Abstract — High DC current interruption system and large energy dissipation system are required for the protection of ITER superconductive coils. The ITER superconducting magnets will store up to 56 GJ of magnetic energy per operation cycle. In case of coil quench and other fault conditions the energy stored in the coils must be discharged quickly and safely within a discharge time constant from 7.5 to 14 second. It can be achieved by Fast Discharge Unit (FDU), which consists of DC circuit breaker and energy dump resistors. The paper will present its peculiarities and challenging issues and the design and simulation results of FDU system.

I. INTRODUCTION

The magnet system for ITER consists of 18 Toroidal Field (TF) coils, a Central Solenoid (CS, six independent winding pack modules), six Poloidal Field (PF) coils and 18 Correction Coils (CCs). All coils are designed using superconductors with high current carrying capability [1]. The TF coils, which provide the toroidal field, operate in a steady-state mode with a current of 68 kA and discharge the stored energy in case of quench with using 9 interleaved FDUs. The Central Solenoid (CS) coils and poloidal Field (PF) coils, which provide the change in poloidal flux, operate in a pulse mode with current of up to 55 kA and each coil has its own FDU system. Also the ITER plasma requires the use of an In-Vessel Coil (IVC) set as part of the system for active control of vertical position and providing magnetic perturbations resonant with edge magnetic field. The IVC consists of 27 picture frame-type copper coils (Edge Localized Modes, ELM coils) and toroidal ring copper coils (Vertical Stabilize, VS coils).

Quench means that the superconductor reverts to the normal, resistive state, as a result of excursions over limits of temperature, magnetic field and current density. Since, in general, the magnets have a low enthalpy in comparison with their stored magnetic energy the magnetic energy from the coil should be removed as soon as possible. When the quench is occurred, the flow of current through a superconductor should be interrupted and the charged coil energy dumped into the resistor to limit the hot spot temperature in the superconductor [2],[3]. The TF coils have the largest stored magnetic energy (41 GJ) compared to other coils systems. It has a large potential to damage other adjacent systems such as vacuum vessel (1st confinement barrier) caused by the uncontrolled release of this energy. The TF FDUs are classified as safety important component (SIC) and perform the safety function of protection of vacuum vessel for confinement and limiting exposure. Fast discharge is performed by bypassing the power converters with Protective Make Switches (PMS) and

introducing resistance, using FDUs, into the superconducting circuit to discharge the stored magnetic energy as heat.

The paper describes the function and requirements of FDU system for ITER superconducting magnets (TF, CS/PF and CCs except IVCs) and each component of FDU. The detailed specification and design also are presented including the results of simulation and calculation.

II. ITER COIL POWER SUPPLY SYSTEM Function of the Coil Power Supply System (CPSS) is to

provide controlled DC current and voltage to each coil for plasma confinement, plasma initiation and shape control. The CPSS includes nine systems to supply the coils; 1) TFPS for the 18 series connected TF coils, 2) one CS1PS for the CS1 upper and lower modules connected in series, 3) four CSPSs for the CS2 upper, CS2 lower, CS3 upper and CS3 lower modules, 4) two PFPSs for individual supply of the PF1 and PF6, 5) one common system for the four outer PF coils (PF2, 3, 4 and 5) including VS converter, 6) one VSPS for the upper and lower VS coils connected in an anti-series arrangement (saddle configuration) and 7) 27 ELMPS for 9 upper, 9 middle and 9 lower ELM coils. In addition, nine relatively small CCPS systems will supply the error field CCs allowing correction of error field harmonics due to position errors. Fig. 1 shows the configuration of CPSS.

PMS

PMS

PMS

PM

SPM

SPM

S

PM

SPM

SPM

S

PMS

PMS

PM

SP

MS

PM

SP

MS

PMS

PM

S

Fig. 1. Configuration of ITER Coil Power Supply System.

In each circuit, there is one or more AC/DC thyristor

converters connected in series, to provide the power needed to charge and stabilise the current in the TF coils and following the Plasma Control System (PCS) to establish and control the

The Fast Discharge System of ITER Superconducting Magnets

Inho Song1, Alexander Roshal2, Victor Tanchuk2, Jeff Thomsen1, Francesco Milani1, Ivone Benfatto1 1ITER Organization, Route de Vinon sur Verdon, 13115 St Paul Lez Durance, France

2 D.V. Efremov Scientific Institute of Electrophysical Apparatus, 189632 St. Petersburg, Russia E-mail: Inho.Song@iter.org

currents in the CS, PF and CC coils during the plasma pulse phases.

The loop voltage required for breakdown is obtained by AC/DC converters and SNUs, causing a very large amount of power (about 2 GW) to be extracted, in the PS systems for CS modules and the coils PF1 and PF6. And in the PF2-5 coils PS system (power to be extracted is less than 100 MW) more thyristor AC/DC converters in series are used instead of SNUs.

Protective Make Switches (PMS) bypass the AC/DC converters and separate the magnets from the power sources.

The whole CPSS will be interconnected between the AC/DC converters, PMS, SNU, FDU and magnets by a huge and complex DC busbar system. A soft earthing via high impedance is provided for all the coils and power supply components. The leakage current to earth will be measured and used for earth fault protection.

A. TF Coil Power Supply All TF coils are supplied with power by one thyristor

converter (TFPS): a 12-pulse, 2-quadrant converter rated for 68 kA, 900 V no-load voltage with a unidirectional, continuous duty external thyristor bypass. The converter is designed as two thyristor 6-pulse bridges connected in parallel via Interphase reactors. In case of a failure in the thyristor converter, it can be bridged by an external thyristor bypass and PMS. The coil fast discharge is provided by the 9 FDUs, which are interleaved with the pairs of the series-connected coils in order to limit the voltage between the coil terminals and to earth.

Fig. 2. Configuration of TF Coil Power Supply System.

The TF system is earthed through a set of identical

resistors associated with the 9 coil groups. The mid points of the resistors connected in parallel to each group are connected to the TF neutral. Due to this arrangement, the potentials of the two terminals of each group are balanced regarding the neutral/earth potential and, therefore, each terminal voltage to earth is reduced ½ of the voltage across the terminals. The maxim voltage across current leads at fast discharge is about 8 kV.

B. CS/PF/CC Coil Power Supply The common feature of CSs, PF1 and PF6 is that all of

them use AC/DC converter units (2.7 kV no-load, 45 kA for CSs and 55 kA for PF1, PF6) and SNU (8.5 kV except for CS1). With the exception of the CS1 PSS all other systems supply individually on CS module (upper and lower) or PF coils. The basic ITER AC/DC converter unit (PF/CS) comprises four of six-pulse bridges decoupled by external inductors performing four-quadrant operation, supplied by two phase-shifted transformers, providing twelve-pulse operation.

Each unit is equipped with a large sized thyristors directly connected in parallel, with a fuse in each arm. Circulating current operation is used for polarity change and an external thyristor bypass (crowbar) is adopted (pulsed duty) to handle fault conditions and to circulate the coil current. The thyristor bypass comprises several devices in parallel, with sufficient impedance in each branch to achieve acceptable current sharing and reliable triggering. A mechanical bypass (PMS) with continuous duty across the coil is designed to commutate the current from the pulsed duty thyristor bypass and to isolate the coils from the power supply in the fault or emergency conditions.

The two CS1 coil modules, upper and lower, are connected in series and are interleaved with the two SNUs (6 kV each) and two FDUs to reduce the coil potential to earth. CS1U&L coil PSS is shown in Fig. 3.

Fig. 3. Configuration of CS1U&L Coil Power Supply System.

The Correction Coil converters (CCS, CCU/L) have the

same topology as the PF/CS converter unit, but with relatively lower power. Acting in inversion mode the CC converters provide fast discharge of coil energy with adequate time constant. In case of a failure of the converter the PMS, shown in Fig. 1, will be switched on and the coil energy will be dissipated in the busbars.

C. Switching Network Unit (SNU) In ITER, high voltage required for the CS modules, PF1

and PF6 coils at the beginning of the plasma pulse to ensure gas breakdown and initial plasma current ramp-up will be obtained by inserting resistors in series with the preliminary energized coils. It is provided by switching networks capable to interrupt current up to 45 kA in few ms and to transfer it to resistor banks at a voltage up to 8.5 kV. These circuits, called Switching Network Unit (SNU), are made up of DC circuit breaker, thyristor switches, make switches and resistor banks. Make switches are used to change by steps the voltage applied to the coils during and after the initial plasma current ramp-up phase on commands from the plasma control system [4].

III. FAST DISCHARGE UNIT

A. Design Features The ITER FDUs are key components of the ITER machine

protection. They are by far the largest system built so far for

extracting energy from superconducting coils in case of quench, and beyond the present practice of the industry. Moreover, stringent requirements are dictated for nuclear safety. ITER has been defined by French Safety Authorities as an INB (Installation Nucléaire de Base or Basic Nuclear Installation). In ITER, confinement of radioactive material (tritium) and limitation of internal/external exposure to humans of ionising radiation are required as main safety functions. TF FDUs are classified as Safety Importance Classification Level 2 (SIC-2) and the safety relevant design criteria must be taken into consideration, in order to assure the capability of withstanding common-mode failures, e.g. earthquake, fire, aircraft impact, flood, storms, etc.

The FDU shall be capable to continuously carry and interrupt the DC current up to 68 kA with a recovery voltage around 10 kV. This led the choice towards mechanical contacts and vacuum circuit breakers, instead of static devices, which would have required a large number of components in parallel and in series to meet the current and voltage requirements, thus leading to considerable on-state losses. In addition, each vacuum circuit breaker contains two vacuum interrupters connected in series to increase reliability.

The FDU resistor banks must discharge the energy from a few GJ to about 46 GJ. The maximum voltage and the total specified energy (I2t) during the discharge are the two critical concerns for the safety of superconductive magnets. The high thermal coefficient resistor is designed allowing for significant decrease in maximum voltage during the beginning of fast discharge. At the same time, the resistance has the significant increase during the discharge period. Therefore, the I2t of the magnet is easily matched within the limit of voltage requirement. In addition, the location of the discharge resistor and the switches are in different buildings with relative long distance, the connection is again made by parallel coaxial cables, which serve to reduce the impedance, avoid transient overvoltage and accelerate current commutation.

In the design of the FDU equipment, sufficient redundancy is employed. The high reliability Pyrobreaker provides the backup of the mechanical switches and the associate CPCs. In addition, in the TF circuit, there are nine FDUs interleaved with the eighteen coils. Even if two of the nine FDUs fail to open, the remainers are still capable of the energy extraction. In the design of the physical layout of the FDUs, full fire segregation shall be implemented not only for the equipment, but also for all the power and control cables. The control system shall also employ different technologies for the purposes of redundancy.

B. Current Commutation Unit (CCU) The EU Home Team undertook the development of a

current commutating unit for multiple operations, based on traditional mechanical switches [5],[6]. CCU consists of mechanical bypass switch (BPS), vacuum circuit breakers (VCB) and counterpulse circuit (CPC). The BPS is based on an existing low voltage, high current switch used in the electrochemistry and made with natural air-cooled main contacts connected in parallel. The duty of the pulsed circuit breaker is performed using VCBs. Two VCBs are connected in series to increase reliability and each able to interrupt the

nominal current and to sustain the full voltage. The ratings of the FDU switching devices are listed in the table I.

Table I Ratings for the FDU Switching Devices

Parameter Unit BPS VCB PB

Continuous Pulse kA (DC) 68

n/a N/A

68(0.4s) 68 n/a

Opening time ms ≤ 300 ≤ 50 ≤ 1.0

Contact resistance μΩ < 1 2*12 11

Arc contact Main Contacts Operation > 100

> 1,000 > 1,000

N/A N/A N/A

Driving method - Spring Spring Pyrodrive

Rated insulation level kV (AC) 12 12 12

Cooling system - Natural Air

Natural Air

Demineralised water

The commutation of the current to the discharge resistors is achieved by means of CPC that creates an artificial current zero in the VCB arc chamber. CPC consists of capacitor, thyristor switch and inductor, which is in series with capacitor and switch and used to limit short circuit current. Unidirectional CPC is designed for the TF FDU and bidirectional CPC for CS/PF FDU. The current commutation in the resistor is accompanied with the overvoltages caused by the stray inductances in the commutation circuit (jumper/lead, cables etc.) and the stray inductance of discharge resistor. The snubber RC circuit (SC) is included in parallel with the discharge resistor to smooth these overvoltages.

C. Pyrobreaker (PB) The pyrobreaker for backup protection was developed by

the Russian Federation Home Team specifically for ITER application. It is a very reliable, single action, component triggered with a pyrocharge. It consists of two parts, each triggered with separate explosive charges: a multi-gap current interrupter and a disconnector able to withstand the high voltage. Their development is based on the large experience existing at the Efremov Institute, St. Petersburg, Russia in that field.

Fig. 4. Photo of Pyrobreaker

The first stage (multi-gap current interrupter) is based on

multi-cuts of a thin, water cooled copper cylinder initially carrying the current. The hydrodynamic energy due to the explosion of first pyrocharge will cut the multi-gap cylinder

and interrupt the current under high voltage. The cooling water helps also to extinguish the arc appeared when the cylinder is being destroyed by the charge explosion. A second stage (a disconnector, initially connected in series with the first stage contact), is a single disk, cut by an insulated cylinder and withstands the high voltage for a long time. The explosion of second pyrocharge pushes down the insulated cylinder and cuts the disk ensuring the interruption. The driving system consists of two pyrocharges fired electrically at the same time. The pyrobreaker is triggered with a special high-voltage pulse, which fires the explosive charges in the switch. The photo of pyrobreaker is shown in Fig. 4.

D. Discharge Resistors The discharge resistors are based on unified resistor

sections made from ST08 steel tapes in a tight serpentine pattern to minimize the stray inductance. Carbon steel of St08 grade [7] (A6222 in USA or St50-2 in Germany), material with high temperature coefficient (~5.8×10-3 1/K for 20 - 300°C), is chosen. For the given discharge time constant this allows significant decrease in maximum voltage and in initial voltage at the beginning of the fast discharge. Another benefit is the reduction of induced current and, hence forces in the vacuum vessel, as compared to an exponential discharge provided with constant resistors.

Table II Ratings for the FDU Discharge Resistors

Parameter Unit TF CS PF

Rated energy GJ 3.9 1.0 ~ 2.1 0.8 ~ 2.9

Discharge time constant S 11 7.5 14

Max. temperature °C 300 300 300

Resistor weight Ton 81 25 ~ 34 19 ~ 77

Rated insulation level kV (AC) 12 12 12

Cooling system - Natural air ventilation

The discharge time constants of TF, CS and PF, given by Magnet are used for designing the initial resistance value and upper limit of I2t, which decide the total required mass of steel. The table II shows the ratings for the discharge resistors.

Fig. 5. Layout of resistors in the building.

The Russian Home Team conducted the study to verify the

applicability of the natural air convection for FDRs cooling on the basis of the performed hydraulic analysis of the developed

ANSYS CFX model of the FDR cooling network [7]. The time necessary for the natural circulation and complete cooling and possible unbalance of cooling of the resistor modules located in different places are checked. The layout of resistors and the simulation result are shown in Fig. 5 and 6.

Fig. 6. Simulation of air temperature distribution at the end of the

establishment phase of air circulation (τ=60 s).

IV. ANALYSIS OF FAST DISCHARGE UNIT

A. Circuit diagram and operational sequence Each FDU is operated when a request is generated by the coil protection system and thus the energy stored in the coils is dissipated as heat in the resistors. In case of the failure of the CCU, the PB is opened as a backup. The simplified circuit diagram of TF FDU and CS/PF FDU are shown in Fig. 7. In the normal operation, BPS, VCB and PB are closed conduct the full rated current. When the fast discharge is required, the BPS is opened and the current flows through the VCB. After the current has been completely commutated, the VCB is opened and the CPC produces an artificial current zero status providing the VCB arc extinction and therefore the current is interrupted and transferred to the discharge resistors.

Fig. 7. Circuit diagram of TF and PF FDU.

The quench detection system monitors the coil voltages

and if the measured signal is over the first threshold value, then it starts to calculate whether it is real quench or not. Finally, this calculated signal is over the second threshold and quench loop is triggered. It is hardwired link between coil quench detection system, Plant Interlock System (PIS) of magnet and AC/DC converters, and FDU systems. The activation time of this loop is less than 10 ms and also, this

information transfers to the Central Interlock System (CIS) through PIS of magnet and PIS of coil power supply system triggers FDUs of other coils. This signal communication uses the PLC and it takes around 100 ms. Operation delay of FDU is below 500 ms and it represents the delay time from the arrival of quench signal through the quench loop to the starting discharge through the dump resistor. It includes the communication delay inside the FDU controller and operation delay of the BPS, VCB and counterpulse circuit. Figure 8 shows the sequence and monitoring signal of FDU system. It takes about 260 ms ~ 350 ms for commutating the current from the BPS to the VCB. The opening of arcing contacts in BPS develops arcing voltage and it provides the transfer of the current. After the opening of the BPS, if the VCB current is same as the coil current then the signal of VCB turn-off is generated and if not, the PB is triggered. The signal of Check Back VCB-OFF is generated after the opening of VCB and the CPC turns on with some time delay in order to guarantee enough contact gap at the arc extinction and to limit the energy dissipation due to arc [6]. Finally, the controller checks the VCB current and then generates the supervision VCB signal. Each FDU controller checks the VCB current at each step and produces the pyrobreaker triggering signal in case of malfunction.

Fig. 8. Sequence and monitoring signal of FDU.

B. Simulation studies

Fig. 9. The equivalent circuit for the callculation of the transient process.

The analysis of the FDU circuit has been performed with the aim to optimize the parameters of the counterpulse and snubber circuits and to verify transient characteristics of the current commutation process and finally to calculate maximum voltages on the coils due to transient processes and fault conditions. The stray parameters of the circuit used for the simulation studies are determined with the presently available data from Russian team. The equivalent circuit used for the analysis is given in fig. 9. The main parameters used for the simulation studies are as follows: LDR = 13 μH, RDR = 0.11 Ω, LSC = 3 μH, RSC = 0.15 Ω, CSC = 2 mF, LC = 15 μH, RC = 11 mΩ, C = 2.7 mF, Lcab = 20 μH, Rcab = 10 mΩ, LVCB = 2 μH, RVCB = 62 μΩ, LPB = 3 μH, RPB = 15 μΩ, LBPS = 3 μH and RBPS = 0.5 μΩ.

The simulations of the currents and voltages on the different circuit elements including the counterpulse capacitors and the snubber circuit elements are conducted. The current transfers from the VCB into the discharge resistor over 1.5 ~ 2.0 ms and the voltage on the coils during the current commutation does not exceed the maximum allowable values. The parameters of the counterpulse circuit and snubber circuit were checked which provided the necessary current for the VCB extinction and in accordance with the requirement to the maximal voltage applied to the coils. The results of the simulation studies are given in Fig. 10. As is seen, the use of the snubber circuit practically eliminates the transient overvoltage on the load and it leads to a significant reduction of reverse voltage on the counterpulse capacitor C. Also, the simulation result shows the negative effect of stray inductances of the discharge resistors and cables and these inductances has to be minimized.

Fig. 10. Result of simulation studies of TF FDU.

The verification of the high voltage design of the magnet system is an important issue for a reliable operation of the ITER machine. Transient peak voltages occur on the TF, CS and PF coils during the operation of FDU in case of a fast discharge. The maximum voltages across a coil (terminal to terminal) or terminal to ground depends on the circuit configuration, on the operating conditions and on the number and type of fault. The maximum coil voltage to ground strongly varies with the number of simultaneous failures and the stray parameters of the system contribute mainly on the transient peak voltages. For proper insulation co-ordination it is important to determine the peak transient and steady-state voltages applied to the coils. The maximum calculated

0.008 0.01 0.012 0.014 0.016 0.018 0.02 0.022 0.024 0.026 0.028-10k

0

20k

40k

60k

80k

Cur

rent

[A]

0.008 0.01 0.012 0.014 0.016 0.018 0.02 0.022 0.024 0.026 0.028

-4k

-2k

0

2k

4k

6k

8k

10k

Time [s]

Vol

tage

[V]

Bypass currentVacuum currentCounterpulse currentSnubber currentDischarge current

w SCw/o SC

voltages of all relevant cases will produce necessary basis for testing parameters for design and selection of insulation. All of coil power supply systems are modeled and stray parameters of key components are introduced for calculating the peak transient and steady-state voltage induced to the coils considering the normal and abnormal conditions [8].

V. CONCLUSIONS

In the ITER fast discharge system for quench protection and safety function the commutation of coil current to the discharge resistors will be accomplished with the current commutation unit and a very reliable pyrobreaker.

The results of simulation studies have shown that within the selected values of the counterpulse and snubber circuit parameters the FDU can provide the reliable transferring of the coil current into the discharge resistors without causing any transient overvoltage on the coils.

DISCLAIMER

The views and opinions expressed herein do not necessarily reflect those of the ITER Organization.

REFERENCES

[1] N. Mitchell, D. Bessette, R. Gallix, C. Jong, P. Libeyre, C. Sborchia, F. Simon, “Electrical Design Requirements on the ITER Coils,” IEEE Trans. Applied Supercond., vol.18, No.2, pp. 68-73, 2008.

[2] C. Neumeyer, “Fast discharge options for tokamak physics experiment toroidal field & poloidal field superconducting magnets,” Princeton Plasma Phy. Lab., Princeton, NJ, Rep. 40-9308270, 1973.

[3] I. Song, C. Choi. M. Cho, “Quench Protection System for the Superconducting Coil of the KSTAR Tokamak,” IEEE Trans. Applied Supercond., vol.17, No.8, pp. 1-6, 2007

[4] A. Roshal, S. Avanesov, E. Koktsinskaya, M. Manzuk, F. Milani, G. Mustafa, A. Nesterenko, I. Song, A. Filippov, A. Frolov, “Design and analysis of Switching Network Units for the ITER coil power supply,” Fusion Engineering and Design, to be published in 2011.

[5] B. Bareyt, “A 170-kA DC circuit breaker for the quench protection of the central solenoid coil of the ITER tokamak,” Fusion Engineering and Design, 54, pp. 49-61, 2001

[6] T. Bonicelli, A. De lorenzi, D. Hrabal, R. Piovan, E. Sachs, E. Salpietro, S.R. Shaw, “The European development of a full scale switching unit for the ITER switching and discharging networks,” Fusion Engineering and Design 75-79, pp. 193-200, 2005.

[7] V. Tanchuk, S. Grigoriev, V. Lokiev, A. Roshal, I. Song, O. Buzykin, “Air-cooled fast discharge resistors for ITER magnets,” Fusion Engineering and Design, 26th SOFT, September 2010.

[8] I. Song, J. Thomsen, F. Milani, J. Tao, I. Benfatto, “Fault Analysis of ITER Coil Power Supply System,” The 10th International Conference on Electric Power Systems, High Voltages, Electric Machines, October 2010.