FIP/2-5Ra

Development of DC ultra-high voltage insulation technology for ITER NBI H. Tobari1), M. Hanada1), K. Watanabe1), M. Kashiwagi1), A. Kojima1), M. Dairaku1), N. Seki1), H. Abe1), N. Umeda1), H. Yamanaka1), T. Maejima1), Y. Terunuma1), T. Kondo1), S. Tanaka2), M. Kadowaki2), K. Yamaguchi2), H. Decamps3), M. Kuriyama3), J. Graceffa3), L. Svensson3), R. Hemsworth3) and D. Boilson3)

1)Japan Atomic Energy Agency (JAEA), 801-1-Mukouyama, Naka, Ibaraki 311-0193, Japan 2) Hitachi, Ltd., 1-1-1 kokubu-cho, Hitachi, Ibaraki 316-8501, Japan 3) ITER Organization, Route de Vinon sur Verdon, 13115 Saint Paul-lez-Durance, France

E-mail contact of main author: tobari.hiroyuki@jaea.go.jp

Abstract. To realize a stable generation and transmission of DC 1 MV for the Neutral Beam (NB) injector on ITER, the key components in the power supply system, namely, an insulating transformer and a high voltage (HV) bushing have been developed. A DC 1 MV insulating transformer with a newly developed composite bushing has been successfully developed and fulfilled the ITER requirement of DC 1.2 MV for 3600 s insulation. To clarify vacuum insulation characteristics of the cylindrical screen electrodes for the ITER HV bushing, a two stage high-voltage bushing mockup with three screens was manufactured and tested. As a result, the database which estimate voltage holding of the large-area cylindrical electrodes equivalent to those in the HV bushing was obtained. And also, the two stage HV bushing mockup demonstrated the DC 480 kV insulation for 3600 s which satisfied the ITER requirement. Through these R&Ds, high voltage components to be procured by JAEA have has been designed and started to be manufactured for ITER NBTF on a baseline schedule.

1. Introduction

To realize ITER neutral beam (NB) system [1, 2] in which 2-5.5 times higher beam energy and 360 times longer pulse than those in the existing negative-ion-based NBIs on LHD and JT-60U are required. The insulation of DC ultra-high voltage are one of critical issue as well as such high-power D- beam acceleration for realization of the ITER NB. In order to verify the performance of the ITER NB system prior to ITER operation, the NB Test Facility (NBTF) to be identical to the ITER NB is now under construction in Padova, Italy [3].

Japan Atomic Energy Agency (JAEA) is in charge of the procurement of key components of the power supply (PS) system such as a DC 1 MV insulating transformer, five DC generators (0.2 MV ~ 1.0 MV), the transmission line and the HV bushing served as the termination of the PS in the NBTF. Those components are required to insulate DC 1 MV for 3600 s. Among the high voltage components to be procured by JAEA, the DC 1 MV insulating transformer is one of critical components in which 2 times higher voltage and 360 times longer pulse than an existing 500 kV negative-ion-based NBI system on JT-60U are required. Especially, 1 MV bushing for extracting high voltage conductor from the inside of the transformer at 1 MV potential to the air was not realized. With the existing porcelain condenser insulator, a φ=2 m, H=10 m big porcelain bushing is required for the ITER NB PS. However, no manufacturing facility exists to manufacture such a large bushing. In order to overcome this problem, JAEA devise a new concept of the bushing. The new composite bushing consist of a small condenser bushing installed inside the fiber reinforced plastic (FRP) tube where SF6 gas is filled for insulation. The 1 MV insulating transformer mockup with the composite bushing was manufactured and tested to verify the insulation capability.

The views and opinions expressed herein do not necessarily reflect those of the ITER Organization.

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A high-voltage (HV) bushing acting as a terminal, should be newly developed. In the NB system on JT-60, the power conductors and cooling water pipes with three-different potential are fed through plural bushings from the PS to ion sources that are insulted in the air. However, since an installation space of the HV bushing in the ITER is strictly restricted to minimize tritium boundary, the conductors and pipes with five-different potentials should be transmitted through one bushing. In addition, the HV bushing acts as a bulkhead between the gas-insulated transmission line with 0.6 MPa SF6 gas and the beam source in vacuum. Hence the HV bushing must withstanding a pressure difference of 0.6 MPa. These requirements lead to the difficulty in insulation of the HV bushing, in particular, the insulation in vacuum that is not sufficiently clarified yet in the field of vacuum discharge. To realize the HV bushing, the mockup bushing with two different high-voltages was manufactured and tested to design and manufacture the HV bushing.

Through the R&D of the key components to be procured by JAEA, design and manufacturing of the high voltage components of the PS system in the ITER NBTF have been started. In this paper, recent progress of R&Ds and the present status of the procurements are reported.

2. Development of a DC 1 MV insulating transformer

Inner insulation structure of transformer winding Compared with existing DC insulating transformer in JT-60, 2 times higher voltage and 360 times longer pulse is required in the ITER NB. Key issue is the DC 1 MV insulation between primary at ground potential and secondary winding at 1 MV in the transformer for long-pulse up to 3600 s. Insulation structure with insulation oil and oil-immersed paper layers is used in the 500 kV, 10 s DC transformer on JT-60U [4, 5]. In case of DC insulation, potential distribution depends on the resistivity of an insulation material. Since the paper has higher resistivity than the oil, higher electric field occurs at the oil-immersed paper layer than that of insulation oil. And also, potential distribution on the paper gradually increases with operation time. Electric field analysis taking into account long-pulse effect found that a time constant of

potential distribution variation is up to 3000 s in case of the same structure as the transformer on JT-60. Further, it was found that five times higher electric field occurs on the oil-immersed paper after attaining a steady state rather than in case of voltage application of 10 s, and hence ten times higher in the ITER NB. Here, as one of countermeasures for such a high electric field, a ten times larger transformer having an efficient insulation distance might be considered. But it is not acceptable from a view point of weight and an installation space in ITER and also manufacturing cost. Therefore, a new inner structure of insulating transformer specializing DC long

Fig.1 Electric field distribution in the transformer winding and output lead.

The views and opinions expressed herein do not necessarily reflect those of the ITER Organization.

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duration insulation is required for the ITER NB PS.

To establish inner structure by optimizing the layout of oil-immersed paper and the insulation oil layers, electric filed analysis was implemented taking into account the effect of DC long-pulse and temperature. Important regions for electric field design are between primary winding and secondary one and also output lead from secondary winding where electric field tends to concentrate as shown in Fig.1. To withstand DC 1 MV long-pulse insulation, electric field on the surface of the oil-immersed paper is required to be less than 16 kV/mm. By combining 30 % thicker oil-immersed paper and L-shaped layer structure, the inner insulation structure between windings and output lead was optimized. As a result of the optimization, maximum electric field is suppressed to 7.4 kV/mm at the corner of winding region, 4.1 kV/mm in the output lead. As shown in Fig.1, both are sufficiently lower than the above criteria.

Development of a composite bushing for extraction of 1 MV A DC 1 MV bushing extracting AC power from the secondary winding of the transformer to the air is a key component of the insulating transformer. A role of the bushing is to feed the AC power from the secondary winding of insulating transformer to the PS for negative ion production with the DC 1 MV insulation. In an original plan, a conventional condenser bushing was considered, which was already utilized in the DC 500 kV insulating transformer on JT-60. The condenser bushing consists of porcelain cylinder with inner insulation structure where oil-immersed thin papers and aluminum films are coaxially layered to suppress the concentration of electric field along the bushing. This configuration forms uniform equi-potential line on the surface of the porcelain cylinder as shown in the left side of Fig.2. Namely the condenser bushing with uniform electric field has a high insulation capability. A design work found that a φ=2 m, H=10 m big porcelain tube is required in case of the condenser bushing for ITER, however, no manufacturing facility exists to manufacture such a big condenser bushing.

To solve that issue, a new concept of the bushing was devised by replacing the porcelain cylinder with a fiber reinforced plastic (FRP) tube. Namely the new composite bushing consisting of the porcelain condenser bushing as an inner insulator and the FRP tube as outer one was devised as shown in the right side of Fig.2. The interspace is filled with SF6 gas at 0.4 MPa (abs.). The porcelain condenser bushing is installed in SF6 gas region, hence a surface insulation distance can be drastically shortened. Namely the small size of the condenser bushing can be utilized. The FRP tube is needed to have enough length to insulate 1 MV on outer surface in air, but its simple structure is useful for

Fig.2 Comparison with conventional and composite bushing for 1 MV extraction.

The views and opinions expressed herein do not necessarily reflect those of the ITER Organization.

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manufacturability of the bushing.

In addition, the small condenser bushing also forms equi-potential lines on the surface of the FRP tube, which gives a good insulation capability on the outer surface of FRP tube. Also, the small porcelain bushing and FRP tube have better seismic adequacy. Those components; the small condenser bushing and the FRP tube can be easily manufactured.

Through electric field analyses, configuration of porcelain condenser bushing, output lead and bushing barrier was optimized. As a result, the small designed was design as 0.7 m in a diameter and 3.7 m in a height, respectively. And

the FRP tube was 1 m in a diameter and 8 m in a height, respectively. The maximum electric field in (a) insulation oil and (b) SF6/air side are found as (a) 5.7 kV/mm and (b) 9.2 kV/mm as shown in Fig.3, respectively. Those are lower than the criteria of electric field of (a) 16 kV/mm, (b) 15.3 kV/mm, respectively. And that on the outer surface of the FRP tube is 1.7 kV/mm which is 56 % of the breakdown electric field in the air. Hence, a realistic design of the bushing for 1 MV transformer has been established. The composite bushing leads to a cost and a weight reduction, which are one third of the conventional porcelain bushing.

DC 1 MV test of insulating transformer mockup In order to verify a performance of the insulating transformer, a mockup was manufactured as shown in Fig.4. The assembled mockup was installed in the ultra-high voltage test facility in

Fig.3 Electric field distribution around new bushing

Fig.4 The insulating transformer mockup and a result of high voltage test

The views and opinions expressed herein do not necessarily reflect those of the ITER Organization.

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Hitachi, Ltd. Discharge sound inside the transformer tank was monitored with acoustic emission (AE) sensors. Applied voltage was measured with a CR voltage divider. Long-pulse insulation test between the primary and secondary winding of transformer and on the composite bushing was examined. Test conditions are 1.06 MV for 5 hours and 1.2 MV for 3600 s in accordance with ITER requirement. The high voltage of 1.06 MV was stably sustained for 5 hours without breakdown. During voltage holding, leak current (drain current of PS) was less than 1 mA. And also, DC high voltage test at 1.2 MV was examined. After reaching at 1.2 MV, it was sustained for 3600 s without breakdowns as shown in Fig.4. No AE noise was observed during the high voltage test. The results satisfy the requirement on the ITER NB system. The results also show critical insulation techniques, that is, the insulation structure between windings in the transformer and the new devised composite bushing for 1 MV extraction are established. Also, this is the first demonstration of 1 MV insulating transformer in the world.

Through the R&D, a critical DC ultra-high voltage technology for NB PS system has been successfully established, that ensures the realization of the ITER NB.

3. Vacuum insulation design in HV bushing

The HV bushing consists of double-layer insulator rings, the inner ring is a brazed ceramic ring with outer diameter of 1.56 m (the world’s largest ceramic ring) and the outer one is FRP ring with outer diameter of 1.76 m. The insulator rings are stacked in five-stage interleaving stainless flanges at intermediate potential as shown in Fig.5. The HV bushing acts as a terminal of the gas-insulated transmission line of the PS and also as a bulkhead between vacuum and insulation gas. For stable power transmission through the HV bushing, 1 MV vacuum insulation and a stiffness to withstand the maximum pressure difference of 0.6 MPa in a limited space are required. To realize the HV bushing, manufacturing technology of the world’s largest ceramic ring and brazing has been established in JAEA [6].

Conductors and cooling pipes at 0.2 MV ~ 1 MV penetrates at top and side of the HV bushing. Penetrating conductors are located inside the ceramic ring, where sufficient insulation distance to sustain 1 MV in total shall be kept. Inside of the HV bushing is separated by coaxial five thin cylindrical electrodes (electrostatic screen) in the present design. Conductors at each potential are electrostatically shielded by them. Then, required voltage to be sustained in vacuum is 0.2 MV in one gap. Voltage holding of a large coaxial electrode (e.g. φ490 mm, H=3.6 m) depends on the gap length and surface area of facing electrodes. The clump theory [7] gives a dependence on gap

Fig.5 Cross sectional view of HV bushing

The views and opinions expressed herein do not necessarily reflect those of the ITER Organization.

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length that is proportional to a square root of gap length. However, an effect of surface area on voltage holding in such a large coaxial electrode has not been clarified yet. Therefore, voltage holding dependence on the area has been examined in order to design the electrostatic screen in the HV bushing.

The database of voltage holding with planar electrode has been obtained

in the previous study [8], in which characteristics of V/g0.5 = (EV)0.5= constant has been confirmed. Here, V is the sustainable voltage, g is a length between faced electrodes and E is electric field on the cathode surface. And dependence of surface area has been also obtained for planar electrodes such as quasi-Rogowski (small) electrode, a meter-class large electrode for JT-60 N-NBI and vacuum-immersed accelerator tested for ITER (VIBS). However, the voltage holding capability in a gap between large coaxial screens is not clarified. Due to the curvature effect, electric field on the surface of the inner cylinder is larger than that of the planar electrodes even when the voltage and gap length are the same.

To clarify this point, two-stage mockup with three-layer cylindrical electrodes full-size insulator rings (ceramic ring and FRP ring), was manufactured as shown in Fig.6. Those electrodes in the mockup were 1/3 in height of the actual HV bushing. High voltage test was examined to investigate voltage holding capability in coaxial electrodes. Figure 7 shows voltage holding capability (VHC, =(EV)0.5) in planar electrodes and also in the coaxial electrode. For the mockup with cylindrical electrodes, the VHC was evaluated with the cathode surface area. The two kinds of VHC shows the same dependence on the surface area. This indicates the voltage holding of electrostatic screens on the HV bushing can be estimate

with this database (scaling). In this configuration, high voltage of 480 kV including 20 % margin of rated voltage of 400 kV was successfully sustained in vacuum without breakdown for 3600 s. That voltage holding capability satisfies the ITER requirement. Followed by results on R&Ds, design tool was obtained for a large coaxial cylindrical electrode and a detailed design of the electrostatic screen determining the gap length and surface area has started in order to obtain 1 MV insulation capability in vacuum.

Fig.6 Two-stage mockup bushing

Fig.7 Dependence of voltage holding capability on surface area of electrode

The views and opinions expressed herein do not necessarily reflect those of the ITER Organization.

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4. Present status of the procurement

JAEA signed the procurement arrangement on high voltage components of 1 MV PS and the HV bushing for ITER NBTF in 2012. As for the PS, the design of five DC generators, a DC filter, 1MV transmission lines, a 1MV insulating transformer and a 1.3 MV testing power supply was completed by September 2014. Among the procured components, manufacturing of the 1.3 MV testing power supply was completed in June, 2014. As for the HV bushing, manufacturing of ceramic ring and brazing work started from the end of 2012. The five-sets brazing of the large ceramic rings. Voltage holding and pressure tests of those rings to verify the performance of the products have been completed in JAEA by June 2014. Manufacturing of FRP rings and the manufacturing design of the high voltage conductors and the electrostatic screen of the HV bushing are now on-going. Manufacturing of the HV bushing is to be completed in the second half of 2015 and delivered to RFX in 2016. The procurement activities in JAEA have made progress as scheduled toward the test of the NBTF to be started in 2017.

5. Summary

Progress on R&Ds for DC ultra-high voltage technologies toward ITER NB in the past two years was reported. For the 1 MV insulating transformer, the inner insulation structure between primary and secondary winding was devised with thicker oil-immersed paper and L-shaped layer structure to sustain 1 MV for 3600 s. The composite bushing with small condenser bushing and FRP tube for 1 MV extraction was also newly developed. Followed by newly developed technique, the DC 1 MV insulating transformer mockup was manufactured and tested. As a result, high voltage insulation at 1.2 MV for 3600 s was successfully achieved, which satisfies the ITER requirement. The critical insulation techniques for realization of the 1 MV insulating transformer are established.

In order to clarify the voltage holding capability in a gap between large coaxial screens, two-stage mockup was manufactured and tested. It was found that the voltage holding capability in the coaxial electrode shows the same dependence on surface area as the previously obtained database with planar electrodes. This indicates the voltage holding of electrostatic screens on the HV bushing can be estimate with this database (scaling) to obtain 1 MV insulation capability in vacuum.

Based on those R&Ds, design and manufacturing of high-voltage components of -1 MV PS and the HV bushing for ITER NBTF has been in progress in JAEA as scheduled.

References [1] ITER Engineering Design Activities, Summary of the Final Design Report ITER EDA

Document Series No.22 (Vienna: IAEA), 2001. [2] R. Hemsworth et.al, “Status of the ITER heating neutral beam system”, Nucl. Fusion 49,

045006 (2009). [3] A. Masciello, Proc. 25th IAEA FEC, FIP-2-4Ra, presented in this conference. [4] T. Ohga、K. Usui, K. Omori, K. Watanabe, K. Ohshima, T. Itoh, M. Kawai, and M.

The views and opinions expressed herein do not necessarily reflect those of the ITER Organization.

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Kuriyama," High voltage power supply of negative ion based NBI for JT-60U", 17th IEEE/NPSS Symp. Fusion Engineering，San Diego, California, Oct.6-10, pp1091-1094, (1997).

[5] K. Omori, K. Usui, K. Ohshima, T. Ohga, M. Kawai, K. Watanabe, T. Itoh, M. Kuriyama, Y. Ono and S. Kawashima “ Improvement of the ion source power supply for JT-60 negative-ion based NBI”, Proc. 20th Symp. on Fusion Technology (Marseille, France, 7–11 September 1998) pp 395–398.

[6] H. Tobari et.al, “Manufacturing and Tests of Full-size Mockup Bushing for ITER NB System”, Proc. 23rd IAEA FEC, ITR/P-15(2010).

[7] L. Cranberg, “The Initiation of Electrical Breakdown in Vacuum”, J. Appl. Phys. 23, 518 (1952).

[8] A. Kojima et al., “Vacuum Insulation of the High Energy Negative Ion Source for Fusion Application”, Review of Scientific Instruments, 83, 02B117-1-02B117-5 (2012).

The views and opinions expressed herein do not necessarily reflect those of the ITER Organization.