China Fusion Engineering Test Reactor (CFETR) is a new tokamak device, which is one design option under the consideration of the China National Integration Design Group employs superconducting magnets. The main target of this project is to build a self-sufficiency fusion engineering tokamak reactor with its fusion power is 50-200 MW. Toroidal Field (TF) coils, Center Solenoid (CS) coils and Poloidal Field (PF) coils, whose rated current range from 52.9 kA to 90 kA, produce the magnetic field for plasma generation and confinement. The core of Quench Protection Unit (QPU) is to divert the coil current into Discharge Resistor (DR) by using commutation technology, which should be determined by the rated current and voltage requirement of magnets. Different kinds of technologies for QPU are considered, and an advanced commutation technology composed of a mechanical Bypass Switch (BPS), Vacuum Circuit Breaker (VCB) and LC commutation circuit based on artificial current zero technology has been worked out to meet the current and voltage requirement (10 kV/100 kA). To provide a faster and more reliable removal of the energy stored in those superconducting coils, this paper detailed analyzes the feasibility of a commutation scheme based on artificial current zero technology by analyzing and simulating each stage of the commutation process under such large rated current and high voltage conditions. Both analysis and simulation results verified that the operating reliability of QPU can be guaranteed by this 100 kA DC commutation scheme.

The computational complexity of the three-level space vector pulse width modulation (SVPWM) limits its application in high-switching-frequency converters. To address this issue, this paper proposes a simplified SVPWM algorithm based on virtual chopping. Firstly, the reference voltage vector (VV) is adjusted to avoid excessive line-to-line voltage jumps. Secondly, boundary VVs in the space vector diagram, with simple switching sequences, are chosen as standards. Thirdly, the switching sequence of the reference VV is determined by virtual chopping the corresponding standard VV’s. Furthermore, to generate it in the microcontroller, the corresponding modulation waves are obtained. Finally, a method to balance DC side capacitor voltages is proposed. Consequently, the proposed algorithm simplifies the process, requiring only four equations. The sector determination, dwell time calculation and allocation are bypassed. Compared with the existing algorithms, the execution time and design complexity of the proposed algorithm in the microcontroller is reduced. Experimental results are presented to verify its effectiveness.

In the high-power superconducting fusion converter, the single-phase neutral point clamped (NPC) three-level converter is applied to replace the traditional thyristor converter using an integrated gate-commutated thyristor (IGCT) as a power device. The operating frequency of IGCT is usually lower, only a few hundred Hz. An optimized hybrid space vector pulsewidth modulation (SVPWM)-8H is proposed to solve the problems of discontinuous output average voltage, low dc bus voltage utilization rate, dead time compensation, and neutral point potential balance. The new sectors are inserted to make the range of output average voltage of adjacent sectors intersect, which improves the continuity of output average voltage. Furthermore, some sectors are composed of four vectors to reduce the minimum working time of redundant small vectors, improving the dc bus voltage utilization ratio and the converter’s performance. Moreover, by analyzing the influence of dead time on redundant small vectors, redundant vectors are rearranged to make redundant small vectors’ working time synchronously in sector and sector switching, which reduces the influence of dead time on neutral point potential balance. Finally, by analyzing the influence of dead time on the output average voltage, dead time compensation is applied, which reduces the difficulty of compensation implementation. Simulations and experiments verify the rationality of the optimized hybrid SVPWM-8H.

When a quench occurs, a fast discharge resistor (FDR) system is required to dissipate the huge energy stored inside a superconducting magnet during the quench protection (QP) process. The FDR system design is a key and hard factor in the QP process due to the necessity of protecting the superconducting magnet, especially under the condition of large current and high energy. In this paper, a novel conceptual design of the FDR system with parameter variance is put forward. The design can meet the stable operating requirement under extremely large currents up to 100kA. The system structure can also be altered to provide specific power characteristics to meet the QP energy transfer demand. In this paper the working condition of the QP process is introduced first and the structure design is illustrated. Then the layout, structure, material and stability of a single resistor module is described. Finally, the simulation process with FEA is presented to verify the feasibility of the FDR system design. By analyzing the conceptual design of the FDR system, it is shown that the conceptual design can be acceptable and economical for the energy transfer and consumption for superconducting magnets in fusion devices.

The quench protection (QP) system is crucial to transfer the quench energy out within specific time when the quench phenomenon occurs in the superconducting magnet. The fast discharge resistor (FDR) system in the QP system is responsible for transferring and consumption of the quench energy. In large-scale superconductor test facility (LSTF), with extreme quench parameters concerning about the large current up to 90 kA, high inductance up to 2 H, and huge energy up to 8.1 GJ, the structure stability, heat controlling, and power feasibility of the FDR system are hard to be guaranteed. Therefore, a novel FDR system based on forced-air cooling method is put forward and discussed. The FDR system is with the characteristic of effectiveness protection, safety operating, and economic efficiency when compared with previous systems. In addition, based on the designed concept, the analysis and test process of the FDR system are carried out to testify the effectiveness and veracity. In particular, the structure stability of the shape deformation and cooling method of the temperature rise are analyzed. With the proof of simulation and testing results, the FDR system can serve for the QP system development of the new fusion device, the LSTF.