H. Chikaraishi

K. Maeba

T. Ise

Y. Miura

National Institute for Fusion Science (NIFS-Repository)

Power supply systems for superconducting coils of magnetic confinement fusion devices are generally composed of ac/dc (alternating current / direct current)power converters, which rectify utility frequency ac to dcfor exciting and de-exciting of coils, and some of them employ superconducting cables because the distancebetween power supplies and coils are relatively long.  In this research, we propose the power supply system thatemploys low frequency power transmission for superconducting coils of fusion devices[1].  In the system, ac/ac power converters convert utility frequency ac to    1 ~ 10 Hz low frequency ac (LFAC) and LFAC power is delivered to ac/dc converters near superconducting coils with superconducting cables.  Advantages of LFAC system over dc system are easy fault protection due to existence of current zero crossing point, easy conversion to large current by using transformers, and on the other hand, an advantageover utility frequency ac system is reduction of ac losses of superconducting cables.An example of LFAC system for a fusion device is shown in Fig. 1.  In this system, power system is connected to ac/ac converter through transformers and LFAC power is delivered to each components by using superconducting cables.  We introduced a 12-phase double-star-connected thyristor rectifier for superconducting coil, as shown in Fig.2.  This rectifier output low voltage and large current.  A 12-phase configuration is applied to achieve reduction of the voltage ripple and short control cycle for high accuratecurrent control.  In addition, it adopts star connections due to reduction of the voltage drop and thyristors are employed as switching devices due to high reliability for the large current control.  Fig.3 shows the block diagram of control scheme for the rectifier.  It employs the proportional control scheme with gain K and a low pass filter (LPF) is applied to detected coil current.  We conducted numerical simulation of charging and discharging the superconducting coil by using an ideal voltage source with a frequency of 10 Hz.  Major parameters of the system are summarized in Table I and a dc filter is designed as the voltage ripple be less than 0.5 V.  The ramp up rate of the current command value is set to 4 A/s and the output voltage is 45 V. Fig.4 shows the simulation results.  It is found that the maximumvoltage ripple is 0.15 V and the tracking error of the current is within 0.01% at the steady state.From these results, the high accurate coil currentcontrol is demonstrated even in the 10 Hz LFAC system.  We confirm that LFAC system is possible for the application of the fusion device.Plasma heating deviceCommercial AC60 HzLFAC1~10 HzDCSuper-conductingcoilsLFAC/DCLFAC/DCLFAC/DCAC/LFAC Helium refrigeratorLFAC/DCGrid77kV/6.6kVFig.2Superconducting cableFig. 1. An LFAC system for a fusion deviceFig. 2. Main circuit configuration of the power supply for the superconducting coilFig. 3. Block diagram of the proportional control schemeTable I Major parametersRated voltage 45 VRated current 6 kALFAC frequency 10 HzProportional gain 1-60-40-2002040600 2000 4000time[s]Superconducting Coil  terminal voltage [V]020004000600080000 2000 4000time [s]Superconducting Coil current [A]Fig. 4. Simulation results (exciting and de-exciting of coil)[1] K. Maeba, Y. Miura and T. Ise “Power Supply System for Superconducting Coils based on Low Frequency Power Transmission”, Kansai-section Joint Convention of IEE , G5-10 (2013.11)295§5. Study on Power Supply System for Superconducting Magnets Using Low Frequency Power TransmissionIse, T., Miura, Y., Maeba, K. (Osaka Univ.),  Chikaraishi, H.

Study on Power Supply System for Superconducting Magnets Using Low Frequency Power Transmission

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