Design and Implementation of Four 20 ka, 5 kv Hybrid Switching Networks for Plasma Ignition in the International Tokamak JT-60SA

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1 Design and Implementation of Four 20 ka, 5 kv Hybrid Switching Networks for Plasma Ignition in the International Tokamak JT-60SA F. Burini, Y. Kuate-Fone, G. Taddia, S.M. Tenconi OCEM Energy Technology Srl Bologna, Italy M. Matsukawa, K. Shimada Japan Atomic Energy Agency, Naka Fusion Institute Mukouyama, Naka-si, Ibaraki-ken, Japan A. Lampasi, P. Zito National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA) Frascati, Italy A. Coletti, L. Novello Fusion for Energy (F4E) Broader Fusion Development Department Garching, Germany Abstract This paper describes the design and implementation of the Switching Network Unit (SNU) for the superconducting Central Solenoid coils of the international nuclear fusion experiment JT-60SA to be built in Naka, Japan. Fusion experiments require an overvoltage in the poloidal coils inducing an overvoltage in the plasma chamber in order to produce the plasma breakdown. In the modern fusion devices, as JT-60SA, this is achieved by a SNU. The designed SNU can interrupt a DC current up to 20 ka in less than 1 ms to create a voltage up to 5 kv. It is realized with a hybrid switch integrating an electro-mechanical device and a solid state static circuit breaker, parallel connected. The SNU resistance can be prearranged and dynamically reduced by a solid state making switch to comply with the experimental requests. Preliminary test results confirmed the current balance of the multiple parallel branches constituting the solid state switch and proper behavior of the devices, confirming simulations results. The developed technical solutions may be employed in many other fields where is required a DC high current interruption, as medium and high voltage DC networks (HVDC systems). To initiate the plasma breakdown in the tokamak, an abrupt current derivative shall be produced almost simultaneously in the four CS coils, in order to produce an adequate voltage transient across them. As the power and voltage ratings of the SCMPSs are limited to about 30 MVA and ±1 kv DC, a Switching Network Unit (SNU) is inserted in series to each of the four CS circuits. The SNU exploits the magnetic energy stored in the superconducting coils to support the required transient response [3]. The basic functional structure of the SNU is schematized in the red rectangle in Fig. 1. Keywords JT-60SA; Switching Network Unit (SNU); Superconducting Magnet Power Supply (SCMPS), Hybrid switch; I. INTRODUCTION JT-60SA is an international research project, involving Japanese and European partners, aiming the investigation of key physics and engineering issues for nuclear fusion reactors. The central solenoid (CS) of the new tokamak JT-60SA will consist of four superconducting coils, classified as CS1, CS2, CS3 and CS4, each connected to a dedicated Superconducting Magnet Power Supply (SCMPS) circuit [1]. The SCMPS circuits are practically identical for the four CS coils. The basic diagram of the CS1 circuit, including its protection devices, is reported in Fig. 1. According to the international agreements, the procurement of each device in Fig. 1 is assigned to a partner of the project JT-60SA [2]. Fig. 1. Configuration of the circuit of JT-60SA Central Solenoid CS1

2 On the basis of the analysis of plasma breakdown and corresponding transient voltage [4-7], an interruption nominal DC current up to 20 ka, a nominal voltage of 5 kv, and an opening/closing time lower than 1 ms are required for the SNU operations. In the implemented design, such performances are obtained by a SNU main switch, hereafter referred to as SS (symbolized by the O/C switch in Fig. 1). The SS represents a function essential for the tokamak operations that cannot be obtained by a single device. In the presented design this function is implemented by a group of components, resulting in a hybrid electromechanical-static switch. By opening the SS, the current is diverted into the resistor bank R1, which is part of the SNUs. The resistance value of R1 can be pre-adjusted by selectors, in order to match the maximum current to be interrupted. Finally, after plasma breakdown, a further resistance R2 can be put in parallel to R1 to support plasma current ramp-up, by means of a solid state making switch (MS). After plasma breakdown and current ramp-up, the SNU SS shall be reclosed and from this time on the SCMPS rectifier bridges will control the current in the central solenoids. II. CHARACTERISTICS AND OPERATION OF THE HYBRID SWITCHING CIRCUIT A. Operation and Ratings of the Switching Network Unit Table 1 reports the main parameters of the SNU. The detailed power circuit diagram as developed in the SNU final design is shown in Fig. 2. TABLE I. SNU MAIN OPERATIONAL PARAMETERS SNU Parameter Description Value Unit Nominal Current ± 20 ka SS Maximum conducting Current ± 23 ka SS Maximum interrupting Current 20 ka Maximum Pulse Length 250 s Minimum repetition time 1800 s Current Interruption Unidirectional Rated Voltage 5 kv Reference highest voltage for equipment (IEC 60071) 7.2 kv SS maximum switch-on/off time 1.0 ms MS maximum switch-on time 1.0 ms SS operation accuracy/repeatability 0.5 ms Accuracy of each breakdown resistor (at 20 C) ±2 % Maximum variation of resistors with temperature ±10 % Number of operations without maintenance (excluding sacrificial contacts) Fig. 2. Functional Scheme of the complete SNU. The SS structure improves and adapts the use of a solid state static circuit breaker (SCB), in parallel with the main electromechanical by-pass switch (BPS). The use of a hybrid circuit breaker for DC applications is in principle not new and has been proposed and developed in the past for some tokamak and naval applications [8-10]. In this application, the reasons of the hybrid configuration come from the specific working cycle of tokamaks: the central solenoid current is ramped up for a maximum of 40 seconds and then, at a predefined time, the voltage across them is abruptly reversed. So, while the mechanical BPS carries the current for most of the operational time, the SCB masks the inadequate velocity and repeatability of the BPS in commutation, giving the hybrid switch the precision timing and velocity of solid state devices. Moreover, the power dissipated in the arcing across the BPS contacts is greatly reduced and the operational life of mechanical contacts is correspondingly increased. This is important as the opening and re-closing of the SS will be repeated every thirty minutes, for about ten hours per day. It is worth pointing out that, even though the Base SCMPSs are able of four-quadrant operation, the SCB and the MS are unidirectional devices, since they operate at the beginning of the tokamak current pulse. Since the BPS mechanical switch is obviously bidirectional, the SNU as a whole is also bidirectional. As mentioned earlier, during the current ramp-up in the CS coils, the BPS is closed and the current flows through it only. When the plasma breakdown is required, the SS is opened. Fig. 3 summarizes the exact sequence and the expected voltage and current during the main SNU operations [3]. The time scales in the figure are adapted to emphasize the SS operations. To implement the first SS commutation (opening) the hybrid operation is started about 200 ms before the plasma breakdown (corresponding to t=0 in the machine timer). In this commutation the SCB is activated, but the current still flows in the BPS, until the mechanical contacts begin to open. Across

3 the opening contacts, an electrical arc develops and this voltage forces the current into the activated SCB; correspondingly, the conduction voltage across the SCB (a few volts) limits the arcing voltage to some tens of volts and greatly reduces the corresponding dissipated power in the contacts. After a time margin ensuring the completion of the current transfer and the de-ionization of the air across the BPS contacts, the SCB can be opened, producing across the resistors R1 the high voltage necessary to generate the plasma breakdown (from a minimum of 4.4 kv to a maximum of 5.5 kv). The described operation assumed a wide margin for the first commutation (about 200 ms) in order to ensure the maximum safety, reliability and repeatability. This margin was used for the design but could be reduced to limit the stress on the components. The energy ratings of resistor R1 have been designed in order to match the maximum insertion duration time of about 300 ms. The experiments could require a lower insertion time, also considering that the peak current could be lower. After this time, the resistors in the bank R2 are inserted by turning-on the MS. This reduces the reverse voltage across the CS coils to about 500 V. The MS command is not show in Fig. 3 but it corresponds to the first reduction in the current derivative and in the SNU voltage. The MS and R2 branches are designed to sustain the coil current even for several seconds (also as last resource in case of fault of other protections). This time is compressed in Fig. 3 to include the second SS commutation (closing). The SS must be closed again by the following sequence. First, the SCB is turned on and most of current returns to flow through it. In practice, the time of this operation coincides with the SS reclosing. Afterwards, the BPS can be closed while the voltage across its closing contacts is reduced to few volts and the arc, if any, sets in when the contacts are so close that its duration and energy are negligible. The times necessary for the BPS reclosing and SCB turning off must be taken into account for the design but are not relevant for the SNU time performance. B. The By-Pass Switch (BPS) The sequence of operation described above is based on a bistable high speed contactor of compact size (1530 mm x 984 mm x 560 mm) and weight (280 kg). It consists of eight main contacts, arranged in two groups of four. Each group is mounted on a common shaft, which is controlled by three independent coils. The two groups of contacts are put in series two by two and the two shafts are mechanically connected to ensure a synchronism as perfect as possible. The contacts opening time is about ms and their closing time is less than 75 ms. The BPS prototype is shown in Fig. 4. The contacts in series can be seen one above the other. As required by the SNU main parameters (Table I), the BPS can carry up to 23 ka for more than 4 minutes every 30 minutes. Since the BPS reclosing is relevant for the SNU safety in case of fault, the BPS can be closed even if two coils out of six were defective and coil commands are duplicated. A mechanical latch prevents any unwanted opening and avoids the use of auxiliary power when in closed or open position. Fig. 3. Operational sequences of the SS with the current and voltage expected across it. The time scales are adapted to emphasize to main commands and operations, as explained in the text. Fig. 4. The BPS cubicle seen from the side of the eight main contactors. The six main operating coils are also visible, together with a number of auxiliary contacts. The connections to the CS circuit are from the top.

4 C. The Static Circuit Breaker (SCB) The SCB (Fig. 5) consists of eight parallel branches based on the ABB 5SHY42L6500 Integrated Gate Commutated Thyristor (IGCT). Each of them is provided with a progressive snubber, to limit both the voltage transient peak at turn-off and the voltage derivative as well. A decoupling diode Dd is put in series to each of them, to avoid the transfer of the snubber current from one branch to the other in case of turn-on delays and to help steady state current balance among the branches. This is also obtained by using parallel connections of calibrated resistance (about 2 m ). This ensures a worst case imbalance lower than 10% at the nominal current of 20 ka, limiting the maximum current to be turned-off by a single device to less than 2750 A, whereas the maximum controllable current is 3800 A. The current and voltage waveforms of the most stressed IGCT are reported in Fig. 6. The time intervals are as follows: A-B: current transfer from the BPS to the SCB; B-C: opening of SCB and current through R1 resistors; C-E: resistors R2 are put in parallel to R1 by turning-on the MS; E-G: the SCB is closed again; D-F: the BPS is commanded to re-close and contacts reclose at point F, transferring the current back from the SCB to the BPS. The SCB is designed to sustain and control the current during the limited time required to transfer the current from the BPS to breakdown resistors R1 and R2 and vice versa. That notwithstanding, they have been provided with water cooling, in order to ensure to the SCB a starting temperature of its devices between 5 and 35 C. The use of the progressive snubber permits to limit the transient voltage peaks. This is obtained first by limiting the dv/dt of the devices at turn-off by a traditional RCD snubber and, when the voltage across the IGCT reaches a determined voltage (around 5 kv in most cases), by switching-in a low impedance; the commutation dv/dt is thus reduced to about 7%. The module layout has been designed to minimize the stray inductances. The effectiveness of this solution has been first simulated and then tested in two prototypes of the branch, as better explained in Section IV. Fig. 6. Calculated current and voltage waveforms in the most stressed element of the SCB. D. Breakdown Resistors Banks R1 and R2 The R1 and R2 resistor banks are among the most critical components for the realization of the desired current waveform in the coils, which can induce the plasma breakdown. In particular, a very low tolerance must be achieved (<2%), which is a difficult task considering the high dump energy which the resistors must absorb in the worst case. The overall resistance of the R1 bank is pre-arranged using motorized selectors in series with the four resistive branches R11, R12, R13 and R14, which have scaled resistance values. To realize them two different resistor modules are used: R13 and R14 (two R13 modules connected in parallel form the R12 resistor, while four R13 modules in parallel form R11). As the SCR switches are in series with four identical R2 branches (operatively divided in two sub resistors R11 and R12), the R2 total resistance is controlled by activating selectively the SCRs of the MS. Other major requirements of the breakdown resistors are described in the following: Maximum variation of resistors with temperature <10%; Different constraints on the stray inductances, to be minimized in R1 in order to reduce its insertion time and to be tailored in R2 to limit the current derivative in the MS (see Section II.E); Total dump energy of 30 MJ for R1 bank and 60 MJ for R2 bank; Maximum temperature of the air exhausted from the banks cubicle <100 C (40 C maximum ambient temperature), using air cooling (fans). Both the banks are realized using AISI 304 stainless steel. Specific type tests have been performed on both the resistor banks, with positive results. Applying the specified dump energy on the resistors, the temperature rise measured is about 80 C, with a resistance variation of about 8%. The temperature of exhaust air from the cubicle is always less than 20 C higher than ambient temperature. Fig. 5. The SCB cubicle

5 Fig. 7. The cubicle containg the R1 resistors bank. E. The Making Switch As shown in Fig. 2, the MS is made with four ABB Thyristors in parallel, selected from the type 5STP27U8500. In fact, each single SCR feeds a single resistor of the R2 bank, so the current balance among them is ensured by the resistors themselves and their tolerances (±2%). The dynamic balance and the di/dt limitation at turn on are obtained by increasing on purpose the stray inductance of the resistors of the R2 bank to a value between 10 and 16 H. They are double side cooled by means of de-mineralized water, and their junction temperature does not exceed the 90 C limit in normal operation. The four thyristors have been arranged in one single stack, inside its own cubicle, as shown in Fig. 8. Even in case of worst-condition faults, the thyristor junction temperatures reach a value lower than the maximum suggested one of 110 C. Fig. 8. The MS cubicle during assembly. III. CONTROL AND PROTECTION OF THE SNU The four SNUs are controlled by a single Local Control Cubicle (LCC), which handles the operative functions and safety of the SNU, remote communication and local interface. This allows to coordinate and synchronize the operations and the protective actions on all the four SNUs. Control hardware is based on a National Instruments PXI platform, which performs the functions of acquisition, control and interface. The PXI system is connected to custom electronic boards which realize signal distribution, level translation, fiber optics insulation and interface functions. The interface with remote control system consists of three interfaces: Reflective memory interface. It is a low latency, high speed deterministic interface, which implements the fast communication (alarms, commands, states, measurements) Fiber Optic Interface, which carries signals J (protection request command from JT-60SA), and HL (light fault) and HS (severe fault), exchanged between the SNU LCC and the JT-60SA Internal Protection System Hard-Wired interface for safety signals, exchanged between the SNU LCC and the JT-60SA Safety interlock System. The optical alarms HL and HS are transmitted both by optical links and by Reflective Memory for redundancy, to abort the pulse and to pilot the CS currents to zero. A complete fault analysis has been performed, which has been included in the general protection strategy of JT-60SA. The main result is that, in case of internal fault, SNU can reach a safe state in maximum 150 ms (total time including detection and protection times), thus not requiring any external intervention by system Crow-Bar and Quench Protection Circuit (see Fig. 1). IV. IMPLEMENTATION AND PRELIMINARY TESTS RESULTS In order to test the SCB design in the most critical conditions, concerning specifically the steady state current balance and the behavior of the devices at turn-off, two SCB modules (S/N 001 and S/N 002) have been assembled. The IGCTs and the associated decoupling diodes Dd were selected in order to obtain the worst possible situation concerning voltage drop in conduction and turn-on/turn-off delay times. Namely, delay times difference was about 220 ns and voltage drop at 2500 A, 6.62 V (SN 001) and 5.85 V (SN 002). The test circuit is based on a capacitor bank of 1.2 mf, discharged through a 70 H inductor into the two SCB modules in parallel; the starting voltage of the capacitor bank can be adjusted to achieve the desired peak current at turn-off. Two resistors of the bank R1, with a combined resistance 1.25 were put in parallel to the two SCB modules. The two modules were first tested separately up to 2750 A and then tested in parallel with a total current peak of 4200 A. The test was first simulated using a PSIM model, which took all the

6 circuit elements into account, including the stray inductances (Fig. 9). As shown in Fig. 10, the experimental waveforms measured during the tests are in very good agreement with the simulation results. V. CONCLUSIONS AND PERSPECTIVES A high power DC hybrid circuit breaker, capable to carry 20 ka for 5 minutes and able to commutate in less than 1 ms with extremely precise repeatability and low maintenance (in spite of the required 20 cycles per day) has been designed and is now in the final stages of testing. The overall footprint is 7.3 m x 4.2 m (excluding the Local Control Cubicle). Preliminary tests have proved compliancy with design simulations and, in particular, the balance of the solid state circuit breaker branches and the efficacy of their snubber circuit to limit voltage peak and derivative. Although designed and produced for the JT-60SA international tokamak, possible application include medium voltage DC networks (either naval or land based). The extension to higher current and voltages is viable by connecting and coordinating several electromechanical and static devices. REFERENCES Fig. 9. Simulation results of the parallel testing of the two SCB modules. Time scales: 0.1 ms/div (upper) and 20 μs/div (lower). Fig. 10. Test results of module 001 and 002 in parallel: blue trace = total current: 700 A/div; yellow trace (SN002), green trace (SN001) = module current, 400 A/div; red trace = IGCT Voltage, 1.25 kv/div. Time scale: 0.1 ms/div and 10 s/div (zoom). [1] A. Coletti, O. Baulaigue, P. Cara, R. Coletti, A. Ferro, E. Gaio et al., JT-60SA power supply system, Fusion Eng. Des. 86 (2011) [2] JT-60SA Plant Integration Document (PID) Version 3.3, [3] A. Lampasi, A. Coletti, L. Novello, M. Matsukawa, F. Burini, G. Taddia, S. Tenconi, Final design of the Switching Network Units for the JT-60SA Central Solenoid, Elsevier Fusion Engineering and Design 89 (2014) , DOI: /j.fusengdes [4] K. Yamauchi, K. Shimada, T. Terakado, M. Matsukawa, P. Cara, E. Gaio et al., Electric circuit analysis for plasma breakdown in JT-60SA, J. Plasma Fusion Res.Series 9 (2010) [5] M. Matsukawa, T. Terakado, K. Yamauchi, K. Shimada, P. Cara, E. Gaio et al., Optimization of plasma initiation scenarios in JT-60SA, J. Plasma Fusion Res.Series 9 (2010) [6] L. Novello, E. Gaio, R. Piovan, M. Takechi, S. Ide, M. Matsukawa, Overcurrent analyses in JT-60SA poloidal circuits due to plasma disruption and quench protection intervention, Fusion Eng. Des. 86 (2011) [7] P. Zito, D. A. Lampasi, G. Maffia, G. Candela, "A Novel Digital Controller for 12-Pulse Back-to-Back AC/DC Converters in Nuclear Fusion Experiments," International Symposium on Power Electronics, Electrical Drives, Automation and Motion (SPEEDAM 2014), Jun [8] E. Gaio, A. Maistrello, M. Barp, M. Perna, A. Coffetti, F. Soso, L. Novello, M. Matsukawa, K. Yamauchi, "Full scale prototype of the JT- 60SA Quench Protection Circuits", Fusion Engineering and Design, Volume 88, Issues 6-8, October 2013, Pages [9] A. Coletti, G. Allegra, A. Cadore, G. Candela, R. Coletti, T. Consani, et al., The IGNITOR electrical pulsed power supply system, in: Twenty- First IEEE/NPS Symposium on Fusion Engineering (SOFE 05), September, [10] B. Roodenburg, A. Taffone, E. Gilardi, S.M. Tenconi, B.H. Evenblij, M.A.M. Kaanders, Combined ZVS ZCS topology for high-current direct current hybrid switches: design aspects and first measurements, Journal of Electric Power Applications, 1, 2, , 2007.