increase with the distance d only with an exponent b = 0.3.
Further tests are planned with increased electrode distances. Additionally tests applying surge voltages and measurements within a closed cryostat shall be object to future research.

This work is funded by the German Federal Ministry of Education and Research (BMBF) and cared by Projektträger Jülich (PtJ). The authors thank Mathias Noe and Stefan Fink from ITEP/KIT and Judith Schramm and Achim Hobl from Nexans for fruitful discussions and their advice to this work.

The progress in 3D-printing opens new ways to produce complex structures and therefore 3D printing may be an appropriate fabrication method for novel cryogenic high voltage devices with special geometrical requirements. One of the 3D printing techniques is the so called “fused deposition modeling” (FDM) which designates the printing of successive layers of material under computer control. The FDM printer for the fabrication of the described plastic samples is an “Ultimaker Original” with heated printbed.
Firstly some simple tests were performed with one sample geometry type using the materials Polyamide (PA), Polystyrene (PS), Polylactic acid (PLA) and Acrylonitrile butadiene styrene (ABS). The results with ABS material concerning mechanical, cryogenic and leak tightness properties were considered as most promising. High voltage breakdown strength of ABS disk samples was examined similar to standard IEC 60243 for alternating, direct and standard lightning impulse with unequal electrodes and liquid nitrogen as surrounding medium. In addition flashover voltage tests were performed. Mechanical properties of printed ABS samples were examined under cryogenic and room temperature conditions.
Finally some prototype high voltage devices were printed and tested and the application of 3D printing is discussed under high voltage, mechanical and leak tightness property aspects for several devices.

Electrical insulation in cryogenic environment can be categorized in two breakdown modes; static breakdown and dynamic breakdown. The static breakdown is the intrinsic or conventional breakdown of liquid nitrogen (LN₂) in steady condition under high voltage operation. On the other hand, the dynamic breakdown is the breakdown of LN₂ in transient condition, such as quench due to an overcurrent larger than the critical current, which is peculiar to HTS power apparatus. The dynamic breakdown is quite important, because the quench of superconductors would reduce the breakdown voltage of LN₂ in the explosive bubble disturbance than the static breakdown voltage of LN₂.
We have been investigating the dynamic breakdown characteristics of LN₂ under quasi-uniform electric field configuration. In this paper, the dynamic breakdown characteristics were investigated for different LN₂ pressures (0.1-0.2 MPa, 77 K). A nichrome sheet as the grounded electrode as well as the bubble generator was placed under a rod or sphere high voltage electrode (diameter: 6-50 mm, gap length: 2-4 mm). The transient bubble behavior in the gap volume was observed by a high-speed video camera (1000 frames/s). Experimental results revealed that the static breakdown strength increased with the LN₂ pressure and decreased against the highly stressed LN₂ volume (SLV). The dynamic breakdown strength under transient electrical/thermal combined stress was lower than the static breakdown strength. The difference between the static and dynamic breakdown strength became smaller with the increase in the LN₂ pressure and SLV. The reduction rate of dynamic breakdown strength against the static breakdown strength was systematized in terms of pressure dependence and size effect of breakdown strength of LN₂, which was interpreted by the reduced size and number of bubbles in LN₂.

In the growing field of cryogenic applications the proper choice of material is an important topic. For some application also high voltage performance at cryogenic regime needs to be investigated. The Cryogenic High Voltage Lab at the KIT-ITEP is able to perform such tests, for this purpose a material is needed to perform flashover voltage investigations at cryogenic temperatures. The following material properties are required for this application: high flashover resistance, smooth surface to enable easy sliding of stainless steel components, and appropriate mechanical properties at RT and 77K. Also, low costs and simple mechanical handling are desired. One possible candidate was found to be PE300. High voltage tests and mechanical investigations were performed on this material to check its applicability as an insulating material in the cryogenic environment. Thermal expansion is also measured down to low temperatures to estimate a possible mismatch between different materials during cool down. As it was expected, mechanical properties differ significantly when temperature changes from RT down to 77K. Flashover voltage values were investigated for alternating, direct and standard lightning impulse voltage with PE300 disks for lengths up to 6mm in slightly boiling liquid nitrogen as well as in nitrogen liquid and gas mixture.
In this report the measurement results are presented, and a usability concept of PE300 post insulators in cryogenic environment is discussed.

The superconducting magnet system is the key part of international thermonuclear experimental reactor (ITER) device which has attracted much attention in many countries. Due to its excellent performance in many aspects, epoxy resin has been selected as the base material of superconducting magnet insulation. Considering the unique operating environment, epoxy resin faces the challenge of liquid helium and nitrogen temperature as well as pulse voltage. The influence of pulse frequency and polarity on the electrical tree characteristics in epoxy resin under low temperature is investigated in this paper. Samples made of epoxy resin were stressed with the different frequencies in the needle-plate geometry electrodes. The frequency of pulse voltage varied from 100 to 500 Hz with positive and negative polarities. The experimental temperature was set to -30, -90 and -196 °C. Accumulated damage and expansion coefficient (D/L) are employed to characterize the electrical tree. The experimental results indicate that low temperature, pulse frequency and polarity have an influence on treeing process. The typical tree structures in epoxy resin are obviously different with the variation of low temperature and pulse frequency. It is revealed that the low temperature is resistant to the growth of electrical tree and there are more small branches generating from the main branches with the decrease of temperature. However, when under the same low temperature, the tree initiation and propagation processes are accelerated by the higher pulse frequency. The density of the tree branches increases with the increase of pulse frequency. The accumulated damage presents a similar trend corresponding to branches density. Meanwhile, electrical tree tends to grow faster with the positive pulse than negative pulse under the low temperature. Moreover, the higher value of D/L at lower temperature implies that tree branches extend at a wide range direction.

This work is supported by the Chinese National Natural Science Foundation under the Grant 51277131. The National Basic Research Program of China (Program 973, Grant 2014CB239501 and 2014CB 239506).

The accurate measurement of the temperature of ITER magnets is of fundamental importance, to make sure that the magnets operate under well controlled and reliable conditions, and allow suitable flow distribution in the magnets through the helium piping. Therefore, the temperature measurements shall be reliable and accurate. In this paper, we present the full thermometric chain of ITER magnets, from the sensor and its attachment, to the electronic conditioning of the signals and the shielding system, as well as a discussion on the electromagnetic immunity of the system. The design is based on the one developed by CERN for the LHC, which has been further optimized thanks to thermal simulations carried out by CEA. The ITER specifications are challenging in terms of accuracy and call for severe environmental constraints, in particular regarding the distance between the sensors and the electronic measuring system. A focus will be made on this device, which has been recently developed by CEA: based on a lock-in measurement and amplification of small signals, and providing a web interface and a software to monitor and record temperatures, this measuring device provides a reliable, accurate, electromagnetically immune, and fast (100 Hz bandwidth) system for resistive temperature sensors between a few ohms to 100 kohms. Measurements are also available through Profinet and Modbus TCP fieldbus for process automation. A cryogenic test bench has been designed and built for the qualification of the full thermometric chain, with a particular attention to the accuracy of the measurement. First results of this qualification will be presented, which give good confidence in the reliability and accuracy of the thermometric system for ITER magnets.

The authors thank C. Mariette, R. Desbuisson, F. Bancel, G. Marzo, T. Jourdan, P. Saint-Bonnet, JM Mathonnet and P. Dalban-Moreynas for their technical support.
The views and opinions expressed herein do not necessarily reflect those of the ITER Organization
 

Fabrum Solutions and Callaghan Innovation have been developing large pulse tube cryocoolers based on Callaghan Innovation’s diaphragm pressure wave generators (DPWG). The pulse tube’s lack of moving parts in combination with the DPWG’s metal diaphragms produces a cost-effective, long life and robust cryocooler. The DPWG has had 10 years of development, resulting in a series of DPWGs ranging in input powers from 0.5 kW to 30 kW that have been coupled to a variety of in-line and coaxial pulse tubes. Two DPWGs have each had in excess of 7000 hours running to date. A 330 cc DPWG was designed and manufactured to run with an inline pulse tube with a target of 330 W at 77 K. Absolut System carried out the pulse tube design, manufacture by Fabrum Solutions with integration and testing by Callaghan Innovation. Over 400 W of cooling power at 77 K was achieved from the cryocooler. A sealed condensation chamber was added to the cold-head and connected to a Dewar via vacuum insulated lines for Nitrogen liquefaction. Three such pulse tubes were combined on a single 1000 cc DPWG to produce over 1000 W at 77 K. This paper details the development of the PT330 and PT1000 cryocoolers from initial lab prototypes through to commercial products, integrated into liquefiers and ready for use in applications such as: Liquid nitrogen production, re-liquefaction of storage tank boil-off, or cooling of High Temperature Superconductor applications.

The conduction cooling with cryocoolers makes cryogenic cooling systems more flexible, user-friendly and compact than the cryogen cooling. The cryogenic conduction cooling system for a 400 MHz HTS NMR magnet is thermally and structurally designed. The two-stage pulse tube cryocooler is used for thermal loads from the magnet system. It is a remote motor type to minimize the vibration in the NMR magnet. The first stage of the cryocooler is in charge of thermal loads from supporters, current leads and radiation shields, and the second stage is used for the cooling of the NMR magnet. The highly thermo-conductive cooling plates are inserted in between HTS double pancake coils to transfer the heat generated in coils, such as Joule heating at lap splice joints, to the cryocooler. The cooling plates and the cryocooler are thermally linked with flexible thermal straps. The thermal and structural analysis of the cooling system is performed for the system design. The temperature and mechanical stress distributions are calculated, and the system geometry for a thermally and structurally stable operation is determined from calculation results.

This work was supported by KBSI grant(D35611) to S.-G.L.

Since the highest temperature of refrigerant liquid nitrogen along the flow channel limits the current characteristics of the cable of the superconducting power transmission, the temperature rise of the liquid nitrogen, which is usually caused by heat leak, is expected to be low. Therefore, only from this point of view, the flow rate of the liquid nitrogen should be high. However, since the frictional resistance between the liquid nitrogen and the pipe surface, which results in the pressure loss of the circulation, increases by the second power of the flow rate, the increase of it demands higher discharge pressure of circulation pumps. This affects not only the circulation pumps themselves, but also the structures of the cryogenic pipes. Therefore the evaluation of the pressure loss with respect to the flow rate is indispensable for the design of the superconducting power transmission systems, in particular, with long transmission lines.
We have performed a circulation test of the liquid nitrogen with the 200 m class superconducting DC power transmission system at Chubu University, which includes pressure loss measurements by changing the flow rate. The results of the measurements are needed for the estimation of the operation of 500 m and 1 km class superconducting DC power transmission systems in Ishikari area in Japan. The circulation system of the 200 m class transmission system was recently modified to increase the flow rate so as to meet the condition of the Ishikari lines, which made experiments with the flow rates twice higher than previously possible. The results of the pressure loss are compared with a scaling calculated theoretically. These results will be presented in the conference together with the other measurements performed during the circulation test.

One of the important components of the ecological stress is the energy mining activity and energy transportation. Transition to renewable energy sources, energy loss reduction, and optimization of delivery systems can improve considerably the ecological situation on the Earth. It is considered that technologies on the basis of effect of superconductivity are the optimal approach to solve this problem. Unfortunately, high cost and complexity of HTS cable cooling systems restrain rapid progress in the field of large-scale application of HTS. Because a cryogen naturally heats up while flows through the cryogenic channel accumulating heat load, its density changes from point to point. Therefore, if it will be possible to achieve low hydraulic resistance and provide sufficient cryogen level difference a natural circulation will arise. Ideally, superconducting power transmission line may not contain the cryogenic pump. In order to approve this approach the experiments were carried out using 200-meter DC HTS facility at Chubu University.
Level difference together with the low hydraulic resistance caused by using direct smooth pipes creates favorable conditions for observation of the thermosiphon effect. The experiment began at the circulation flow rate of about 11 L/min. After a short-time preheating of liquid nitrogen, cryopump was turned off. Decaying positive oscillations of the flow rate were observed. The steady state at 2 L/min was achieved after about 40 min. The experiment lasted more than 6 hours without trouble, and then the cryopump was started again. The maximum output temperature was 80.3 K. Although the level difference was insignificant (2.6 m), the effect was strong enough to keep HTS cable temperature at an acceptable level.

KEPCO (Korea Electric Power Corporation) installed the HTS cable with the specification of 22.9 kV, 50 MVA, and 410 m length at the 154 kV Icheon substation with collaboration of LS cable & system in 2011. During the successful real grid operation of one and a half year, we have addressed the practical applicability, stepping into the modification of cryogenic system from decompression cooling system to closed system. The construction of closed cooling system composed of 4 kW Stirling cryocooler and 2 kW Brayton cryocooler is now in progress, and the grid operation is scheduled to energize on the fourth quarter of 2015. We present the cryogenic system design, installation and some results of preliminary tests in this paper.

This work was supported and funded by Korea Electric Power Corporation (KEPCO).

Viability of HTS devices in electrical engineering is mainly determined by their cryo-consumption. Integral losses of various devices such as HTS based FCLs, cables, transformers, current leads are analysed as a function of “critical” time-period during which the transport current approaches the critical current or even exceeds this value.
Unexpectedly high cryo-losses found when particular losses caused e.g. by supports cryo-vessel, current feed throughs, ac losses, and fractions of active impedance were summarized.
Especially high fraction of cryo-losses is caused by time-extended quench when spreading of the normal zone in the device exhibits a slow behaviour allowing the internal impedance to be almost equal to the external impedance for longer time. Within this time-period maximum energy/power absorption takes place causing not only huge increase of cryo-loss but also resulting in many cases damage of the entire device because of high local dissipation in the HTS element.
Representative example here is a well-known effect of “slow quench” which represents a significantly more dangerous event in comparison with “quick quench” which happens at much higher level of over-current.
A number concepts to be employed for improvement of cryo-performance of electrical devised include
- a principle of “quench quantization” where quenchable elements are split into a number of sub-elements each of which exhibits accelerated quick quench behaviour,
- measures for suppression of cryo-losses in the cryostats providing potentially a stand-alone function.
- advances in design of SFCL cryo-vessel,
- advanced cryo-cable and cryo-lead designs,
- novel methods for suppression of ac losses.
Application of these concepts in HTS based SMES and motors/generators are also considered.
Criteria that define applicability of HTS based electrical devices regarding their cryo-consumption are suggested.

The Central Solenoid Model Coil (CSMC) of China Fusion Energy Test Reactor (CFETR) is currently in the design and manufacture process. CSMC assembly consists of the winding pack, an outer NbTi coil，a middle Nb3Sn coil, an inner Nb3Sn coil and a pre-load structure. The highest field of the model coil is 12T, while the highest change rate of magnetic field of the conductor is 1.5T/s. Due to the AC losses during charging, a huge heat load will be produced in the model coil.In order to make the coil work properly in normal condition, a well-designed and precisely-analyzed cooling loop plays an important role.
In this paper, the design of the cooling loops is based on the calculation results of the AC losses deposited on the model coil. The length of the cooling channels, together with the thermohydraulic parameters such as inlet pressure, temperature, mass flow rate are optimized.In addition,thermal hydraulic analysis for the cooling loop located in the worst condition of the model coil was conducted to recognize the temperature and mass flow rate change over time. The hydraulic model, the material properties and the heat loads involved in the analysis are given, and the results of the analysis are presented.
Keywords:CFETR,CS model coil,Superconducting magnet, Thermal hydraulics.

We designed the cryogenic system of the 154 kV class SFCL. Our cryogenic system consists of a cold box, main cryostat and pressure builder. The cold box which has a cryocooler, heat exchanger, and pump cools liquid nitrogen and supplies cooled liquid nitrogen to the main cryostat. The cryocooler controls liquid nitrogen temperature, 71 K by helium pressure and compressor motor RPM change. In order to keeping dielectric strength of the liquid nitrogen and sub-cooled state, the pressure builder makes operation pressure, 0.5 MPa of the main cryostat using electrical heater and solenoid valve.
Test results for example supplied liquid nitrogen temperature and pressure of the main cryostat show appropriate performance correspond to the design goal.


This work was supported by the Power Generation and Electricity Delivery of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy.

The superconducting magnet test facility in the National Institute for Fusion Science (NIFS) has been upgraded for excitation tests at a wide temperature range and a higher magnetic field of 13 T. As part of the upgrade, the helium refrigerator/liquefier operated for 25 years was replaced with a temperature variable helium refrigerator/liquefier. The nominal refrigeration capacity is 600 W at 4.5 K (corresponding to liquid helium of 250 liters per hour or supercritical helium of 50 g/s) as the previous one. Besides, it has a new feature that can supply helium gas of a wide temperature range. The typical design cooling capacity is 1 kW under the condition of 20 K supply/30 K return and 1.5 kW under the condition of 40 K supply/50 K return.
After the replacement, a series of commissioning tests were performed under the various operational conditions. As the results, the satisfactory thermodynamic performance was confirmed. In the future, it is expected to progress the development of superconducting magnets with advanced superconductors such as MgB2, high temperature superconductors and so on. In the present paper, the design of the temperature variable helium refrigerator/liquefier and the results of the commissioning tests will be reported in detail.

The Large Helical Device (LHD)-type helical fusion reactor is well suited as a fusion power plant because it has attractive features such as steady-state operation in the absence of a plasma current drive. The National Institute for Fusion Science is developing a conceptual design of the LHD-type helical reactor, FFHR-d1. The superconducting magnet system of FFHR-d1 includes one pair of helical coils and two pairs of vertical field coils. The helical coil has major and minor radii of 15.6 m and 3.744 m, respectively. Several cooling schemes, such as forced flow with a cable-in-conduit conductor with a low temperature superconductor (LTS), indirect cooling with LTS, and helium gas cooling with a high temperature superconductor (HTS), have been proposed for the coils. Candidates for use as the superconductor for the coils are Nb₃Sn and Nb₃Al for the LTS, and YBCO for the HTS. A multi-scale stress analysis was used to assess various cooling schemes and superconductors. For example, assuming that 40 HTS tapes are stacked and covered with a copper block and a stainless steel jacket, and under the condition that they are completely in contact with each other, the maximum tensile strain and local shear stress of the conductor stack are estimated to be 0.145% and 32 MPa, respectively. In contrast, the coil using an LTS conductor can be wound continuously, while a segmented fabrication is possible using an HTS conductor. In the case of segmented fabrication, the length of the segment and the number of joints within the section are important from the perspectives of fabrication and thermal properties. Winding feasibility investigations using the 3D CAD system and additive manufacturing (3D printing) were conducted. The 3D printing model revealed the gap width required to insert the conductor segment into the coil casing. As a result, it was found that a maximum of one helical pitch could be inserted into the coil casing without deformation.

Today NbTi is the undisputed superconductor for use in the Poloidal Field coils (PF), Correction Coil (CC), graded Toroidal Field (TF) coils and Feeders of a fusion machine. However, a major advantage of MgB2 is it’s higher operating temperature and larger temperature margin. The larger temperature margin allows cost reduction of the cryogenic system and improvement of reliability. A Cable-In-Conduit Conductor (CICC) concept is adopted with a strand cable pattern designed for minimum interstrand coupling loss to limit the heat load and maximum strand mechanical support to avoid degradation from thermal and electromagnetic stress. The lead for the design and eventual test is a full-size MgB2 Poloidal Field conductor. The prospective to use state of the art MgB2 strands for the PF and CC superconductors of a fusion device is analyzed with the code JackPot-ACDC. The strand critical parameters for second-generation multifilamentary HyperTech MgB2 strand with a diameter of 0.83 mm serve as a critical input for the analysis of the computed conductor performance. The predicted MgB2 PF CICC performance is compared with the requirement of the present ITER PF design with maximum operating current of 45 kA and nominal peak field of 6 T.

Nb3Sn is considered to be one of the best candidate low temperature superconducting materials for the fusion superconducting magnet system. However, in consideration of the optimization design work of fusion device magnet system, higher volt seconds under a presupposition of a much more economical device size needs to be achieved. Hybrid center solenoid magnet which consists of low temperature superconducting coil and high temperature superconducting coil is one of the best way to improve the capacity of the volt seconds.
In our recent work, a new type of hybrid magnet has been designed for the central solenoid of tokamak device. It can provide a maximum volt seconds of 200 V.s with the maximum magnetic field of 17 T. The inner HTS part of the magnet is designed with YBCO conductor while the outer LTS part is designed with Nb3Sn conductor. The HTS conductor is optimized as multi-layer structure with smaller skeleton and lower AC losses. It can meet the requirements of high magnetic field and operation current. Lots of analysis work for the HTS part has been carried out based on its brittle ceramic configuration and difficult bending and twisting. This paper presents the study of hybrid magnet which includes electromagnetic and mechanical analysis, eddy current calculation, stability analysis work and so on. In addition, the experiment of HTS conductor such as critical current test has been carried out.

The work was supported by China National Magnetic Confinement Fusion Science Program (Grant No.2011GB114000).

The coil PF1 is part of the ITER superconducting magnet system. It is designed for positioning and shaping of the poloidal component of the ITER magnetic field. According to the specifications a number of technological operations on vacuumpressure impregnation (VPI) and subsequent compound polymerization should be carried out during manufacturing of the coil PF1, including technology optimization on a full-scale model of the coil.
With the aim to prepare for vacuum-pressure impregnation of the superconducting coil PF1 under manufacturing the authors have developed and manufactured the working mock-ups of the coil double pancakes and the proper assembled coil and designed the heating system. Numerical and experimental investigations have been carried out on the working mock-ups and numerical models so as to test the required temperature scenario for heating of the mock-ups of pancakes and coil, which will ensure the insulation with the required properties.
The paper presents the results of the numerical simulation and in-situ testing, which reveal the feasibility of the proposed technologies for impregnation of mock-ups of the ITER superconducting coil PF1.

A compact spherical tokamak (ST) has been developed and demonstrated 24-hour operation with the primary objective to test out the feasibility of the first fully superconducting HTS tokamak device. The ST25-HTS device is a 0.25m major radius tokamak of toroidal field 0.1T fitted with a six-limb TF magnet and two PF coils, all made from HTS flat tape YBCO conductors. Cryogenic cooling to ~20K is provided by a combination of helium gas flow and mechanical cooler. The ST25-HTS coils are a route to improving our understanding of the performance of HTS material in tokamak coils and plasma environment and will demonstrate many of the techniques required to construct high field compact spherical tokamak HTS coils. Results of TF coil tests on ST25-HTS are presented and major learning and challenges are discussed. ST25-HTS will be the first tokamak to be made entirely using HTS magnets, and is a valuable step in evaluating the engineering challenges for a spherical tokamak coils and structure.

The ITER Tokamak is surrounded by a magnet feeder system consisting of 31 units, which connect the magnets to their power supplies and also house the cryogenic lines and instrumentation cables needed to operate and monitor the magnet system.
As a part of this feeder system the superconducting busbar system for ITER magnets consists of NbTi cable-in-conduit conductors, the joints for electrical and hydraulic connection between the busbar sections and the coil terminals, and the structural rigid ducts and clamps to take the reaction electromagnetic loads by the busbars.
All 234 feeder joints utilize the twin-box “shaking hands” concept: inside a helium tight joint box a bare cable is compressed against an indium-tinned copper plate. The copper plates of two boxes are tightly bonded by compressed indium wires to form the joint. One of the challenges to the design is the weld between the joint boxes and the busbars without overheating the in-conduit NbTi cable during the welding. On top of that these welds are submitted to large operational electromagnetic loads in combination with differential displacements from the different coils to which the feeders are connected and/or supported. This paper presents the analyses and the R&D which have been performed to get the qualification for these welds. This paper will also show the results of the fatigue tests at cryogenic temperature which will be performed to qualify the design and the manufacturing process for two different types of joint welds

Disclaimer: The views and opinions expressed herein do not necessarily reflect those of the ITER Organization

The ITER tokamak includes 18 superconducting D-shaped toroidal field (TF) coils. Unavoidable shape deformations as well as assembly errors will lead to error fields in the final configuration, which can be modeled with the knowledge of the current center line (CCL) to validate the winding geometry of these large scale magnetic field coils. Geometrical survey during manufacturing will be combined with magnetic survey of finished assemblies. We are developing a flux meter system, capable of sampling magnetic flux density values (or gradients thereof) at many locations distributed around the assembly. The measurement method uses low frequency AC excitation of the coil under study and records induced voltage sensed by each flux meter. The sampling strategy leverages on the use of arrays of calibrated flux meter coils, manufactured with printed circuit board technology. One array has 8 flux meters. The magnetic survey system uses 6 of these arrays mounted on a dedicated support. During a measurement campaign, magnetic (6 x 8 coils = 48 channels) and geodesic data (3-6 optical targets) are recorded. Combined with the knowledge of the precise position of each flux meters in space, a 3D map of induced voltages is obtained. This data can be compared with forward computed values using the nominal (or deformed) CCL and known experimental parameters. On the other hand, it can be used as input for an inversion procedure, delivering the deviations from the nominal winding situation, using a least squares regression scheme iteratively. The precision obtained by both approaches depends critically on the experimental error budget; eddy currents must be taken into account as well. In this contribution, we will present the flux meter calibration procedure and experimental results obtained on the first available european prototype of a double pancake (22 turns) for the toroidal field coils.

To make full use of the high energy collisions produced by the Future Circular Collider (FCC), detector magnets are needed providing higher magnetic fields over substantially larger volumes in comparison to the detector magnets of the LHC. A promising detector magnet system is the twin solenoid. The twin solenoid design incorporates a 5 or 6 T main inner solenoid with a bore of 10 to 12 meter and a coil length of over 20 to 23 meter. An outer solenoid with oppositely flowing current is placed around the inner solenoid for the purpose of shielding the magnetic field that is generated by the inner solenoid. In addition magnet systems are present at both ends of the twin solenoid that generate bending power for high-pseudo-rapidity particles. The space inside the inner solenoid, in between the inner and outer solenoid, and at the end sections of the twin solenoid is available for particle tracking. The design of the twin solenoid magnet system comprises challenges due to high stress in the windings and structure, the stored magnetic energy of about 50 GJ, and large forces resulting from misalignments between the inner and outer solenoid. In this presentation, the preliminary design is presented and discussed in terms of the geometry, material choices, quench behaviour, mechanical properties and magnetic field profiles.

The ITER High Temperature Superconducting Current Leads (HTS-CL) connect the room temperature (RT) bus bars and the 4.5 K feeder bus bars to convey the large currents to the superconducting ITER magnets. The TF and PF current leads are the larger ones, supplying the respective coils with up to 68 and 55 kA. The design of the two types of leads are very similar. So are the manufacturing approaches. The Institute of Plasma Physics, Chinese Academy of sciences (ASIPP) is responsible for the supply of the ITER current leads, based on a design jointly developed with the ITER Organization (IO). To supply the ITER leads on time, ASIPP started a technology development program on some mock-ups in 2013. In 2014 and 2015 some critical features were further developed for the prototypes. In this paper the technology improvements introduced for the HV insulation, fin type Heat eXchanger (HX), LTS linker, HTS-shunt and twin-box joint of the PF and TF-type HTS lead prototypes for ITER are discussed, including the results of the prototype tests. Some further improvements of the manufacturing technique and current lead design are proposed in preparation of the next step, series production.

A magnetic field is a very powerful thermodynamic parameter to influence the state of any material system. Consequently magnetic fields serve as an experimental tool in very diverse research areas like condensed matter physics, molecular physics, chemistry and, with increasing importance, in biology and biotechnology. Conventional commercially available magnets can nowadays provide field up to 18-20 T. To go beyond, very large infrastructures are necessary as the generation of intense magnetic fields requires the use of copper alloy based resistive coils with forced cooling which consume large amounts of electrical current and cooling water.

The increasing energy cost of operating high field resistive magnets strongly stimulates the interest in HTS for magnetic field generation. The specific property of HTS for high field generation is that at low temperatures (below 20 K) and under high magnetic induction (> 20 T), they remain in the superconducting state while keeping a transport capacity of strong electrical currents. In fact, they offer opportunities to go beyond 25 T impossible with conventional low temperature superconductors (LTS).

The aim of the present 3 years project is the fabrication of an HTS insert based on the strong potential of coated conductor tapes made of REBaCuO (RE=Rare Earth) superconductors, and capable of providing at least 10 T at 4.2 K in a stable and protected manner in a background field of 20 T, to produce at least a total of 30 T. The background field will be produced by a resistive magnet available at the LNCMI. This combination offered at the Grenoble High Field Magnet Laboratory (LNCMI) provides a robust way to focus on the operation and the protection modes under high magnetic field of such an HTS insert. The HTS insert is intended to be a stack of pancake coils. As a first step, the same resistive environment is used to test available commercial HTS tapes for predesigning these coils and developping enabling coil technologies.

The project NOUGAT is granted by the french research agency ANR under the convention ANR-14-CE05-0005. We acknowledge the support of the LNCMI - CNRS, member of the European Magnetic Field Laboratory (EMFL).

This paper presents a flux pumping method and results gained when it is used to magnetize a range of different YBCO coils. The device consists of an iron magnetic circuit with eight copper coils to apply a traveling magnetic field to the superconductor. The pole pieces are arranged vertically with air gap length of 1 mm and the iron core is made of laminated electric steel plates to reduce eddy-current losses instead of iron bulk. we have used this arrangement to investigate the best possible pumping result when parameters such as frequency and amplitude are varied. we have successfully pumped current into the superconducting coil up to a value of 85% Jc and achieved resultant magnetic field in excess of 1.5 T.

The NOUGAT project is the first step before a 30 T all superconducting high-field magnet. It combines a 20 T resistive outsert, which is one of the test facilities at LNCMI, and a 10 T HTS insert. The design of this insert has been studied in details, including magnetic, mechanical and protection aspects.
After a brief presentation of the HTS insert, we will detail the protection principle which is based on the extraction of the energy simultaneously in a discharge resistance as well as in copper rings located within the insert. The role of these rings is double: they act as transformer and as passive heaters. FEM computations have been performed and showed the benefits of such rings to reduce the hot spot temperature and the temperature gradients across the coil.
Analytical calculations have also been led on a simple two loops circuit and criteria regarding the flux linkage and mutual coupling have been defined to check the efficiency of the protection principle.

A laboratory-scale magnet for tokamak application using YBCO tape is designed and tested. The magnet consists of 13 D-shaped double-pancake sub-coils which are arranged on the circumference of 80 mm in radius. The outer radius is 280 mm. The cooling method is conduction-cooled by two cryocoolers. The magnet is tested at both liquid nitrogen temperature and about 20 K. A series of experiments were conducted to investigate the properties of every coil and the magnet. The results show that the maximum current is 200 A and the storage energy is 6.4 kJ at 20 K.

This work was supported by Specialized Research Fund for ITER under Grant 2011GB113004.

This paper presents the conceptual design and analysis of a compact superconducting magnet for Lorentz Force Electrical Impedance Tomography (LFEIT). In order to establish a LFEIT system, a strong and uniform magnetic field is essentially needed, which can enhance the signal detected from the LFEIT system and increase the signal to noise ratio (SNR). This magnet is based on type-II High Temperature Superconducting (HTS) coils due to their high critical current (I_C) capacity. The size and geometry of this HTS magnet are specifically designed for the clinical requirements, which guarantee a LFEIT is able to scan the whole subject. The modelling of this magnet design was achieved by using COMSOL Multiphysics PDE mathematical model based on H-formulation. A magnetic field of 1 Tesla over the scanning area required for LFEIT was achieved according to the simulation results.

High critical current densities, high critical magnetic fields, improved mechanical properties together with high operating temperatures have made the application of 2G HTS superconductors in high field magnets attractive. In this paper, a rectangular tunnel magnet system using many HTS coils as magnets is simulated. The configuration is designed to produce a uniform magnetic field, which can be used as a MRI magnet or for an accelerator. The rectangular configuration is composed of two separate sets, the top and bottom plate magnets and the left and right side magnets. The combined field generated by the side magnets and plate magnets produces a region of uniform field. The HTS coils are all connected in series and the magnetic field produced by each coil is varied by its turns. Several optimization methods, like genetic algorithm based on MATLAB, are applied to find the geometry of the magnet system to produce the uniform region of required field strength and uniformity with the smallest magnet tunnel size and thereby smallest material cost. Finite element analysis is performed to verify the optimal design parameters.

We report the design, manufacture and testing of a compact magnet for neutron time-of-flight (TOF) scattering experiments. The magnet provides scattering angles up to 150˚x40˚, plus four conical 40˚ access ports – two in the scattering plane and two perpendicular to the scattering plane, for a 25 mm diameter target. The minimum room-temperature diameter of each of the ports is 80 mm. The maximum sample field is 2.2 T, at an operating current of 208 A. The magnet plus cryocooler weighs only 186 kg.
A partial iron circuit limits fringe fields to less than 0.5 mT at a distance of 1.8 m from the magnet centre. The iron also forms the vacuum cryostat and furnishes part of the support structure for the HTS coils. The compact dimensions and large apertures require a high engineering current density which was achieved with a split-pair coil-pack constructed from stacked double-pancakes of Bi-2223 wire (77 K self-field critical current greater than 190 A). The inner and outer diameter of each double-pancake was optimised to maximise room-temperature bore and scattering angles.
The magnet incorporates a 2-stage GM cryocooler for cryogen-free operation. The HTS coil-packs were designed for a maximum operating temperature of 20 K but in testing the coils remained below 17 K, even at maximum operating current. The insensitivity of cooler performance to orientation allows the magnet to be tilted or rotated to any orientation during use. The relatively high safe operating temperature of the coils allows for a very compact and robust support structure and minimises clearances required for radiation shielding. The magnet exemplifies the compact high performance magnets that are possible with HTS technology.

Considerable advances have been made in recent years in HTS tape. Critical currents have been raised, longer continuous lengths have been fabricated and structural strength has been improved. Progress towards ever lower resistance and even no resistance joints has been made but the presence of resistance in the joints (together with price) still means that LTS is preferred in applications such as MRI. The Electrical Power and Energy Conversion (EPEC) superconductivity group at Cambridge University has been working on the use of flux pumping to activate superconducting coils. In this paper we report the current status of the flux pumping technology. The paper will show first the fundamental principles of the flux pump, second the performance and finally the potential applications both in the near and the far term. Flux pumping enables the magnetisation of superconducting permanent magnets by the application of time varying magnetic fields. Using this technique means that superconducting coils can be operated in persistent mode without the need for either current leads or a persistent current switch and that magnetisation can be achieved without the need for high current power supplies or external sources of magnetic fields. We will present both experimental and modelling results showing the present and expected future behaviour of flux pumped coils.

CFETR (China Fusion Engineering Test Reactor) is a test reactor which shall be constructed by National Integration Design Group for Magnetic Confinement Fusion Reactor of China. The present conceptual design of CFETR magnets are based on the ITER magnet technology with Nb3Sn and NbTi superconductor. But considering the reactor operation mode in the future, the magnet with larger current-carrying and stronger magnetic-field-bearing capacity should be need. The 2G high temperature superconductor YBCO with outstanding properties of larger critical current density and magnetic field under 20K, is an option of superconductor material for CFETR TF conceptual design. The new design of TF will make the center magnetic field up to 8T, which is helpful to the physics design with higher operation parameters.

In order to effectively improve the output current of HTS pulsed power supply and lower technical requirements for the circuit breaker, this paper introduced a method of multi-module discharge. Based on the electromagnetic characteristic of a single-module HTS pulsed power magnet, discharge process and characteristics of the multi-module pulsed power supply are studied by simulation and experiments. As the number of modules is changing to meet application requirements, parameters like the system’s equivalent inductance and mutual inductance between each module will also be varied. The existence of mutual inductance will lead to a discharge module transferring energy to other superconducting windings, which brings the problems of energy loss, overvoltage and damage of superconducting windings. This paper intends to analyse electromagnetic field distribution of multi-module HTS pulsed power supply to seek for appropriate coil structure and optimal control strategy to reduce the impact of mutual inductance in the discharge process. Further experiments will been taken to verify the feasibility of the control strategy optimization.

This paper is concerned with the analysis of thermal and magnetic design of a high uniformity magnetic field generator using superconducting coils in Helmholtz arrangement, aimed for neutrons scattering experiments for condensed matter investigation. The sample, exposed to the neutron beam, is placed in the middle of the coils system inside the pressure cell. The detectors of spectrometers are registering the emerging neutrons.
The aim of the project is the design and construction of a 4T dipole magnet system, cooled by a closed-cycle cryocooler Gifford-McMahon type. A second cryocooler should be used for cooling of the pressure cell with a sample installed inside the magnet. The pressure onto a sample is up to 15 GPa and the temperature range is 4.2-300K.
The coils of the magnet are made of High Temperature Superconductor (HTS) tape of YBCO type mounted in a special shaped cryostat.
The paper presents a mathematical model and the numerical experiments performed on the dipolar Helmholtz magnet for high magnetic field uniformity. The study is based on a three dimensional CAD model of the system prototype. The numerical simulation results outline the magnetic field spectrum and the heat transfer paths within the aggregate structure of the magnet system. The temperature of the HTS field winding must be kept within safe limits or the HTS would exit the superconductive state. In this regard, the „warm” channel where the probe is placed crosses axially through the magnet, is a menace that has to be treated adequatly. Consequently, cryo-cooling is used to remove the heat influx from the ambient towards HTS coils.
Test results are also presented and compared with the numerical simulations results.

The authors acknowledge the project funded by Romanian Government through National Authority for Scientific Research under contract no 08626319/1420986-74 /2014, with Joint Institute for Nuclear research- Dubna, Russia.

In the frames of the laboratory superconducting high-field magnets development program 7 T @ 4.2 K (background field is 13 T) 2G HTS solenoidal insert coil with 45 mm clear bore was fabricated and preliminary tested. HTS solenoid consists of two separate layer-wound sections with several joints inside. Reliable quench detection and fast energy evacuation are crucial for solenoid survivability. Power source operation was synchronized with data acquisition system for noise reduction and better voltage resolution. High voltage (up to 600 V) discharge is needed for fast and safe energy evacuation. The discharge rate was additionally accelerated by good inductive coupling between HTS sections and its copper mandrels. The HTS solenoid and its quench protection system were preliminary tested at 64 K.

The high temperature superconducting (HTS) magnet has superior features for NMR/MRI application than the low temperature superconducting (LTS) ones. But in case of second-generation (2G) HTS coils, there is no way to make superconducting joints that have no electrical resistance, until now. We have suggested and made joint-less 2G HTS coils by so-called “Wind & Flip Method” and succeeded in magnetizing them up to 1 T by field cooling. As we know, the magnetic field profile in the joint-less HTS coil after field cooling is different from the one before field cooling. We need another solution for fine-tuning of the magnetic field from the joint-less coil. The flux pump has been a good power supply to energize superconducting coils/magnets without contacts and heat leaks since LTS magnets. We have tried to energize the joint-less HTS coil with a permanent magnet flux pump. Though the magnetic field from the permanent magnet was very weak and so the increasing rate of the field in the joint-less HTS coil was very low, we have confirmed that the persistent current mode operation with the flux pump was possible in liquid nitrogen. In this paper, we suggested several designs of the flux pumps for the joint-less 2G HTS coils and compared their characteristics.

This research was supported by Korea Electric Power Corporation Research Institute through Korea Electrical Engineering & Science Research Institute. [grant number : R14XA02-23]

Development of High-Temperature Superconducting (HTS) levitation coils with conduction cooling have been conducted to realize a liquid hydrogen container supporting by magnetic levitation using HTS coils. The container is high efficiency due to low heat invention. In addition, it has robustness against earthquake by controlling HTS levitation coil to suppress vibration. During the operation to suppress vibration, large ac losses are generated in the windings of the coils. As a result, the winding temperature rises. In this study, heat pipes are applied to the HTS coils. Heat pipes have greater heat transport property than solid heat conduction. In order to show the feasibility of HTS coils with conduction cooling by using heat pipes, calculation of both ac loss and temperature distribution of the windings on test coil wound by Bi-2223 multi filamentary tape have been carried out under operation to suppress the vibration. A critical currents of the Bi-2223 tape is 180 A at 77.3 K. The test coil is composed of two single pancake-coils. Inner and outer diameters of the test coil are 150 mm and 400 mm, respectively. Total turn number of the test coil is 880 turns. Heat pipes are formed into a plate shape, whose dimensions are 95 mm in width, 210 mm in length, and 5 mm in thickness. Four heat pipes with plate shapes are put between the two pancake-coils. These heat pipes are arranged radially. Both edge of inner and outer side of the heat pipes are cooling edge. In this paper, the details of the test coil and its performances are reported.

A 200 m high temperature superconducting dc power cable was constructed in 2009 and since then we have performed six cooling cycle tests to evaluate its performance. Current feeding test is one of the most important issues for this evaluation. This cable has a three-layer coaxial structure. The inner two layers are used as positive (+) polarity and only one layer in the outmost is used as negative (-) polarity for DC electric energy power, respectively. In the summer of 2014, we started the sixth cooling cycle test on the 200-m cable. After cooling down to liquid nitrogen temperature, we applied a DC transport current to the 200-m cable. In this report, we will present the current feeding test in the 200-m cable together with the critical current measurements.

Parts of this work were supported by a grant of Strategic Research Foundation Grant-aided Project for Private Universities, by Ministry of Education, Culture, Sports, Science and Technology (MEXT).

One of expected system on the power transmission aspect of high-capacity and lower loss is high temperature superconducting (HTS) DC cable system. In HTS DC cable system, the components with insulation system mainly consist of superconducting power cable, cable joint, and termination bushing at cryogenic temperature. The superconducting power cable is composed of a former, superconducting conductor, polypropylene laminated paper (PPLP) as insulation material impregnated with liquid nitrogen (LN2). The cable joint, which is used to connect cables in joint boxes, have central copper conductor surrounded by an insulating spacer such as epoxy and PPLP. The cryogenic parts of termination bushings are immersed in LN2 with glass-fiber reinforced plastics (GFRP) used as insulator. This paper reports basic dielectric characteristics of HTS DC cable system, that is, cable, cable joint and termination bushing. We discuss the breakdown characteristics of PPLP-insulated mini-model cable under dc, impulse and dc polarity reversal voltage. The surface flashover characteristics of PPLP, epoxy, and composite of epoxy-PPLP are studied for cable joint insulation. For insulator of termination bushing, the surface flashover characteristics of GFRP are performed according to directions of glass-fiber, diameter, length, width and height under lightning impulse voltage.

This research was supported by Korea Electrotechnology Research Institute(KERI) Primary Research Program through the National Research Council of Science & Technology (NST) funded by the Ministry of Science, ICT and Future Planning(MSIP) (No. 15-12-N0101-09)

The Superconducting Link project at CERN aims at developing helium gas cooled high-current electrical transfer lines for the powering of the superconducting magnets which are being developed in the framework of the High-Luminosity upgrade of the Large Hadron Collider. A candidate conductor for the cables is Magnesium Diboride (MgB2) round wire operated at temperatures of up to 25 K. One of the most important factors to be taken into account during the design phase is quench protection. In this work we investigate the quench behaviour of high-current cables made from MgB2 wires produced by Columbus Superconductors. The cables are operated in the Superconducting Link Test Station at CERN, cooled by helium gas at temperatures between 10 K and 30 K. The propagation of the quench front is monitored by means of voltage and temperature sensors installed on the cables. Results from the measurements are compared to numerical simulations, in order to validate the models and to quantify the parameters related to quench propagation and protection. The ultimate goal is to devise a quench protection strategy suitable for the safe and reliable operation of the Superconducting Link circuits.

High-temperature superconducting (HTS) power cables are expected as one of solutions to expansion of the future electrical power transmission system, because HTS power cables have higher current-density limits, lower losses, and smaller cross-sections than conventional power cables. The safety evaluation of HTS power cables through a fault current should be needed for practical use.
A computer program to estimate the transient temperature and pressure distributions in a HTS power cable cooled by a forced flow of sub-cooled LN2 with a fault current has been developed. Temperature and pressure profiles of the LN2 coolant and the cable cores temperature were analyzed by solving the heat-conduction and heat-transfer equations using the finite-difference method. Cryodata’s GASPAK software package was used to estimate the fluid properties. When a fault occurs, an excessive current of 31.5 kA may flow in a cable for maximal 2 seconds. It is important to estimate the temperature and pressure profiles in a cable cooled by the forced flow of LN2 in order to assess the effects of short-circuit accidents for realizing a practical HTS power cable.
Evaluation on stability of a 1500m HTS cable with a fault current of 31.5kA-2.0sec was carried out using our computer program. According to the results, the pressure in a cable rises sharply because the temperature of sub-cooled LN2 reached up to the saturated temperature and suddenly vaporization happen to occur. In this case, safety valves of a refrigeration system will open.
We evaluate a thermal stability of a cable for several parameters in order to suppress the temperature rise of sub-cooled LN2 lower than the saturated temperature. From the results, we found out that it's effective to reduce duration of a fault current or to suppress the value of the fault current small using such as a fault current limitter.
If duration of a fault current reduces from 2 seconds to 1 second, the temperature of sub-cooled LN2 in a cable can be suppressed lower than the saturated temperature, and a refrigeration system can be operated continuously.

The high Tc superconducting (HTS) tri-axial cable fits long-distance power transmission rather than the 3-in-One superconducting cable, since the tri-axial HTS cable has low ac loss and large cooling channel, and the tape length used for a cable is much shorter. We investigated a suitable cable structure of HTS tri-axial cable with counter cooling for long-distance power transmission using the numerical analysis which considered the heat transfer from the cable outside and heat generation caused by to the ac loss inside the cable. Our results show that it is very important to increase the flow rate of a coolant by enlarging an outside channel and the flow velocity in order to operate the tri-axial HTS cable over long-distance. In the design of a long-distance tri-axial HTS cable, reducing the diameter of outermost layer of conductor layer is very important to reduce the friction loss, the pressure loss of a coolant, and the heat transfer from the outside of cable even if ac loss in HTS cable is increased by decreasing the cross-sectional area of HTS layer. More calculated results will be presented in detail.

A hybrid energy transfer line (HETL) of hydrogen and electricity has been studied. The rated current of the current transmission cable is 10 kA and delivery capacity of the liquid hydrogen is 100 tons per day. The HETL consists of the SC cable, electrical insulation layer, space for liquid hydrogen, inner corrugated tube of low temperature, vacuum space for thermal insulation and outer corrugated tube. High Jc performance in around 20 K requires for the SC cable. The MgB2 wire is one of the potential candidates for this system as well as Bi2212 wire. It should be robust for the repetition of the bend and stretch of following manufacture process of; the heat treatment, transfer to the real, twist and bundle, transportation by cable drums and installation on site. A special multi-filamentary MgB2 wire was developed to suppress the transport current degradation against bending repetition. When the cable is wound to the cable dram, tensile stress and compressive stress are induced to the outside and inside of the bending portion of the cable. Structure of a coaxial stranded cable is suitable to relieve the bending stress for the large bore cable. In the loose-twisted cable, bending strain will decrease, because the slip among the strands to the axial direction will compensate the outside tensile stress with inside compressive stress. In this report, Ic performance for coaxial stranded MgB2 cables was examined as a function of the bending strain. Effect of the surface treatment by molybdenum-disulfide for the assist of slipping among the strands was also investigated.

The development of a superconducting cable aimed at railways applications has commenced by assuming a DC transmission cable used for electric trains. A cable has been fabricated based upon the results of current testing of superconducting wires, along with a variety of evaluation tests undertaken to determine cable characteristics. A superconducting transmission cable having zero electrical resistance characteristics and applicable for railway use, is anticipated to bring about enhanced regeneration efficiencies, reduced power losses, load leveling and integration of sub-stations, and the suppression of rail potential.
Compact and high reliability systems are required of the superconducting cables for railway systems. In the cooling system, many of other superconducting cable projects individualize each function, such as cryocooler unit, pump unit, reservoir unit and so on, so that the system must connect among individual units by cryogenic piping. As a result, cooling systems become huge with low reliability. Therefore we devised the new cooling system for railway systems which unify all the units.
Based on such various examination results, prototypes of cooling system of superconducting cables were produced. Cooling systems were tested using superconducting cable installed in railway test track of RTRI. Results of cooling tests under various conditions will be discussed.

Turbulent flow is encountered in most of the thermal systems including heat exchangers. However, estimation of friction factors and heat transfer coefficients in turbulent flow regime is still a challenge. A numerous experiments have to be conducted in order to estimate the same. This challenge can be overcome with the help of numerical and computational techniques which are economic to be implemented. In recent years, conventional power transmission systems are recommended to be replaced by superconducting power transmission systems with High Temperature Superconducting (HTS) cables. However, in retaining the superconductivity of these HTS cables need identification of appropriate cryogenic coolants. For futuristic HTS cables, Super Critical Nitrogen (SCN) is proposed as one such coolant having desirable thermopyhsical properties such as density, viscosity, specific heat and thermal conductivity. However, flow of SCN in the corrugated steel pipe (former) of HTS cable is found to be turbulent. Hence, the present work aims at estimating pressure drop and heat transfer in turbulent flow regime using Computational Fluid Dynamics (CFD). The CFD method involves development of 3D computational geometry using GAMBIT 2.2.30, a preprocessing package. This geometry is imported to commercial solver FLUENT 6.3.26, reflects the flow domain of SCN. Different corrugation geometries such as rectangular, triangular and circular are modeled and flow through these geometries is considered to be turbulent. Various turbulent schemes such as K-epsilon, K-omega and Shear Stress Transport (SST) are compared with each other to signify the validity of implemented turbulence model to suit practical experimental conditions. Heat fluxes ranging from 0.8W/m2 to 1.2W/m2 are imposed on the fluid domain to understand the flow behavior at various operating conditions. The results of simulations are also validated with the experimental results available in the literature.

The Authors would like to acknowledge the support extended by IIT Kharagpur, India

For the minimization of AC-losses in a superconducting power cable their experimental determination is highly desirable with respect to the optimization of the design. However measurements on short samples are strongly hampered by the inhomogeneous current distribution between the superconducting tapes. A recently developed experimental setup gets rid of this problem by using a serial connection of the tapes with mutual field compensation of the backwards conductors.
With this experimental method we now obtained first results of AC-losses in a one phase superconducting cable equipped with 22 YBCO-tapes (Superpower SCS 4050, width 4mm, Ic = 112 A). The losses were measured with an s-shaped voltage lead geometry and good reproducibility on the different tapes of the cable. Compared to the losses of a single tape, which are well described with the Norris-model (strip), the specific losses within the cable are reduced by a factor of about two, this in spite of the increased local field. This result is explained with the nearly perfect azimuthal direction of the local magnetic field. In additional experiments we studied the AC-losses for cases in which one or more tapes carry no current, a situation identic to application cases with damaged tapes. As a result the losses in the neighbouring tapes are increased by up to an order of magnitude, due to the increased vertical components of the self-field. This is confirmed by dedicated finite-element method simulations.
Finally the experiments were compared to previous measurements on cables based on BSCCO 2223 tapes. The AC-losses for both materials are comparable, however the increased critical DC-current observed in the 1G – cable configuration could not be confirmed for 2G – cables.

The work was supported by the German government under grant# 03ET1055D (project Ampacity)

Authors would like to acknowledge the support extended by IIT Kharagpur, India.

The losses encountered while transmitting power can be reduced drastically by using high temperature superconducting (HTS) cables. However in retaining the superconductivity the coolants such as supercritical nitrogen (SCN), supercritical hydrogen (SCH), and supercritical argon (SCAR) are to be maintained at and above critical pressure (Pc) and critical temperature (Tc). This results in the development of stresses in the former material such as corrugated steel pipe (CSP) through which coolant is passed. In the present work, an investigation is performed to find mechanical behavior of CSP with different geometries of corrugations such as Circular (C-type), triangular (T-type) and rectangular(R-type). It is observed that, in the CSP with T- type corrugation experiences large stresses as compared to other corrugation geometries. Further, it is also found larger deformations in T-type corrugations lead to failure of CSP.

Authors gratefully acknowledege the support by IIT Kharagpur,India. Authors also acknowledege the financial support extended by Lovely Professional University Phagwara, India

An axial HTS generator with an innovative topology has been designed. Both HTS wire and bulks are used in the realization on the inductor. The three-phase winding armature is made from copper. An HTS solenoid is cooled using liquid nitrogen and is excited by a DC current. HTS bulks are used as barriers of magnetic flux density created by the HTS solenoid. This configuration allows a spatial variation of the flux density in the air-gap of the generator when the minimal values are obtained behind each HTS bulk and the maximal ones are obtained in the middle regions between two HTS bulks. Studies for extrapolated topologies show that the torque-volume ratio increases for the megawatt ranges. This result is very important for the power wind generation when the generator is placed at a height of several tens of meters.

Calculation, design simulations and design performance of 1 MW HTS 2G Generator for Wind Turbines were done in 2013-2014 [1]. The results of manufacturing and testing of 1 MW generator are presented in the article.
HTS 2G field coils for the rotor were redesigned, fabricated and tested. The tests have shown critical current 41-45 A (self field within the ferromagnetic core, T = 77 K) which corresponds to the current of short samples in self field. Application of the copper inner frame on the pole has improved internal cooling conditions of HTS coil windings and reduced the magnetic field in the area, thereby to increase the critical current value.
The original construction of the rotor with rotating cryostat was developed which decreases the thermal flow to the rotor.
The stator of 1 MW HTS 2G generator has been manufactured. In order to improve the specific weight of the generator the wave multiplier was used, which provides increasing of RPM from 15 RPM up to 600 RPM.
The final test of assembled synchronous generator with HTS 2G field coils for Wind Turbines with output power 1 MW is scheduled for autumn 2015.
1. K Kovalev, L Kovalev, V Poltavets, S Samsonovich, R Ilyasov, A Levin, M Surin. Synchronous Generator with HTS-2G field coils for Windmills with output power 1 MW. 11th European Conference on Applied Superconductivity (EUCAS2013) IOP Publishing Journal of Physics: Conference Series 507 (2014) 032023 doi:10.1088/1742-6596/507/3/032023

This work is done within the frames of supported by Rosatom national program “Superconducting Industry”.

Comparative Study on Electrical Design of 10 MW HTS Wind Turbine Generators
Yuanyuan Xu, Naoki Maki, Mitsuru Izumi
Wind energy is being widely exploited due to its effective cost and abundant source. And it has been a trend to develop larger wind turbines because the cost of wind energy decreases with respect to enlarging sizes of wind turbines. However, the generator weight becomes a serious problem when the capacity of conventional wind turbines is more than 5 MW. In addition, a speed up gearbox is necessary for a 10 MW class conventional wind turbine, therefore, the reliability and maintenance of a gearbox is another severe challenge.
High temperature superconducting (HTS) generators for 10 MW class wind turbine systems are being favored due to their light weight, small size, and high efficiency. In this paper, the study on electrical design of large-scale salient-pole wind turbine HTS generators is introduced briefly, which mainly consist of a novel electrical design method, the optimization of main generator parameters, the calculation of the field leakage coefficient and electric heat losses of HTS field windings based on the magnetic field simulation results, the introduction of a cryogenic vessel for HTS field windings, and the calculation of various heat losses on the cryogenic vessel in our previous work [1]. Then, the other electrical designs of HTS generators for 10 MW class wind turbine systems which were conducted in several institutes are summarized. And the advantages and disadvantages for all types of electrical designs are discussed and compared. In addition, some crucial issues for HTS wind turbine generators are studied, which include HTS field windings, stator structures, and the cryogenic cooling systems.
[1] Y. Xu, N. Maki, and M. Izumi, “Study of Key Parameters and Cryogenic Vessel Structure of 10 MW Salient-pole Wind Turbine HTS Generators,” IEEE Trans. Appl. Supercond, vol. 25, no. 2, Art. ID 5200406, Apr. 2015.

Homopolar machines have the advantage that the magnet system operates independently from the stator and rotor allowing a large amount of flexibility in the rotor, stator and magnet arrangement. Multiple rotors in the same magnet system are magnetically coupled creating a simple energy converter. They can be configured to run as a motor and a generator in the same magnet system with either a coupled shaft (DC to DC power converter) or coupled electrical circuit (electromagnetic gearbox). An experimental test machine was built to validate our DC/DC converter, electromagnetic gearboxes and counter rotating motor concepts.
This test machine used the same magnet system as our 200 kW prototype electromagnetic turbine to demonstrate the concept. The magnet system, that produces a DC magnetic field up to 2.7 Tesla, consists of a pair of superconducting magnets in a spilt coil design. Each magnet has a mirror-inverted coil arrangement of one main coil and two Guina’s invented cancelling coils. A Liquid Metal Current Collector (LMCC) system was developed in-house that allows a stable electric contact between the rotor and stator at surface speeds of up to 100 m/s and current densities up to 250 A/cm2 for two discs next to each other. The copper rotors and LMCC system are fully enclosed within an air-tight composite envelope to maintain an inert atmosphere preventing oxidisation of the liquid metal.
The DC to DC power converter allows DC voltages and current to be efficiently stepped up or down or to separate electrical circuits without the need for additional power electronics. The electromagnetic gearbox configuration allows the conversion of mechanical energy to and from different speeds and torques electromagnetically without physical contact. This can be also used as an overload and mechanical failure protection. The theory and test results of these two configurations of homopolar machines for electro-mechanical energy conversion will be presented and discussed.

There was significant interest in homopolar machines 5-6 decades ago due to their numerous advantages such as pure DC operation, high efficiency and power-to-weight ratios. However a number of engineering challenges and lack of technology held back mass production and utilization of these machines. In the beginning of the 21st along with all the advances in superconductors, magnet technology, cryogenics and the emerging need for renewable energy and efficient energy usage, homopolar machines have again become extremely attractive. Again, fundamental absence of both alternating fields and reaction forces on the excitation coils makes them a perfect application for the use of high temperature superconductors in rotating machines, eliminating the problems of AC-loss and mechanical stress in the superconducting wires.
In this paper we present the experimental results gathered from comprehensive testing of our 200 kW superconducting motor equipped with liquid metal current collectors (LMCC). We discuss a number of critical parameters of the machine such as efficiency, potential scalability to a multi-MW range, service life and faultless continuous operation along with dynamic response of the motor to slow, fast and periodic changes of external load and its robustness in such “real-world” conditions. Some considerations regarding control of the homopolar motor will also be presented.
Special attention is paid to the LMCC which is an advanced alternative to conventional solid brushes brining a number of advantages including superior current carrying capacity even when subjected to extreme surface speeds, ensuring low-loss and low-wear operation in comparison with conventional brush technology. In many designs of a high performance homopolar machines LMCC are the only feasible and/or economically viable solution. It is a dynamic system and relies solely on the centrifugal forces acting upon conducting liquid for its operation. Operation in external magnetic fields with high current flowing through the liquid makes stable and robust operation of rotating machine a challenging engineering task. We will present a number of advanced technological solutions implemented in order to achieve reliability and robustness of the superconducting motor.

This work was supported by National Natural Science Foundation of China (51377020), Specialized Research Fund for the Doctoral Program of Higher Education of China (20130092130005), and the Fundamental Research Funds for the Central Universities (KYLX_0120).

This paper proposed an effective design process for 10MW high temperature superconductor (HTS) wind turbine generator using analytical method, which consists of space harmonic analysis and image method. In order to get 3-D flux distribution. First, by using harmonic analysis, 2-D radial flux distributions are calculated. Second, by using image method, 3-D winding factors for radial flux component according to position in z-axis direction are calculated. HTS direct drive generator has wider air-gap between field and armature winding since it have a damper and cooling p. Due to aforementioned reason, 3-D magnetic field analysis is required. However, finite element analysis (FEA) has setbacks which are complex preprocess and long computation time. By integrating space harmonic and image method, electrical parameters for the generator is reflected on the design faster and easier than the FEA.
To reduce cost of energy, wind generators of larger sizes are under research. For increase of stability and reduction of construction cost, size and weight reduction of the generator is important design factor.
In this paper, design to increase torque density for 10MW HTS wind turbine generator which has advantages in efficiency increase, size and weight reduction, is conducted using analytic method. Operating point for HTS conductor was determined through testing and reliability was satisfied by HTS conductor operating under critical current and critical flux density.

This research was supported by the MKE (The Ministry of Knowledge Economy), Korea, under the CITRC (Convergence Information Technology Research Center) support program (NIPA-2013-H0401-13-1008) supervised by the NIPA (National IT Industry Promotion Agency).

AS part of an ARPAe REACT sponsored project, an improved HTS tape exhibiting a 4 fold (4X) increase of current density at 30 K and T was developed. The availability of an improved conductor can have a significant impact on the design of HTS wind turbine generators. A design for a 10 MW 8 RPM generator was generated for maximum levelized cost of energy (LCOE); the generator relies on iron poles at the rotor and an iron based stator. The paper presents the impact of the use of 4X conductors on the design of the generator. The analysis is done through a Monte Carlo design space exploration and leads to interesting results. The conductor performance is found to have a limited impact on the LCOE in the case of an iron core configuration. The analysis was repeated for an air-cored generator configuration for which the impact of conductor performance is significant.

This work was partially funded by the ARPAe REACT program

Cyclone Initiative is a student led organization at the University of Houston that strives to explore new and innovative ways to effectively utilize and generate energy through the application of superconductors. The team is working on designing and developing a modified wind turbine by applying superconductor technology to the power generating unit. The preliminary design of each component of a 100 kW wind generator will be presented, along with cost analysis and market research. A major goal of this project will be to gather real world data on reliability of long term use of superconductors. This data will also help to eventually optimize the generator and turbine design. This project is expected to be completed over the course of 2 years.