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Final design of the High Voltage Deck 1 and Bushing for MITICA: The ITER Heating Neutral Beam Injector prototype
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Final design of the High Voltage Deck 1 and Bushing for MITICA: The ITER Heating Neutral Beam Injector prototype Marco Boldrin a,∗ , Tullio Bonicelli b , Hans Decamps c , Claudio Finotti a , Elena Gaio a , Gerard Escudero Gomez b , Michael Krohn d , Edgar Sachs d , Muriel Simon b , Vanni Toigo a a
Consorzio RFX (CNR, ENEA, INFN, Università di Padova, Acciaierie Venete SpA), Corso Stati Uniti 4, 35127 Padova, Italy Fusion For Energy, c/Josep Pla 2, 08019 Barcelona, Spain c ITER Organization,Route de Vinon-sur-Verdon, CS 90 046, 13067 St. Paul Lez Durance Cedex, France d Siemens AG,Process Industries and Drives Division, Large Drives, Industrial Applications PD LD AP S TA EL, Gleiwitzer Str. 555, 90475 Nürnberg, Germany b
h i g h l i g h t s • • • •
HVD1 and HVBA are unique devices for size, electrostatic and mechanical features. Main design choices to satisfy the challenging technical requirements are presented. The supporting electrostatic, seismic and thermal analyses are reported. Next procurement steps and tests to verify the key design choices are also reported.
a r t i c l e
i n f o
Article history: Received 26 September 2016 Received in revised form 20 March 2017 Accepted 22 March 2017 Available online xxx Keywords: ITER Heating Neutral Beam Injector (HNB) High voltage deck 1 (HVD1) High Voltage Bushing Assembly (HVBA) Insulation design Seismic design Thermal design
a b s t r a c t The Ion Source of the ITER Heating Neutral Beam Injector (HNB) is polarized at −1MVdc to ground. A large-1MVdc air-insulated Faraday cage (12.5 m (L) × 8.4 m (W) × 9.6 m (H)), called High Voltage Deck 1 (HVD1), hosts the Ion Source and Extractor Power Supplies (ISEPS) and the associated HNB diagnostics; the HVD1 is interfaced with a SF6 insulated Transmission Line (connecting the main power supplies to the HNB) through a High Voltage (HV) Bushing Assembly (HVBA) carrying inside ISEPS power conductors and diagnostics. The HVD1 and HVBA are unique devices for size, electrostatic and mechanical features. This paper presents the main design choices adopted to comply with the challenging technical requirements and the supporting electrostatic, seismic and thermal analyses. Finally, next procurement steps and definition of special tests to verify the key design choices are also reported. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Two Heating Neutral Beam Injectors (HNBs) will be installed in ITER, designed to deliver to the plasma a power up to 16.5 MW each, with a pulse lasting up to 1 h [1]. The realization and future operation of ITER HNB is very demanding both from engineering and physics point of view. For this reason, a full scale HNB prototype called MITICA (Megavolt ITER Injector & Concept Advancement) is being built at the Consorzio RFX premises in Padua (Italy), hosted in a test facility called PRIMA (Padua Research on Injector with Megavolt Acceleration) [2]; its design has been developed with the aim
∗ Corresponding author. E-mail address: [email protected] (M. Boldrin).
to be as close as possible to that one for ITER so as to validate the technological choices and benefit of the feedbacks from the commissioning and first operation. The Power Supply (PS) system is very complex, too. To solve some issues related to the accessibility for maintenance and feasibility of the implementation of the RF driven Ion Source it was chosen to host the Ion Source and Extractor Power Supplies (ISEPS) [3] in a large air insulated platform [4], called High Voltage Deck 1 (HVD1), hosted inside a High Voltage Hall (HVH). The connection from the HVD1 to the SF6 insulated Transmission Line (TL) is provided by a High Voltage (HV) Bushing Assembly (HVBA, see Fig. 1) carrying inside ISEPS power and diagnostics conductors. The HVD1 and the HVBA are unique components in terms of size and complexity. The main challenges of their design are the very high electric insulation (the maximum voltage level for indus-
http://dx.doi.org/10.1016/j.fusengdes.2017.03.133 0920-3796/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: M. Boldrin, et al., Final design of the High Voltage Deck 1 and Bushing for MITICA: The ITER Heating Neutral Beam Injector prototype, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.133
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Fig. 1. Overall view of the MITICA HV components. Table 1 Power supply system voltage withstand requirements. Rated voltage Short duration (3600 s) DC test voltage Long duration (5 h) DC test voltage Pulsed voltage test starting from −1060 kV dc Polarity reversal (up to 200 times per hour; total number of 450,000 during lifetime)
−1000 kVdc −1200 kVdc −1060 kVdc –1265 kV +300 kV
trial applications is presently about 800 kVdc), Electro-Magnetic Interferences (EMI) produced by breakdown events between the acceleration grids, the mechanical constraints (huge components with strict seismic requirements) and thermal aspects (related to Joule losses in the HVBA inner power conductors). The first studies to analyse the issues and outline possible design solutions are described in [5] while in [6] deeper analyses and some basic design choices are presented. The HVD1 and HVBA for MITICA are presently being procured by SIEMENS AG, via a contract with Fusion for Energy (F4E) started in December 2014. This paper presents the final design of the HVD1 and HVBA for the MITICA experiment and the relevant supporting analyses. Finally, next procurement steps and definition of special tests to verify the key design choices are also reported. 2. Main technical design specifications The HVD1 and HVBA will be installed indoor, inside the MITICA HVH (26 m (L) × 29 m (W) × 21,4 m (H)) with controlled ambient conditions (air temperature, humidity and dust level). The maximum pulse duration is 3600 s with a duty cycle of 25% and the voltage withstand requirements are reported in Table 1. It can be observed that some test requirements are not so far from current industrial practice, while the polarity reversal test is peculiar of the HNB application and derives from the transient voltage induced by grid breakdowns in the power supply system. Other main requirements driving the design are: • the maximum electrical field around the HVD1, set lower than 1 kV/mm (i.e. a safety factor of 3 with respect to the dry air dielectric strength) when the short duration test voltage of 1.2 MV is applied; • the function of a Faraday cage containing all the ISEPS components; • the large diameter for the air to SF6 interface to be provided by the HVBA; • the capability to sustain the seismic conditions of the PRIMA and of the ITER sites. 3. HVD1 design The HVD1 is a −1 MVdc air insulated Faraday cage, which contains all the ISEPS components distributed on two floors. The
Fig. 2. View of main components inside MITICA HVH.
HVD1 outer cage dimensions are 12.5 m (L) × 8.4 m (W) × 9.6 m (H) (Fig. 2). The descriptions of the main components are reported in the following sections. 3.1. Supporting structure An inner mechanical structure (see Fig. 2) is designed to support the ISEPS equipment (50t), the auxiliary systems, the electrical screen, the connection to the insulating transformer and the structure itself (additional 52t). Horizontal and vertical standard steel beams constitute the main frame whilst additional reinforcing connections (e.g. diagonal bracing among vertical and horizontal beams) guarantee the necessary stiffness to the whole construction. Smaller beams and plates are arranged for the anchoring of the ISEPS components and of the electrical screen panels. The HVD1 is supported by eight hollow composite insulators based on Fibre-Reinforced Plastic (FRP) tube technology, manufactured with e-glass and epoxy resin, externally covered by silicone rubber sheds to increase the creepage distance and internally filled with N2 ; each of these post insulators is about 6 m high and weighs approximately 0.7 t. They are able to sustain static and seismic loads without the need for additional insulated transversal bracing, as originally foreseen in the conceptual design [5]. Decoupling elements are installed at their top to reduce the bending load. Finally, all the post insulators are connected to a base frame composed of steel beams and fixed to the HVH concrete floor via anchor bolts. The clearance of 6 m from the floor and of 5 m from the walls of HVH allows withstanding the steady state and transient electrical voltage levels. 3.2. Electrical screen The electrical screen is composed of two nested metallic shells. The inner screen, conceived to minimize the EMI produced by the grid breakdowns, is made of modular hot-dip galvanized steel panels; the screwed interconnections are equipped with a special soft metal sealing and any outward aperture (for heat dissipation and cables entrance) is provided with honeycomb caps, thus maintaining the EMI screening property of the shell. The outer electrostatic screen, made of AlMg3 panels, is designed to avoid high electrical fields around the HVD1, which could cause corona phenomena.
Please cite this article in press as: M. Boldrin, et al., Final design of the High Voltage Deck 1 and Bushing for MITICA: The ITER Heating Neutral Beam Injector prototype, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.133
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To this aim, the corner panels are bent with a radius of 0.5m, optimized through electrostatic analyses, and the apertures are designed avoiding sharp edges in order to maintain the electrical field smoothness. With the same purpose, metal panels are installed above the HVD1 base frame beams and around the connection to the insulating transformer. 3.3. Additional equipment The Ventilation System (VS) is designed to keep the HVD1 maximum inner temperature below 40 ◦ C, given a maximum ambient temperature of 35 ◦ C in the HVH and maximum ISEPS heat losses of about 100 kW. Air intakes and exhausts apertures, provided with fans, are foreseen respectively at the bottom and at the ceiling of the HVD1. The VS is regulated to keep constant the differential pressure in between the HVD1 electrical screens; a control for regulation based on the actual heat dissipation from ISEPS will be implemented. The Grounding Switch (GS) ensures the grounding of the external screen for safe access inside the HVD1 outside operation. It is a specially designed component: the mobile contact consists in a metallic ball, connected to a metallic rope, moved in vertical direction by an electrical motor installed underneath the HVD1. At ground level, the fixed contact consists in a metallic basket for the ball housing. The GS operates in “fail safe” logic: in case of loss of the auxiliary power supply the GS closes, thanks to the gravity, thus earthing the HVD1; the GS is equipped with a passive magnet brake to slow down the vertical movement of the mobile contact in this case. The insulated breaks, carrying the demineralized cooling water (18.5 l/s) for ISEPS components from ground to −1MVdc, consist in DN100 insulating pipes arranged in spiral coils to increase the total resistance of the water, thus reducing the drain current to less than 5 mA. A third pipe hosts the optical fibres. These three pipes are supported by composite post insulators and located underneath the HVD1 in order not to disturb the electrostatic field inside the HVH.
Fig. 3. HVBA three-dimensional view.
ing the space, increasing the sections of high current conductors and, consequently, reducing the Joule losses.
4. HV Bushing Assembly design
5. HVH Integrated electrostatic analysis
The HVBA connects the HVD1 to the insulated SF6 TL, as shown in Fig. 1; it will have an overall height of about 12 m and a total weight of 19t. The chosen layout foresees the installation of the HVBA in a pit underneath the HVD1, with the HVBA base (Interface Box) aside of the TL (Fig. 2). This integrated layout allows simplifying the overall design [5,6], because the HVBA takes advantage of the HVD1 shielding effect, avoiding the installation of additional screens; all the electrical connections at the interfaces are flexible, so the HVBA is mechanically decoupled from the HVD1 and the TL. As a result of previous analyses of different technological solutions [6], ungraded SF6 fully insulated technology was finally selected, as the most reliable insulating structure able to withstand the very frequent voltage stresses due to grid breakdowns. Similarly to the HVD1 supporting insulators, the HVBA insulator (Fig. 3) is made of a fibre glass reinforced epoxy resin tube with external silicone elastomer sheds vulcanized onto it. The top of the insulator is closed by an aluminium flange from which are hanged all the internal conductors. The flange, provided with electrical feedthroughs for all conductors, assures the tight closure for the SF6. At the bottom, the HVBA is supported by an Interface Box, in which the conductors transition from HVBA to TL design is ensured. The connection to the TL is made through a lateral flange, thus resulting in a unique SF6 volume, filled at 0.6 MPa rated pressure. The HVBA carries inside the ISEPS power conductors and diagnostic cables, summarized in Table 2. A coaxial configuration was chosen, in which one of the high current busbar is the external conductor and the other conductors are arranged inside. This allows optimiz-
An integrated three-dimensional (3D) Finite Element (FE) electrostatic analysis has been carried out by the Supplier (using getdp 2.5.1 code) to identify possible critical positions inside the HVH (i.e. with an electrical field higher than 1 kV/mm). To this aim, the maximum test voltage of 1.2MVdc has been applied to the HVD1 screen, to the HVBA inner conductors and to the tip of the insulating transformer; the HVH floor, ceiling and pillars and the insulating transformer tank, as well as the fire protection walls presently foreseen in front of the insulating transformer bushing, have been set at ground potential. In Fig. 4 the positions where the calculated electrical field is higher than 1 kV/mm are highlighted. On the HVD1 surface the electric field is lower than the given threshold, confirming therefore a proper design of the electrical screen. Hot spots near the post insulators derive from some model and mesh simplifications. Other hot spots are present inside the HVBA, on SF6 volume where a greater electric field is admissible, while around the HVBA (in air) the electric field decreases below 1 kV/mm threshold in few centimetres. The critical electrostatic condition pointed out on the top edge of the fire protection wall needs to be addressed by providing it with a suitable electrostatic screen. Moreover, the case of a surface irregularity on building construction has been simulated by modelling a 10 mm protrusion (cone) on a pillar nearby HVD1: also in this case the electrical field increases to dangerous level (∼5 kV/mm). In principle, no protrusions should be present on the building walls but the areas facing the HVD1 will be inspected with particular care to remove or screen any irregularity before onsite acceptance tests.
Please cite this article in press as: M. Boldrin, et al., Final design of the High Voltage Deck 1 and Bushing for MITICA: The ITER Heating Neutral Beam Injector prototype, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.133
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4 Table 2 ISEPS conductors characteristics. Conductor type
Quantity
1 MHz RF coaxial lines 3 1/8 inches Busbars HV cable Signal cables Other cables Fibre optics
4 (200 kW each) 2 1 1 5 6 12
Ratings Voltage [kV]
Current [A]
<0.1 <0.1 12 <0.3 <0.1 –
∼5000∼1000 140 ≤2 ≤140 –
static loads applied finding the main stresses and displacements (by ANSYS code). The safety factors for each material (mainly steel, FRP, aluminium) of the HVD1 and HVBA for the DLS and ULS spectra, calculated with respect to the yield and ultimate strengths respectively, resulted lower than admissible limits given in the relevant standards: IEC 1463 Bushings Seismic (1996-07), NTC 2008 (Italian Norm), Eurocode (EC0, EC3, EC8). The eigenfrequencies with the higher mass participations are below 20 Hz for the HVBA and 5 Hz for the HVD1. The maximum DLS and ULS displacements of the conductors at the top of the HVBA are 12 mm and 27 mm respectively while at the interface with the TL are 18 mm and 42 mm. The maximum lateral displacement in the upper part of the HVD1 has been estimated as 63 mm in DLS case; it increases to 201 mm in ULS case.
8. Next procurement steps
Fig. 4. Electric field map (values in V/mm).
6. HV Bushing Assembly thermal analysis A 3D thermal analysis (by Comsol Multiphysics 5.2) was carried out to identify the maximum temperature reached in the HVBA conductors (not actively cooled), after three daily operation cycles, each of them consisting in 1 h ON phase followed by 3 h OFF. The nominal Joule losses of the main power conductors and of the RF lines were considered as heat sources. As boundary conditions, free ambient air convection around the HVBA was considered; for the inner conductors only thermal exchanges by conduction were allowed whilst radiation and SF6 convection were precautionary neglected. The ambient temperature was set at 40 ◦ C. Simulation results showed that the hottest inner conductor heats up at 85 ◦ C, which complies with the allowed maximum temperature of 130 ◦ C reported in the relevant standard IEC 60137. Moreover, in the following 12 h the HVBA was able to cool down to initial conditions. 7. Seismic analysis The HVD1 and the HVBA must comply with seismic requirements of PRIMA and ITER sites. Two spectra were provided for each site: for PRIMA, the former is the Damage Limit State (DLS), i.e. after such earthquake the facility should be able to restart immediately the operation; the latter is the Ultimate Limit State (ULS), i.e. the facility can suffer damages but it does not collapse. Seismic analyses have been carried out independently for the HVD1 and the HVBA, because they are seismically decoupled from each other and from the other equipment inside the HVH. The main eigenfrequencies and mass participations were calculated and the spectra and pre-
The detailed design phase of HVD1 and HVBA was concluded in May 2016. The manufacturing design is being finalized, with the objective to conclude the manufacturing and factory testing within 2016. Where available, certificates and standard test will be supplied for the single components but special tests are also foreseen for the assemblies. A HVD1 mock-up with only one floor and a reduced size of 4 m × 4 m × 4 m, supported by 4 full size post insulators, was built to perform factory tests based on the ratings reported in Table 1; for the HVBA the same tests are planned in December, together with additional insulation tests on the HVBA inner conductors. Seismic tests are under definition, accounting for the fact that no facilities provided with shaking (vibrational) table of suitable dimensions are available for these big size components. To perform such tests, HVBA and HVD1 once installed will be duly instrumented with accelerometers and then stimulated to derive the main eigenfrequencies in order to validate the models used for the seismic design of the structures. The modalities of execution of these tests as well as the acceptance criteria are under discussion. For pressurized components, like the HVBA insulator and HVD1 post insulators, dedicated material and consequent hydraulic pressure tests are required for the homologation according to Italian National Body (INAIL) instructions. The pressure tests of the postinsulators were successfully held in June in the presence of INAIL, while the pressure tests of the HVBA are scheduled for September. The installation of the HVD1 and of the HVBA for MITICA is foreseen to start in the first quarter of 2017.
9. Conclusions The HVD1 and HVBA design and relevant analyses have been finalized proving the feasibility and compliance with technical specifications. The manufacturing phase is in progress and factory tests are ongoing, in view of installation at PRIMA site foreseen beginning 2017.
Please cite this article in press as: M. Boldrin, et al., Final design of the High Voltage Deck 1 and Bushing for MITICA: The ITER Heating Neutral Beam Injector prototype, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.133
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Acknowledgement The work leading to this publication has been funded partially by Fusion for Energy under the contract F4E-OPE-083. This publication reflects the views only of the author, and F4E cannot be held responsible for any use which may be made of the information contained therein. The views and opinions expressed herein do not necessarily reflect those of the ITER Organization. References
5
[2] V. Toigo, et al., Progress in the realization of the PRIMA neutral beam test facility, Nucl. Fusion 55 (2015). [3] M. Bigi, et al., Design manufacture and factory testing of the Ion Source and Extraction Power Supplies for the SPIDER experiment, Fusion Engineering and Design 96-97 (2015) 405–410. [4] E. Gaio, et al., The alternative design concept for the ion source power supply of the ITER neutral beam injector, Fusion Engineering and Design 83 (2008) 21–29. [5] M. Boldrin, et al., Design issues of the High Voltage platform and feedthrough for the ITER NBI ion source, Fusion Engineering and Design 84 (2009) 470–474. [6] M. Boldrin, et al., Design status High Voltage Deck 1 and Bushing for the ITER Neutral Beam Injector and procurement activities of the, Fusion Engineering and Design 88 (2013) 985–989.
[1] ITER technical basis, IAEA, Vienna 2002, ITER EDA, Doc. Series N. 24, Plant Description Document, Sec. 2.5.1.
Please cite this article in press as: M. Boldrin, et al., Final design of the High Voltage Deck 1 and Bushing for MITICA: The ITER Heating Neutral Beam Injector prototype, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.133
ARTICLE IN PRESS
FUSION-9314; No. of Pages 5
Fusion Engineering and Design xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Final design of the High Voltage Deck 1 and Bushing for MITICA: The ITER Heating Neutral Beam Injector prototype Marco Boldrin a,∗ , Tullio Bonicelli b , Hans Decamps c , Claudio Finotti a , Elena Gaio a , Gerard Escudero Gomez b , Michael Krohn d , Edgar Sachs d , Muriel Simon b , Vanni Toigo a a
Consorzio RFX (CNR, ENEA, INFN, Università di Padova, Acciaierie Venete SpA), Corso Stati Uniti 4, 35127 Padova, Italy Fusion For Energy, c/Josep Pla 2, 08019 Barcelona, Spain c ITER Organization,Route de Vinon-sur-Verdon, CS 90 046, 13067 St. Paul Lez Durance Cedex, France d Siemens AG,Process Industries and Drives Division, Large Drives, Industrial Applications PD LD AP S TA EL, Gleiwitzer Str. 555, 90475 Nürnberg, Germany b
h i g h l i g h t s • • • •
HVD1 and HVBA are unique devices for size, electrostatic and mechanical features. Main design choices to satisfy the challenging technical requirements are presented. The supporting electrostatic, seismic and thermal analyses are reported. Next procurement steps and tests to verify the key design choices are also reported.
a r t i c l e
i n f o
Article history: Received 26 September 2016 Received in revised form 20 March 2017 Accepted 22 March 2017 Available online xxx Keywords: ITER Heating Neutral Beam Injector (HNB) High voltage deck 1 (HVD1) High Voltage Bushing Assembly (HVBA) Insulation design Seismic design Thermal design
a b s t r a c t The Ion Source of the ITER Heating Neutral Beam Injector (HNB) is polarized at −1MVdc to ground. A large-1MVdc air-insulated Faraday cage (12.5 m (L) × 8.4 m (W) × 9.6 m (H)), called High Voltage Deck 1 (HVD1), hosts the Ion Source and Extractor Power Supplies (ISEPS) and the associated HNB diagnostics; the HVD1 is interfaced with a SF6 insulated Transmission Line (connecting the main power supplies to the HNB) through a High Voltage (HV) Bushing Assembly (HVBA) carrying inside ISEPS power conductors and diagnostics. The HVD1 and HVBA are unique devices for size, electrostatic and mechanical features. This paper presents the main design choices adopted to comply with the challenging technical requirements and the supporting electrostatic, seismic and thermal analyses. Finally, next procurement steps and definition of special tests to verify the key design choices are also reported. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Two Heating Neutral Beam Injectors (HNBs) will be installed in ITER, designed to deliver to the plasma a power up to 16.5 MW each, with a pulse lasting up to 1 h [1]. The realization and future operation of ITER HNB is very demanding both from engineering and physics point of view. For this reason, a full scale HNB prototype called MITICA (Megavolt ITER Injector & Concept Advancement) is being built at the Consorzio RFX premises in Padua (Italy), hosted in a test facility called PRIMA (Padua Research on Injector with Megavolt Acceleration) [2]; its design has been developed with the aim
∗ Corresponding author. E-mail address: [email protected] (M. Boldrin).
to be as close as possible to that one for ITER so as to validate the technological choices and benefit of the feedbacks from the commissioning and first operation. The Power Supply (PS) system is very complex, too. To solve some issues related to the accessibility for maintenance and feasibility of the implementation of the RF driven Ion Source it was chosen to host the Ion Source and Extractor Power Supplies (ISEPS) [3] in a large air insulated platform [4], called High Voltage Deck 1 (HVD1), hosted inside a High Voltage Hall (HVH). The connection from the HVD1 to the SF6 insulated Transmission Line (TL) is provided by a High Voltage (HV) Bushing Assembly (HVBA, see Fig. 1) carrying inside ISEPS power and diagnostics conductors. The HVD1 and the HVBA are unique components in terms of size and complexity. The main challenges of their design are the very high electric insulation (the maximum voltage level for indus-
http://dx.doi.org/10.1016/j.fusengdes.2017.03.133 0920-3796/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: M. Boldrin, et al., Final design of the High Voltage Deck 1 and Bushing for MITICA: The ITER Heating Neutral Beam Injector prototype, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.133
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Fig. 1. Overall view of the MITICA HV components. Table 1 Power supply system voltage withstand requirements. Rated voltage Short duration (3600 s) DC test voltage Long duration (5 h) DC test voltage Pulsed voltage test starting from −1060 kV dc Polarity reversal (up to 200 times per hour; total number of 450,000 during lifetime)
−1000 kVdc −1200 kVdc −1060 kVdc –1265 kV +300 kV
trial applications is presently about 800 kVdc), Electro-Magnetic Interferences (EMI) produced by breakdown events between the acceleration grids, the mechanical constraints (huge components with strict seismic requirements) and thermal aspects (related to Joule losses in the HVBA inner power conductors). The first studies to analyse the issues and outline possible design solutions are described in [5] while in [6] deeper analyses and some basic design choices are presented. The HVD1 and HVBA for MITICA are presently being procured by SIEMENS AG, via a contract with Fusion for Energy (F4E) started in December 2014. This paper presents the final design of the HVD1 and HVBA for the MITICA experiment and the relevant supporting analyses. Finally, next procurement steps and definition of special tests to verify the key design choices are also reported. 2. Main technical design specifications The HVD1 and HVBA will be installed indoor, inside the MITICA HVH (26 m (L) × 29 m (W) × 21,4 m (H)) with controlled ambient conditions (air temperature, humidity and dust level). The maximum pulse duration is 3600 s with a duty cycle of 25% and the voltage withstand requirements are reported in Table 1. It can be observed that some test requirements are not so far from current industrial practice, while the polarity reversal test is peculiar of the HNB application and derives from the transient voltage induced by grid breakdowns in the power supply system. Other main requirements driving the design are: • the maximum electrical field around the HVD1, set lower than 1 kV/mm (i.e. a safety factor of 3 with respect to the dry air dielectric strength) when the short duration test voltage of 1.2 MV is applied; • the function of a Faraday cage containing all the ISEPS components; • the large diameter for the air to SF6 interface to be provided by the HVBA; • the capability to sustain the seismic conditions of the PRIMA and of the ITER sites. 3. HVD1 design The HVD1 is a −1 MVdc air insulated Faraday cage, which contains all the ISEPS components distributed on two floors. The
Fig. 2. View of main components inside MITICA HVH.
HVD1 outer cage dimensions are 12.5 m (L) × 8.4 m (W) × 9.6 m (H) (Fig. 2). The descriptions of the main components are reported in the following sections. 3.1. Supporting structure An inner mechanical structure (see Fig. 2) is designed to support the ISEPS equipment (50t), the auxiliary systems, the electrical screen, the connection to the insulating transformer and the structure itself (additional 52t). Horizontal and vertical standard steel beams constitute the main frame whilst additional reinforcing connections (e.g. diagonal bracing among vertical and horizontal beams) guarantee the necessary stiffness to the whole construction. Smaller beams and plates are arranged for the anchoring of the ISEPS components and of the electrical screen panels. The HVD1 is supported by eight hollow composite insulators based on Fibre-Reinforced Plastic (FRP) tube technology, manufactured with e-glass and epoxy resin, externally covered by silicone rubber sheds to increase the creepage distance and internally filled with N2 ; each of these post insulators is about 6 m high and weighs approximately 0.7 t. They are able to sustain static and seismic loads without the need for additional insulated transversal bracing, as originally foreseen in the conceptual design [5]. Decoupling elements are installed at their top to reduce the bending load. Finally, all the post insulators are connected to a base frame composed of steel beams and fixed to the HVH concrete floor via anchor bolts. The clearance of 6 m from the floor and of 5 m from the walls of HVH allows withstanding the steady state and transient electrical voltage levels. 3.2. Electrical screen The electrical screen is composed of two nested metallic shells. The inner screen, conceived to minimize the EMI produced by the grid breakdowns, is made of modular hot-dip galvanized steel panels; the screwed interconnections are equipped with a special soft metal sealing and any outward aperture (for heat dissipation and cables entrance) is provided with honeycomb caps, thus maintaining the EMI screening property of the shell. The outer electrostatic screen, made of AlMg3 panels, is designed to avoid high electrical fields around the HVD1, which could cause corona phenomena.
Please cite this article in press as: M. Boldrin, et al., Final design of the High Voltage Deck 1 and Bushing for MITICA: The ITER Heating Neutral Beam Injector prototype, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.133
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To this aim, the corner panels are bent with a radius of 0.5m, optimized through electrostatic analyses, and the apertures are designed avoiding sharp edges in order to maintain the electrical field smoothness. With the same purpose, metal panels are installed above the HVD1 base frame beams and around the connection to the insulating transformer. 3.3. Additional equipment The Ventilation System (VS) is designed to keep the HVD1 maximum inner temperature below 40 ◦ C, given a maximum ambient temperature of 35 ◦ C in the HVH and maximum ISEPS heat losses of about 100 kW. Air intakes and exhausts apertures, provided with fans, are foreseen respectively at the bottom and at the ceiling of the HVD1. The VS is regulated to keep constant the differential pressure in between the HVD1 electrical screens; a control for regulation based on the actual heat dissipation from ISEPS will be implemented. The Grounding Switch (GS) ensures the grounding of the external screen for safe access inside the HVD1 outside operation. It is a specially designed component: the mobile contact consists in a metallic ball, connected to a metallic rope, moved in vertical direction by an electrical motor installed underneath the HVD1. At ground level, the fixed contact consists in a metallic basket for the ball housing. The GS operates in “fail safe” logic: in case of loss of the auxiliary power supply the GS closes, thanks to the gravity, thus earthing the HVD1; the GS is equipped with a passive magnet brake to slow down the vertical movement of the mobile contact in this case. The insulated breaks, carrying the demineralized cooling water (18.5 l/s) for ISEPS components from ground to −1MVdc, consist in DN100 insulating pipes arranged in spiral coils to increase the total resistance of the water, thus reducing the drain current to less than 5 mA. A third pipe hosts the optical fibres. These three pipes are supported by composite post insulators and located underneath the HVD1 in order not to disturb the electrostatic field inside the HVH.
Fig. 3. HVBA three-dimensional view.
ing the space, increasing the sections of high current conductors and, consequently, reducing the Joule losses.
4. HV Bushing Assembly design
5. HVH Integrated electrostatic analysis
The HVBA connects the HVD1 to the insulated SF6 TL, as shown in Fig. 1; it will have an overall height of about 12 m and a total weight of 19t. The chosen layout foresees the installation of the HVBA in a pit underneath the HVD1, with the HVBA base (Interface Box) aside of the TL (Fig. 2). This integrated layout allows simplifying the overall design [5,6], because the HVBA takes advantage of the HVD1 shielding effect, avoiding the installation of additional screens; all the electrical connections at the interfaces are flexible, so the HVBA is mechanically decoupled from the HVD1 and the TL. As a result of previous analyses of different technological solutions [6], ungraded SF6 fully insulated technology was finally selected, as the most reliable insulating structure able to withstand the very frequent voltage stresses due to grid breakdowns. Similarly to the HVD1 supporting insulators, the HVBA insulator (Fig. 3) is made of a fibre glass reinforced epoxy resin tube with external silicone elastomer sheds vulcanized onto it. The top of the insulator is closed by an aluminium flange from which are hanged all the internal conductors. The flange, provided with electrical feedthroughs for all conductors, assures the tight closure for the SF6. At the bottom, the HVBA is supported by an Interface Box, in which the conductors transition from HVBA to TL design is ensured. The connection to the TL is made through a lateral flange, thus resulting in a unique SF6 volume, filled at 0.6 MPa rated pressure. The HVBA carries inside the ISEPS power conductors and diagnostic cables, summarized in Table 2. A coaxial configuration was chosen, in which one of the high current busbar is the external conductor and the other conductors are arranged inside. This allows optimiz-
An integrated three-dimensional (3D) Finite Element (FE) electrostatic analysis has been carried out by the Supplier (using getdp 2.5.1 code) to identify possible critical positions inside the HVH (i.e. with an electrical field higher than 1 kV/mm). To this aim, the maximum test voltage of 1.2MVdc has been applied to the HVD1 screen, to the HVBA inner conductors and to the tip of the insulating transformer; the HVH floor, ceiling and pillars and the insulating transformer tank, as well as the fire protection walls presently foreseen in front of the insulating transformer bushing, have been set at ground potential. In Fig. 4 the positions where the calculated electrical field is higher than 1 kV/mm are highlighted. On the HVD1 surface the electric field is lower than the given threshold, confirming therefore a proper design of the electrical screen. Hot spots near the post insulators derive from some model and mesh simplifications. Other hot spots are present inside the HVBA, on SF6 volume where a greater electric field is admissible, while around the HVBA (in air) the electric field decreases below 1 kV/mm threshold in few centimetres. The critical electrostatic condition pointed out on the top edge of the fire protection wall needs to be addressed by providing it with a suitable electrostatic screen. Moreover, the case of a surface irregularity on building construction has been simulated by modelling a 10 mm protrusion (cone) on a pillar nearby HVD1: also in this case the electrical field increases to dangerous level (∼5 kV/mm). In principle, no protrusions should be present on the building walls but the areas facing the HVD1 will be inspected with particular care to remove or screen any irregularity before onsite acceptance tests.
Please cite this article in press as: M. Boldrin, et al., Final design of the High Voltage Deck 1 and Bushing for MITICA: The ITER Heating Neutral Beam Injector prototype, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.133
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4 Table 2 ISEPS conductors characteristics. Conductor type
Quantity
1 MHz RF coaxial lines 3 1/8 inches Busbars HV cable Signal cables Other cables Fibre optics
4 (200 kW each) 2 1 1 5 6 12
Ratings Voltage [kV]
Current [A]
<0.1 <0.1 12 <0.3 <0.1 –
∼5000∼1000 140 ≤2 ≤140 –
static loads applied finding the main stresses and displacements (by ANSYS code). The safety factors for each material (mainly steel, FRP, aluminium) of the HVD1 and HVBA for the DLS and ULS spectra, calculated with respect to the yield and ultimate strengths respectively, resulted lower than admissible limits given in the relevant standards: IEC 1463 Bushings Seismic (1996-07), NTC 2008 (Italian Norm), Eurocode (EC0, EC3, EC8). The eigenfrequencies with the higher mass participations are below 20 Hz for the HVBA and 5 Hz for the HVD1. The maximum DLS and ULS displacements of the conductors at the top of the HVBA are 12 mm and 27 mm respectively while at the interface with the TL are 18 mm and 42 mm. The maximum lateral displacement in the upper part of the HVD1 has been estimated as 63 mm in DLS case; it increases to 201 mm in ULS case.
8. Next procurement steps
Fig. 4. Electric field map (values in V/mm).
6. HV Bushing Assembly thermal analysis A 3D thermal analysis (by Comsol Multiphysics 5.2) was carried out to identify the maximum temperature reached in the HVBA conductors (not actively cooled), after three daily operation cycles, each of them consisting in 1 h ON phase followed by 3 h OFF. The nominal Joule losses of the main power conductors and of the RF lines were considered as heat sources. As boundary conditions, free ambient air convection around the HVBA was considered; for the inner conductors only thermal exchanges by conduction were allowed whilst radiation and SF6 convection were precautionary neglected. The ambient temperature was set at 40 ◦ C. Simulation results showed that the hottest inner conductor heats up at 85 ◦ C, which complies with the allowed maximum temperature of 130 ◦ C reported in the relevant standard IEC 60137. Moreover, in the following 12 h the HVBA was able to cool down to initial conditions. 7. Seismic analysis The HVD1 and the HVBA must comply with seismic requirements of PRIMA and ITER sites. Two spectra were provided for each site: for PRIMA, the former is the Damage Limit State (DLS), i.e. after such earthquake the facility should be able to restart immediately the operation; the latter is the Ultimate Limit State (ULS), i.e. the facility can suffer damages but it does not collapse. Seismic analyses have been carried out independently for the HVD1 and the HVBA, because they are seismically decoupled from each other and from the other equipment inside the HVH. The main eigenfrequencies and mass participations were calculated and the spectra and pre-
The detailed design phase of HVD1 and HVBA was concluded in May 2016. The manufacturing design is being finalized, with the objective to conclude the manufacturing and factory testing within 2016. Where available, certificates and standard test will be supplied for the single components but special tests are also foreseen for the assemblies. A HVD1 mock-up with only one floor and a reduced size of 4 m × 4 m × 4 m, supported by 4 full size post insulators, was built to perform factory tests based on the ratings reported in Table 1; for the HVBA the same tests are planned in December, together with additional insulation tests on the HVBA inner conductors. Seismic tests are under definition, accounting for the fact that no facilities provided with shaking (vibrational) table of suitable dimensions are available for these big size components. To perform such tests, HVBA and HVD1 once installed will be duly instrumented with accelerometers and then stimulated to derive the main eigenfrequencies in order to validate the models used for the seismic design of the structures. The modalities of execution of these tests as well as the acceptance criteria are under discussion. For pressurized components, like the HVBA insulator and HVD1 post insulators, dedicated material and consequent hydraulic pressure tests are required for the homologation according to Italian National Body (INAIL) instructions. The pressure tests of the postinsulators were successfully held in June in the presence of INAIL, while the pressure tests of the HVBA are scheduled for September. The installation of the HVD1 and of the HVBA for MITICA is foreseen to start in the first quarter of 2017.
9. Conclusions The HVD1 and HVBA design and relevant analyses have been finalized proving the feasibility and compliance with technical specifications. The manufacturing phase is in progress and factory tests are ongoing, in view of installation at PRIMA site foreseen beginning 2017.
Please cite this article in press as: M. Boldrin, et al., Final design of the High Voltage Deck 1 and Bushing for MITICA: The ITER Heating Neutral Beam Injector prototype, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.133
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Acknowledgement The work leading to this publication has been funded partially by Fusion for Energy under the contract F4E-OPE-083. This publication reflects the views only of the author, and F4E cannot be held responsible for any use which may be made of the information contained therein. The views and opinions expressed herein do not necessarily reflect those of the ITER Organization. References
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[2] V. Toigo, et al., Progress in the realization of the PRIMA neutral beam test facility, Nucl. Fusion 55 (2015). [3] M. Bigi, et al., Design manufacture and factory testing of the Ion Source and Extraction Power Supplies for the SPIDER experiment, Fusion Engineering and Design 96-97 (2015) 405–410. [4] E. Gaio, et al., The alternative design concept for the ion source power supply of the ITER neutral beam injector, Fusion Engineering and Design 83 (2008) 21–29. [5] M. Boldrin, et al., Design issues of the High Voltage platform and feedthrough for the ITER NBI ion source, Fusion Engineering and Design 84 (2009) 470–474. [6] M. Boldrin, et al., Design status High Voltage Deck 1 and Bushing for the ITER Neutral Beam Injector and procurement activities of the, Fusion Engineering and Design 88 (2013) 985–989.
[1] ITER technical basis, IAEA, Vienna 2002, ITER EDA, Doc. Series N. 24, Plant Description Document, Sec. 2.5.1.
Please cite this article in press as: M. Boldrin, et al., Final design of the High Voltage Deck 1 and Bushing for MITICA: The ITER Heating Neutral Beam Injector prototype, Fusion Eng. Des. (2017), http://dx.doi.org/10.1016/j.fusengdes.2017.03.133