JT-60SA power supply system

JT-60SA power supply system

Fusion Engineering and Design 86 (2011) 1373–1376 Contents lists available at ScienceDirect Fusion Engineering and Design journal homepage: www.else...

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Fusion Engineering and Design 86 (2011) 1373–1376

Contents lists available at ScienceDirect

Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes

JT-60SA power supply system A. Coletti a,∗ , O. Baulaigue b , P. Cara a , R. Coletti c , A. Ferro d , E. Gaio d , M. Matsukawa e , L. Novello d , M. Santinelli c , K. Shimada e , F. Starace c , T. Terakado e , K. Yamauchi e a

F4E Broader Fusion Development Dept. – Garching, Germany CEA – Cadarache, France Associazione EURATOM-ENEA – Frascati, Italy d Consorzio RFX – Padova, Italy e JAEA – Naka, Japan b c

a r t i c l e

i n f o

Article history: Available online 8 March 2011 Keywords: JT-60SA Superconducting Magnets Power Supply Broader Approach

a b s t r a c t The paper describes the main features of the Superconducting Magnets Power Supply to generate the toroidal and poloidal magnetic fields in JT-60SA tokamak, with special regard to coil current regulation mode and magnets protection. © 2011 Published by Elsevier B.V.

1. Introduction JT-60SA [1] is a joint international project involving Japan and Europe, in the frame of the “Broader Approach Agreement”, for the construction and operation of a new tokamak intended to prepare and support ITER operation. SA, as “super advanced”, refers to the use of Superconducting Coils Magnets (SCM) and to the study of advanced modes in plasma operation. The SCM system includes Toroidal and Poloidal Field Coils (TFC and PFC, respectively). In addition the machine features a number of normal conducting coils: Fast Plasma Control Coils, Resistive Wall Mode Control Coils and Error Field Correction Coils. JT-60SA is to be built in Naka, Japan, using existing infrastructures and subsystems of the former JT-60U experiment, as much as possible. As a consequence, the JT-60SA SCM Power Supply (PS) should re-use, as much as possible, actual JT-60U PS system [2]. In the following paragraphs the new components are described. 2. JT-60SA toroidal field coil circuit The TFC system [3] is composed by 18 coils grouped in three sections interconnected through three quench protection circuits (QPC), Fig. 1. The ac/dc converter has to provide 25.7 kA at 80 Vdc and it is composed by a six-pulse, unidirectional thyristor bridge. The TFC is protected against overvoltages by the crowbar unit, against fault to

∗ Corresponding author. Tel.: +39 3388779022; fax: +49 8932994198. E-mail address: [email protected] (A. Coletti). 0920-3796/$ – see front matter © 2011 Published by Elsevier B.V. doi:10.1016/j.fusengdes.2011.02.042

ground by monitoring the fault current in the crowbar grounding resistor, against quench of the SCM inserting the QPC (see Section 5). Due to the very low voltage of the SCM in steady state conditions, fault to ground current detection is not a trivial matter. For this, two electrical models of JT-60SA TFC circuit have been developed (using MATHLAB SIMPOWER© and PSIM© codes) including detail model of SCM. Preliminary results show that, in case of Rfault = 0, the fault current (Fig. 2) has an average value of ∼15 mA, with a significant 300 Hz component, substantially higher than in normal situation. Then, the fault to ground in one of the TF coils seems to be detectable (also improving the accuracy of the measurement using a 300 Hz notch filter). At the moment of QPC operation, a V = 2.8 kV is applied across each group of coil. The voltage distribution across the 72 turns of each TF coil has been evaluated to check the coil insulation specification. The electrical modeling of one coil has been created (Fig. 3, a quarter of the total model is shown) including: the inductances of any turn (L1 n); the stray capacitances among the turns (C1 , C1 ); the stray capacitance (C2 and C2 ) among the turns and the coil conductive layer (Rpaint , in Fig. 3), applied on the external surface of each coil to homogenize the electrical field, and stray capacitances among turns and the coil casing (C4 , C4 ob, C4 ib). Related resistances of stray capacitances are also included. Due to the fact that only voltage at t = 0+ is checked, the effect of mutual inductances is neglected to reduce time of calculation. Using the single coil model, one group QPC+ TFC section +QPC has been modeled. Detail in Fig. 3 shows the voltage between two adjacent turns assuming a voltage ramp up time of 5 ms. The resulting peak value (∼71 V) meets the TFC insulation specification (80 V turn-toturn).

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Fig. 1. Electrical scheme of TFC circuit.

Fig. 4. Typical JT-60SA PFC circuit with booster PS.

Fig. 2. Detected current in case of JT-60SA TFC fault to ground (Rfault = 0).

3. JT-60SA PFC circuits Each PFC PS system includes: a 12 pulses/4 quadrant thyristor converter (PS), a Switching Network Units (Fig. 5) or Booster PS (Fig. 4) to generate plasma breakdown and a QPC unit. In order to

re-use the existing JT-60 transformers and to reduce the overall costs, the Back-to-Back (BtB) reverse thyristor scheme has been selected. Two BtB bridges (conv. 1 and conv. 2) are connected to the two secondaries of the transformer, each one able to carry out half of the total load current IL (Fig. 6). When I > 0.5IL both BTB bridges are operating in parallel. When I < 0.5IL one BtB is switched off and the other one carries all IL . When I ≈ 2 kA, circulating current operation starts in order to reverse the load current without any

Fig. 3. Cross-section of one JT-60SA TFC, showing the winding direction, and details of the related electrical scheme. Difference of voltage between to adjacent turns is also showing.

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Fig. 7. Typical reference signals studied to control JT-60SA BtB converter.

Fig. 5. Typical JT-60SA PFC circuit with SNU units.

ii. Switching Network Units (SNU) to perform the same action in the remaining PFC circuits (Fig. 8). Each SNU provides up to 5 kV at 20 kA by inserting a resistor R1. Current derivative can be changed by inserting the resistor R2 through MS2. SNU is switched ON/OFF as shown in Fig. 9. Taking into account that

discontinuity. When IL < 0 a symmetric sequence is followed. To perform this procedure, the power supply control system has to generate specific reference signals for the two BtB converters. A typical example is reported in Fig. 7. To optimize this, a model of the converter shall be realized by the manufacturer to fully test the power supply regulation. 4. Booster PS/switching network units To generate the sufficiently high electric field in the vacuum chamber to start plasma breakdown, two different systems are used to make a step voltage variation: i. Booster PS, to re-use existing JT-60U PS, in the circuit where lower current is requested (Fig. 4). The Booster PS are normally by-passed and they will be inserted into the circuit for plasma breakdown and during the first phase of plasma rump-up.

Fig. 6. Typical sequence in BtB thyristor converter to reverse current with circulating current procedure.

Fig. 8. General scheme of JT-60SA SNU.

Fig. 9. Typical SNU operation sequence.

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Hybrid CB

BPS

QPC

Static CB Pyrobreaker DC Power Supply

Superconducting Magnet

Dump Resistor

Fig. 10. QPC simplified scheme.

BPS delay time BPS complete opening time On

BPS opening command

Off On

Static breaker turn-on command

Off In

BPS current 0 In

Static breaker current

0 In

Dump resistor current

0 t1

t2 t3 t4 t5

time

Fig. 11. QPC operation sequence.

SNUs operate at every plasma shot, a small resistance R is put in series at BPS to reduce its contacts erosion/maintenance. Operation and preliminary design of SNUs have been successfully checked through a detailed MATHLAB SIMPOWER© model and the result is shown in Fig. 9.

tion, composed of a mechanical bypass switch paralleled to a static one, based on Integrated Gate Commutated Thyristor (IGCT) technology, has been worked out; the preliminary feasibility studies are described in [5]. The resulting design allows benefiting from the fast circuit breaking and very low maintenance requirement of the static switches, besides maintaining the advantage of the much lower power losses of the mechanical bypass in normal operation. The simplified scheme of the QPC and the operation sequence are shown in Figs. 10 and 11, respectively. In normal operation the continuous current flows through the BPS, capable to sustain high DC currents and to assure low conduction losses due to its very small resistance value. In case of QPC intervention, an opening command for the BPS and a turn-on command for the static breaker are generated (time t1 in Fig. 10). After a delay due to mechanical inertia, the BPS contacts open (time t2 ) and an arc is formed, whose voltage forces the current to commutate from the BPS into to the static breaker. The static breaker is maintained in the on-state until the current commutation is over (time t3 ) and the BPS contacts are fully open (time t4 ); in this way, the BPS opening occurs with zero voltage applied. The full voltage is applied across BPS only when the static breaker is turned-off and the current is transferred to the discharge resistor (time interval t4 –t5 ). The time interval between the opening command of BPS t1 and the contacts fully open t4 can range from tens to hundred of milliseconds depending on the switching technology and. On the contrary, the IGCT opening time is very short, of in the order of microseconds. 6. Conclusions The main features of the JT-60SA SCM PS have been shown. Presently, the technical specifications for the related procurements have been agreed between the JT-60SA European and Japanese Home Teams (EU HT, JA HT). The first call for tender, regarding the Quench Protection Circuits (QPC), is expected to be launched within 2010, while the procurement of all majority of the other units is expected to start within the first half of 2011. Further analyses are on going to optimize the values of grounding resistors and to check the max value of the fault resistance (Rfault ) still allowing fault detection. Acknowledgments

5. Quench protection circuits The conceptual design of the Quench Protection Circuits (QPC) of the JT-60SA superconducting magnets is described in [4]. The QPCs must provide a fast discharge of the energy stored in the superconducting coils, in case of quench or other circuit faults, by opening a DC Current Breaker (CB), which diverts the coil current into a discharge resistor circuit faults, by opening a DC Current Breaker (CB), which diverts the coil current into a discharge resistor. Due to the relevance of the SCM, a pyrobreaker is added as a backup protection. Each one of the 3 TFC QPCs (Fig. 1) generates up to 2.8 kV at 25.7 kA with a max I2 t of 4.6GA2 s. Each PFC QPC generates up to 4.2 kV at 20 kA with a max I2 t of 2GA2 s. In both cases, the maximum delay time from the command to the QPC CB is t = 1 s and the maximum delay for the backup protection operation is tbck = 0.5 s. After having considered different design alternatives, a hybrid solu-

This work was undertaken under the Broader Approach Agreement between the European Atomic Energy Community and the Government of Japan. The views and opinions expressed herein do not necessarily state or reflect those of the Parties to this Agreement. References [1] P. Barabaschi, et al., Status of Design and Procurement Activities in JT-60SA (this issue, invated talk). [2] K. Shimada, et al., Design Study of an AC Power Supply System in JT-60SA (this issue). [3] G. Phillips, et al., JT-60SA-Design of TF Magnet System (this issue). [4] E. Gaio, L. Novello, R. Piovan, K. Shimada, T. Terakado, K. Kurihara, et al., Fusion Engineering and Design 84 (2009) 804–809. [5] L. Novello, E. Gaio, R. Piovan, IEEE Transactions on Applied Superconductivity 19 (April (2)) (2009).