MJ Coil Research Articles

Summary of a 1 MJ Conduction-Cooled LTS Pulse Coil Developed for 1 MW, 1 s UPS-SMES

The development study of a 1 MJ conduction-cooled low temperature superconducting (LTS) pulse coil used for a 1 MW, 1 s UPS-SMES is summarized. We have developed a conduction-cooled LTS pulse coil as a key technology for the UPS-SMES. The AC loss reduction and the high stability are required for the SC conductor for a LTS pulse coil because of a limited cooling capacity of 4 K cryocooler. The conductor of a NbTi/Cu compacted strand cable extruded with an aluminum was designed to have the anisotropic AC loss properties to minimize the coupling loss. The coil was wound, utilizing a specially developed automatic winding machine which enables an innovative twist-winding method. The Dyneema FRP (DFRP) spacers and the Litz wires (braided wires of insulated copper strands) were inserted in each layer in order to enhance the heat transfer in the coil windings. The coil was installed in the test cryostat and was connected to three GM cryocoolers, which have a total cooling capacity of 4.5 W at 4 K and 240 W at 50 K. The coil was cooled conductively without liquid helium by attaching the end of the Litz wires directly to the cold heads of the cryocoolers. The cooling and excitation test of the 1 MJ coil has been done successfully. The test results validated the high performance of the conduction-cooled LTS pulse coil, because the high thermal diffusivity resulted in the rapid temperature stabilization in the coil.

AC Losses in a Conduction-Cooled LTS Pulse Coil With Stored Energy of 1 MJ for UPS-SMES as Protection From Momentary Voltage Drops

AC losses in the conduction-cooled low temperature superconducting (LTS) pulse coil with stored energy of 1 MJ are estimated. The 1 MJ coil is a superconducting pulse coil for 1 MW, 1 sec UPS-SMES. UPS-SMES is an uninterruptible power supply (UPS) with superconducting magnetic energy storage (SMES) for protection of production lines of an industrial plant or large-scale experimental devices such as a fusion device, from a momentary voltage drop and an instant power failure. The winding conductor for the 1 MJ coil is a NbTi/Cu Rutherford cable, which is extruded with aluminum. The 1 MJ coil was wound by a new twist winding method. A 1 MJ coil was fabricated and cooling and excitation tests were carried out. In this paper, two methods to estimate ac losses in conduction-cooled LTS coils are proposed. One method estimates ac losses in the coil under steady-state conditions. The other method estimates ac losses in the coil under transient-state conditions. For estimation of ac losses in the 1 MJ coil, measurement of temperature in the coil during tests and thermal analysis using the two-dimensional finite element method are compared. These procedures clarified that the 1 MJ coil has low losses.

The Design to Downsize a Conduction-Cooled LTS Pulse Coil for UPS-SMES as Protection From Momentary Voltage Drops

The stability margin of a conduction-cooled LTS pulse coil with stored energy of 1 MJ, which is used for UPS-SMES as protection from momentary voltage drops, is estimated from thermal analyses using the two-dimensional finite elementary method. How much the 1 MJ coil is able to be downsized is discussed. The winding conductor of this coil is a NbTi/Cu Rutherford cable, which is extruded with aluminum. Dyneema FRP and Litz wires are used as the spacers of this coil. In the design of this 1 MJ coil, the effects of spacers were neglected. In order to estimate the stability margin, thermal analyses was carried out by taking into account the thermal effects of spacers, for the coil during overload operation. And it was checked whether the maximum temperature The results showed that the 1 MJ coil has a large stability margin of 8 times the rated ac loss. In addition, new coils with a stability margin of about two times were designed. Possibilities for reducing the size and the cost of a 1 MJ LTS pulse coil are also shown.

Development of 1 MJ Conduction-Cooled LTS Pulse Coil for UPS-SMES

A 1 MW, 1 s UPS-SMES is being developed for a protection from a momentary voltage drop and an instant power failure. As a key technology of the UPS-SMES, we developed a prototype LTS pulse coil with a stored energy of 100 kJ and conducted cooling and excitation tests in 2005. The operation test of the prototype UPS-SMES using this 100 kJ coil with power converters have been performed in 2006. A 1 MJ coil was designed before the fabrication of the 100 kJ prototype coil. The superconductor, the electric insulation technique, the winding method, and the cooling structure used for the 100 kJ coil were based upon the 1 MJ coil design. The successful performance test results of the prototype 100 kJ coil validated the design concept and fabrication technique of the 1 MJ coil. According to the achievement of the prototype 100 kJ UPS-SMES, the 1 MJ conduction-cooled LTS pulse coil has been fabricated successfully. The successful experimental results of the 100 kJ prototype coil with power converters and the fabrication procedure of the 1 MJ full size coil are described.

Test Results of Power System Control by Experimental SMES

The authors have developed a solenoid model coil used for superconducting magnetic energy storage (SMES) for power system control, aimed at drastically reducing the costs of the SMES system. The single solenoid model coil of 2.9 MJ is designed with the rated current of 9.6 kA, a maximum magnetic field of 5.5 T and coil charge rates equivalent to those of the practical 100 MW/54 MJ class SMES. The coil is characterized by the use of an aluminum stabilized NbTi CIC (Cable-in-conduit) conductor. The experimental SMES consists mainly of a 2.9 MJ coil, the cooling system for the CIC conductor and the 1 MW class AC/DC converter. The SMES is connected to 6 kV distribution lines at a substation. In the field test, the SMES's 17 ms responses for the step input of active and reactive power were ascertained. Furthermore, its function of compensating for load fluctuation in the 6 kV distribution line was confirmed. The test results show the realization of a practical SMES system for power line control

Prediction of the quench current of a large magnet with a small model test

The quench current of any large nonsteadily stabilized magnet under design can be predicted by the testing of a relatively small model. The model should reproduce the main features of the design and the manufacturing technique as well as the temperature margin and the amplitude of thermo-mechanical excitations inherent in the magnet being designed and dependent on the stress level. Usually it is unfeasible to reach a high stress level in a small model. However, it is possible when external forces are used. An experiment with a superconducting force generator is described here. It has allowed the testing of two different models of a 50 MJ coil under high compressive load, to choose the best and to determine the necessary temperature margin for the large coil.

Development and test results of a double 0.5 MJ coil SMES system

The design and test results of a SMES system built for the Korea Electrical Power Company are described. The system consists mainly of two superconducting (SC) coils, two cryostats, a primary power supply and a DC-AC-DC power converter to transfer the stored energy from one coil to another and vice versa. Each of two almost identical coils can store more than 0.5 MJ at about 1.55 kA. Minimum energy transfer time is about 2 s. The conductor is made by cabling of several SC, copper and stainless steel strands around a copper core. At 2 s transfer time mode, the AC losses in each coil are about 500 J. The cryostats are manufactured mainly of thin-walled stainless steel. Their eddy current losses at liquid helium temperatures are lower than AC losses in the coils. After preliminary tests in Moscow, the system was delivered to Republic of Korea and successfully tested at Korea Electrotechnology Research Institute.

Superconductor design and loss analysis for a 20 MJ induction heating coil

The design of a 50 kAmpere conductor for use in a 20 MJ Induction Heating Coil is described. The conductor is a wide flat cable of 36 subcables, each of which contains six Nb-Ti strands around a stainless steel core strand. It is cryostable, with a pool-boiling, fully-normal equivalent heat transfer from the unoccluded strand surface of 0.26 Watts/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . A thin, tough polyester amide-imide insulation enhances heat transfer and prevents interstrand coupling. The tightly-twisted strands are configured using Cu-Ni elements to provide low AC losses with Nb-Ti filaments in an all-copper matrix. AC losses are expected to be approximately: (1) 0.3% of 20 MJ for a -7.5 T to 7.5 T one-second 1/2-cosinusoidal bipolar operation in a 20 MJ coil, and (2) for additional reference, 0.1% of 100 MJ for a 1.8 second -8 T to +8 T ramped operation in a 100 MJ coil with a heat transfer of 0.36 Watts/ cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The design is based on the manufacture and testing of prototype strands and subcables.

Quench detector for large pulsed coils and quench analysis for the LASL/Westinghouse 20 MJ coil

A unique detection scheme has been devised for possible use in the test of the 20 MJ Induction Heating Coil. This scheme allows the sensing of plus or minus voltages less than 320 millivolts resistive in magnitude in coils which will have inductive voltage components as high as ±2.5 kVolts. The network which achieves this sensitivity is stable to less than 12.8 ppm. The method adopted involves the bucking out of the inductive voltage with two secondary co-wound flux sensing coils tapped at locations adjacent to voltage taps in the main superconducting coil. The detection scheme is recommended especially if large ripple or control voltages exist subsequent to the coil pulse. The most severe credible event which might quench the coil and/or damage the winding is exposure of the operating coil to gaseous cooling through lack of proper liquid level control. The detection scheme described will protect the coil against this and other abnormal conditions that could conceivably damage the coil.

Performance tests of a 1.5 MJ pulsed superconducting coil and its cryostat

To demonstrate cryostability of large pulsed superconducting ohmic heating (OH) coils for Tokamak power reactors a 1.5 MJ coil was developed at Argonne National Laboratory (ANL). The coil was first charged up to 11.2 kA dc, which is the critical current of short sample cables at 4.5 T. Above the critical current, an unbalance voltage of the coil increased smoothly indicating stable current sharing between the superconducting filaments and the copper stabilizer. The coil was then pulsed with a 7 MW power supply. The highest ramping rate was 11 T/s. AC losses, mainly eddy current losses in the copper stabilizer, were proportional to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\dot{B}^{2}</tex> with about 2.5 kW at 9 T/s. After 3000 pulses no degradation of the coil performance has been observed. A non-metallic cryostat was developed for testing of the coil to avoid eddy currents in the cryostat.

La protection des grands bobinages supraconducteurs

When a quenching occurs in a large coil the dissipation of the energy will increase the temperature of the superconductor and the pressure in the cryostat. The study of these phenomena showed that, using the usual protection device, pressure and temperature can be hold at a low value and make the coil safe even if the protection is switched on one second after detection of the quenching. Experimental results obtained on a 10 MJ coil confirm the theory.