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Circuit analysis and control of the power supply system for the testing of ITER TF model coil
Fusion Engineering and Design 58 – 59 (2001) 69 – 73 www.elsevier.com/locate/fusengdes
Circuit analysis and control of the power supply system for the testing of ITER TF model coil V. Marchese *, S.M. Darweschsad, G. No¨ther, A. Ulbricht, F. Wu¨chner Forschungszentrum Karlsruhe, Institute for Technical Physics, P.O. Box 3640, D-76021 Karlsruhe, Germany
Abstract The TOSKA test facility of the Forschungszentrum Karlsruhe, which was especially built for testing large superconducting coils, has been upgraded for the test of the ITER Toroidal Field Model Coil (TFMC) in the framework of the ITER-EDA R&D programme. The upgrade includes the extension of the cryogenic supply system, a new 20 kA power supply for the background field generated by the existing LCT Coil as well as two 80 kA current leads and a 80 kA dump circuit for the TFMC. At this stage the power supply systems, including their dump circuits, can be tested only up to 10 kA with a 25 tons Cu coil. Therefore, suitable calculation tools have been developed for circuit analysis, control optimisation and simulation of possible scenarios at full current. © 2001 Elsevier Science B.V. All rights reserved. Keywords: ITER; Toroidal Field Model Coil (TFMC); TOSKA
1. Introduction TFMC is a large racetrack shaped model coil of 31 tons of weight, built to test the design principles of the ITER TF Coil system. The coil is made of Nb3Sn superconducting cable inserted in a thin steel jacket placed in machined groves of radial plates [1]. The mechanical, thermal and electrical performances of TFMC will be assessed in the TOSKA facility early next year [2]. The tests will be performed first with TFMC alone, with a current flow of up to 80 kA, that generates a maximum self-magnetic field of 7.8 T, and after-
* Corresponding author. Tel.: + 49-7247-6248; fax: +497247-7248. E-mail address: [email protected] (V. Marchese).
wards at 70 kA with a background field of 2 T produced by a current flow of 16 kA in the LCT coil. The maximum combined field in the latter case is 9.02 T. The TFMC coil is fed by two existing thyristor power supplies-nominal currents 30 and 50 kA — connected in parallel (see Fig. 1). Two new especially designed current leads connect the power supplies to the coil. The current flowing in the LCT coil is provided by a new 20-kA thyristor power supply with nominal voltage, at the load terminals, of 30 V. A new Dump Circuit, based on conventional power switches and a stainless steel resistor has been installed for the Safety Discharge (SD) of TFMC which is invoked in case of transition to the normal conducting state (current quench) or for plant internal faults.
0920-3796/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 3 7 9 6 ( 0 1 ) 0 0 3 0 4 - 0
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V. Marchese et al. / Fusion Engineering and Design 58–59 (2001) 69–73
Fig. 1. TOSKA power system configuration.
Early testing of the 30 and 50 kA power supplies with the Polo coil steamed out the need of a detailed analysis of the current sharing especially during the safety discharge sequence and the optimisation of the feedback loops. The study shows that the optimum current balance in steady state and in transient conditions is achieved if the two power supplies contribute to the load current proportionally to their nominal current. The circuit analysis has been performed with computer models built using Simulink and Power System Blockset (PSB) developed both for MATLAB®. The models have been validated with experimental data during the tests of the power supplies with a Cu coil. The main results of the predictions for the normal operation and during a safety discharge will be presented.
2. Power tests with Cu coil The new 20 kA power supply has been tested up to 20 kA in short-circuit and up to 10 kA with a Cu coil (resistance 2.75 mV and self-inductance of 7 mH). A typical SD sequence is shown in Fig. 2. The sequence starts with a full inversion command sent to the power supply. The current in the coil IL starts to decay with a time constant of the order of 2.5 s. At time t1 the make switch S1 is
Fig. 2. LCT power system safety discharge test pulse.
closed and most of the load current is diverted in the short circuit. For this test the switch S3 was always closed and therefore, the dump resistor R, with resistance of 125 mV, is always inserted in the circuit. After the power supply current reaches zero the by-pass switch Sn is closed (time t2) to protect the power supply in case of a.c. main loss. At time t3 the power supply is isolated from the rest of the circuit by opening the isolation switches S01 and S02. At time t4 the circuit breaker S2 is open thus initiating the fast decay phase (time constant 55 ms). Similar commissioning pulses have been performed for the TFMC power system to test the new 80-kA dump circuit. A bad current sharing between the 30 and the 50-kA power supply was observed during a test of the safety discharge at 10 kA (see Fig. 3). During the full inversion phase, when the power supplies operate in open loop, the 50 kA power supply current (I2) is shifted to the 30 kA power supply (I1) due to a delayed closing of the fast make switches. The problem should be solved with a better synchroni-
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sation of the control and by the adjustment of the negative voltage limits. The current sharing of the two power supplies would not be a problem when they are operated in closed loop control mode but, the relatively low voltage of the power supplies, makes this type of discharge not suitable for faults in superconducting coils.
3. TFMC Power System model The magnetic coupling of the TFMC and LCT coil is quite low (k =0.6 p.u.). It has been evaluated, with such a magnetic coupling, that during a simultaneous safety discharge of the TFMC and LCT circuits– —maximum foreseeable dB/dt — the overshoot of the LCT current at 16 kA is only 1.5% and therefore, it is possible to analyse the two circuits separately [3]. A computer system, specially developed to simulate power systems in the Simulink environment, called Power System Blockset (PSB), was selected for this purpose. The package, supplied as standard Matlab product, provides a graphical user interface for building models as block diagrams and a library containing a large variety of blocks to simulate power system components.
Fig. 3. TFMC power system safety discharge test pulse.
Fig. 4. Simulink model of the 30 kA power supply.
The PSB model implemented for the TFMC Power System uses five state variables for the electric network. The eigenvalues of the state matrix are [− 1.3484, − 0.5491, − 0.3273, − 0.0257, − 0.0001]× 106. The coil is feed from the 30 and 50 kA power supplies working in parallel. The two thyristor converters are simulated (see Fig. 4) with a simplified model based on a controlled d.c. voltage source with amplitude Vd, function of the h-angle and the mains ac voltage Vc (Vd = 1.4 Vc cos(h), with Vc =30 V rms). The time constant of 3.3 ms is introduced to take into account the commutation time of the thyristor converter. A diode in series limits the operation to two quadrants only. A reduction of a factor 50 in the number of integration steps has been achieved with respect a full a.c. model of the thyristor converter. A current controller, of PI type, physically installed in the 30-kA power supply controls the current of each converter, which act as a master. The circuit breaker S2 and the power supply isolation switches S0 shown in Fig. 1 are simulated with a GTO block with 10 ms current fast drop (90%) and 10 ms tail. The remaining switches are simulated with an ‘ideal switch’ block. The solver adds an additional state variable for each switching device. The integration of the system equations is performed with a solver suitable for stiff systems and with a tolerance set at 1.0e− 3. The outputs of the model have been compared with the experimental data obtained during the tests on dummy load with good results. Besides, the PSB model has allowed the discovery of exces-
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V. Marchese et al. / Fusion Engineering and Design 58–59 (2001) 69–73
sive filtering to the current measurements provided by the new current transducers supplied with the dump circuit.
4. TFMC circuit operation at full current After the validation, the model has been used to simulate the behaviour of the power system at the nominal test current of 80 kA. The parameters of the current controller have been selected to have a bandwidth of the system in closed loop of about 8 Hz with a phase shift of 75°. The load current is supplied in the proportion 3/8 and 5/8 by the 30 and 50 kA, respectively. A possible scenario for the current ramp-up with a dI/dt 700 A/s, in feedback control mode, is shown in Fig. 5. The proportional and integral gains of the current controller are 0.025 and 0.5, respectively. As can be observed from the voltage transients shown in Fig. 5 the dominant mode of the system has a frequency of 1.5 Hz and is well damped. For the test of TFMC alone the dI/dt is limited by the inductance of the TFMC coil (27 mH) and the eddy current in the coil cases and in the inter-coil structure. In combined operation with the LCT coil, which will be operated at 16
Fig. 6. Computed safety discharge at 80 kA (TFMC alone).
kA, the dI/dt is limited by the large inductance of the LCT Coil (1.58 H). Therefore, in the combined operation it will take between 15 and 20 min to reach the flat top. The steady-state measurements at the end of the previous simulation are passed automatically to the following run, shown in Fig. 6, which simulates a safety discharge of TFMC alone. The fast make switch S1 is closed 50 ms after the start of the full inversion thus limiting the overshoot in the 30 kA power supply current to 3.3%, which is quite acceptable. The voltage across the coil at the opening of the circuit breaker (S2) is less than 600 V against a nominal voltage across the terminals of 10 kV, which is the voltage of ITER TF Coil at the commutation stage.
5. Conclusions
Fig. 5. Computed current start up (TFMC alone).
The test of the TFMC coil, planned for 2001 at the TOSKA facility has required an upgrade of the power supply system, which is now in its final stage of commissioning. The commissioning of the newly installed 80-kA dump circuit was possible only up to 10 kA with a Cu coil. Therefore, it
V. Marchese et al. / Fusion Engineering and Design 58–59 (2001) 69–73
was considered appropriate to spend some efforts to develop computer models for the simulation of different scenarios at full current both in normal operation, controlled by feedback, and during a safety discharge. Two independent models were developed the first for the LCT power system and the second for the TFMC power system. The models have been validated during the tests with a Cu coil up to 10 kA performed recently and are now used for fault analysis and to make predictions at higher current levels.
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References [1] E. Salpietro, R. Maix, G. Bevilacqua, N. Mitchell, B. Truck, A. Ulbricht, M. Spadoni, The ITER toroidal field model coil (TFMC) program development programme, Proc. 17th IAEA Fus. Energy. Conf., Yokohama, Japan, 19 – 24 October 1998. [2] P. Komarek, E. Salpeitro, The test facility for the ITER TF model coil, Fus. Eng. Design 41 (1998) 213 – 221. [3] V. Marchese, Parallel operation of the 50 kA and 30 kA power supplies with the 80-kA dump circuit: preliminary circuit analysis and model validation, Proc. 10th TFMC TEST and Analysis Group meeting, CEA/Cadarache, 16 December 1999.
Circuit analysis and control of the power supply system for the testing of ITER TF model coil V. Marchese *, S.M. Darweschsad, G. No¨ther, A. Ulbricht, F. Wu¨chner Forschungszentrum Karlsruhe, Institute for Technical Physics, P.O. Box 3640, D-76021 Karlsruhe, Germany
Abstract The TOSKA test facility of the Forschungszentrum Karlsruhe, which was especially built for testing large superconducting coils, has been upgraded for the test of the ITER Toroidal Field Model Coil (TFMC) in the framework of the ITER-EDA R&D programme. The upgrade includes the extension of the cryogenic supply system, a new 20 kA power supply for the background field generated by the existing LCT Coil as well as two 80 kA current leads and a 80 kA dump circuit for the TFMC. At this stage the power supply systems, including their dump circuits, can be tested only up to 10 kA with a 25 tons Cu coil. Therefore, suitable calculation tools have been developed for circuit analysis, control optimisation and simulation of possible scenarios at full current. © 2001 Elsevier Science B.V. All rights reserved. Keywords: ITER; Toroidal Field Model Coil (TFMC); TOSKA
1. Introduction TFMC is a large racetrack shaped model coil of 31 tons of weight, built to test the design principles of the ITER TF Coil system. The coil is made of Nb3Sn superconducting cable inserted in a thin steel jacket placed in machined groves of radial plates [1]. The mechanical, thermal and electrical performances of TFMC will be assessed in the TOSKA facility early next year [2]. The tests will be performed first with TFMC alone, with a current flow of up to 80 kA, that generates a maximum self-magnetic field of 7.8 T, and after-
* Corresponding author. Tel.: + 49-7247-6248; fax: +497247-7248. E-mail address: [email protected] (V. Marchese).
wards at 70 kA with a background field of 2 T produced by a current flow of 16 kA in the LCT coil. The maximum combined field in the latter case is 9.02 T. The TFMC coil is fed by two existing thyristor power supplies-nominal currents 30 and 50 kA — connected in parallel (see Fig. 1). Two new especially designed current leads connect the power supplies to the coil. The current flowing in the LCT coil is provided by a new 20-kA thyristor power supply with nominal voltage, at the load terminals, of 30 V. A new Dump Circuit, based on conventional power switches and a stainless steel resistor has been installed for the Safety Discharge (SD) of TFMC which is invoked in case of transition to the normal conducting state (current quench) or for plant internal faults.
0920-3796/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 3 7 9 6 ( 0 1 ) 0 0 3 0 4 - 0
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V. Marchese et al. / Fusion Engineering and Design 58–59 (2001) 69–73
Fig. 1. TOSKA power system configuration.
Early testing of the 30 and 50 kA power supplies with the Polo coil steamed out the need of a detailed analysis of the current sharing especially during the safety discharge sequence and the optimisation of the feedback loops. The study shows that the optimum current balance in steady state and in transient conditions is achieved if the two power supplies contribute to the load current proportionally to their nominal current. The circuit analysis has been performed with computer models built using Simulink and Power System Blockset (PSB) developed both for MATLAB®. The models have been validated with experimental data during the tests of the power supplies with a Cu coil. The main results of the predictions for the normal operation and during a safety discharge will be presented.
2. Power tests with Cu coil The new 20 kA power supply has been tested up to 20 kA in short-circuit and up to 10 kA with a Cu coil (resistance 2.75 mV and self-inductance of 7 mH). A typical SD sequence is shown in Fig. 2. The sequence starts with a full inversion command sent to the power supply. The current in the coil IL starts to decay with a time constant of the order of 2.5 s. At time t1 the make switch S1 is
Fig. 2. LCT power system safety discharge test pulse.
closed and most of the load current is diverted in the short circuit. For this test the switch S3 was always closed and therefore, the dump resistor R, with resistance of 125 mV, is always inserted in the circuit. After the power supply current reaches zero the by-pass switch Sn is closed (time t2) to protect the power supply in case of a.c. main loss. At time t3 the power supply is isolated from the rest of the circuit by opening the isolation switches S01 and S02. At time t4 the circuit breaker S2 is open thus initiating the fast decay phase (time constant 55 ms). Similar commissioning pulses have been performed for the TFMC power system to test the new 80-kA dump circuit. A bad current sharing between the 30 and the 50-kA power supply was observed during a test of the safety discharge at 10 kA (see Fig. 3). During the full inversion phase, when the power supplies operate in open loop, the 50 kA power supply current (I2) is shifted to the 30 kA power supply (I1) due to a delayed closing of the fast make switches. The problem should be solved with a better synchroni-
V. Marchese et al. / Fusion Engineering and Design 58–59 (2001) 69–73
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sation of the control and by the adjustment of the negative voltage limits. The current sharing of the two power supplies would not be a problem when they are operated in closed loop control mode but, the relatively low voltage of the power supplies, makes this type of discharge not suitable for faults in superconducting coils.
3. TFMC Power System model The magnetic coupling of the TFMC and LCT coil is quite low (k =0.6 p.u.). It has been evaluated, with such a magnetic coupling, that during a simultaneous safety discharge of the TFMC and LCT circuits– —maximum foreseeable dB/dt — the overshoot of the LCT current at 16 kA is only 1.5% and therefore, it is possible to analyse the two circuits separately [3]. A computer system, specially developed to simulate power systems in the Simulink environment, called Power System Blockset (PSB), was selected for this purpose. The package, supplied as standard Matlab product, provides a graphical user interface for building models as block diagrams and a library containing a large variety of blocks to simulate power system components.
Fig. 3. TFMC power system safety discharge test pulse.
Fig. 4. Simulink model of the 30 kA power supply.
The PSB model implemented for the TFMC Power System uses five state variables for the electric network. The eigenvalues of the state matrix are [− 1.3484, − 0.5491, − 0.3273, − 0.0257, − 0.0001]× 106. The coil is feed from the 30 and 50 kA power supplies working in parallel. The two thyristor converters are simulated (see Fig. 4) with a simplified model based on a controlled d.c. voltage source with amplitude Vd, function of the h-angle and the mains ac voltage Vc (Vd = 1.4 Vc cos(h), with Vc =30 V rms). The time constant of 3.3 ms is introduced to take into account the commutation time of the thyristor converter. A diode in series limits the operation to two quadrants only. A reduction of a factor 50 in the number of integration steps has been achieved with respect a full a.c. model of the thyristor converter. A current controller, of PI type, physically installed in the 30-kA power supply controls the current of each converter, which act as a master. The circuit breaker S2 and the power supply isolation switches S0 shown in Fig. 1 are simulated with a GTO block with 10 ms current fast drop (90%) and 10 ms tail. The remaining switches are simulated with an ‘ideal switch’ block. The solver adds an additional state variable for each switching device. The integration of the system equations is performed with a solver suitable for stiff systems and with a tolerance set at 1.0e− 3. The outputs of the model have been compared with the experimental data obtained during the tests on dummy load with good results. Besides, the PSB model has allowed the discovery of exces-
72
V. Marchese et al. / Fusion Engineering and Design 58–59 (2001) 69–73
sive filtering to the current measurements provided by the new current transducers supplied with the dump circuit.
4. TFMC circuit operation at full current After the validation, the model has been used to simulate the behaviour of the power system at the nominal test current of 80 kA. The parameters of the current controller have been selected to have a bandwidth of the system in closed loop of about 8 Hz with a phase shift of 75°. The load current is supplied in the proportion 3/8 and 5/8 by the 30 and 50 kA, respectively. A possible scenario for the current ramp-up with a dI/dt 700 A/s, in feedback control mode, is shown in Fig. 5. The proportional and integral gains of the current controller are 0.025 and 0.5, respectively. As can be observed from the voltage transients shown in Fig. 5 the dominant mode of the system has a frequency of 1.5 Hz and is well damped. For the test of TFMC alone the dI/dt is limited by the inductance of the TFMC coil (27 mH) and the eddy current in the coil cases and in the inter-coil structure. In combined operation with the LCT coil, which will be operated at 16
Fig. 6. Computed safety discharge at 80 kA (TFMC alone).
kA, the dI/dt is limited by the large inductance of the LCT Coil (1.58 H). Therefore, in the combined operation it will take between 15 and 20 min to reach the flat top. The steady-state measurements at the end of the previous simulation are passed automatically to the following run, shown in Fig. 6, which simulates a safety discharge of TFMC alone. The fast make switch S1 is closed 50 ms after the start of the full inversion thus limiting the overshoot in the 30 kA power supply current to 3.3%, which is quite acceptable. The voltage across the coil at the opening of the circuit breaker (S2) is less than 600 V against a nominal voltage across the terminals of 10 kV, which is the voltage of ITER TF Coil at the commutation stage.
5. Conclusions
Fig. 5. Computed current start up (TFMC alone).
The test of the TFMC coil, planned for 2001 at the TOSKA facility has required an upgrade of the power supply system, which is now in its final stage of commissioning. The commissioning of the newly installed 80-kA dump circuit was possible only up to 10 kA with a Cu coil. Therefore, it
V. Marchese et al. / Fusion Engineering and Design 58–59 (2001) 69–73
was considered appropriate to spend some efforts to develop computer models for the simulation of different scenarios at full current both in normal operation, controlled by feedback, and during a safety discharge. Two independent models were developed the first for the LCT power system and the second for the TFMC power system. The models have been validated during the tests with a Cu coil up to 10 kA performed recently and are now used for fault analysis and to make predictions at higher current levels.
73
References [1] E. Salpietro, R. Maix, G. Bevilacqua, N. Mitchell, B. Truck, A. Ulbricht, M. Spadoni, The ITER toroidal field model coil (TFMC) program development programme, Proc. 17th IAEA Fus. Energy. Conf., Yokohama, Japan, 19 – 24 October 1998. [2] P. Komarek, E. Salpeitro, The test facility for the ITER TF model coil, Fus. Eng. Design 41 (1998) 213 – 221. [3] V. Marchese, Parallel operation of the 50 kA and 30 kA power supplies with the 80-kA dump circuit: preliminary circuit analysis and model validation, Proc. 10th TFMC TEST and Analysis Group meeting, CEA/Cadarache, 16 December 1999.