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Components and system tests on the RFX toroidal power supply
Fusion Engineering and Design 75–79 (2005) 49–53
Components and system tests on the RFX toroidal power supply V. Toigo a,∗ , L. Zanotto a , E. Gaio a , M. Perna b , P. Bordignon b , A. Coffetti b , R. Novaro b , P. Bertolotto c , E. Rinaldi c , G. Villa c a
Consorzio RFX, Associazione EURATOM-ENEA sulla Fusione, Corso Stati Uniti 4, 35127 Padova, Italy b ASIRobicon, Viale Sarca 336, 20126 Milano, Italy c Passoni & Villa, Viale Suzzani 229, 20162 Milano, Italy Available online 6 September 2005
Abstract The paper deals with the component and system tests performed on the new toroidal power supply system of the RFX experiment. The high technological innovation of the system required a deep experimental characterization and validation campaign; special factory tests were performed on prototypes of single components aimed at verifying the most critical design aspects. Consequently an articulated series of tests were performed, based on a step-by-step approach to achieve the desired coordinate operation of the whole system. The test procedures and the most significant results are described in the paper. © 2005 Elsevier B.V. All rights reserved. Keywords: RFX experiment; Toroidal power supply; Tests and commissioning
1. Introduction The new toroidal power supply system of RFX [1] is the first example of a completely static power supply in a reversed field pinch (RFP) fusion experiment, where circuits are usually based on large energy storage devices such as capacitor banks. Forty-eight coils uniformly distributed along the torus and divided into 12 winding sectors are supplied by this system to generate and control the toroidal magnetic field waveforms for the RFP configuration setting up and the production of m = 0 field harmonics in order to ∗ Corresponding author. Tel.: +39 049 8295010; fax: +39 049 8700718. E-mail address: [email protected] (V. Toigo).
0920-3796/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2005.06.291
apply a torque to the plasma or special poloidal current drives [2]. The toroidal power supply system is composed of two identical groups, each made of six equal sections, feeding the corresponding toroidal winding sector; Fig. 1 shows the electric scheme of half the circuit and Table 1 reports the ratings of the main devices. The system design was uncommon not only because of the single components, which have been utilized for the first time especially for this application and characterized by a high degree of technological innovation [3], but also for their coordinated interactions, which are supervised by a sophisticated digital control system [4] able to satisfy the high flexibility level required in the RFX operation. Therefore, a prototype of 1/12 of the system was developed and factory tests were
V. Toigo et al. / Fusion Engineering and Design 75–79 (2005) 49–53
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2. Static breakers special tests
Fig. 1. Electric scheme of one of the two toroidal power supply groups.
The static circuit breaker TCIS represents a remarkable example of static dc current interruption technology at high power; it is capable of interrupting dc currents up to 16 kA, with up to 4 kV of reapplied voltage; the high I2 t value (128 kA2 s) was achieved by connecting five integrated gate commutated thyristors (IGCTs) branches in parallel, arranged into two power modules made of three and two IGCTs branches, respectively. At the time of the design phase, the behaviour of IGCTs when connected in parallel was not known, as the component is quite recent and only applications with series connections of IGCTs existed. A very careful design layout of the power module was performed to optimize the current sharing both in steady state and transient conditions. Factory tests were performed on a prototype before proceeding with the module production. The current sharing tests were carried out separately for the two types of module and then for the whole circuit breaker at various current levels. The test arrangement consisted of a dc voltage source, which supplied the IGCT module and an inductance in series. When the IGCTs are fired, the dummy inductance is charged; then, by opening the IGCTs, the current in the load is commutated to a freewheeling diode in parallel. An example is shown in Fig. 2 for a total current of about 5000 A; the currents measured in the three branches are shown. The current sharing
performed to check the critical aspects of the design of each component, before starting with the production of the other units. Successively, a consistent sequence of integrated tests were performed to verify the stresses on the components in normal and fault conditions, which were not possible to reproduce when the components operate individually. Such tests optimized their coordinated operation. Table 1 Ratings of the toroidal power supply main devices Number
Label
Device
Ratings
2 12 12 12 12 14
TFAT TCDB TCCB TCCH TCAC TCIS
ac/dc converter Blocking diodes Capacitor banks Chopper groups Inverters Static breakers
3 kV at 16 kA 4 kV at 5.5 kA 4 kV at 16 mF 3 kV at 3 kA/4.6 kA 3 kV at 6 kA 4 kV at 16 kA at 128 MA2 s
Fig. 2. Current sharing in a circuit breaker module in transient conditions. CH1, firing command; CH2, CH3, CH4, IGCTs branches currents (500 A/div); time scale (100 s/div).
V. Toigo et al. / Fusion Engineering and Design 75–79 (2005) 49–53
among them is very satisfactory, being the maximum current difference with respect to the mean value at the peak lower than 3.5%. The tests on the other module and on the complete circuit breaker at different current levels gave similar results. The factory tests showed that the parallel operation of IGCTs is facilitated by their very small jitter both at turn-on and turn-off and by their positive temperature coefficient, not requiring decoupling inductances among the IGCTs in parallel. Another important issue in the static breaker development is related to the overvoltages arising during the opening phase; simulations of the whole power supply system, carried out during the design phase, showed that they are quite large due to the stray inductance of the connection cables. Special filters and snubber networks have been integrated in the static breaker to mitigate the problem. Their effectiveness was first evaluated by means of the simulations and was then verified during integration tests on site. The overvoltages across the static breaker at its opening always remained under the 3800 V threshold.
3. Tests on the dc/ac inverter The dc/ac inverter is composed of three IGCTs Hbridges connected in parallel. It is one of the first prototypes of single-phase inverter for such high-pulsed power levels (18 MVA). The parallel connection of three elementary H-bridges (3 kV at 2 kA each) is realized here for the first time, and consequently an experimental verification of the design was required. The design has focused in particular on optimizing the current sharing among the bridges in parallel, on limiting the overvoltages during IGCTs commutations and on reducing as much as possible the energy and the peak current (about 100 kA) in case of IGCTs or freewheeling diodes failures. The factory tests have been performed inserting the three H-bridge modules into a prototype cubicle and connecting them in parallel by means of temporary busbar links. The current sharing was checked by supplying the inverter with a dc voltage of 2000 V, using a dummy inductance of 190 H as load and firing the IGCTs at the nominal pulse width modulation frequency (720 Hz) with 11 repetitive pulse commands lasting for 65 s to raise the current up to the nominal value. A zoom on one pulse is shown in Fig. 3, where
51
Fig. 3. Current sharing among the inverter bridges in parallel. CH1, firing command; CH2, upper bridge current (500 A/div); CH3, medium bridge current (500 A/div); CH4, load current (1000 A/div); time scale (20 s/div).
the currents referring to the same IGCT of two bridges are reported. The current sharing is very good; the measured currents in the bridges (1680, 1730 and 1710 A) showed an unbalance lower than 3%; the results are very satisfactory also in the firing transient phase. The overcurrent peak at the turn-on is due to the energy stored in the snubber and clamp capacitive filters. As for the overvoltages across the IGCTs, the tests showed that they remained well within the safe values (4100 V) and are sufficiently dumped, validating the module design.
4. Fast protection fuse development and characterization Another noteworthy technology in the circuit is that related to the devices for the capacitor banks internal fault protection. Each capacitor bank comprises 16 parallel-connected elementary capacitors designed to minimize stray inductances and to have a high energy density. As a consequence, transients due to external and internal faults can be extremely fast and characterized by high current peaks; a resistor was provided in series to each capacitor for limiting the possible fault currents. Moreover, to avoid dramatic degeneration of such faults, extra rapid fuses (4 kV dc, 350 A, 77 kA2 s prearc energy) have been selected and connected in
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V. Toigo et al. / Fusion Engineering and Design 75–79 (2005) 49–53
series with each single capacitor. In order to effectively protect the capacitor banks, the fuses must operate as quick as possible whenever an internal fault occurs, i.e. the simultaneous short circuit of the capacitor and of the resistance connected in series with it. The fuse design was particularly critical due to the high selectivity level required; the operation of the thyristor crowbar in parallel to the capacitor bank is foreseen both in the case of overvoltages due to external faults, but also in particular normal operating conditions, therefore the fuses have neither to intervene nor to have significant ageing in these cases. Moreover, the capacitor bank can be configured to 8 or 16 mF and charged from 2 to 4 kV; as a result the stored energy varies very much, complicating the fuse coordination design. The fuse type was selected on the basis of detailed system simulations, and the choice was validated by means of tests in which all different types of internal faults were reproduced in conditions similar to those of the operative scenarios. In general, the results of the tests have been satisfactory, showing that this type of fuse behaves as expected; during the most onerous test (4000 kV and short circuit of both capacitor and resistance), the fuse broke the circuit in less than 35 s, limiting the peak current to 85 kA and the prearc I2 t to about 80 kA2 s. Fig. 4 reports an example of fuse intervention during a short circuit of a capacitor charged at 4 kV. It can be noted that the fuse breaks the current in less than 70 s, when the I2 t reaches about 80 kA2 s.
Fig. 4. Fuse current in the case of a short circuit of a capacitor charged at 4 kV.
5. System tests The purpose of these tests was to verify the coordinated operation of all the devices comprising the power supply, analysing in particular all the aspects which could not be tested during factory tests on the prototype, such as the synchronous operation of the static breakers, the TFAT converter operation in presence of the sudden load variations (from full inductive to full capacitive load) occurring when the static breakers open, the correct sharing of the current provided to the sectors by TFAT converters when the choppers are regulating, and the critical transit from the reversal phase to the flat-top phase. As the circuit operation is quite complex, it was clear from the beginning of the project that integrating the operation of circuit devices was a major task. Therefore, the strategy selected for the system tests consisted in a step-by-step approach; a special part of the control system was designed in particular to simplify this progressive test scheme [4]. After the first dry tests, the protection logic and devices of the system were individually tested. In particular, the correct intervention of the thyristor crowbars was verified by charging the capacitor bank TTCB (Fig. 1) up to 3800 V. Then, power tests were carried out, testing the single devices of each sector. The capability, both of the slow and fast choppers TCCH, to control the capacitor voltage up to the maximum operating value of 3 kV was verified by charging the capacitor banks with the TFAT converters and maintaining the static breakers open and the inverter turned off; these tests were in particular addressed to set up the chopper’s hysteresis control and in particular to confirm the threshold levels selected during the design analyses to guarantee a satisfactory control of the capacitor voltages and maintain limited commutation losses. A further step was the integration between the static breakers and choppers. The synchronized closing and opening of all static breakers was tested, discharging the energy stored in the load inductance into the capacitors through the inverter diodes and regulating the voltage with the choppers. Such tests were also necessary to optimize the control of the TFAT converter, which is subjected to a sudden load variation for every static breaker operation. The integrated sector operation was then set up and after having performed the same sequence for all the sectors, the whole toroidal power supply group
V. Toigo et al. / Fusion Engineering and Design 75–79 (2005) 49–53
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tests, lasted for about six months, were concluded at the end of 2003 and the system is now ready for the integration with the RFX machine. The special factory tests on prototypes of single components were performed up to the nominal rating; they gave good results and confirmed the validity of the main design choices and the expectations derived from the numerical simulations performed. The step-by-step approach adopted for the system integration proved to be suitable to face gradually and to identify more easily the technical difficulties; the whole system coordinated operation has been verified at low current level, the next step will be the optimization up to the full load current. Fig. 5. Current in the dummy load during an integration test.
operation was tested. The integrated tests allowed to verify the capability of the power supply to achieve the desired control of the load current in the various discharge phases. In Fig. 5, the waveform of the current in a dummy load during the final integration test is shown; during the flat-top phase, an oscillating poloidal current drive (OPCD) [5] at 50 Hz was required in the system, which satisfied the request.
6. Conclusions and future work The integrated operation of the new RFX toroidal power supply system was achieved. The necessary
References [1] G. Rostagni, RFX: an expected step in RFP research, Fusion Eng. Des. 25 (1995) 301–313. [2] R. Piovan, New power supply to generate a rotating toroidal field in RFX, in: Proceedings of the 20th SOFT, 1998, pp. 853–856. [3] V. Toigo, R. Piovan, L. Zanotto, A. Coffetti, M. Perna, F. Poletti, E. Rinaldi, G. Villa, New technological solutions for the power supply system of the RFX toroidal circuit, in: Proceedings of the 20th IEEE/NPSS Symposium on Fusion Engineering, San Diego, CA, USA, October 14–17, 2003. [4] V. Toigo, R. Piovan, L. Zanotto, M. Perna, A. Coffetti, M. Freghieri, M. Povolero, The control system of the RFX toroidal power supply, in: Proceedings of the 23rd Symposium on Fusion Technology, Venice, Italy, September 12–15, 2004. [5] S. Martini, T. Bolzonella, A. Canton, A. Intravaia, L. Marrelli, R. Piovan, et al., OPCD experiments in RFX, in: Proceedings of the 26th EPS Conference, Maastricht, 1999, pp. 1137– 1140.
Components and system tests on the RFX toroidal power supply V. Toigo a,∗ , L. Zanotto a , E. Gaio a , M. Perna b , P. Bordignon b , A. Coffetti b , R. Novaro b , P. Bertolotto c , E. Rinaldi c , G. Villa c a
Consorzio RFX, Associazione EURATOM-ENEA sulla Fusione, Corso Stati Uniti 4, 35127 Padova, Italy b ASIRobicon, Viale Sarca 336, 20126 Milano, Italy c Passoni & Villa, Viale Suzzani 229, 20162 Milano, Italy Available online 6 September 2005
Abstract The paper deals with the component and system tests performed on the new toroidal power supply system of the RFX experiment. The high technological innovation of the system required a deep experimental characterization and validation campaign; special factory tests were performed on prototypes of single components aimed at verifying the most critical design aspects. Consequently an articulated series of tests were performed, based on a step-by-step approach to achieve the desired coordinate operation of the whole system. The test procedures and the most significant results are described in the paper. © 2005 Elsevier B.V. All rights reserved. Keywords: RFX experiment; Toroidal power supply; Tests and commissioning
1. Introduction The new toroidal power supply system of RFX [1] is the first example of a completely static power supply in a reversed field pinch (RFP) fusion experiment, where circuits are usually based on large energy storage devices such as capacitor banks. Forty-eight coils uniformly distributed along the torus and divided into 12 winding sectors are supplied by this system to generate and control the toroidal magnetic field waveforms for the RFP configuration setting up and the production of m = 0 field harmonics in order to ∗ Corresponding author. Tel.: +39 049 8295010; fax: +39 049 8700718. E-mail address: [email protected] (V. Toigo).
0920-3796/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2005.06.291
apply a torque to the plasma or special poloidal current drives [2]. The toroidal power supply system is composed of two identical groups, each made of six equal sections, feeding the corresponding toroidal winding sector; Fig. 1 shows the electric scheme of half the circuit and Table 1 reports the ratings of the main devices. The system design was uncommon not only because of the single components, which have been utilized for the first time especially for this application and characterized by a high degree of technological innovation [3], but also for their coordinated interactions, which are supervised by a sophisticated digital control system [4] able to satisfy the high flexibility level required in the RFX operation. Therefore, a prototype of 1/12 of the system was developed and factory tests were
V. Toigo et al. / Fusion Engineering and Design 75–79 (2005) 49–53
50
2. Static breakers special tests
Fig. 1. Electric scheme of one of the two toroidal power supply groups.
The static circuit breaker TCIS represents a remarkable example of static dc current interruption technology at high power; it is capable of interrupting dc currents up to 16 kA, with up to 4 kV of reapplied voltage; the high I2 t value (128 kA2 s) was achieved by connecting five integrated gate commutated thyristors (IGCTs) branches in parallel, arranged into two power modules made of three and two IGCTs branches, respectively. At the time of the design phase, the behaviour of IGCTs when connected in parallel was not known, as the component is quite recent and only applications with series connections of IGCTs existed. A very careful design layout of the power module was performed to optimize the current sharing both in steady state and transient conditions. Factory tests were performed on a prototype before proceeding with the module production. The current sharing tests were carried out separately for the two types of module and then for the whole circuit breaker at various current levels. The test arrangement consisted of a dc voltage source, which supplied the IGCT module and an inductance in series. When the IGCTs are fired, the dummy inductance is charged; then, by opening the IGCTs, the current in the load is commutated to a freewheeling diode in parallel. An example is shown in Fig. 2 for a total current of about 5000 A; the currents measured in the three branches are shown. The current sharing
performed to check the critical aspects of the design of each component, before starting with the production of the other units. Successively, a consistent sequence of integrated tests were performed to verify the stresses on the components in normal and fault conditions, which were not possible to reproduce when the components operate individually. Such tests optimized their coordinated operation. Table 1 Ratings of the toroidal power supply main devices Number
Label
Device
Ratings
2 12 12 12 12 14
TFAT TCDB TCCB TCCH TCAC TCIS
ac/dc converter Blocking diodes Capacitor banks Chopper groups Inverters Static breakers
3 kV at 16 kA 4 kV at 5.5 kA 4 kV at 16 mF 3 kV at 3 kA/4.6 kA 3 kV at 6 kA 4 kV at 16 kA at 128 MA2 s
Fig. 2. Current sharing in a circuit breaker module in transient conditions. CH1, firing command; CH2, CH3, CH4, IGCTs branches currents (500 A/div); time scale (100 s/div).
V. Toigo et al. / Fusion Engineering and Design 75–79 (2005) 49–53
among them is very satisfactory, being the maximum current difference with respect to the mean value at the peak lower than 3.5%. The tests on the other module and on the complete circuit breaker at different current levels gave similar results. The factory tests showed that the parallel operation of IGCTs is facilitated by their very small jitter both at turn-on and turn-off and by their positive temperature coefficient, not requiring decoupling inductances among the IGCTs in parallel. Another important issue in the static breaker development is related to the overvoltages arising during the opening phase; simulations of the whole power supply system, carried out during the design phase, showed that they are quite large due to the stray inductance of the connection cables. Special filters and snubber networks have been integrated in the static breaker to mitigate the problem. Their effectiveness was first evaluated by means of the simulations and was then verified during integration tests on site. The overvoltages across the static breaker at its opening always remained under the 3800 V threshold.
3. Tests on the dc/ac inverter The dc/ac inverter is composed of three IGCTs Hbridges connected in parallel. It is one of the first prototypes of single-phase inverter for such high-pulsed power levels (18 MVA). The parallel connection of three elementary H-bridges (3 kV at 2 kA each) is realized here for the first time, and consequently an experimental verification of the design was required. The design has focused in particular on optimizing the current sharing among the bridges in parallel, on limiting the overvoltages during IGCTs commutations and on reducing as much as possible the energy and the peak current (about 100 kA) in case of IGCTs or freewheeling diodes failures. The factory tests have been performed inserting the three H-bridge modules into a prototype cubicle and connecting them in parallel by means of temporary busbar links. The current sharing was checked by supplying the inverter with a dc voltage of 2000 V, using a dummy inductance of 190 H as load and firing the IGCTs at the nominal pulse width modulation frequency (720 Hz) with 11 repetitive pulse commands lasting for 65 s to raise the current up to the nominal value. A zoom on one pulse is shown in Fig. 3, where
51
Fig. 3. Current sharing among the inverter bridges in parallel. CH1, firing command; CH2, upper bridge current (500 A/div); CH3, medium bridge current (500 A/div); CH4, load current (1000 A/div); time scale (20 s/div).
the currents referring to the same IGCT of two bridges are reported. The current sharing is very good; the measured currents in the bridges (1680, 1730 and 1710 A) showed an unbalance lower than 3%; the results are very satisfactory also in the firing transient phase. The overcurrent peak at the turn-on is due to the energy stored in the snubber and clamp capacitive filters. As for the overvoltages across the IGCTs, the tests showed that they remained well within the safe values (4100 V) and are sufficiently dumped, validating the module design.
4. Fast protection fuse development and characterization Another noteworthy technology in the circuit is that related to the devices for the capacitor banks internal fault protection. Each capacitor bank comprises 16 parallel-connected elementary capacitors designed to minimize stray inductances and to have a high energy density. As a consequence, transients due to external and internal faults can be extremely fast and characterized by high current peaks; a resistor was provided in series to each capacitor for limiting the possible fault currents. Moreover, to avoid dramatic degeneration of such faults, extra rapid fuses (4 kV dc, 350 A, 77 kA2 s prearc energy) have been selected and connected in
52
V. Toigo et al. / Fusion Engineering and Design 75–79 (2005) 49–53
series with each single capacitor. In order to effectively protect the capacitor banks, the fuses must operate as quick as possible whenever an internal fault occurs, i.e. the simultaneous short circuit of the capacitor and of the resistance connected in series with it. The fuse design was particularly critical due to the high selectivity level required; the operation of the thyristor crowbar in parallel to the capacitor bank is foreseen both in the case of overvoltages due to external faults, but also in particular normal operating conditions, therefore the fuses have neither to intervene nor to have significant ageing in these cases. Moreover, the capacitor bank can be configured to 8 or 16 mF and charged from 2 to 4 kV; as a result the stored energy varies very much, complicating the fuse coordination design. The fuse type was selected on the basis of detailed system simulations, and the choice was validated by means of tests in which all different types of internal faults were reproduced in conditions similar to those of the operative scenarios. In general, the results of the tests have been satisfactory, showing that this type of fuse behaves as expected; during the most onerous test (4000 kV and short circuit of both capacitor and resistance), the fuse broke the circuit in less than 35 s, limiting the peak current to 85 kA and the prearc I2 t to about 80 kA2 s. Fig. 4 reports an example of fuse intervention during a short circuit of a capacitor charged at 4 kV. It can be noted that the fuse breaks the current in less than 70 s, when the I2 t reaches about 80 kA2 s.
Fig. 4. Fuse current in the case of a short circuit of a capacitor charged at 4 kV.
5. System tests The purpose of these tests was to verify the coordinated operation of all the devices comprising the power supply, analysing in particular all the aspects which could not be tested during factory tests on the prototype, such as the synchronous operation of the static breakers, the TFAT converter operation in presence of the sudden load variations (from full inductive to full capacitive load) occurring when the static breakers open, the correct sharing of the current provided to the sectors by TFAT converters when the choppers are regulating, and the critical transit from the reversal phase to the flat-top phase. As the circuit operation is quite complex, it was clear from the beginning of the project that integrating the operation of circuit devices was a major task. Therefore, the strategy selected for the system tests consisted in a step-by-step approach; a special part of the control system was designed in particular to simplify this progressive test scheme [4]. After the first dry tests, the protection logic and devices of the system were individually tested. In particular, the correct intervention of the thyristor crowbars was verified by charging the capacitor bank TTCB (Fig. 1) up to 3800 V. Then, power tests were carried out, testing the single devices of each sector. The capability, both of the slow and fast choppers TCCH, to control the capacitor voltage up to the maximum operating value of 3 kV was verified by charging the capacitor banks with the TFAT converters and maintaining the static breakers open and the inverter turned off; these tests were in particular addressed to set up the chopper’s hysteresis control and in particular to confirm the threshold levels selected during the design analyses to guarantee a satisfactory control of the capacitor voltages and maintain limited commutation losses. A further step was the integration between the static breakers and choppers. The synchronized closing and opening of all static breakers was tested, discharging the energy stored in the load inductance into the capacitors through the inverter diodes and regulating the voltage with the choppers. Such tests were also necessary to optimize the control of the TFAT converter, which is subjected to a sudden load variation for every static breaker operation. The integrated sector operation was then set up and after having performed the same sequence for all the sectors, the whole toroidal power supply group
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tests, lasted for about six months, were concluded at the end of 2003 and the system is now ready for the integration with the RFX machine. The special factory tests on prototypes of single components were performed up to the nominal rating; they gave good results and confirmed the validity of the main design choices and the expectations derived from the numerical simulations performed. The step-by-step approach adopted for the system integration proved to be suitable to face gradually and to identify more easily the technical difficulties; the whole system coordinated operation has been verified at low current level, the next step will be the optimization up to the full load current. Fig. 5. Current in the dummy load during an integration test.
operation was tested. The integrated tests allowed to verify the capability of the power supply to achieve the desired control of the load current in the various discharge phases. In Fig. 5, the waveform of the current in a dummy load during the final integration test is shown; during the flat-top phase, an oscillating poloidal current drive (OPCD) [5] at 50 Hz was required in the system, which satisfied the request.
6. Conclusions and future work The integrated operation of the new RFX toroidal power supply system was achieved. The necessary
References [1] G. Rostagni, RFX: an expected step in RFP research, Fusion Eng. Des. 25 (1995) 301–313. [2] R. Piovan, New power supply to generate a rotating toroidal field in RFX, in: Proceedings of the 20th SOFT, 1998, pp. 853–856. [3] V. Toigo, R. Piovan, L. Zanotto, A. Coffetti, M. Perna, F. Poletti, E. Rinaldi, G. Villa, New technological solutions for the power supply system of the RFX toroidal circuit, in: Proceedings of the 20th IEEE/NPSS Symposium on Fusion Engineering, San Diego, CA, USA, October 14–17, 2003. [4] V. Toigo, R. Piovan, L. Zanotto, M. Perna, A. Coffetti, M. Freghieri, M. Povolero, The control system of the RFX toroidal power supply, in: Proceedings of the 23rd Symposium on Fusion Technology, Venice, Italy, September 12–15, 2004. [5] S. Martini, T. Bolzonella, A. Canton, A. Intravaia, L. Marrelli, R. Piovan, et al., OPCD experiments in RFX, in: Proceedings of the 26th EPS Conference, Maastricht, 1999, pp. 1137– 1140.