2011 IEEEINPSS 24th Symposium on Fusion Engineering SOlC-2

ITER Coil Power Supply and Distribution System J. Tao, I. Benfatto, J. K. Goff, A. Mankani, F. Milani, I. Song, H. Tan, J. Thomsen

Dept. of ITER Project, Central Engineering and Plant Directorate, Electrical Engineering Division ITER Organization, 13115 Saint Paul Lez Durance, France

Jun. Tao@ITER.org

Abstract - The ITER Coil Power Supply and Distribution System (CPSDS) will be installed to receive power from the

French 400 kV transmission grid and to provide controlled DC power to the superconducting tokamak magnets for plasma operation.

The Coil Power Supply System (CPSS) consists of ACIDC

converter units connected in series with Switching Network Units (SNU) and Fast Discharge Units (FDU) to provide the

controlled DC for plasma initiation, current, shape and position

control, error field correction, as well as superconductive magnet quench protection.

The installed power of the ITER CPSS is nearly 2 GV A.

Thyristor based technology is used for the ACIDC converters, with four-quadrant operation and circulating current

capability introduced as a basic design feature due to the high

current rating and DC voltage and current characteristics. The SNU and FDU comprise of large resistor banks inserted into the magnet coil circuits by means of opening appropriate

circuit breakers. A complex DC busbar system will be integrated through various ITER buildings to connect all the

components into the circuit.

The CPSS will draw large power pulses from the grid, with short rise times and steps in active and reactive power. Rapid

reactive power control is necessary for voltage stabilisation and compensation together with filters to eliminate harmonics generated by the power converters. Thus a Reactive Power

Compensation and Harmonic Filtering (RPC&HF) system will be installed with an installed total power of 750 Mvar.

As an integral system, the ITER CPSDS poses significant technical challenges due to the large scale, special requirements and complicated interfaces.

The conceptual design of the ITER CPSDS has been

successfully completed. Further engineering design work and fabrication will be performed under the responsibility of the ITER Domestic Agencies in China, Korea and Russia as part of the ITER procurement arrangements.

Keywords - Coil Power Supply; AC/DC Converter; FDU; SNU;

RPC&HF; System Integration

I. INTRODUCTION

The ITER coil power supply system (CPSS) is supplied from a double circuit incoming 400 kV line, and connected to three identical 3-winding, step-down transformers, providing two levels of voltage, 66 kV and 22 kV. The Heating and Current Drive power supplies (H&CD) are connected to both 66 kV and 22 kV busbars.

The Reactive Power Compensator and Harmonic Filtering system (RPC&HF), based on TCR (Thyristor Control

978-1-4577-0668-4/111$26.00 ©20 11 IEEE

Reactor) technology, is connected to each 66 kV busbar, and provides the dynamic reactive power compensation to minimise the voltage variation and eliminate the harmonic distortion.

The CPSS consists of ACIDC converters, SNU (Switching Network Unit), FDU (Fast Discharge Unit), and complex DC busbar and earthing circuits. It provides controlled DC voltage/current for plasma initiation, current, shape and position control and the protection of the superconductive magnets in case of quench or other external fault.

Thyristor-based converters are selected for the ITER power supply due to the high current rating and the DC voltage/current characteristics. The SNUs are connected in series with the ACIDC converters, and are operated at a specific time to generate a high loop voltage to breakdown the fuelling gas and thus initiate the plasma. The principle is based on the sudden introduction of a resistor in the inductive circuit which produces a high voltage across the coil, and simultaneously extracts a very large amount of power (2 GW) which would otherwise have to be absorbed by the grid. The current interruption is performed using a multi-commutation procedure to provide arcless interruption and meet the requirement for repetitive operation. Similarly, the FDUs consist of switches and parallel resistor banks, the resistor banks will be inserted in the circuit to dissipate the energy stored in the superconductive magnets in case of magnet quench. The current interruption for FDU is accomplished in a manner similar to the SNU. A complex DC busbar system is integrated with electrical components in various ITER buildings. By means of the appropriate earthing circuits and their connection, the coil terminal voltage is limited during operation.

Due to the huge stored energy, some coil power supply components are classified as SIC (Safety Importance Classification) or SR (Safety Relevant) components. Special safety requirements (reliability, redundancy) and qualification are essential for these safety components.

A unique control system integrates the CPSS and the RPC&HF. Due to the large installed power for RPC&HF, the high dynamic demands from the plasma control and some abnormal operating conditions, e.g. plasma disruption and magnet quench, the integration with the load becomes more and more critical. The control system regulates the power supply voltage/current in real time following the plasma control system (PCS). It also provides the self-protection and interlock functions for the power supply and other plant systems. Moreover, the control system manages the safety function as well.

Amongst the parties developing the CPSS, uniform design tools have been adopted and are essential for the successful integration of CPSDS due to the complexity of the plant and the mUltiple procurements.

II. ITER PULSED AC DISTRIBUTION SYSTEM AND LOAD

A. AC Distribution System

Figure I is a simplified representation of the ITER pulsed AC distribution, which receives power from a double 400kV line and distributes it to 66kV and 22kV busbars.

300/300/1 00 MVA 400/66122 kV

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Figure I. ITER Pulsed AC Distribution

The 400kV lines provide a short circuit power of around 11.7GV A. It has the capacity to provide the required active pulsed power (up to 500MW). However, only around 200Mvar pulsed reactive power can be supplied because of the voltage variation constraint at 400kV line (less than 3%). Therefore, substantial reactive power compensation is needed.

There identical 3-winding main step-down transformers, rated at 300/2501150 MVA, will feed three independent 66 kV and 22 kV distribution systems. The large and dynamic load will be equally distributed as much as possible among the three 66 kV busbars, while loads of less than 20 MV A will be distributed on the three 22 kV busbars. The short circuit voltages between the primary winding and the secondary winding, and between the primary winding and tertiary winding are II % and 25 % respectively. The step­down transformer has relative small short circuit impedance compared with the standard production, which is to ensure sufficient short circuit power on the 66 kV and 22 kV busbars to improve the operation of the line-commutation converters and minimise the voltage variation. An On Load Tap Changer (OLTC) is installed at the primary winding, which shall be able to operate manually and automatically. The OL TC will compensate the daily and seasonal voltage fluctuations outside the ITER operation pulse, and it can be also integrated with the RPC system in order to fully explore the compensation capacity during the pulse, and to avoid oversizing the RPc.

During the ITER pulse, due to the RPC and the OL TC, the voltage variation of 66 kV can be kept within -6 % and 10 %. As for the 22 kV busbar, it can be maintained within -10 % and 10 %.

B. ITER Pulsed Load

ITER represents a heavy pulsed load. A typical pulsed power demand cycle will have a duration of 1000s with a repetition period of 1800s.

The total pulsed load is composed of the following major components: power required for the PF scenarios, power needed for the plasma current, position and shape control, including the vertical stabilisation control and power to supply the H&CD systems. In addition, resistive losses, e.g. in the normal conducting busbars, and power to supply the correction coils (CC) have also to be taken into account.

A typical load profile seen by the 400 kV grid, corresponding to the 15 MA plasma inductive scenario with 73 MW heating power, is given in Figure 2. Disturbance of the plasma is assumed, e.g. H and L mode transition, minor vertical displacement event (VDE), modulation of the heating power. In reality, the P&Q profile may differ from the figure shown. However, it gives the approximate operation envelope from the electrical power point of view. It can be shown that with the RPC system (rated at 750 Mvar), the reactive power supplied from grid is limited to around 200 Mvar, and the 400 kV, 66 kV and 22 kV busbar voltage variation is kept within the limits as shown in Figure 3.

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Figure 3. Voltage Variation during the Pulse

III. COIL POWER SUPPLY SYSTEM (CPSS)

The ITER CPSS is composed of various circuits to provide controlled DC power to the superconductive magnets, and to perform self-protection and magnet protection. It comprises the AC/DC converter, SNU, FDU and complex DC busbar system in each circuit.

A. Configuration of the Coil Power Supply System

The CPSS includes the following subsystems:

• One for the 18 series connected Toroidal Field (TF) coils; • One for the Central Solenoid (CS) CS 1 upper and CS 1

lower modules connected in series; • Four for the CS 2 upper, CS 2 lower, CS 3 upper and CS

3 lower modules; • Two for individual supplies of the Poloidal Field (PF) PF

1 and PF 6 coils; • One common system for the four outer PF coils, i.e. PF 2,

PF 3, PF 4 and PF 5, used for plasma vertical stabilisation.

• In addition, nine relatively small power supply systems with very similar configurations will supply the error field Correction Coils (CCs)

The circuits are shown in Figure 4 which highlights their relationship to the magnets.

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Figure 4 Outline Scheme of Coil Power Supply System

The CPSS incorporates the following design concepts: • 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 currents in the CS, PF and CC coils during the plasma pulse phases;

• SNUs in series in the circuits of CS, PFI and PF6, to generate high voltage, superimposed with that from the AC/DC converter, for plasma initiation;

• Resistors, normally bridged by circuit breakers, are included in series with the coils to absorb their stored magnetic energy in case of a quench or other abnormal events which could potentially damage the coils. These circuits are called Fast Discharge Units (FDU);

• Protective Make Switches (PMS) to short the AC/DC converters in case of the fault condition, to preclude the energy supplied from AC/DC converters;

• FDUs or SNUs are interleaved with coils when possible in TF and CS 1 circuits, to reduce the coil potential to earth. 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;

• Operation will be maintained within design limits by real time evaluation of the various parameters measured, e.g. coil current, coil voltage, helium temperature, etc.

B. ACIDC Converter

The ITER AC/DC converter is based on thyristor technology, and modular approach is taken for the design, with higher output voltage requirements being met by series connection of converters. Three types of four-quadrant converter units are required in the CS and PF circuits. A further three types of converters are required for the TF and CC circuits. A summary of the converter module connections is shown in Table I.

Table I Connection of ACIDC Converter Modules

Circuit U (kV)

I (kA) Configuration (Upgrade) No. No-load

CS ±I.35 ±45 1 per coil circuit 6 (2 per coil circuit) (12) 1 for PF 1, PF 6 14

PF ±I.35 ±55 3 for PF 2, 3,4, 5 (16) (2 for PF 1, PF 6)

VS ±I.35 ±22.5 2 for VS control 2 (6 for VS control) (6)

TF ±O.9 68 1 for TF coils 1 CCS ±O.45 ±1O 1 for each circuit 3

CCUIL ±O.O9 ±1O 1 for each circuit 6

1) Design Features of ACIDC Converters

The basic ITER AC/DC converter unit (PF/CS) comprises four six-pulse bridges decoupled by external inductors performing four-quadrant operation, supplied by two phase-shifted transformers, providing 12-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. Moreover, in cases of more than one unit connected in series in one circuit, sequential control will be used for reactive power reduction. The topology of the basic unit for the PF and CS circuit converters is shown in Figure 5.

Figure 5. PF/CS Converter Topology

The VS unit is a four-quadrant, six-pulse thyristor converter but providing a much faster response compared with the PF/CS converter. The basic scheme is similar to one

of the PF ICS units, except there is no phase shift for the two parallel bridges.

The TF converter is a two-quadrant converter, with a unidirectional, continuous duty external thyristor bypass. The TF rectifier transformer can provide two levels of secondary voltage with adjustment at the transformer primary side (optionally using an autotransformer or no-load tap changer), i.e. a high voltage for accelerated charging and discharging of the TF magnet, and a lower voltage for maintaining the steady-state TF current of �68 kA, thus providing a reduction in reactive power consumption. Figure 6 shows the topology of the TF converter.

Figure 6. TF Converter Topology

The Correction Coil converter (CCS, CCU/L) has the same topology as the PF/CS converter unit, but with relatively lower power.

2) Design Criteria and the Technical Solutions

Some special design criteria will be adopted for the ITER AC/DC converter due to the very large installed power, unprecedented short circuit level, high reliability and availability requirements, characteristics of the superconductive load and dynamic performance for plasma control.

a) Reduction of Reactive Power Production

The large reactive power produced by the phase­controlled thyristor converters is an issue for the ITER CPSS. The presence of large reactive power compensation is essential to limit the system voltage variation. However, with the large capacitive installed power the system stabilisation is critical since a significant overvoltage would occur in case of load rejection. To minimize the consideration is taken at the level of the ACIDC converter, i.e. sequential control is proposed for the series converter units to make the reactive power saving. In order to fully explore the saving by the sequential control, gamma control is assumed, which will limit the margin angle during the inversion mode.

b) Avoidance of Current Blocking

Current blocking must be avoided due to the high inductance characteristics of the superconductive load. Four­quadrant operation (excepting the TF converter) is necessary to achieve the required current for plasma operation. The current reversal shall be smooth without a dead-time zone. Circulating current operation is used for the ITER AC/DC four-quadrant converters for polarity change. When a converter operates in a low current level, typically at 10 % to 20 % of the full current, both the two anti-parallel bridges

(supplied by different converter transformers) will be in conduction to drive a circulating current. Such virtual current shall be greater than the current variation due to a plasma disruption to avoid any possibility of current blocking.

Moreover, the thyristor bypass (crowbar) with separated command is fitted to each converter, to provide the additional and selective current path to circulate the coil current in the fault condition and emergency cases.

c) High Reliability and Availability

The ACIDC converter may be subjected to various internal or external faults. Reliability and availability are essential for the ITER AC/DC converters. Fault analysis shows that the levels of fault current for some converters are beyond usual industrial practices. A stringent fault withstand capability is thus required for the converters to properly qualify the components. In addition, the electronic protection; activation of thyristor bypass, intervention of the arm fuse, and tripping of the AC circuit breaker must be well differentiated in order to protect the system. Depending on the severity of the fault, a different fault response will be activated to return the system to a safe or shutdown mode.

In cases of internal short circuits of the AC/DC converter or DC short circuit at the converter bridge terminals, fuse intervention is the main protection action, with tripping of the AC circuit breaker as a backup. There is no fault propagation or deformation of the mechanical structure foreseen.

In cases of a DC short circuit, downstream of the DC reactor, electronic protection is the main action to protect the converter, with no fuse intervention foreseen. The tripping of the AC circuit breaker is a backup. Under such fault conditions, there are no failures of electronic devices.

In addition, an N+ 1 approach is used to size the number of parallel thyristors; this is intended to increase the availability of the system.

The bypass may also be subjected to a short circuit current due to the failure of the bypass itself and some other external faults. The bypass shall be designed such that it can withstand fault current caused by the converter external fault. As for the bypass internal faults, this depends on the type of fault, the bypass shall survive with the electronic protection from the AC/DC converter and avoid damage of the mechanical structure.

d) High Dynamic Characteristics and EMI Issues

To anticipate the requirements of plasma control, the voltage for the PFICS converter can vary full-scale within two electrical cycles or less, whereas the VS converter can accomplish the full-scale voltage change within one electrical cycle.

In order to fulfil this requirement, open loop voltage control with feedforward of the AC supply voltage and DC load current is used. The current balance control between two parallel bridges and circulating current control between two anti-parallel bridges is utilised using an internal control feedback loop.

Due to the very high dynamic characteristics of the ITER converter, electro-magnetic interference (EMI) becomes a challenge. It requires a sophisticated design of the physical structure of the AC/OC converter and optimisation of the plant layout to minimise EMI and to achieve good current sharing among the parallel thyristors, not only in normal operation, but also in fault conditions. Moreover, the CPSS is insulated to a high potential, the interface of the control system is generally anticipated using an optical fibre solution to provide galvanic isolation, which will also minimise EM!.

C. Switching Network Unit (SNU)

The SNU is used during every pulse for plasma initiation in each of the CS, PFI and PF6 circuits; the current is interrupted and then transferred into a resistor bank, giving rise to a maximum voltage of 8.5 kV across the coils.

/) Design Feature and Operation Principle

As shown in Figure 7, each of the eight SNU units comprises one Current Commutation Unit (CCU) consisting of both mechanical switches (FOS & FOS) and thyristor circuit breakers (TCB), connected directly in parallel to a large resistor bank; a second resistor bank can be connected in parallel to the first one by means of a mechanical Make Switch (MS); Finally, another mechanical MS and an Explosively-actuated Protective Make Switch (EPMS) are in parallel to the whole system. The EPMS will activate in fault conditions as the backup of the MS. Change of current polarity is achieved by mean of bolted busbar links. The initial resistance is adjustable off-line using busbar link selection of the parallel resistor elements.

The operation principle is as follows: before the pulse, the MS and MS 1 are open and the CCU is closed; when the CCU is operated, the current is diverted into the first resistor bank, thus giving rise to the high voltage required for gas breakdown; then, when lower voltage is required during the plasma current ramp-up, the MS is closed and the second bank is connected in parallel to the first; finally, at the end of the SNU operation, i.e. after 5 to 15 seconds, the whole resistive circuit is shunted by means of the MS I. The design parameters of the SNU are given in Table II.

Table II SNU Parameters Parameter Value

Rated current 45 kA (CS, PFI); 35 kA (PF6)

Max voltage 8.5 kV (CS2, CS3, PFI, PF6); 6 kV (CSI)

Insulation level 12 kV AC

Max. operation time 4 s (CS2, CS3, PFI, PF6); 15 s (CSI)

Rated energy 2.1 GJ (CS, PFI); 2.7 GJ (PF6) Cool down time 30 minutes

2) Design Challenges and Solutions

The repetitive operation and the high energy to be dissipated are the design challenges for the SNU.

a) Repetitive Operation

The SNUs operate during each plasma pulse. This repetitive operation requires a long lifetime which is the

challenge for the SNU. As a result of the two-stage mechanical switch (FOS & FOS) design, the erosion of the mechanical switch due to arcing is limited. The high recovery voltage is withstood by the FOS. The opening of FOS is assisted by the TCB at a very low voltage « 20 V), practically without arcing. The discharge of the capacitor C I will block the thyristor in parallel with FOS, thus the opening of FOS is under no load conditions, and the current is transferred to TH I. The discharge of the capacitor C2 interrupts the current in TH I, then, the current is transferred to the resistor bank. With this multi-stage current commutation, the SNU is foreseen to operate more than 5000 times without significant maintenance.

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b) Large Dissipated Energy

The maximum energy to be dissipated in the SNU is up to 8.5 GJ in total. Each operation will induce a maximum 250 °c temperature for the resistor conductor. With forced air cooling for the resistor bank, 30 minutes cooling time is required before the next operation. Heat-resistant insulation material is required.

The stray parameters of the SNU will induce transients for the current commutation. A well-designed snubber circuit is employed in SNU to absorb any unexpected voltage overshoot. Again, due to the different location of the switches and the resistor banks, priority will be given to minimise the impedance of the connection and parallel coaxial cable is foreseen.

D. Fast Discharge Unit (FDU)

FOUs are included in the TF circuit, the six PF circuits and the six CS circuits. They dissipate the energy from the superconducting coils in case of quench. There are in total twenty-one units: nine in the TF circuit (each interleaved with pairs of series-connected coils, in order to limit the voltage to earth), and one unit for each of the CS and PF coils.

/) Design Feature and Operation Principle

Similar to the SNU, each FOU unit consists of a CCU, a pyrobreaker (PB) and discharge resistors. The CCU, comprising of mechanical switches and a counterpulse circuit (CPC), 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.

As shown in Figure 8, the CCU is composed of the Mechanical Bypass Switch (BPS) and a Yacuum Circuit Breaker (YCB) connected in parallel. The BPS carries the DC current continuously (up to 68 kA) due to the very low contact resistance compared with that of the YCs. The CPC is parallel connected to the mechanical switches. When an open request arrives, the BPS opens transferring the high current to the YCs. The capacitor bank will discharge into the YCB, providing commutation current opposite to the YCB current direction; an artificial zero-crossing is achieved. Once the YCB current is interrupted, the DC current diverts to the discharge resistor bank (DR). The BPS and YCB will withstand the high recovery voltage of around 10 kY.

The resistor modules will be made from steel tapes in a tight serpentine pattern to minimise inductance, and are cooled by means of natural air convection. The FDUs for CS and PF circuits are of a similar design; except that the CPC can provide bi-directional commutation current. The main parameters of the FDUs are given in Table III.

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3) Design Challenges and Solutions

The ITER FDUs are a key component 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 practices of the fusion industry. Moreover, stringent requirements are dictated for nuclear safety.

a) High Current and High Recovery Voltage

The FDU shall interrupt the DC current up to 68 kA with a recovery voltage around 10 kY. This led the choice towards vacuum circuit breakers, instead of static devices, which would have required a large number of components in parallel (to share the current) and in series (to withstand the voltage), thus leading to considerable on-state losses. In addition, each vacuum circuit breaker contains two bottles connected in series to increase reliability.

Table III. FDU Parameters Parameter Value

68 kA (TF) Rated current 48-55 kA (PF)

45 kA (CS) Rated volta�e 10 kV Insulation level 12 kV AC

11 s (TF) Time constant 14 s (PF)

7.5 s (CS)

Max. operation time 0.5 s 35 GJ (total TF)

Energy to be dissipated 12.2 GJ (total PF) 8.4 GJ (total CS)

b) Constraints of Magnet Discharge

The FDU resistor banks must discharge the energy from a few GJ to 35 GJ. The maximum voltage and the total specified energy (ft) 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 fast discharge. At the same time, the resistance has the si�nificant increase during the discharge period. Therefore, the I t of the magnet is easily matched. 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.

c) Safety Function Requirements

ITER has been defined by French Safety Authorities as an INB (Installation Nucleaire 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. Due to the huge energy stored in the TF magnets (41 GJ), the TF FDUs are required to protect and support the (vacuum vessel) confinement and thus limit exposure, therefore, they are classified as Safety Importance Classification Level 2 (SIC-2). 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.

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 remainder 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.

During the discharge, the AC/DC converter shall be bridged by a switch, a Protective Make Switch (PMS). The PMS comprises a fast mechanical switch and an EPMS in

parallel connection. The EPMS operates as a backup. Due to its participation in the TF fast discharge circuit, the TF PMS is also classified at SIC-2. All the safety design criteria shall be applied. In the PF and CS circuits, due to the level of stored energy, the associated FDU is not classified as SIC, however, the PMS is designed to Safety Relevant (SR) criteria. The concern is to avoid the delivery of electrical energy from AC/DC converter to further damage the confinement barrier.

E. DC Busbars

Water cooled aluminium DC busbar is used for ITER CPSS. Three main cross sections have been developed. The TF and CC will have a dedicated cross section with respectively 48,000 mm2 and 5,000mm2 conductive, while the CS and PF will have the same cross section with 24,000 mm2• Both polarities will be encapsulated in a steel case in order to minimise EMC emission as well as electrical protection. To accommodate thermal expansion, each single busbar made of 12m long piece will be interconnected via copper flexible joints.

The TF busbar system is designed to operate in continuous duty, whereas, the PFICS and CC busbar will take advantage of the thermal capacity of aluminium and are designed for pulsed operation. Nevertheless they all will have to withstand loss of cooling during fast discharge of the magnets.

The main challenge is the integration and installation of nearly 9 km of water cooled aluminium bars over more than 300 meters of the compact designed and crowded buildings.

IV. REACTIVE POWER COMPENSATOR AND HARMONIC FILTERING (RPC&HF)

During the ITER pulse, the CPSS requires significant reactive power. The periodic ITER dynamic pulses of the load reactive power demand have an amplitude of around 900�950 Mvar. Taking account of the effective contribution of the grid and constraints of the voltage variation, the total installed RPC will be 750 Mvar.

A. Design Features

Due to the large amplitude and short rise times of the pulsed power and for likely power step changes, rapid reactive power control is necessary for voltage stabilisation and compensation together with large inductor capacitor (LC) filters to eliminate harmonics generated by the power converters.

Three RPC&HF units are connected, one to each of three 66 kV busbars, based on Static Var Compensation (SVC) technology. A symmetrical 3 x 250 Mvar RPC&HF unit solution based on TCR+FC offers continuous and fast voltage control, able to compensate the rapid changes in reactive power demand, and to provide a high level of operational requirement compliance. Such a solution is the optimum choice as it is now both industrially feasible and cost effective. Furthermore, due to the symmetry of the solution, an individual unit is standardised for other reasons, improving spare parts management and optimising engineering, manufacturing and installation.

Hence, the RPC&HF system considered comprises a TCR rated at 250 Mvar and six harmonic filters tuned to the 3rd, 5th, 7th, 11th, 13th and 23rd harmonics of 50 Hz, connected to the 66 kV busbar. The filters generate a total fundamental reactive power of 250 Mvar. The 3rd, 5th, 7th filters are grouped with the TCR and other filers are grouped. They are standard LC filters.

There is no compensation at the 22 kV busbars, due to relatively low contribution to total harmonic distortion and meeting the compatibility levels, IEC limits and acceptable operational range.

B. Design Challenges and Solutions

Due to the enormous installed power and the characteristics of the ITER load, design challenges have been introduced.

i) High Voltage Valve

The thyristor valves are to be connected to the 66 kV busbar directly. Worldwide, there are very few installations thus far at such voltage level, but with relative low power. The voltage sharing in steady and dynamic state is critical for secure operation in a very complicated electro-magnetic environment. An N+2 or N+3 approach is used to size the number of series thyristors; the LTT (light trigger thyristor) integrated with BOD (Breakover Diode) is a preferred option, which can minimise the interfaces to the earth potential; moreover, careful consideration shall be taken into account of the physical structure design to minimise the unequal distribution of stray capacitance.

2) Low Frequency Oscillation

The ITER load is regulated according to the needs of the Plasma Control System. The firing angles of ACIDC converters are randomly varied. Therefore, a continuous harmonic spectrum will appear. The inherent oscillation frequency caused by the installed capacitor banks and the impedance of the system is likely between 100 Hz to 150 Hz. Low frequency oscillation is a potential risk. A well-designed filter with sufficient damping capability is thus required. Good comprise must be made between the filtering performance and the capability of damping the low frequency oscillation. The 3rd order harmonic filters have been included to compensate for instabilities due to parallel resonance of the filter capacitors with the power system.

3) Fast Response and High integration

The TCR has infinite response due to phase control technology. Even if its response has been fully explored, it cannot well match the possible load variation in the normal and abnormal ITER operation conditions. The main control of the ITER RPC is thus foreseen as an open loop of Q control to facilitate the very stringent real time requirement. The RPC will absorb or generate current to decrease or eliminate the reactive power absorbed from the grid. With such control, the RPC can regulate power in full scale within one electrical cycle. In addition, voltage feedback can be also applied to increase the control accuracy.

During normal plasma operations, the ACIDC converter will generate very high reactive power variation. The individual load reactive power from the ACIDC converter will be varied in full scale in half or one electrical cycle. In the case of abnormal conditions, the load variation is even faster, especially in the case of quench, the reactive power disconnection will be accomplished within few milliseconds. In order to limit the voltage variation, the load status and the control of RPC shall be integrated. The philosophy is to introduce the load status into the RPC control.

The reactive power provided by RPC will be reduced according to the square of the line voltage. The RPC may be saturated in the heavy load condition (flat top of plasma), thus the voltage drop and the reactive power supplied by the grid may exceed the limits. The OLTC installed on the main step down transformer is normally controlled outside the pulse. However, the control of the RPC can also change the tap to provide sufficient voltage support.

V. CONTROL SYSTEM

The CPSS and the RPC will be capable of autonomous 'local' control for commissioning and dummy load testing as well as operation under supervisory control by the central COntrol Data Access and Communication (CODAC) system. CODAC will facilitate the archiving, monitoring, logging and visualisation from the ITER central control room. Whereas, the coil power supply and RPC will have their own Local Control at earth potential with input output signals handled as remote 110, with or without intelligence.

Interlocks for equipment protection and emergency shutdown will be implemented within the Plant Interlock System (PIS) of the coil power supply and RPC, being coordinated by the Central Interlock System (CIS).

Similarly, interlocks for equipment and personnel safety and emergency shutdown will be implemented within the Plant Safety System (PSS), being coordinated by the Central Safety System (CIS). The PSS also implements the safety function, such as the TF fast discharge (TF FDU) and the isolation of PF and CS power supply (PF, CS-PMS).

VI. SYSTEM INTEGRATION

Owing to the huge scale, special requirements of a superconductive tokamak, needs of plasma control, multi­procurements and complicated interfaces involved, the system level integration is critical for the ITER CPSDS including the RPC&HF.

A. Tools for the System Integration

A substantial proportion of the detailed design work will be performed by the Domestic Agencies (DA) with the support of their contractors/suppliers. To ensure efficient collaboration between ITER Organization, DAs and the contractors/suppliers, the 10 has selected a set of tools for the execution of electrical engineering analyses, production of circuit diagrams and design of cable routing.

B. Layout Integration

The ITER CPSDS including the RPC&HF system are located in various ITER buildings and site locations, with complicated busbar and cable routing. The ITER buildings and plant systems are represented by a 3D model called the Configuration Management Model (CMM) to check consistency and to manage layout.

C. Control Integration

The CPSDS including RPC&HF must be well coordinated across different plant systems, across different and inside individual coil circuits. The control system must be highly integrated. A master or supervisory level control system is located downstream of the top level CODAC, CIS and CSS, to manage the entire system control, protection and associated interfaces.

D. Interfaces Integration

The CPSDS including RPC&HF have multiple interfaces with other systems, e.g. plasma control system, CODAC, CIS, CSS, cooling water, compressed air, auxiliary power, etc. Those interfaces must be well managed during the whole lifecycle of the manufacture, installation, commissioning and operation.

VII. CONCLUSION

The ITER CPSDS including RPC&HF will be procured by the Chinese, Korean and Russian Federation DAs. The conceptual design was completed by ITER Organization during 2010. The further detailed engineering design work and fabrication will be performed in the three DAs. The current conceptual design demonstrates the technical feasibility, manufacturability, and the compliance of the system requirements, even though significant challenges and integration issues still remain.

DISCLAIMER

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

ACKNOWLEDGMENT

The authors wish to thank the technical teams of the Chinese, Korean and Russian Federation DAs that contributed to the Conceptual Design of the ITER CPSDS and RPC&HF.

REFERENCES

[1] F. Milani, et al. "System integration of the ITER switching networks, fast discharge units and busbars", 26th SOFT, 2010.

[2] C. Neumeyer, "Sequential Control for TFTR Power Supplies", 14th SOFT,1986.

[3] 1. Goff, "The ITER Magnet Power Supplies and Control System", International Conference on Electrical Machines and Systems (ICEMS) Incheon, Korea, 2010.

[4] A. Mankani, "The ITER Reactive Power Compensation and Harmonic Filtering (RPC & HF) System", 26th SOFT, 2010.

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