- Email: [email protected]
Conceptual design of the power supply systems for the Divertor Tokamak Test facility
Fusion Engineering and Design 146 (2019) 937–941
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Conceptual design of the power supply systems for the Divertor Tokamak Test facility
T
⁎
Alessandro Lampasia, , Andrea De Santisb, Simone Minuccic, Fabio Staracea, Pietro Zitoa a
National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA), Frascati, Italy University of Rome Sapienza, Italy c University of Tuscia, Italy b
A R T I C LE I N FO
A B S T R A C T
Keywords: Divertor Tokamak Test (DTT) Power supply AC/DC converter Energy storage Pulsed load
The Divertor Tokamak Test (DTT) facility will be the link between ITER and DEMO as regards the study of the power exhaust in the divertor structure. In order to accomplish this goal, the Italian DTT will be a complete superconducting tokamak with 45 MW of additional power coupled to the plasma. According to the last requirements, the DTT coil power supplies are designed to achieve a toroidal field of 6 T and a 5.5 MA plasma in single-null configuration, but also to achieve double-null (with plasma at 5.5 MA), snow-flake (4.5 MA), super-X (3 MA) and other advanced configurations. This paper presents the conceptual design of the DTT power supply and electrical systems, including the site layout, the distribution networks, the expected scenarios and the mechanisms for breakdown and emergency discharge. In particular, the main rationales for the selection of the power supply topology are summarized. The power demand from the grid, presented in the various design options, can reach values above 300 MVA, including the contribution of the additional heating and current drive systems and of the auxiliary services.
1. Introduction The design of the power supply (PS) systems of a new tokamak starts with many degrees of freedom and options involving many technical and economical aspects. This is a stimulating and rare opportunity, but also a complex process requiring many successive iterations with all the other tokamak systems and external stakeholders, that may even lead to continuous modifications in the previous choices. The Divertor Tokamak Test (DTT) facility will be a link between ITER and DEMO as regards the power exhaust in the divertor structure [1]. In order to accomplish this main goal, the Italian DTT will be a fullworking tokamak. Two important events occurred after the previous DTT design presented in [1] and [2]: 1 Since the best option for the DEMO divertor is still far to be identified, an agreement between ENEA and EUROfusion established that DTT shall be enough “flexible” to host different types of divertors and magnetic configurations. Accordingly, the original design [1] was modified to have vertically symmetrical vessel and coils, as shown in Fig. 1, slightly reducing the major radius from 2.15 to 2.10 m. 2 After a national Call for Interests, with the participation of 9
⁎
qualified candidates, in 2018 the ENEA Center in Frascati was selected as the DTT site. As the Frascati Center is presently hosting two tokamaks (FTU and PROTO-SPHERA) and many minor research facilities, some electrical systems and supporting services are already available or can be updated. On the other hand, the existing buildings involve several constraints in term of space and layout. This paper presents the conceptual design of the DTT PS and electrical systems. The DTT PSs are designed to achieve a toroidal field of 6 T and a 5.5 MA plasma in single-null configuration, but also to achieve double-null (still at 5.5 MA), snow-flake (4.5 MA), super-X (3 MA) and other advanced magnetic configurations [3]. The final power demand from the grid, including the additional heating and current drive (H&CD) systems and the auxiliary services, is presented in the various design options. 2. Classification of the electrical loads Even though DTT will be upgraded in successive phases, the distribution system, the input cables and the connection to the national grid must be sized for the maximum power demand in the final
Corresponding author. E-mail address: [email protected] (A. Lampasi).
https://doi.org/10.1016/j.fusengdes.2019.01.118 Received 8 October 2018; Received in revised form 14 January 2019; Accepted 23 January 2019 Available online 28 January 2019 0920-3796/ © 2019 Elsevier B.V. All rights reserved.
Fusion Engineering and Design 146 (2019) 937–941
A. Lampasi et al.
Fig. 1. Three-dimensional view of the DTT facility with emphasis to the magnetic system. The plasma is sketched during a typical single-null scenario.
Fig. 2. Layout of the main electrical systems of the DTT site in the ENEA Center in Frascati.
only during plasma operations, that are about 100 s for DTT). While the auxiliary and the H&CD loads clearly belong to the former and latter group respectively, the situation of the coil PSs is fuzzy and depends on the design choices, as addressed in Section 4. In order to achieve the DTT power exhaust target levels, at least 45 MW of additional power shall be coupled to the plasma by mixing
configuration. The resulting site layout and medium voltage (MV) distribution are sketched in Fig. 2. The DTT loads, as in standard tokamaks, came from three general systems: auxiliary services, additional H&CD systems and coil PSs [4]. For the design purpose, they can be classified in two groups: steadystate (requiring a rather constant power) and pulsed (requiring power 938
Fusion Engineering and Design 146 (2019) 937–941
A. Lampasi et al.
independent superconducting modules of the Central Solenoid (CS), 6 Poloidal Field (PF) superconducting coils, 2 in-vessel copper coils for real-time vertical stabilization (VS) of the plasma, 4 in-vessel copper coils to shape the magnetic configuration close to the divertor (DS). Further coils are not considered in the present design, but some spare space and power is available for plasma control (ELM, RWM, etc.) and for shaping applied to an upper divertor. New scenarios for the coil currents were defined after the design modifications. With respect to [2], the CS and PF currents were increased to range in the interval ± 28 kA. The voltages are computed from the currents through the mutual inductances [2]. The voltages at the plasma breakdown (zero time) are not achieved by the PS but by the support of a switching network unit (SNU) [2] in series between the PS and the coil. The voltages requested at the breakdown are lower than in other tokamaks [4], even though a good loop electric field of 0.8 V/m is expected to be achieved [3]. This is possible mainly because the current is not left uncontrolled to decay with the circuit time constant, but it is also dynamically compensated by increasing the voltage (and exploiting the contribution of the VS PSs). Such capability, as the fine tuning of the breakdown voltages, is fully available using converters with fully-controlled commutation and accurate SNUs. The bars in Fig. 4 summarize the maximum voltage amplitude needed for each CS and PF PS during a reference single-null (5.5 MA), double-null (5.5 MA), snow-flake (4.5 MA) scenario, respectively. The fourth red bar displays the voltage, identical for all the configurations, necessary for the dynamic compensation at breakdown, namely the voltage variation to be covered around the value provided by the SNU. This bar is absent for PF2-5 because the simulations showed that the breakdown is more robust without SNU in these coils [3]. Also the voltages in Fig. 4 are relatively small [4] thanks to an optimization activity, excepting for the snow-flake case, because this configuration was obtained without the support of the DS coils that could be different or absent for an alternative divertor configuration.
Table 1 Expected characteristics of the DTT H&CD systems. H&CD system
Initial mix
Maximum upgrade
Wall-plug efficiency
cosφ
EC IC N-NBI
15 MW 3 MW 7 MW
30 MW 9 MW 15 MW
40% 40% 45%
0.9 0.9 0.9
Fig. 3. Assumed scenarios for the H&CD systems in the final DTT setup at full power.
three different H&CD systems: radiofrequency at ion and electron cyclotron (IC and EC) resonance frequencies and at least two negative neutral beam injectors (N-NBI). The optimal mixing composition will be identified after a first experimental phase coupling 25 MW from all the three systems, as reported in Table 1. Table 1 reports also the maximum upgrade possible for each system, under the constrain that the total power will be 45 MW. The DTT electrical system and the external grid should be ready to supply the most demanding H&CD final configuration. The H&CD electrical power depends on the specific source. Table 1 summarizes the expected (target) wall-plug efficiency and power factor (cosφ) of the H &CD systems that will be present at the initial and final phases of the DTT project. The scenario in Fig. 3 derived by simulations [5] can be used as a reference case for the power calculations (fast and small power modulations are not considered). The Frascati Center is presently connected to a 150-kV line that is now providing less than 10 MVA of power and cannot be upgraded also due to the presence of other loads connected to the same line. Moreover, the three FTU flywheels are sized to deliver energy for few seconds. Therefore, a new high-voltage line is necessary for DTT. The preliminary analyses on the grid stability indicated that the power ramps in Fig. 3 can be sustained only by the Italian 400-kV grid, having a strong node located at about 15 km from Frascati. In order to simplify the infrastructures and to reduce the costs, the voltage is reduced to 150 kV immediately after the connection at 400 kV. A new 150-kV line will be fitted between the grid node and an electrical substation inside the ENEA Center. The old high voltage line and substation will be kept for the offices and to ensure an emergency backup in case of fault of the new systems. Since the voltage drops produced by the pulsed loads are critical for the other ones, the DTT power is distributed through two separated subnetworks, classified as steady-state (SSEN) and pulsed power electrical network (PPEN), each supplied by two (for redundancy) dedicated 150kV/20-kV transformers in the substation. In this way, the SSEN simulations achieved voltage drops within 3% and a cosφ > 0.9. The PPEN is able to feed for less than 100 s every 30 min (instead of 60 min reported in [2]) the H&CD systems. Even though the EC, IC and N-NBI operations typically need very higher voltages [4], these systems are connected to the 20-kV PPEN.
4. Rationales for the selection of the PS topology The actual scheme selected for each coil PS will depend on the final project requirements and budget. However, also in order to reduce the costs, the PSs were grouped in the three topologies sketched in Fig. 5. In any case, the design was focused on modularity to attain up to 50 kA (needed for DS coil PSs) in 4-quadrant operations. The SNU and the fast discharge unit (FDU) [2] are shown only in Fig. 5(a). Some considerations for the optimal selection among the schemes in Fig. 5 are summarized in Table 2. For example, the design of the TF PS circuit (sketched in Fig. 6) moves from the following considerations. All the coils are supplied in series to reduce the magnetic field toroidal ripple. However, the FDUs must be divided in three groups to limit the voltage across coils in case
3. Ratings of the PSs for the magnetic system Fig. 4. Maximum voltage required at the CS and PF PSs during different reference scenarios and at breakdown.
The coil PSs includes 18 toroidal field (TF) superconducting coils, 6 939
Fusion Engineering and Design 146 (2019) 937–941
A. Lampasi et al.
Fig. 5. Alternative topologies for the CS, PF, VS, DS PSs.
power analyses. The global results are very similar for the double-null and snow-flake configurations, even though two relevant power peaks are present in the middle of the snow-flake scenario. After a revision of the electrical load list, the DTT auxiliary power was estimated to be around 60 MVA, including a safety margin and the TF PS contribution. The H&CD power, represented by the dashed red line in Fig. 7, can be derived directly by Fig. 3 and Table 1. The power of the CS and PF coils is very variable. Even though the active power to the coils is very low (< 10 MW with the SNU support), the reactive power is relevant if the Fig. 5(a) topology is adopted [2,4], even when the scenario currents are lower than the rated values. This leads to the dashed blue line in Fig. 7. The contributions of the dashed red and blue lines, together with the auxiliary power, produce the total DDT power represented by the
of protection opening. The topology in Fig. 5(c) allows to recovery a good portion of the energy if the coils are charged/discharged at each shot, but such cycles would be detrimental for the Nb3Sn superconductors. The Fig. 5(a) topology is adequate as the control can be slow (charge in minutes, flat-top for days). The resulting cosφ is low, but also the power (≈1 MVA) and, as it is constant, the correction is simple. Therefore, the TF PS can be included in the SSEN.
5. DTT power contributions and total power Fig. 7 shows the different contributions to the DTT power and the total demand from the national grid under some hypothesis on the PS implementation, as described in the following. The single-null scenario at 5.5 MA was used as reference for the
Table 2 Summary of main (dis)advantages for each topology in Fig. 5 to be adopted in DTT coil PSs. Topology Fig. 5(a)
Fig. 5(b)
Fig. 5(c)
Advantages
Disadvantages
known in fusion (proven reliability) • Well “local” cost • Low • High power with less components control • Fast compensation at breakdown • Dynamic power quality (no cosφ correction and harmonic filters) • High control and dynamic breakdown compensation • Fast recovery (lower energy bill) • Energy distribution network (SSEN instead of PPEN) and protections • Simpler upstream costs • Low oil in transformers (safety and pollution) • No • Intrinsic modularity and possible upgrades
940
(and complex) control • Slow power quality (reactive, harmonics, …) • Poor cosφ and harmonic correction • Mandatory power from the external grid • Huge • Cost power (backward power may be unacceptable for the grid) • Huge to be discharged in case of fault • DC-link known technology • Less know costs (but expected to decrease in next years) • Less energy losses in the supercapacitors • Internal energy stored in supercapacitors • Huge • Hundreds of supercapacitor modules
Fusion Engineering and Design 146 (2019) 937–941
A. Lampasi et al.
Fig. 6. Scheme selected for the TF PS circuit. The inductance of each superconducting coil is ≈100 mH, while the total load inductance of ≈5 H. In this case, the crow-bar is unidirectional.
hand, since the coil powers are managed only by the energy storage banks, the power profile is smoothed and the cosφ is not affected by the coil contribution. Accordingly, the power performances are similar to those obtained with a large cosφ correction. The input requests of the chargers can be even adapted to compensate the H&CD ramps, reducing their slopes. The resulting load is less critical for the external grid. For these reasons, present studies are devoted to the feasibility of alternative supercapacitor-based solutions. Of course, to ensure a safety margin and to allow future upgrades, the connection was requested for a power up to 300 MVA. 6. Conclusions Fig. 7. Analysis of the contributions to the DTT power.
Many aspects and options must be taken into account in the design of a new large fusion facility as DTT. Some considerations and results of this process were presented in the paper. Further simulations are being carried out to assess the impact (voltage oscillations, flicker and so on) of the DTT facility on the transmission grid and the provisions required to cope with it. The Calls for Tender for the first DTT PS and electrical procurements are expected to start in 2019 to complete the commissioning by 2025.
dashed purple line. The external transmission grid would not be able to handle this total power, mainly due to the large reactive power (intrinsic in the coil PSs) and the fast derivatives. The reactive power issue can be addressed by means of conventional correction techniques. According to the national grid rules (that would be similar in other countries), the cosφ must be continuously corrected to 0.9, transforming the dashed purple line in Fig. 7 in the solid green line. A further reduction of the cosφ and of the resulting apparent power during all the scenario can be achieved only increasing the overall costs. The green line also corresponds to the total power if the topology in Fig. 5(b) is adopted for all the coils. In this case, the cosφ correction can be limited to the H&CD contributions. The power derivatives and peaks can be smoothed by means of an intermediate energy storage. The solid red line in Fig. 7 exemplifies the power profile when the coil PSs are based on the topology in Fig. 5(c). The SSEN baseline is higher, due the input powers of the chargers, but without problems for the cable and protection design. On the other
References [1] A. Pizzuto, DTT Divertor Tokamak Test Facility – Project Proposal, ENEA, 2015. [2] A. Lampasi, et al., The DTT device: power supplies and electrical distribution system, Fusion Eng. Des. 122 (2017) 356–364. [3] R. Albanese, et al., Design review for the Italian divertor tokamak test facility, 30th Symposium on Fusion Technology (SOFT) (2018). [4] A. Lampasi, S. Minucci, Survey of electric power supplies used in nuclear fusion experiments, 17 IEEE International Conference on Environment and Electrical Engineering (EEEIC 2017) (2017). [5] G. Granucci, et al., The heating systems capability of DTT, 30th Symposium on Fusion Technology (SOFT) (2018).
941
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Conceptual design of the power supply systems for the Divertor Tokamak Test facility
T
⁎
Alessandro Lampasia, , Andrea De Santisb, Simone Minuccic, Fabio Staracea, Pietro Zitoa a
National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA), Frascati, Italy University of Rome Sapienza, Italy c University of Tuscia, Italy b
A R T I C LE I N FO
A B S T R A C T
Keywords: Divertor Tokamak Test (DTT) Power supply AC/DC converter Energy storage Pulsed load
The Divertor Tokamak Test (DTT) facility will be the link between ITER and DEMO as regards the study of the power exhaust in the divertor structure. In order to accomplish this goal, the Italian DTT will be a complete superconducting tokamak with 45 MW of additional power coupled to the plasma. According to the last requirements, the DTT coil power supplies are designed to achieve a toroidal field of 6 T and a 5.5 MA plasma in single-null configuration, but also to achieve double-null (with plasma at 5.5 MA), snow-flake (4.5 MA), super-X (3 MA) and other advanced configurations. This paper presents the conceptual design of the DTT power supply and electrical systems, including the site layout, the distribution networks, the expected scenarios and the mechanisms for breakdown and emergency discharge. In particular, the main rationales for the selection of the power supply topology are summarized. The power demand from the grid, presented in the various design options, can reach values above 300 MVA, including the contribution of the additional heating and current drive systems and of the auxiliary services.
1. Introduction The design of the power supply (PS) systems of a new tokamak starts with many degrees of freedom and options involving many technical and economical aspects. This is a stimulating and rare opportunity, but also a complex process requiring many successive iterations with all the other tokamak systems and external stakeholders, that may even lead to continuous modifications in the previous choices. The Divertor Tokamak Test (DTT) facility will be a link between ITER and DEMO as regards the power exhaust in the divertor structure [1]. In order to accomplish this main goal, the Italian DTT will be a fullworking tokamak. Two important events occurred after the previous DTT design presented in [1] and [2]: 1 Since the best option for the DEMO divertor is still far to be identified, an agreement between ENEA and EUROfusion established that DTT shall be enough “flexible” to host different types of divertors and magnetic configurations. Accordingly, the original design [1] was modified to have vertically symmetrical vessel and coils, as shown in Fig. 1, slightly reducing the major radius from 2.15 to 2.10 m. 2 After a national Call for Interests, with the participation of 9
⁎
qualified candidates, in 2018 the ENEA Center in Frascati was selected as the DTT site. As the Frascati Center is presently hosting two tokamaks (FTU and PROTO-SPHERA) and many minor research facilities, some electrical systems and supporting services are already available or can be updated. On the other hand, the existing buildings involve several constraints in term of space and layout. This paper presents the conceptual design of the DTT PS and electrical systems. The DTT PSs are designed to achieve a toroidal field of 6 T and a 5.5 MA plasma in single-null configuration, but also to achieve double-null (still at 5.5 MA), snow-flake (4.5 MA), super-X (3 MA) and other advanced magnetic configurations [3]. The final power demand from the grid, including the additional heating and current drive (H&CD) systems and the auxiliary services, is presented in the various design options. 2. Classification of the electrical loads Even though DTT will be upgraded in successive phases, the distribution system, the input cables and the connection to the national grid must be sized for the maximum power demand in the final
Corresponding author. E-mail address: [email protected] (A. Lampasi).
https://doi.org/10.1016/j.fusengdes.2019.01.118 Received 8 October 2018; Received in revised form 14 January 2019; Accepted 23 January 2019 Available online 28 January 2019 0920-3796/ © 2019 Elsevier B.V. All rights reserved.
Fusion Engineering and Design 146 (2019) 937–941
A. Lampasi et al.
Fig. 1. Three-dimensional view of the DTT facility with emphasis to the magnetic system. The plasma is sketched during a typical single-null scenario.
Fig. 2. Layout of the main electrical systems of the DTT site in the ENEA Center in Frascati.
only during plasma operations, that are about 100 s for DTT). While the auxiliary and the H&CD loads clearly belong to the former and latter group respectively, the situation of the coil PSs is fuzzy and depends on the design choices, as addressed in Section 4. In order to achieve the DTT power exhaust target levels, at least 45 MW of additional power shall be coupled to the plasma by mixing
configuration. The resulting site layout and medium voltage (MV) distribution are sketched in Fig. 2. The DTT loads, as in standard tokamaks, came from three general systems: auxiliary services, additional H&CD systems and coil PSs [4]. For the design purpose, they can be classified in two groups: steadystate (requiring a rather constant power) and pulsed (requiring power 938
Fusion Engineering and Design 146 (2019) 937–941
A. Lampasi et al.
independent superconducting modules of the Central Solenoid (CS), 6 Poloidal Field (PF) superconducting coils, 2 in-vessel copper coils for real-time vertical stabilization (VS) of the plasma, 4 in-vessel copper coils to shape the magnetic configuration close to the divertor (DS). Further coils are not considered in the present design, but some spare space and power is available for plasma control (ELM, RWM, etc.) and for shaping applied to an upper divertor. New scenarios for the coil currents were defined after the design modifications. With respect to [2], the CS and PF currents were increased to range in the interval ± 28 kA. The voltages are computed from the currents through the mutual inductances [2]. The voltages at the plasma breakdown (zero time) are not achieved by the PS but by the support of a switching network unit (SNU) [2] in series between the PS and the coil. The voltages requested at the breakdown are lower than in other tokamaks [4], even though a good loop electric field of 0.8 V/m is expected to be achieved [3]. This is possible mainly because the current is not left uncontrolled to decay with the circuit time constant, but it is also dynamically compensated by increasing the voltage (and exploiting the contribution of the VS PSs). Such capability, as the fine tuning of the breakdown voltages, is fully available using converters with fully-controlled commutation and accurate SNUs. The bars in Fig. 4 summarize the maximum voltage amplitude needed for each CS and PF PS during a reference single-null (5.5 MA), double-null (5.5 MA), snow-flake (4.5 MA) scenario, respectively. The fourth red bar displays the voltage, identical for all the configurations, necessary for the dynamic compensation at breakdown, namely the voltage variation to be covered around the value provided by the SNU. This bar is absent for PF2-5 because the simulations showed that the breakdown is more robust without SNU in these coils [3]. Also the voltages in Fig. 4 are relatively small [4] thanks to an optimization activity, excepting for the snow-flake case, because this configuration was obtained without the support of the DS coils that could be different or absent for an alternative divertor configuration.
Table 1 Expected characteristics of the DTT H&CD systems. H&CD system
Initial mix
Maximum upgrade
Wall-plug efficiency
cosφ
EC IC N-NBI
15 MW 3 MW 7 MW
30 MW 9 MW 15 MW
40% 40% 45%
0.9 0.9 0.9
Fig. 3. Assumed scenarios for the H&CD systems in the final DTT setup at full power.
three different H&CD systems: radiofrequency at ion and electron cyclotron (IC and EC) resonance frequencies and at least two negative neutral beam injectors (N-NBI). The optimal mixing composition will be identified after a first experimental phase coupling 25 MW from all the three systems, as reported in Table 1. Table 1 reports also the maximum upgrade possible for each system, under the constrain that the total power will be 45 MW. The DTT electrical system and the external grid should be ready to supply the most demanding H&CD final configuration. The H&CD electrical power depends on the specific source. Table 1 summarizes the expected (target) wall-plug efficiency and power factor (cosφ) of the H &CD systems that will be present at the initial and final phases of the DTT project. The scenario in Fig. 3 derived by simulations [5] can be used as a reference case for the power calculations (fast and small power modulations are not considered). The Frascati Center is presently connected to a 150-kV line that is now providing less than 10 MVA of power and cannot be upgraded also due to the presence of other loads connected to the same line. Moreover, the three FTU flywheels are sized to deliver energy for few seconds. Therefore, a new high-voltage line is necessary for DTT. The preliminary analyses on the grid stability indicated that the power ramps in Fig. 3 can be sustained only by the Italian 400-kV grid, having a strong node located at about 15 km from Frascati. In order to simplify the infrastructures and to reduce the costs, the voltage is reduced to 150 kV immediately after the connection at 400 kV. A new 150-kV line will be fitted between the grid node and an electrical substation inside the ENEA Center. The old high voltage line and substation will be kept for the offices and to ensure an emergency backup in case of fault of the new systems. Since the voltage drops produced by the pulsed loads are critical for the other ones, the DTT power is distributed through two separated subnetworks, classified as steady-state (SSEN) and pulsed power electrical network (PPEN), each supplied by two (for redundancy) dedicated 150kV/20-kV transformers in the substation. In this way, the SSEN simulations achieved voltage drops within 3% and a cosφ > 0.9. The PPEN is able to feed for less than 100 s every 30 min (instead of 60 min reported in [2]) the H&CD systems. Even though the EC, IC and N-NBI operations typically need very higher voltages [4], these systems are connected to the 20-kV PPEN.
4. Rationales for the selection of the PS topology The actual scheme selected for each coil PS will depend on the final project requirements and budget. However, also in order to reduce the costs, the PSs were grouped in the three topologies sketched in Fig. 5. In any case, the design was focused on modularity to attain up to 50 kA (needed for DS coil PSs) in 4-quadrant operations. The SNU and the fast discharge unit (FDU) [2] are shown only in Fig. 5(a). Some considerations for the optimal selection among the schemes in Fig. 5 are summarized in Table 2. For example, the design of the TF PS circuit (sketched in Fig. 6) moves from the following considerations. All the coils are supplied in series to reduce the magnetic field toroidal ripple. However, the FDUs must be divided in three groups to limit the voltage across coils in case
3. Ratings of the PSs for the magnetic system Fig. 4. Maximum voltage required at the CS and PF PSs during different reference scenarios and at breakdown.
The coil PSs includes 18 toroidal field (TF) superconducting coils, 6 939
Fusion Engineering and Design 146 (2019) 937–941
A. Lampasi et al.
Fig. 5. Alternative topologies for the CS, PF, VS, DS PSs.
power analyses. The global results are very similar for the double-null and snow-flake configurations, even though two relevant power peaks are present in the middle of the snow-flake scenario. After a revision of the electrical load list, the DTT auxiliary power was estimated to be around 60 MVA, including a safety margin and the TF PS contribution. The H&CD power, represented by the dashed red line in Fig. 7, can be derived directly by Fig. 3 and Table 1. The power of the CS and PF coils is very variable. Even though the active power to the coils is very low (< 10 MW with the SNU support), the reactive power is relevant if the Fig. 5(a) topology is adopted [2,4], even when the scenario currents are lower than the rated values. This leads to the dashed blue line in Fig. 7. The contributions of the dashed red and blue lines, together with the auxiliary power, produce the total DDT power represented by the
of protection opening. The topology in Fig. 5(c) allows to recovery a good portion of the energy if the coils are charged/discharged at each shot, but such cycles would be detrimental for the Nb3Sn superconductors. The Fig. 5(a) topology is adequate as the control can be slow (charge in minutes, flat-top for days). The resulting cosφ is low, but also the power (≈1 MVA) and, as it is constant, the correction is simple. Therefore, the TF PS can be included in the SSEN.
5. DTT power contributions and total power Fig. 7 shows the different contributions to the DTT power and the total demand from the national grid under some hypothesis on the PS implementation, as described in the following. The single-null scenario at 5.5 MA was used as reference for the
Table 2 Summary of main (dis)advantages for each topology in Fig. 5 to be adopted in DTT coil PSs. Topology Fig. 5(a)
Fig. 5(b)
Fig. 5(c)
Advantages
Disadvantages
known in fusion (proven reliability) • Well “local” cost • Low • High power with less components control • Fast compensation at breakdown • Dynamic power quality (no cosφ correction and harmonic filters) • High control and dynamic breakdown compensation • Fast recovery (lower energy bill) • Energy distribution network (SSEN instead of PPEN) and protections • Simpler upstream costs • Low oil in transformers (safety and pollution) • No • Intrinsic modularity and possible upgrades
940
(and complex) control • Slow power quality (reactive, harmonics, …) • Poor cosφ and harmonic correction • Mandatory power from the external grid • Huge • Cost power (backward power may be unacceptable for the grid) • Huge to be discharged in case of fault • DC-link known technology • Less know costs (but expected to decrease in next years) • Less energy losses in the supercapacitors • Internal energy stored in supercapacitors • Huge • Hundreds of supercapacitor modules
Fusion Engineering and Design 146 (2019) 937–941
A. Lampasi et al.
Fig. 6. Scheme selected for the TF PS circuit. The inductance of each superconducting coil is ≈100 mH, while the total load inductance of ≈5 H. In this case, the crow-bar is unidirectional.
hand, since the coil powers are managed only by the energy storage banks, the power profile is smoothed and the cosφ is not affected by the coil contribution. Accordingly, the power performances are similar to those obtained with a large cosφ correction. The input requests of the chargers can be even adapted to compensate the H&CD ramps, reducing their slopes. The resulting load is less critical for the external grid. For these reasons, present studies are devoted to the feasibility of alternative supercapacitor-based solutions. Of course, to ensure a safety margin and to allow future upgrades, the connection was requested for a power up to 300 MVA. 6. Conclusions Fig. 7. Analysis of the contributions to the DTT power.
Many aspects and options must be taken into account in the design of a new large fusion facility as DTT. Some considerations and results of this process were presented in the paper. Further simulations are being carried out to assess the impact (voltage oscillations, flicker and so on) of the DTT facility on the transmission grid and the provisions required to cope with it. The Calls for Tender for the first DTT PS and electrical procurements are expected to start in 2019 to complete the commissioning by 2025.
dashed purple line. The external transmission grid would not be able to handle this total power, mainly due to the large reactive power (intrinsic in the coil PSs) and the fast derivatives. The reactive power issue can be addressed by means of conventional correction techniques. According to the national grid rules (that would be similar in other countries), the cosφ must be continuously corrected to 0.9, transforming the dashed purple line in Fig. 7 in the solid green line. A further reduction of the cosφ and of the resulting apparent power during all the scenario can be achieved only increasing the overall costs. The green line also corresponds to the total power if the topology in Fig. 5(b) is adopted for all the coils. In this case, the cosφ correction can be limited to the H&CD contributions. The power derivatives and peaks can be smoothed by means of an intermediate energy storage. The solid red line in Fig. 7 exemplifies the power profile when the coil PSs are based on the topology in Fig. 5(c). The SSEN baseline is higher, due the input powers of the chargers, but without problems for the cable and protection design. On the other
References [1] A. Pizzuto, DTT Divertor Tokamak Test Facility – Project Proposal, ENEA, 2015. [2] A. Lampasi, et al., The DTT device: power supplies and electrical distribution system, Fusion Eng. Des. 122 (2017) 356–364. [3] R. Albanese, et al., Design review for the Italian divertor tokamak test facility, 30th Symposium on Fusion Technology (SOFT) (2018). [4] A. Lampasi, S. Minucci, Survey of electric power supplies used in nuclear fusion experiments, 17 IEEE International Conference on Environment and Electrical Engineering (EEEIC 2017) (2017). [5] G. Granucci, et al., The heating systems capability of DTT, 30th Symposium on Fusion Technology (SOFT) (2018).
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