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Design issues of the High Voltage platform and feedthrough for the ITER NBI Ion Source
Fusion Engineering and Design 84 (2009) 470–474
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Design issues of the High Voltage platform and feedthrough for the ITER NBI Ion Source M. Boldrin ∗ , M. Dalla Palma, F. Milani Consorzio RFX, Associazione EURATOM-ENEA sulla Fusione, Corso Stati Uniti 4, I-35127 Padova, Italy
a r t i c l e
i n f o
Article history: Available online 31 December 2008 Keywords: Neutral Beam Injector (NBI) High Voltage Deck (HVD) High voltage bushing Insulation design Structural design Seismic analysis
a b s t r a c t In the ITER heating Neutral Beam Injector (NBI), a High Voltage air-insulated platform (named High Voltage Deck, HVD) will be installed to host the Ion Source and Extractor Power supply system and associated diagnostics referred to −1 MV DC potential. All power and control cables are routed from the HVD via a feedthrough (HV bushing) to the gas insulated transmission line which feeds the Injector. The paper focuses on insulation and mechanical issues for both HVD and HV bushing which are very special components, far from the present industrial standards as far as voltage (−1 MV DC) and dimensions are concerned. For this purpose, a preliminary design of the HVD has been carried out as concerns the mechanical structure and external shield. Then, the structure has been verified with a seismic analysis applying the seismic load excitation specified for the ITER construction site (Cadarache) and carrying out verifications according to relevant international standards. As regards the HV bushing design, proposals for the complex inner conductor structure and for interfaces to the HVD and transmission line are outlined; alternative installation layouts (aside or underneath the HVD) are compared from both mechanical and electrical points of view. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The Neutral Beam Injectors (NBI) for the International Thermonuclear Experimental Reactor (ITER) have to supply 16.7 MW each of additional heating power to ignite the plasma, accelerating negative ions up to 1 MeV with a beam current up to 40 A [1]. Negative ions are generated in the radio frequency ion source referenced at −1 MV DC potential, then they are extracted and finally accelerated up to the ground potential of the last accelerating grid. The Ion Source and Extractor Power Supplies (ISEPS), whose output voltages range from tens of volts to some kilovolts, are referred to −1 MV DC potential and will be hosted inside a metallic air insulated platform, named High Voltage Deck (HVD, see Fig. 1), that is shielded and therefore acts as a Faraday cage. As far as the HVD insulation design is concerned, the shape of its external shell and of the inner building wall metallic shield have been optimized by means of electric field analysis, in order to avoid the occurrence of corona/breakdown events. On the basis of weight estimation of the components hosted inside the HVD, the feasibility study for the structure is discussed carrying out seismic analyses of the preliminary design. The
∗ Corresponding author. Tel.: +39 49 829 5681; fax: +39 49 829 0718. E-mail address: [email protected] (M. Boldrin). 0920-3796/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2008.11.013
obtained seismic displacements and forces are verified according to Eurocode 8 [2]. The high voltage bushing, which connects the HVD with the −1 MV DC central conductor of the gas insulated transmission line which supplies the NBI, has to carry all the ISEPS power and measurement-control conductors. A preliminary design of the complex inner conductor layout of this bushing and a solution for the interfaces to the HVD and to the transmission line is proposed. Concerning the installation, two solutions have been studied and compared under both electrical and mechanical aspects: the first (the reference one) envisages the bushing installed aside the HVD, whilst the second underneath. 2. HVD description and insulation design 2.1. General description The HVD will contain all devices forming the ISEPS (such as transformers, power and control cubicles), the related diagnostics and also all the auxiliary equipments (lighting, low voltage AC distribution, compressed air plant, ventilation system and water cooling circuits). All devices will be disposed on two floors. Fig. 2 presents sketched views of the HVD with indication of maximum allowed dimensions. The HVD will be installed indoor. During NBI operation, the equipment installed inside the HVD, polarized at −1 MV DC with respect to the ground, is fed by an
M. Boldrin et al. / Fusion Engineering and Design 84 (2009) 470–474
471
Fig. 1. Simplified scheme of NBI electric connections.
insulation transformer which provides main AC power supply via a three-phase cable (plus neutral). In order to comply with the high insulation level, the overall structure (including main and auxiliary equipment, steel frame and mechanical completions, outer shield) has to be sustained by means of insulating supports with a net insulating height of 6 m, in order to withstand steady state and transient voltage associated to grids breakdown events. A proper insulators creepage distance will be chosen according to installation condition. An air earthing switch, rated for −1 MV voltage, shall be installed in order to permit personnel accessibility in safe conditions, for installation or maintenance operations. Moreover, the construction has to satisfy ITER seismic requirements as described in paragraph 3. 2.2. Electrical design issues The external surface of the HVD structure will be covered by a metallic shell in order to screen the internal volume like in a Faraday cage. The shell will be composed of aluminium panels, which have to be removable in order to allow easy access to both floors in case of equipment installation or maintenance. To assure a good screening action against Electro Magnetic Interferences originated by accelerator grids breakdowns, care must be taken to guarantee electrical continuity between panels by using soft metal seals at all joints. In order to avoid discharges from the HVD towards the surroundings (i.e. the ceiling and the building walls), a net clearance of 5 m around the HVD shall be guaranteed; in addition, all surfaces facing the HVD shall be screened by means of a
Fig. 3. Electric field magnitude [V/m] on the HVD surface.
metallic mesh, in order to have a precise reference for the ground potential. The external shape of the shield shall be rounded; as a general rule, the electric field on the screen surface has not to be greater than 1 kV/mm (∼ =1/3 of dry air insulating strength) when system test voltage (fixed at 20% more than rated voltage) is applied. Three-dimensional finite element analyses allowed to derive the minimum radius of curvature of the shell (Rmin ) able to fulfil the aforementioned criterion. The result is represented in Fig. 3: the HVD is modelled as a round-cornered metallic box, whereas the ground electrode consists in a surrounding box, 5 m distant from the HVD, representing the building walls. For symmetry reason, only one eighth of geometry has been studied. Several simulations were performed leading to Rmin = 0.75 m. As mentioned above, the building inner walls will be covered by a metallic mesh, aimed to screen the unavoidable protrusions (lamps, windows, etc.); in order to optimize the surrounding mesh, 3D analyses were carried out calculating the deformation of the electric field in proximity of the grounded mesh. For this purpose, the electric field magnitude was plotted along the direction perpendicular to the mesh, taking as limit on the wire surface an electric field value not greater than 1 kV/mm. A satisfactory solution
Fig. 2. HVD lateral views.
472
M. Boldrin et al. / Fusion Engineering and Design 84 (2009) 470–474
Fig. 4. Horizontal displacements [m] of the structure (magnification factor 100). On the left: short side model, mode 1, f = 2.3 Hz; on the right: long side model, mode 1, f = 2.6 Hz.
consists in a net with 10 cm square mesh made by 5 mm diameter wires. As for the floor of the building, in this case it seems more reasonable to employ a compact surface, instead of a mesh. The final choice will derive from further technical and economical evaluations.
3. HVD seismic analysis The seismic analysis of the HVD was carried out to evaluate the feasibility of the insulating structure. A preliminary design of the steel structure has been carried out considering the dead weight of
Fig. 5. High voltage bushing and conductors flange.
M. Boldrin et al. / Fusion Engineering and Design 84 (2009) 470–474
the power supply and diagnostic equipment on each floor. Then, a solution to support the platform guarantying electrical insulation has been proposed and the seismic action effects on the overall structure have been evaluated. The conceptual design of the HVD building has been carried out considering HEB 120 beams for the steel frame structure and 90 mm side, 4 mm thick, square tubes as bracing elements. To support the overall structure, the following elements have been selected: Fibre Reinforced Plastic (FRP) tubes having 250 mm external diameter and 240 mm internal diameter for the columns and two in-series 345-kV AC commercial suspension insulators for each bracing element. The initial preload strain of the insulating bracing elements was set to 0.001 as a result of an iterative optimization process to minimize the building lateral displacements without excessive axial loads in the guy-wires and FRP supporting columns. The structure was assumed to consist of a number of vertical and lateral load resisting systems, connected by horizontal diaphragms. The floor diaphragms of the building were conceived with a very large stiffness in their planes and the masses of each floor were lumped at the floor nodes. The building was also considered satisfying the criteria for regularity in plan [2], then the analyses were performed using two plan models (“short” = 8 m side, “long” = 12 m side) for both the main directions (see Fig. 2). 3.1. Finite element seismic analysis The general seismic requirement for non-nuclear plant (SL-0 spectrum) is not yet officially defined. However, the horizontal (vertical) acceleration SL-0 is assumed corresponding to 1/2 (1/3) of the acceleration of safety required earthquake (SL-2) at the ITER construction site (Cadarache) [3]. The method used for determining the seismic effects is the modal response spectrum analysis, using a linear-elastic model of the structure and applying the design spectrum SL-0. The inertial effects of the design seismic action have been evaluated by taking into account the persistent presence of the masses associated with all gravity loads: facilities on the first floor 22 ton, on the second floor 20 ton, 2 mm thick aluminium shell 3 ton, building structure 14 ton and 6 ton of distributed masses for auxiliary systems, services and building completions. The stress state due to static loads applied to the structure was evaluated with a prestress analysis. Then, a modal analysis was carried out applying the prestresses as initial condition. In correspondence of the fully constrained nodes at the ground level, the base excitation spectrum was applied and the response of the structure to the seismic action (simultaneous application of both horizontal and vertical acceleration spectra) was calculated; the seismic action effects (displacements or forces) corresponding to all modes were then combined together obtaining the seismic response for each plan model of the structure. The resultant seismic action effects were considered separately and without combinations for each plan model as the building satisfies the plan regularity criteria. As a result of the spectral analysis, the maximum displacements (12 mm) were found at the upper nodes of the structure, as shown in Fig. 4.
calculated at the cross section of the supporting FRP columns, were evaluated referring to the material compressive strength and they resulted large (SF ≥7) for the seismic verification. The total axial forces on the bracing insulators ensure the persistence of tensile state without slackening; the minimum SF was 2.3, calculated with respect to the routine test load. The worst loading scenario in the steel bracing bars yielded a SF of 13. The most loaded bracing elements have been verified against buckling, leading to a SF on the axial load of 3.2. In the steel frame structure, the minimum SF resulted 16. The most loaded frame element has been verified against buckling with a SF on the axial load of 5.5. The second-order effects were not taken into account because the calculated interstorey drift sensitivity coefficient  satisfies the ULS condition at all storeys [2]. The damage limitation requirement for DLS is also satisfied and the fatigue resistance does not need to be verified under the seismic design situation [2]. 4. HV bushing preliminary design and installation solutions All ISEPS power and control cables will be routed from the HVD to the Ion Source and Extractor via a feedthrough (HV bushing) which connects the HVD to the −1 MV potential central conductor of the gas insulated transmission line. Both HVD and HV bushing will be installed indoor, in the so called High Voltage Hall under controlled (humidity and dust) air conditions. In the reference layout, the HV bushing is located aside the HVD (∼6 m distance). Therefore, it is necessary: • to shield the bushing tip to control the local electric field (for example using a spherical screen); • to connect the bushing tip to the HVD by means of a metallic link containing all ISEPS conductors; in order to reduce the electric field, a series of shields can be used (metallic rings) distributed along the link (see Fig. 5). This link will be sustained by means of insulator chains attached to the building ceiling. An alternative layout for the HV hall can be obtained inserting the bushing directly below the HVD. In this way it is possible to take advantage of the platform shielding function, which provides a uniform electric field in the footprint area, and therefore it is no longer necessary to shield the bushing tip and link it with the HVD by means of the heavy and cumbersome link. In order to host the bushing under that platform, its arcing distance length has to be coordinated with that of HVD supporting insulators. Table 1 compares the two layouts under different mechanical and electric Table 1 HVD layouts comparison. Reference layout
New layout
Area & building size
Large area; large building
HVD design
Independent from bushings Each component shall be designed to hold 1 MV “alone” Heavy and cumbersome HVD-bushing link Allows testing of the system without HVD and Insulation Transformer (needs disconnection of the heavy link) High impact on the bushing design (presence of heavy link)
Reduced area; smaller building Needs integrated design
Insulation issue
3.2. Seismic verifications Electrical connections
Ultimate Limit State (ULS) and Damage Limitation State (DLS) verifications were carried out for both short and long side models in accordance with Eurocode 8 [2]; minimum values of safety factors and worst loading conditions are herein reported. The resistance conditions in ULS for supporting and bracing elements were verified taking into account the maximum actions on each element resulting from the superposition of prestress analysis values with the spectral analysis values. The safety factors (SF),
473
Commissioning
Seismic
Coordination of the insulation design Simplified HVD–bushing connection Allows testing of the system only without Insulation Transformer
Simpler design (direct link to HVD)
474
M. Boldrin et al. / Fusion Engineering and Design 84 (2009) 470–474
aspects. It can be remarked that the proposed solution presents some characteristics that lead to a more simplified and flexible realization. From the electrostatic point of view, the electric field distribution with the bushing underneath the HVD is almost similar to the original solution. As assessed by means of electrostatic analyses, the insulator surface is subjected to a tangential electric field much smaller than 1 kV/mm (with a reasonable safety margin with respect to the dry air dielectric strength); however, in both configurations the electric field has to be controlled by installing electrostatic guard rings. 4.1. The central conductor The bushing central conductor, polarized at −1 MV DC potential, contains all power conductors and control cables; for this purpose, an inner conductor of at least 400 mm diameter is required. The transition section between the bushing and the gas insulated transmission line consists in a metallic flange embedded in an epoxy spacer, as sketched in Fig. 5. This assembly realizes a tightness barrier between bushing and gas insulated line environments. All conductors and cables pass through this barrier by means of dedicated feedthroughs mounted on the flange and rated for different insulation voltage and current levels. At the HVD side, the bushing will terminate with a metallic flange having the same structure described above. 4.2. Insulation issues From the insulation point of view, typically in high voltage bushings the electric potential is capacity-graded. Oil impregnated paper wounded around the central conductor is the predominant technology, nevertheless Epoxy-Resin impregnated paper and SF6 gas are also available options. The external insulator could be made of porcelain or could be a FRP tube eventually covered wyth silicone rubber sheds. The latter, different to porcelain, has the advantage to realize an “explosion safe” bushing. In consideration of the large wounded dimension required, much larger than the present industrial standard, problems may be encountered for the component realization. If necessary, after discussion with industries, the pos-
sibility of splitting the ISEPS conductors in more parallel bushings with reduced diameters should be considered. 5. Conclusions A conceptual design of the High Voltage Deck (HVD) for the ITER NBI Ion Source and Extractor Power Supply (ISEPS) has been proposed based on mechanical and electrical requirements. Preliminary seismic analyses allowed to identify a feasible and reliable design for the support structures. The seismic verifications of the HVD are satisfied according to Eurocode 8 [2] for two plan models of the structure. Further dedicated analyses have to be developed in order to take into account more detailed informations on mass distributions and the mechanical connection with the gas insulated transmission line. As regards the high voltage bushing, which carries all the ISEPS conductors and associated diagnostics from the HVD to the Ion Source via the gas insulated line, a solution for the conductor interfaces with the gas insulated line central conductor and some technological issues have been outlined. Concerning the installation, the solution with the bushing underneath is under development. Acknowledgments This work, supported by the European Communities under the contract of Association between EURATOM/ENEA, was carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European Commission. References [1] ITER Design Requirements and Guidelines (DRG2), Neutral Beam Heating & Current Drive System and Diagnostic Neutral Beam, G A0 GDRD 3 01-07-19 R 1.0. [2] Eurocode 8: Design of structures for earthquake resistance—Part 1: General rules, seismic actions and rules for buildings, EN 1998-1:2004. [3] G. Sannazzaro, G. Mazzone, L. Patisson, Seismic requirement for Tokamak Cooling Water System, Project Office-System Analysis Section Civil Construction and Site Support, Cadarache, 16 January 2008.Figure captions.
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Design issues of the High Voltage platform and feedthrough for the ITER NBI Ion Source M. Boldrin ∗ , M. Dalla Palma, F. Milani Consorzio RFX, Associazione EURATOM-ENEA sulla Fusione, Corso Stati Uniti 4, I-35127 Padova, Italy
a r t i c l e
i n f o
Article history: Available online 31 December 2008 Keywords: Neutral Beam Injector (NBI) High Voltage Deck (HVD) High voltage bushing Insulation design Structural design Seismic analysis
a b s t r a c t In the ITER heating Neutral Beam Injector (NBI), a High Voltage air-insulated platform (named High Voltage Deck, HVD) will be installed to host the Ion Source and Extractor Power supply system and associated diagnostics referred to −1 MV DC potential. All power and control cables are routed from the HVD via a feedthrough (HV bushing) to the gas insulated transmission line which feeds the Injector. The paper focuses on insulation and mechanical issues for both HVD and HV bushing which are very special components, far from the present industrial standards as far as voltage (−1 MV DC) and dimensions are concerned. For this purpose, a preliminary design of the HVD has been carried out as concerns the mechanical structure and external shield. Then, the structure has been verified with a seismic analysis applying the seismic load excitation specified for the ITER construction site (Cadarache) and carrying out verifications according to relevant international standards. As regards the HV bushing design, proposals for the complex inner conductor structure and for interfaces to the HVD and transmission line are outlined; alternative installation layouts (aside or underneath the HVD) are compared from both mechanical and electrical points of view. © 2008 Elsevier B.V. All rights reserved.
1. Introduction The Neutral Beam Injectors (NBI) for the International Thermonuclear Experimental Reactor (ITER) have to supply 16.7 MW each of additional heating power to ignite the plasma, accelerating negative ions up to 1 MeV with a beam current up to 40 A [1]. Negative ions are generated in the radio frequency ion source referenced at −1 MV DC potential, then they are extracted and finally accelerated up to the ground potential of the last accelerating grid. The Ion Source and Extractor Power Supplies (ISEPS), whose output voltages range from tens of volts to some kilovolts, are referred to −1 MV DC potential and will be hosted inside a metallic air insulated platform, named High Voltage Deck (HVD, see Fig. 1), that is shielded and therefore acts as a Faraday cage. As far as the HVD insulation design is concerned, the shape of its external shell and of the inner building wall metallic shield have been optimized by means of electric field analysis, in order to avoid the occurrence of corona/breakdown events. On the basis of weight estimation of the components hosted inside the HVD, the feasibility study for the structure is discussed carrying out seismic analyses of the preliminary design. The
∗ Corresponding author. Tel.: +39 49 829 5681; fax: +39 49 829 0718. E-mail address: [email protected] (M. Boldrin). 0920-3796/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2008.11.013
obtained seismic displacements and forces are verified according to Eurocode 8 [2]. The high voltage bushing, which connects the HVD with the −1 MV DC central conductor of the gas insulated transmission line which supplies the NBI, has to carry all the ISEPS power and measurement-control conductors. A preliminary design of the complex inner conductor layout of this bushing and a solution for the interfaces to the HVD and to the transmission line is proposed. Concerning the installation, two solutions have been studied and compared under both electrical and mechanical aspects: the first (the reference one) envisages the bushing installed aside the HVD, whilst the second underneath. 2. HVD description and insulation design 2.1. General description The HVD will contain all devices forming the ISEPS (such as transformers, power and control cubicles), the related diagnostics and also all the auxiliary equipments (lighting, low voltage AC distribution, compressed air plant, ventilation system and water cooling circuits). All devices will be disposed on two floors. Fig. 2 presents sketched views of the HVD with indication of maximum allowed dimensions. The HVD will be installed indoor. During NBI operation, the equipment installed inside the HVD, polarized at −1 MV DC with respect to the ground, is fed by an
M. Boldrin et al. / Fusion Engineering and Design 84 (2009) 470–474
471
Fig. 1. Simplified scheme of NBI electric connections.
insulation transformer which provides main AC power supply via a three-phase cable (plus neutral). In order to comply with the high insulation level, the overall structure (including main and auxiliary equipment, steel frame and mechanical completions, outer shield) has to be sustained by means of insulating supports with a net insulating height of 6 m, in order to withstand steady state and transient voltage associated to grids breakdown events. A proper insulators creepage distance will be chosen according to installation condition. An air earthing switch, rated for −1 MV voltage, shall be installed in order to permit personnel accessibility in safe conditions, for installation or maintenance operations. Moreover, the construction has to satisfy ITER seismic requirements as described in paragraph 3. 2.2. Electrical design issues The external surface of the HVD structure will be covered by a metallic shell in order to screen the internal volume like in a Faraday cage. The shell will be composed of aluminium panels, which have to be removable in order to allow easy access to both floors in case of equipment installation or maintenance. To assure a good screening action against Electro Magnetic Interferences originated by accelerator grids breakdowns, care must be taken to guarantee electrical continuity between panels by using soft metal seals at all joints. In order to avoid discharges from the HVD towards the surroundings (i.e. the ceiling and the building walls), a net clearance of 5 m around the HVD shall be guaranteed; in addition, all surfaces facing the HVD shall be screened by means of a
Fig. 3. Electric field magnitude [V/m] on the HVD surface.
metallic mesh, in order to have a precise reference for the ground potential. The external shape of the shield shall be rounded; as a general rule, the electric field on the screen surface has not to be greater than 1 kV/mm (∼ =1/3 of dry air insulating strength) when system test voltage (fixed at 20% more than rated voltage) is applied. Three-dimensional finite element analyses allowed to derive the minimum radius of curvature of the shell (Rmin ) able to fulfil the aforementioned criterion. The result is represented in Fig. 3: the HVD is modelled as a round-cornered metallic box, whereas the ground electrode consists in a surrounding box, 5 m distant from the HVD, representing the building walls. For symmetry reason, only one eighth of geometry has been studied. Several simulations were performed leading to Rmin = 0.75 m. As mentioned above, the building inner walls will be covered by a metallic mesh, aimed to screen the unavoidable protrusions (lamps, windows, etc.); in order to optimize the surrounding mesh, 3D analyses were carried out calculating the deformation of the electric field in proximity of the grounded mesh. For this purpose, the electric field magnitude was plotted along the direction perpendicular to the mesh, taking as limit on the wire surface an electric field value not greater than 1 kV/mm. A satisfactory solution
Fig. 2. HVD lateral views.
472
M. Boldrin et al. / Fusion Engineering and Design 84 (2009) 470–474
Fig. 4. Horizontal displacements [m] of the structure (magnification factor 100). On the left: short side model, mode 1, f = 2.3 Hz; on the right: long side model, mode 1, f = 2.6 Hz.
consists in a net with 10 cm square mesh made by 5 mm diameter wires. As for the floor of the building, in this case it seems more reasonable to employ a compact surface, instead of a mesh. The final choice will derive from further technical and economical evaluations.
3. HVD seismic analysis The seismic analysis of the HVD was carried out to evaluate the feasibility of the insulating structure. A preliminary design of the steel structure has been carried out considering the dead weight of
Fig. 5. High voltage bushing and conductors flange.
M. Boldrin et al. / Fusion Engineering and Design 84 (2009) 470–474
the power supply and diagnostic equipment on each floor. Then, a solution to support the platform guarantying electrical insulation has been proposed and the seismic action effects on the overall structure have been evaluated. The conceptual design of the HVD building has been carried out considering HEB 120 beams for the steel frame structure and 90 mm side, 4 mm thick, square tubes as bracing elements. To support the overall structure, the following elements have been selected: Fibre Reinforced Plastic (FRP) tubes having 250 mm external diameter and 240 mm internal diameter for the columns and two in-series 345-kV AC commercial suspension insulators for each bracing element. The initial preload strain of the insulating bracing elements was set to 0.001 as a result of an iterative optimization process to minimize the building lateral displacements without excessive axial loads in the guy-wires and FRP supporting columns. The structure was assumed to consist of a number of vertical and lateral load resisting systems, connected by horizontal diaphragms. The floor diaphragms of the building were conceived with a very large stiffness in their planes and the masses of each floor were lumped at the floor nodes. The building was also considered satisfying the criteria for regularity in plan [2], then the analyses were performed using two plan models (“short” = 8 m side, “long” = 12 m side) for both the main directions (see Fig. 2). 3.1. Finite element seismic analysis The general seismic requirement for non-nuclear plant (SL-0 spectrum) is not yet officially defined. However, the horizontal (vertical) acceleration SL-0 is assumed corresponding to 1/2 (1/3) of the acceleration of safety required earthquake (SL-2) at the ITER construction site (Cadarache) [3]. The method used for determining the seismic effects is the modal response spectrum analysis, using a linear-elastic model of the structure and applying the design spectrum SL-0. The inertial effects of the design seismic action have been evaluated by taking into account the persistent presence of the masses associated with all gravity loads: facilities on the first floor 22 ton, on the second floor 20 ton, 2 mm thick aluminium shell 3 ton, building structure 14 ton and 6 ton of distributed masses for auxiliary systems, services and building completions. The stress state due to static loads applied to the structure was evaluated with a prestress analysis. Then, a modal analysis was carried out applying the prestresses as initial condition. In correspondence of the fully constrained nodes at the ground level, the base excitation spectrum was applied and the response of the structure to the seismic action (simultaneous application of both horizontal and vertical acceleration spectra) was calculated; the seismic action effects (displacements or forces) corresponding to all modes were then combined together obtaining the seismic response for each plan model of the structure. The resultant seismic action effects were considered separately and without combinations for each plan model as the building satisfies the plan regularity criteria. As a result of the spectral analysis, the maximum displacements (12 mm) were found at the upper nodes of the structure, as shown in Fig. 4.
calculated at the cross section of the supporting FRP columns, were evaluated referring to the material compressive strength and they resulted large (SF ≥7) for the seismic verification. The total axial forces on the bracing insulators ensure the persistence of tensile state without slackening; the minimum SF was 2.3, calculated with respect to the routine test load. The worst loading scenario in the steel bracing bars yielded a SF of 13. The most loaded bracing elements have been verified against buckling, leading to a SF on the axial load of 3.2. In the steel frame structure, the minimum SF resulted 16. The most loaded frame element has been verified against buckling with a SF on the axial load of 5.5. The second-order effects were not taken into account because the calculated interstorey drift sensitivity coefficient  satisfies the ULS condition at all storeys [2]. The damage limitation requirement for DLS is also satisfied and the fatigue resistance does not need to be verified under the seismic design situation [2]. 4. HV bushing preliminary design and installation solutions All ISEPS power and control cables will be routed from the HVD to the Ion Source and Extractor via a feedthrough (HV bushing) which connects the HVD to the −1 MV potential central conductor of the gas insulated transmission line. Both HVD and HV bushing will be installed indoor, in the so called High Voltage Hall under controlled (humidity and dust) air conditions. In the reference layout, the HV bushing is located aside the HVD (∼6 m distance). Therefore, it is necessary: • to shield the bushing tip to control the local electric field (for example using a spherical screen); • to connect the bushing tip to the HVD by means of a metallic link containing all ISEPS conductors; in order to reduce the electric field, a series of shields can be used (metallic rings) distributed along the link (see Fig. 5). This link will be sustained by means of insulator chains attached to the building ceiling. An alternative layout for the HV hall can be obtained inserting the bushing directly below the HVD. In this way it is possible to take advantage of the platform shielding function, which provides a uniform electric field in the footprint area, and therefore it is no longer necessary to shield the bushing tip and link it with the HVD by means of the heavy and cumbersome link. In order to host the bushing under that platform, its arcing distance length has to be coordinated with that of HVD supporting insulators. Table 1 compares the two layouts under different mechanical and electric Table 1 HVD layouts comparison. Reference layout
New layout
Area & building size
Large area; large building
HVD design
Independent from bushings Each component shall be designed to hold 1 MV “alone” Heavy and cumbersome HVD-bushing link Allows testing of the system without HVD and Insulation Transformer (needs disconnection of the heavy link) High impact on the bushing design (presence of heavy link)
Reduced area; smaller building Needs integrated design
Insulation issue
3.2. Seismic verifications Electrical connections
Ultimate Limit State (ULS) and Damage Limitation State (DLS) verifications were carried out for both short and long side models in accordance with Eurocode 8 [2]; minimum values of safety factors and worst loading conditions are herein reported. The resistance conditions in ULS for supporting and bracing elements were verified taking into account the maximum actions on each element resulting from the superposition of prestress analysis values with the spectral analysis values. The safety factors (SF),
473
Commissioning
Seismic
Coordination of the insulation design Simplified HVD–bushing connection Allows testing of the system only without Insulation Transformer
Simpler design (direct link to HVD)
474
M. Boldrin et al. / Fusion Engineering and Design 84 (2009) 470–474
aspects. It can be remarked that the proposed solution presents some characteristics that lead to a more simplified and flexible realization. From the electrostatic point of view, the electric field distribution with the bushing underneath the HVD is almost similar to the original solution. As assessed by means of electrostatic analyses, the insulator surface is subjected to a tangential electric field much smaller than 1 kV/mm (with a reasonable safety margin with respect to the dry air dielectric strength); however, in both configurations the electric field has to be controlled by installing electrostatic guard rings. 4.1. The central conductor The bushing central conductor, polarized at −1 MV DC potential, contains all power conductors and control cables; for this purpose, an inner conductor of at least 400 mm diameter is required. The transition section between the bushing and the gas insulated transmission line consists in a metallic flange embedded in an epoxy spacer, as sketched in Fig. 5. This assembly realizes a tightness barrier between bushing and gas insulated line environments. All conductors and cables pass through this barrier by means of dedicated feedthroughs mounted on the flange and rated for different insulation voltage and current levels. At the HVD side, the bushing will terminate with a metallic flange having the same structure described above. 4.2. Insulation issues From the insulation point of view, typically in high voltage bushings the electric potential is capacity-graded. Oil impregnated paper wounded around the central conductor is the predominant technology, nevertheless Epoxy-Resin impregnated paper and SF6 gas are also available options. The external insulator could be made of porcelain or could be a FRP tube eventually covered wyth silicone rubber sheds. The latter, different to porcelain, has the advantage to realize an “explosion safe” bushing. In consideration of the large wounded dimension required, much larger than the present industrial standard, problems may be encountered for the component realization. If necessary, after discussion with industries, the pos-
sibility of splitting the ISEPS conductors in more parallel bushings with reduced diameters should be considered. 5. Conclusions A conceptual design of the High Voltage Deck (HVD) for the ITER NBI Ion Source and Extractor Power Supply (ISEPS) has been proposed based on mechanical and electrical requirements. Preliminary seismic analyses allowed to identify a feasible and reliable design for the support structures. The seismic verifications of the HVD are satisfied according to Eurocode 8 [2] for two plan models of the structure. Further dedicated analyses have to be developed in order to take into account more detailed informations on mass distributions and the mechanical connection with the gas insulated transmission line. As regards the high voltage bushing, which carries all the ISEPS conductors and associated diagnostics from the HVD to the Ion Source via the gas insulated line, a solution for the conductor interfaces with the gas insulated line central conductor and some technological issues have been outlined. Concerning the installation, the solution with the bushing underneath is under development. Acknowledgments This work, supported by the European Communities under the contract of Association between EURATOM/ENEA, was carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European Commission. References [1] ITER Design Requirements and Guidelines (DRG2), Neutral Beam Heating & Current Drive System and Diagnostic Neutral Beam, G A0 GDRD 3 01-07-19 R 1.0. [2] Eurocode 8: Design of structures for earthquake resistance—Part 1: General rules, seismic actions and rules for buildings, EN 1998-1:2004. [3] G. Sannazzaro, G. Mazzone, L. Patisson, Seismic requirement for Tokamak Cooling Water System, Project Office-System Analysis Section Civil Construction and Site Support, Cadarache, 16 January 2008.Figure captions.