3.4 Poloidal Field Power Supply Systems for the EAST Steady State Superconducting Tokamak

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1 3.4 Poloidal Field Power Supply Systems for the EAST Steady State Superconducting Tokamak FU Peng Introduction The EAST superconducting tokamak is an advanced steady state experimental device being built at ASIPP in China from 1998 to around The device consists of toroidal field (TF) and poloidal field (PF) superconducting coil systems, a vacuum system, a power supply (PS) system, a cryogenic system, a data acquisition and plasma control system, a microwave heat system, and a plasma diagnosis system. Its main parameters are listed in Table 1. Table 1. EAST tokamak parameters Major radius R 0 1.7m Plasma current 1.0MA Minor radius a 0.4m Toroidal field 3.5T Elongation K x Pulse length s Triangularity d x Volt second 8 10vs As an electrical device, a tokamak is made up of four main electrical loads: (a) the toroidal coils, which establish the stabilizing toroidal magnetic field; (b) the central solenoid coils, which induce the voltage for gas breakdown and build up, maintain and ohmically heat the plasma; (c) the plasma equilibrium coils, which control the position and the shape of the plasma; and (d) the additional heating devices, which increase the plasma temperatures and drive the plasma current. All these loads require alternating-current/direct-current (AC/DC) power conversion and voltage, current, and power profiles suitable to meet the tokamak operational requirements. In the EAST tokamak, the main loads are the PF (including the central solenoid coils) power supply, the TF power supply, the plasma position control power supply, the LHCD power supply, the ICRH power supply, the ECRH power supply, the NBI power supply, and the auxiliary load, including the cryostat system. With the PF power supply system working in sequential control mode to minimize the reactive power taken from the grid, the active and reactive power that the EAST system requires are shown in Table 2 and Fig.1. Table 2. Power required by EAST equipment Phase PF (MW) TF (MW) Heating (MW) Other (MW) Sum (MW) Magnetization Initiation Slow ramp up Burn Ramp down

2 Q Ip 10 0 P P--Active power (MW) -10 Q--Reactive power(mvr) Ip--Plasma current(50ka) TIME( S ) Fig.1 Active and reactive power required by EAST tokamak All the TF and PF coils in the EAST tokamak are superconducting. Therefore, compared with a normal tokamak, the power required by the magnets will be considerable smaller, especially to the TF system: its power decreases from hundreds of megawatts to hundreds of kilowatts. In EAST, the TF power supply is realized by a 30V/16KA and 12-pulse converter, which takes the electric power directly from the local grid. The PF coils in the EAST require a large amount of power during the plasma initiation and fast ramp up phase, which lasts for about 1s. Such a high pulse electric power, having a peak value of more than 100MW, cannot be taken directly from the grid because of unacceptable electric disturbances to the local utility network. To reduce the disturbance within an allowable limit, two options can be chosen. One is to use an AC flywheel generator and converter to supply the power required during plasma initiation and ramp up; another is to use a switch and resistor network to produce the high power required. IPP, Chinese Academy of Science, now has a 120MVA/400MJ AC flywheel generator, but considering its operating and maintenance cost, the commutation switch with resistor network is preferred to realize the plasma initiation and fast ramp up, and the converter is used to achieve the slow plasma ramp up. Therefore a new 81.5 MVA substation is being built. Two transformers, whose rated continuous power are respectively 50 MVA and 31.5 MVA, will be installed, they are supplied by a double bus-bar and distant power connection points 14 km away from the 110 kv power grid in eastern China. The short-circuit capacity of each 110 kv interconnecting point is 1980 MVA, the short circuit impedance of each transformer between primary and secondary is 10.5%. A schematic of the power distribution system in the EAST tokamak is shown in Fig. 2, [1]. Basically, the load of the tokamak consists of AC/DC converters, which are pulsed. Therefore the load will have a large harmonic content and high reactive pulse power. These factors will produce a large effect on the power system. Due to excessive repetitive mechanical and thermal stresses on the local turbo-generators in the grid, a maximum level for the power step is required. The overall power derivative must also be limited, due to the time response 171

3 Fig.2 The power distribution system of EAST tokamak capability of the power-frequency regulation system. Since there is a maximum voltage excursion guaranteed to customers, a maximum V/V is permitted to the pulse, leading to a limitation in the reactive power swing, while the maximum active power is limited by the system power capability at the time of the pulse. Finally, when making substantial use of rectifiers, such as for a tokamak power supply system, the voltage and current harmonic level must be minimized. In the EAST tokamak system, a reactive compensation of the 30MVA and a harmonic filter has been included. And the negative active power is limited to not more than 5MW by using a suitable operation mode of the PF power supply [2] Outline of the EAST PF Power Supply The poloidal field system consists of 14 upper and lower superconducting coils arranged around the plasma in the EAST tokamak. The configuration is shown in Fig. 3. During the first few years, EAST will operate in double-null mode, the PF power supply system is designed according to this operation mode; and in the same time the power supply can also operate in signal null operation of tokamak. Therefore the PF coils set is reduced to 6 coil pairs by suitable series combination. The equivalent inductance parameters are shown in Table PF9 PF7 PF11 PF5 PF3 PF13 PF1 PF2 PF4 PF14 PF6 PF8 PF10 PF12 Fig.3 Configuration of the PF coils in EAST tokamk 172

4 3 and the configuration of the PF system and its power supply is shown in Fig.4. Because of the special characteristics of the superconducting PF coils, their operating requirements are listed in Table 4. Table 3. The equivalent inductance parameters of the PF coil pairs L(mH) Coil1 Coil2 Coil3 Coil4 Coil5 Coil6 Plasma Coil Coil Coil Coil Coil Coil Plasma Fig.4 PF coils and their power supply system in EAST Table 4. The parameters required by PF coils PF rated current Maximum voltage between terminal and terminal Maximum voltage to ground Delay time of quench protection Max. I 2 dt in quench Maximum PF field ramp rate 15 ka V V 1 s A 2 s 7 T/s The power supply system that supplies the CS and PF coils shall provide the following functions: (1) to provide controlled current in the CS and PF coils for plasma initiation and plasma current, shape, and position control, (2) to provide a fast discharge for the CS and PF coils in case of quench of the CS, PF or TF coils, (3) to protect the PF coils against overvoltage and over-current due to abnormal or faulted operation of the power supplies, (4) to measure the voltage and current in the PF circuits, (5) to provide grounding and ground 173

5 leakage current sensing in the PF circuits, (6) to isolate or ground the PF coils and the electrical equipment of the PF power systems for safe access to the areas where the both the loads and the power supplies are installed by personnel, as required, during a maintenance period. In addition, provision shall be made to reverse the direction of the current in all the coils. The required current rating of the PF circuits has been determined based on (a) the reference plasma scenario; (b) other PF current distributions due to plasma equilibrium conditions different from the nominal plasma scenario; (c) the current variations due to control; (d) the current variations due to plasma disruption. In addition, the design of the PF power supply must satisfy the performance requirements given in Table 5. Table 5. PF power supply system performance requirements Parameter Nominal cycle period PF magnetization PF dwell time to keep magnetization Plasma initiation Plasma current ramp up Maximum plasma flat top Plasma current ramp down Value 2800 s 20 s 20 s 0.06 s 4 s 1000 s 4 s The PF system consists of 6 coil pairs, with each pair connected to its own power supply system. The required voltage and current to be applied to the CS and PF coils has been determined by consideration of three nominal plasma shape (elongated, round and large volume), along with voltage required for control of the plasma current, position and shape. The maximum voltage and current required for plasma current initiation and control are given in Table 6 and Table 7. Each EAST PF power supply is composed of rectifier transformers, converters which provide the slow ramp up and control of plasma, thyristor DC circuit breaker (TDCB) which are used both to initiate the plasma and as the first action for quench protection, and explosive breakers (EB) that are used as a backup for quench protection. A distributed control system (DCS) is used for the control and measurement of all PF power supply system. A typical PF power supply circuit is shown in Fig.5. Table 6. Maximum PF voltage required for EAST operation Coil pair Phase 1 (kv) 2 (kv) 3 (kv) 4 (kv) 5 (kv) 6 (kv) Magnetization Current initiation Other phase

6 Table 7. Maximum PF current and current ramp rate provided by converter in EAST operation Coil pair Rated current (ka) Max. ramp rate (ka/s) Sensor Fig.5 PF power supply circuit in EAST The EAST AC/DC Converter System The EAST PF magnets are supplied by thyristor AC/DC converters, which provide the current necessary to produce the scenario and to control the plasma shape and position. This PF converter of the power supply is rated at a total installed power of about 210MVA. The key issues in the design of the AC/DC conversion system are low cost, high availability and reliability, along with reactive power reduction. The 110kV substation is located at about 200m from the PF converter building. A main transformer, 110 kv/10 kv, 50 MVA continuous rating, supplies the additional heating power supply, the TF power supply, and PF power supply, at 10kV intermediate voltage. Inside the PF power supply building, 12 dry rectifier transformers associated with the converters and other switches are installed. The typical circuit diagram for the PF power supply converter is shown in Fig.6. Fig.6 Simplified diagram of PF converter 175

7 Each converter operates in four quadrants with circulating current between the head and tail sets through the inductance L 1, L 2, L 3 in series. Converter G 1 consists of two half bridges G 11 and G 12 in parallel, G 11 and G 12 s angle difference is 180 degrees, and they share one phase angle controller. As do the converters G 2, G 3, and G 4. For G 1 and G 2, G 3 and G 4 upper and lower, each set head or tail is made of two series identical Graetz bridges with a 30 degree angle difference between the two transformer outputs, and the 12-pulse output voltage. G 1 and G 2, G 3 and G 4, which are respectively paralleled for one tail and head set by inductance L 1 and L 2, make use of the same transformer secondary. The operating principle is based on the simultaneous conduction of only two of the four bridges selected automatically according to the amplitude and the direction of the total load current. An integrated cooling water exchanger extracts the heat energy produced in the bridges. When the coil current is in the range from 10% to 100% of the maximum operating current, the main AC/DC converters in the PF coil circuit operate in two quadrants mode, in which G1, G3 drive the positive load current and G2, G4 drive the negative load current. When the coil current is below the 10% level, the converters operate in the four-quadrant mode, in which only G11, G31, G22, and G42 operate. This operation mode of the PF converters can save half of the capacity of the required transformer, and also ensure that the load current crosses zero smoothly from positive current direction to negative current direction. The synchronous operation of the two series power bridges of each converter permits a twelve-pulse waveform on the AC network. At low load voltage, the reactive power consumption is minimized in the converters by a firing offset between the two series power bridges. In this case, the twelve- pulse reaction can approximately decrease the maximum reactive power 50 MVAR to 25 MVR in the PF power supply system of EAST [3] Direct-current Circuit Breaker The function of the DC circuit breaker is to insert, in the tokamak coil circuit, discharge resistors, which are used to absorb the stored magnetic energy from the coils. In each PF power supply of EAST, DC breakers are used during the plasma initiation and current rampup, its parameters are shown in Table 8. Table 8. The rated parameters of TDCB in EAST Rated current Rated voltage Rated switching time Dwell time before repeatable switching Energy dissipated in plasma initiation Energy dissipated in quench protection 15 ka 2.4 kv 2 ms 200 ms 8 MJ 20 MJ 176

8 Since this DC circuit breaker system is subjected to very frequent operation, both the electrical and mechanical life of the system are of prime importance. In the plasma initiation phase, each PF circuit must contribute initiating plasma. Therefore each of the DC breakers in each power supply must be well synchronized. In the EAST power supply, the required current and voltage parameters of the DC circuit breaker are not extremely high, therefore a TDCB associated with resistor network is chosen, which can also switch bi-directional current and act as the main switch for quench protection. The TDCB consists of thyristor, diode, capacitor, resistor and inductor. Its parameters are given in Table 7,and the circuit is shown in Fig.7. The two thyristor sets and two diode sets (Th1, Th2, D1, and D2) permit bi-directional current and voltage. Fig.7 the TDCB circuit diagram The fast discharge circuit is composed of capacitor C1, L1, th3 and th4, C1 is pre-charged to 2.4 kv. When the positive current of power supply flows through Th1 and D1, while Th3 is triggered to discharge the capacitor C1 through the series circuit including Th1, D1 and L1. The discharge produces an artificial current zero of current in Th1, which turns off Th1. The full current of the power supply is transferred into the C1 branch. When C1 is recharged in the opposite direction, the current is progressively transferred into the discharge resistor connected in parallel with the switch. When the opposite current flows through Th2 and D2, the commutation is realized by triggering Th4. During the commutation, the resistor network produces a high voltage to initiate the plasma discharge, and dissipate the energy stored in superconducting coil in quench protection. The resistor is made from stainless steel tube in a tight serpentine pattern to 177

9 minimize the inductance, and it is cooled by water. Each resistor unit consists of several modules connected in parallel or in series. This arrangement allows the resistor value to be adjusted according to the requirement. A RC snubber circuit is employed to absorb the instantaneous over-voltage during TDCB operation. When TDCB opens a small current load, it makes the coil current to increase, because of excess energy in capacitor C1. Therefore D1 and D2 are respectively paralleled with Th1 and Th2. A dynamic and static current distribution in the parallel thyrisor is also ensured by careful layout of the structure. Quench protection for the superconducting magnets is of critical importance because of the large amount of stored energy. In each power supply, one TDCS is used as the main quenching protection, and two explosive breakers (EB) are as stand-by switches PF Power Supply Control System The local control system of PF power supply provides the routine control, real-time control, communication of control data, a timing system for precise control of event initiation, and data acquisition. The PF power supply system has more than 140 analog signals and 600 digital signals, which will be measured, supervised and controlled. A distributive control system (DCS) consisting of industrial personal computer (IPC) computers has been used, and all computers are connected as real time Ethernet network by the QNX real time operation system. In addition, a field-bus network is also used on the spot. The overall structure of the PF power supply control system is shown in Fig.8. NET:TCP/IP INTERNET System State Display Fault Diagnosis Waveform Analysis Supervisory Control System QNX Server PF Current Feedback controller Center Contol system NET:FLEET AD,DI& Database CAN Controller PF1 subsystem controller PF2 subsystem controller PF6 subsystem controller NET:CANBUS AC components PF1 components PF2 components PF6 components 178

10 QNX is a real time operating system, developed by QNX Software System Limited (QSSL) in Canada, and is widely used in industrial PC s. Besides its high performance in real time, reliability, and embedded characteristics, the unique feature of the QNX real time operating system is its network technology. Its FLEET is an ultra-light, high-speed networking protocol. Its innovative and feature-rich design turns isolated machines into a single logical supercomputer. Because FLEET is built on the message passing architecture of the QNX OS, it offers the very high flexibility. Its features are fault-tolerant networking, load-balancing on the fly, efficient performance, extensible architecture, and transparent distributed processing. Via the QNX all IPC computers in PF power supply control system can be connected as a single large computer, and all IPC share the data and signal in a high speed and transparent network. The distance between the power- supply building and its control room is 40m. To decrease the transmitting cable and disturbance, A network consisted of CANBUS has been built. Many CAN modules work at the location of the power supply. All supervised signals including a majority of control and protection signals, and a majority of analog signals are transmitted by CAN BUS. Other critical signals required for high real time performance are Fig.8 Overall structure of the PF power supply control system transmitted directly by wires. All logical signals and A/D conversion of analog signals measured by the IPC and fieldbus modules is transmitted to a database computer [4] PF power supply R&D Simulation studies by Simplorer software and laboratory tests of PF power supply in EAST have been completed. They show that the design of the PF power supply system is feasible and reliable. All of the equipment of the PF power supply is being fabricated by industry. Now one set of PF power supply has arrived at the institute and been installed [5]. Reference [1] E.Bertolini, et all., The JET magnet power supplies and plasma control systems. Fusion Technology. vol.11, pp71-119, Jan [2] R.Shimada, et all, JT-60 power supplies. Fusion Engineering and Design. pp47-68, May, 1987 [3] Benfatto I. et al., AC/DC Converters for the ITER poloidal system in Proc. of the 16 th SOFE, Champaign, IL, USA, pp [4] C.Sihler, P.Fu, M.Huart, B.streibl and W.Treutterer. Paralleling Of two large Flywheel Generation for the Optimization Of the ASDEX upgrade Power Supply. 21 st Symposium on Fusion Technology, Madrid, Spain, Oct., [5] SIMEC GmbH. Simplorer. Version 4.2, Chemnitz,