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Poloidal Field Power Supply Systems for the HT-7U Steady State Superconducting Tokamak P.Fu, Z.Z.Liu, J.Z.Xu, G.Gao, J.L.Wen, Y.Cao, Z.Q.Song, L.J.Tang, L.S.Wang, and X.Y.Liang Institute of Plasma Physics, Chinese Academy of Science Box. 1126, Hefei 230031, China. Email: [email protected] Abstract—The paper gives a description of the poloidal As an electrical device, a tokamak is made up of four field power supplies and the control system of the HT-7U main electrical loads: (a) the toroidal coils, which establish superconducting tokamak required to energize the magnetic the stabilizing toroidal magnetic field; (b) the central field coils for plasma excitation and confinement. A soloiend coils, which induce the voltage for gas breakdown original configuration of AC/DC converter, thyristor DC and build up, maintain and ohmically heat the plasma; (c) circuit breaker ( TDCB ) , and power supply control the plasma equilibrium coils, which control the position system are introduced in detail. and the shape of the plasma; and (d) the additional heating devices, which increase the plasma temperatures and drive Index Terms— tokamak, power supply, AC/DC converter, the plasma current. All these loads require alternating- TDCB, control, and quench protection. current/direct-current (AC/DC) power conversion and voltage, current, and power profiles suitable to meet the tokamak operational requirements. I. INTRODUCTION In the HT-7U tokamak, the main loads are the PF The HT-7U superconducting tokamak is an advanced (including the central solenoid coils) power supply, the TF steady state experimental device being built at ASIPP in power supply, the plasma position control power supply, China from 1998 to around 2004. The device consists of the LHCD power supply, the ICRH power supply, the toroidal field (TF) and poloidal field (PF) superconducting ECRH power supply, the NBI power supply, and the coil systems, a vacuum system, a power supply (PS) auxiliary load, including the cryostat system. With the PF system, a cryogenic system, a data acquisition and plasma power supply system working in sequential control mode control system, a microwave heat system, and a plasma to minimize the reactive power taken from the grid, the diagnosis system. Its main parameters are listed in Table 1: active and reactive power that the HT-7U system requires are shown in Table 2 and Fig.1. Table 1:HT-7U tokamak parameters Major radius R0 1.7m Plasma current 1.0MA Minor radius a 0.4m Toroidal field 3.5T Elongation Kx 1.6-2 Pulse length 1-1000s Triangularity dx 0.6-0.8 Volt second 8~10vs Table 2 Power required by HT-7U equipment Phase Magnetization PF (MW) 25 TF (MW) 0.2 Heating (MW) 0.8 Other (MW) 4 Sum (MW) 30 Initiation -106 0.2 0.8 4 -101 Slow ramp up -35 0.2 0.8 4 -30 Burn 15 0.2 15.8 4 35 Ramp down -15 0.2 0.8 4 -10 1 All the TF and PF coils in the HT-7U tokamak are installed, they are supplied by a double busbar and distant superconducting. Therefore, compared with a normal power connection points 14 km away from the 110 kV tokamak, the power required by the magnets will be power grid in eastern China. The short-circuit capacity of considerable smaller, especially to the TF system: its power each 110 kV interconnecting point is 1980 MVA, the short decreases from hundreds of megawatts to hundreds of circuit impedance of each transformer between primary and kilowatts. In HT-7U, the TF power supply is realized by a secondary is 10.5%. A schematic of the power distribution 30V/16KA and 12-pulse converter, which takes the electric system in the HT-7U tokamak is shown in Fig. 2, [1]. power directly from the local grid. 50 Q 40 30 Ip 20 10 P 0 P--Active power (MW) Q--Reactive power(MVR) Ip--Plasma current(50kA) -10 -20 0 5 Fig. 2 The power distribution system of HT-7U tokamak 10 15 TIME( S ) Fig.1 The active and reactive power required by HT-7U tokamak Basically, the load of the tokamak consists of AC/DC converters, which are pulsed. Therefore the load will have The PF coils in the HT-7U require a large amount of a large harmonic content and high reactive pulse power. power during the plasma initiation and fast ramp up phase, These factors will produce a large effect on the power which lasts for about 1s. Such a high pulsed electric power, system. Due to excessive repetitive mechanical and thermal having a peak value of more than 100MW, can not be taken stresses on the local turbogenerators in the grid, a directly from the grid because of unacceptable electric maximum level for the power step is required. The overall disturbances to the local utility network. To reduce the power derivative must also be limited, due to the time disturbance within an allowable limit, two options can be response capability of the power-frequency regulation chosen. One is to use an AC flywheel generator and system. Since there is a maximum voltage excursion converter to supply the power required during plasma guaranteed to customers, a maximum ΔV/V is permitted 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 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 HT-7U tokamak system, a reactive compensation of the 30MVA and a harmonic filter have 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]. 2 (2) provide a fast discharge for the CS and PF coils in case II. OUTLINE OF THE HT-7U PF POWER of quench of the CS, PF or TF coils. (3) protect the PF coils SUPPLY against overvoltage and overcurrent due to abnormal or faulted operation of the power supplies. (4) measure the The poloidal field system consists of 14 upper and lower superconducting coils arranged around the plasma in the HT-7U tokamak. The configuration is shown in Fig. 3. During the first few years, HT-7U will operate in doublenull mode, the PF power supply system is designed voltage and current in the PF circuits. (5) provide grounding and ground leakage current sensing in the PF circuits. (6) 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 according to this operation mode; and in the same time the power supply can also operate in signal–null operation of PF9 tokamak. Therefore the PF coils set is reduced to 6 coil PF11 PF7 pairs by suitable series combination. The equivalent PF5 inductance parameters are shown in Table 3 and the PF13 PF3 configuration of the PF system and its power supply is PF1 shown in Fig.4. Because of the special characteristics of PF2 the superconducting PF coils, their operating requirements PF4 are listed in Table 4. PF14 PF6 PF8 Table 3. The equivalent inductance parameters of the PF coil pairs L(mH) Coil1 Coil 1 69.06 Coil2 Coil3 Coil4 Coil5 Coil6 Coil2 22.58 52.40 Coil3 8.240 19.90 50.90 Coil4 12.30 17.75 32.03 358.3 Coil5 6.020 6.060 6.024 32.99 110.0 Coil 6 3.806 3.644 3.326 16.10 30.82 41.85 Plasma 0.122 0.095 0.065 0.197 0.170 0.119 PF12 PF10 Plasma installed by personnel, as required, during a maintenance period. Fig. 3 The configuration of the PF coils in HT-7U tokamk 0.004 Table 4. The parameters required by PF coils PF rated current 15 kA Maximum voltage between terminal and terminal 10000 V Maximum voltage to ground 10000 V Delay time of quench protection 1s 2 8 2 Max. I dt in quench 5.110 A s Maximum PF field ramp rate 7 T/s The power supply system that supplies the CS and PF Fig.4 PF coils and their power supply system in HT-7U coils shall provide the following functions: (1) provide controlled current in the CS and PF coils for plasma In addition, provision shall be made to reverse the initiation and plasma current, shape, and position control. direction of the current in all the coils. The required current 3 the reference plasma scenario; (b) other PF current Table 6. Maximum PF voltage required for HT-7U operation distributions due to plasma equilibrium conditions different Phase from the nominal plasma scenario; (c) the current rating of the PF circuits has been determined based on (a) Coil pair Magnetization 1 (kV) 0.25 2 (kV) 0.27 3 (kV) 0.24 4 (kV) 0.6 5 (kV) 0.15 6 (kV) 0.1 variations due to control; (d) the current variations due to Current initiation 2.0 2.1 1.9 4.7 1.2 0.6 plasma disruption. In addition, the design of the PF power Other phase 0.46 0.49 0.42 0.98 0.39 0.18 supply must satisfy the performance requirements given in Table 7. Maximum PF current and current ramp rate provided by converter in HT-7U operation Table 5. Table5. PF power supply system performance requirements Parameter Value Nominal cycle period 2800 s PF magnetization 20 s PF dwell time to keep magnetization 20 s Plasma initiation 0.06 s Plasma current ramp up 4s Maximum plasma flat top 1000 s Plasma current ramp down 4s Coil pair 1 2 Rated current (kA) 12 Max. ramp rate (kA/s) 18.2 3 4 5 6 13 12 13 14.2 14.1 20.3 18.6 9.2 4.9 3.5 Sensor The PF system consists of 6 coil pairs, with each pair connected to its own power supply system. The required Fig.5 PF power supply circuit in HT-7U 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 III. THE HT-7U AC/DC CONVERTER SYSTEM voltage required for control of the plasma current, position and shape. The maximum voltage and current required for The HT-7U PF magnets are supplied by thyristor plasma current initiation and control are given in Table 6 AC/DC converters which provide the current necessary to and Table 7. produce the scenario and to control the plasma shape and Each HT-7U PF power supply is composed of position. This PF converter of the power supply is rated at rectifier transformers, converters which provide the slow a total installed power of about 210MVA. The key issues in ramp up and control of plasma, thyristor DC circuit the design of the AC/DC conversion system are low cost, breakert (TDCB) which are used both to initiate the plasma high availability and reliability, along with reactive power and as the first action for quench protection, and explosive reduction. breakers (EB) that are used as a backup for quench The 110kV substation is located at about 200m from protection. A distributed control system (DCS) is used for the PF converter building. A main transformer, 110 kV/10 the control and measurement of all PF power supply kV, 50 MVA continuous rating, supplies the additional system. A typical PF power supply circuit is shown in heating power supply, the TF power supply, and PF power Fig.5. 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 4 typical circuit diagram for the PF power supply converter is save half of the capacity of the required transformer, and shown in Fig.6. also ensure that the load current crosses zero smoothly Each converter operates in four quadrants with circulating current between the head and tail sets through from positive current direction to negative current direction. the inductance L1, L2, L3 in series. Converter G1 consists of The synchronous operation of the two series power two half bridges G11and G12 in parallel, G11 and G12’s bridges of each converter permits a twelve-pulse waveform angle difference is 180 degrees, and they share one phase on the AC network. At low load voltage, the reactive power angle controller. As do the converters G2, G3, and G4. consumption is minimized in the converters by a firing For G1 and G2, G3 and G4 upper and lower, each set head or offset between the two series power bridges. In this case, tail is made of two series identical Graetz bridges with a 30 the twelve pulse reaction can approximately decrease the degree angle difference between the two transformer maximum reactive power 50 MVAR to 25 MVR in the PF outputs, and the 12-pulse output voltage. G1 and G2, G3 and power supply system of HT-7U [3]. G4, which are respectively paralleled for one tail and head sets by inductance L1 and L2, make use of the same IV. DIRECT-CURRENT CIRCUIT BREAKER transformer secondary. The operating principle is based on the simultaneous conduction of only two of the four The function of the DC circuit breaker is to insert, in bridges selected automatically according to the amplitude the tokamak coil circuit, discharge resistors, which are used and the direction of the total load current. An integrated to absorb the stored magnetic energy from the coils. In cooling water exchanger extracts the heat energy produced each PF power supply of HT-7U, DC breakers are used in the bridges. during the plasma initiation and current ramp-up, its parameters are shown in Table 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 HT-7U 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 Fig.6 Simplified diagram of PF converter can also switch bi-directional current and act as the main switch for quench protection. The TDCB consists of 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 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. The fast discharge circuit is composed of capacitor C1, L1, th3 and th4, C1 is precharged to 2.4kV. When the positive current of power supply flows through Th1 and D1, while Th3 is triggered to discharge the capacitor C1 5 through the series circuit including Th1, D1 and L1. The A RC snubber circuit is employed to absorb the discharge produces an artificial current zero of current in instantaneous overvoltage during TDCB operation. When Th1, which turns off Th1. The full current of the power TDCB opens a small current load, it makes the coil current supply is transferred into the C1 branch. When C1 is to increase, because of excess energy in capacitor C1. recharged in the opposite direction, the current is Therefore D1 and D2 are respectively paralleled with Th1 progressively transferred into the discharge resistor and Th2. A dynamic and static current distribution in the connected in parallel with the switch. When the opposite parallel thyrisor is also ensured by careful layout of the current flows through Th2 and D2, the commutation is structure. realized by triggering Th4. Quench protection for the superconducting magnets is of critical importance because of the large amount of Table 8. The rated parameters of TDCB in HT-7U stored energy. In each power supply, one TDCS is used as Rated current 15 kA the main quenching protection, and two explosive breakers Rated voltage 2.4 kV (EB) are as stand-by switches. Rated switching time 2 ms Dwell time before repeatable switching 200 ms Energy dissipated in plasma initiation 8 MJ Energy dissipated in quench protection 20 MJ V. 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 to 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 fieldbus network is also used on the spot. The NET:TCP/IP INTERNET overall structure of the PF power supply control system is shown in Fig. 8. Fig.7 the TDCB circuit diagram During the commutation, System State theDisplay resistor QNX is a real time operating system, developed by QNX Software System Limited (QSSL) in Canada, and is Fault network Diagnosis Waveform Supervisory SystemPC’s. Analysis widely used in Control industrial Besides its high produces a high voltage to initiate the plasma discharge, performance in real time, reliability, and embedded and dissipate the energy stored in superconducting coil in characteristics, the unique feature of the QNX real time quench protection. The resistor is made from stainless steel Center Contol system QNX Server PF Current controller operating systemFeedback is its network technology. Its FLEET is NET:FLEET tube in a tight serpentine pattern to minimize the an inductance, and it is cooled by water. Each resistor unit innovative and feature-rich design turns isolated machines consists of several modules connected AD,DI& in parallel or in subsystem controller into aPF1 single logical supercomputer. Because FLEET is controller controller Database CAN Controller ultralight, high-speed networking PF2 subsystem series. This arrangement allows the resistor value to be protocol. Its PF6 subsystem NET:CANBUS adjusted according to the requirement. AC components PF1 components PF2 components PF6 components 6 [2] R.Shimada, et all, “JT-60 power supplies”, Fusion Engineering and Design, pp47-68, May, 1987 built on the message passing architecture of the QNX OS, [3] Benfatto I. et al., “AC/DC Converters for the ITER it offers the very high flexibility. Its features are fault- poloidal system” in Proc. of the 16th SOFE, 1995. tolerant networking, load-balancing on the fly, efficient Champaign, IL, USA, pp658-661. performance, extensible architecture, and transparent [4] C.Sihler, P.Fu, M.Huart, B.streibl and W.Treutterer distributed processing. Via the QNX all IPC computers in “ PF power supply control system can be connected as a Optimization Of the ASDEX upgrade Power Supply”, single large computer, and all IPC share the data and signal 21st Symposium on Fusion Technology, Madrid, Spain, in a high speed and transparent network. Oct., 2000. The distance between the power supply building and its control room is 40m. To decrease the transmitting cable Paralleling Of two large Flywheel Generation for the [5] SIMEC GmbH, “Simplorer”, Version 4.2, Chemnitz, 2000. and disturbance, A network consisted of CANBUS has been built. Many CAN modules work at the location of the power supply. All supervised signals, 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 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]. VI. PF power supply R&D progress summary in HT-7U Simulation studies by Simplorer software and laboratory tests of PF power supply in HT-7U 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]. VII. Reference [1] E.Bertolini, et all., “The JET magnet power supplies and plasma control systems”, Fusion Technology, vol.11, pp71-119, Jan.1987. 7