Acceleration Grid Power Supply Research Articles

Development of Acceleration Grid Power Supply of NNBI System for CRAFT

Development of Acceleration Grid Power Supply of NNBI System for CRAFT

Model improvement and analysis of NBI power supply under breakdown condition

Breakdown commonly occurs in the acceleration grids of the NBI system. It will cause destructive effects on the accelerating grids, and the generated transient overvoltage and current will threaten the normal operation of the acceleration grid power supply (AGPS). Therefore, it is very necessary to model the circuit to analyze the effects of breakdown on the NBI system. However, the current models are not accurate enough and are likely to have insufficient knowledge of the consequences of breakdown. In this paper, a series of improvements are made to the model by analyzing in detail the change process of AGPS during breakdown and correcting the model parameters in conjunction with the existing AGPS design scheme. Subsequently, the key indicators of the NBI system under breakdown are listed. The impact of breakdown and the need for some protection are assessed based on the results of these indicators. The results of this paper are instructive for the design and breakdown protection of future NBI systems.

Direct current measurements of the SPIDER beam: a comparison to existing beam diagnostics

For negative ion beam sources there are several methods of measuring the accelerated beam current, most commonly electrical measurements at the power supply and calorimetric measurements. On SPIDER, the ITER Heating Neutral Beam full-scale beam source prototype, electrical measurements at the acceleration grid power supply (AGPS) are complemented by polarizing the diagnostic calorimeter STRIKE to provide an additional electrical measurement of the accelerated current. This is in addition to the calorimetric measurements provided by STRIKE. These diagnostics give differing measurements of the beam current. Exploiting the reduced number of open apertures on SPIDER a new beam diagnostic has been installed to measure the individual beamlet currents directly. The so called Beamlet Current Monitor (BCM) has been used to measure the current of five beamlets during the most recent SPIDER campaign. This work compares the BCM current to the electrical measurements at the AGPS and STRIKE. The average BCM current agrees well with the STRIKE electrical measurements, indicating that the AGPS overestimates the beam current. The individual beamlets are compared to the STRIKE calorimetric measurements, showing similar current trends with the source parameters.

DC active power filter based on model predictive control for DC bus overvoltage suppression of accelerator grid power supply

The China Fusion Engineering Test Reactor plans to build a 200 kV/25 A acceleration grid power supply (AGPS) for the negative-ion-based neutral beam injector prototype system. The AGPS uses a rectifier-inverter-isolated step-up structure. There is a DC bus between the rectifier and the inverter. In order to limit DC bus voltage ripple and transient fluctuations, a large number of capacitors are used, which degrades the reliability of the power supply and occupies a large amount of space. This work finds that due to the difference in the turn-off time of the rectifier and the inverter, the capacitance mainly depends on the rectifier current when the inverter is turned off. On this basis, an active power filter (APF) scheme is proposed to absorb the current. To enhance the dynamic response ability of the APF, model predictive control is adopted. In this paper, the circuit structure of the APF is introduced, the prediction model is deduced, the corresponding control strategy and signal detection method are proposed, and the simulation and experimental results show that APF can track the transient current of the DC bus and reduce the voltage fluctuation significantly.

Development of HVDC Gas-Insulated Components for the Power Supply of Neutral Beam Injectors

The development of High Voltage Direct Current Gas-Insulated components that will be employed for the power supply of Neutral Beam Injectors, requires specific numerical tools to analyze the electric field distribution, with the purpose of supporting the engineering and operational phases.The tools must comply with the nonlinearity of the material properties of solid insulators and the transport mechanisms in the dielectric gas. Despite its high Global Warming Potential, SF6 will be considered as the baseline dielectric gas for the development of HVDC gas-insulated components to be used for the Acceleration Grid Power Supply of Neutral Beam Injectors. Attention especially to the mechanisms driving to an increase of leakage current flowing in the gas, thus giving rise to possible discharge phenomena is here placed, mainly focusing on radiation and injection regimes. The described methodologies are adopted for the numerical modeling of the 500 kV DC gas-insulated components that will be used for the power supply of the Neutral Beam Injector of the Divertor Tokamak Test facility. In particular, a conceptual design of the SF6 Transmission Line and the SF6-air bushing is conceived.

A strategy to identify breakdown location in MITICA test facility: results of high voltage test campaign

The Acceleration Grid Power Supply of the MITICA test facility in Padova (Italy) is currently under commissioning. The power conversion system, the DC generator, and the High Voltage equipment have been individually commissioned, whereas the integration tests are ongoing. It is a challenging process due to the unconventional application, to the variety of different electrical technologies involved and to the complexity of the interfaces.During the integrated tests of the power supplies the achievement of 700 kV stable operation has been demonstrated for the first time in a Neutral Beam Injector, but an unexpected event occurred, most likely a breakdown in the HV part, which resulted in a fault of the DC generator. A subsequent test using an auxiliary power supply was performed to check the voltage withstanding capability of the HV plant, but another breakdown occurred at around 1MV.This paper describes the activity performed to identify the location of the breakdowns affecting the integrated tests. A test campaign has been devised with increased diagnostic capabilities and specific strategy conceived to trigger intentional breakdowns in specific locations and collect measurement patterns for different cases. The results of the campaign will be presented and the current understanding of the issue will be described, with a view on future tests and further improvements of diagnostics.

Studies on high voltage dc cable connection to supply the acceleration grids of the Neutral Beam Injector for DTT

The Divertor Tokamak Test facility (DTT) is the new tokamak under construction in Frascati (Italy), with the main aim of testing different divertor magnetic configurations and technologies in view of DEMO. The Heating and Current Drive function will be provided also by one Neutral Beam Injector (NBI), accelerating negative Deuterium ions with an energy of 510 keV, giving an output power of 10 MW maximum. The Acceleration Grid Power Supply (AGPS), with 3 stages rated 167 kV each, will feed the NBI accelerator. For optical reasons, the insulating gap among the acceleration grids is at the limit of the voltage holding in vacuum, therefore frequent arc breakdowns (BD) among the grids will occur, requiring the quick switch-off of the AGPS to limit the arc energy. For the AGPS, beside the ITER-like scheme including gas-insulated HV components, an innovative approach based on the Modular Multilevel Converter (MMC) technology is being considered. This would include air-insulated HV converters, requiring a huge building volume, which at present is not available in close proximity to the tokamak building. The alternative of installing the MMC converters in a new building to be erected in an available area located far from the tokamak, and connecting the AGPS to the load with HV coaxial cables, is under study. This paper presents the studies carried out to verify the impact of the cables length on the arc energy, in case of breakdown among the grids, and to estimate the magnitude of the voltage oscillations due to resonance effects. These oscillations can occur not only between each hot pole and the return conductor, endangering the cable insulation, but also between the return conductor and local ground, with potential consequences on the laying method, the human safety and the electromagnetic compatibility with sensitive apparatus nearby, which have all been evaluated.

Fault diagnosis algorithm for 3-phase passive rectifiers based on frequency-domain analysis for acceleration grid power supply in CFETR NBI system

The acceleration grid power supply (AGPS) is a crucial part of the Negative-ion Neutral Beam Injection system in the China Fusion Engineering Test Reactor, which includes a 3-phase passive (diode) rectifier. To diagnose and localize faults in the rectifier, this paper proposes a frequency-domain analysis-based fault diagnosis algorithm for the rectifier in AGPS. First, time-domain expressions and spectral characteristics of the output voltage of the TPTL-NPC inverter-based power supply are analyzed. Then, frequency-domain analysis-based fault diagnosis and frequency-domain analysis-based sub-fault diagnosis algorithms are proposed to diagnose open circuit (OC) faults of diode(s), which benefit from the analysis of harmonics magnitude and phase-angle of the output voltage. Only a fundamental period is needed to diagnose and localize exact faults, and a strong Variable-duration Fault Detection Method is proposed to identify acceptable ripple from OC faults. Detailed simulations and experimental results demonstrate the effectiveness, quickness, and robustness of the proposed algorithms, and the diagnosis algorithms proposed in this article provide a significant method for the fault diagnosis of other rectifiers and converters.

Studies on Alternative Schemes for the MMC-Based Acceleration Grid Power Supply of DEMO Neutral Beam Injector

The European DEMO will make use of a significant additional heating power, which could be partly provided by neutral beam injectors (NBIs), which accelerate negative ions by means of grids placed at increasing potentials. These grids will be fed by the acceleration grid power supply (AGPS), divided in a number of stages, which has to provide an overall dc voltage down to −1 MV and currents in the order of tens of Amperes. The AGPS will have to satisfy a set of static and dynamic requirements, mainly in terms of ripple, accuracy, and rise time. In addition, during the NBI operation, frequent losses of insulation between the grids, called breakdowns (BDs), are expected. The AGPS will be able to handle such events by nullifying the output currents as fast as possible in order to limit the energy discharged onto the grids. Adopting the modular multilevel converter (MMC) technology for the AGPS of DEMO and future tokamaks seems promising, due to its intrinsic properties of modularity, high efficiency, fast dynamic response and small energy transferred to the arc in case of BD. Since the converter will be air-insulated, one of the main drawbacks is the large volume occupied. This can be partly reduced by adopting alternative MMC schemes, to minimize the number of components and optimize the counter-voltage applied by the converter at BD. In this article, alternative topologies for the MMC submodules (SMs) or combinations of different schemes [full-bridge (FB), half-bridge (HB)] are investigated. After a preliminary design of the converter, the results of numerical simulations carried out with circuit models are shown, with control schemes customized for the NBI operation. The performance of the different solutions in steady-state, dynamic and anomalous conditions are discussed, with particular focus on BD events. Finally, thermal analyses on the converter are carried out, to verify whether natural convection of air can be a suitable cooling method for the power components.

The Full-Voltage Operation of the Acceleration Grid Power Supply for SPIDER Experiment

In SPIDER, the full-scale prototype of the negative ion source of the ITER Heating Neutral Beam Injectors, negative ions of hydrogen or deuterium are extracted and accelerated by a set of grids. This article describes the process and the technical solutions that allowed achieving the asseveration to operate the Acceleration Grid Power Supply of SPIDER at full voltage (−96 kV), reports the first experimental results, and discusses some possible enhancements that have been identified.

An improved DC component suppression control strategy applied to high voltage power supply for N-NBI accelerator grid

The acceleration grid power supply (AGPS) is a key component of the negative-ion neutral beam injector (NNBI) system. DC offset is one of the pivotal problems in the construction of the inverter for the AGPS. An optimized control scheme is proposed in this paper to minimize DC current flowed in the isolated step-up transformer. It is based on the idea of optimizing the measured value through a Kalman filter and making it converge to a limited value through a PI controller. Theoretical analysis and simulation results confirm the concept.

Dynamic Voltage-Balancing Method of Series Diode Rectifier in Inverter-Type AGPS

In negative ion-based neutral beam injector (N-NBI) system, acceleration grid power supply (AGPS) provides ultrahigh dc voltage for accelerator. Because the power supply often occurs in the ignition fault similar to the load short circuit, it will produce complex and violent electromagnetic transient process. The diode rectifier is the last stage of it and the diode rectifier is directly connected to the load of the AGPS. The working voltage of the rectifier reaches hundreds of kV, and a large number of diodes are used in the device in series. Therefore, it is necessary to fully consider the influence of distributed capacitance on voltage balancing and improve the static and dynamic voltage-balancing reliability of diodes. However, discussions about solutions for the dynamic voltage imbalance in the reverse recovery process caused by parasitic capacitance are not sufficient. This article presents a voltage-balancing method in the form of parameter relationships between non-isoparametric resistance-capacitance ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RC</i> ) snubber circuits. Under the assumption of uniform voltage-sharing, the equivalent parallel circuit of each diode is obtained through constructing isoelectric points, accordingly, the parameter relationships are deduced. In this article, both the distributed capacitance to ground and the distributed capacitance to high voltage are considered in the derivation process; the derived non-equal parameter <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RC</i> voltage-balancing method includes not only the buffer capacitance parameter relationship, but also the damping resistance parameter relationship and upper limit, which innovatively explores the important influence of damping resistance on the voltage-balancing effect. Dual-pulse tests on series-connected diodes prototype are carried out; experimental results show that voltage balance ratio <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha $ </tex-math></inline-formula> of the prototype is only 0.80 with the traditional isoparametric <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RC</i> circuit in parallel, while <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha $ </tex-math></inline-formula> is up to 0.99 with the non-isoparametric <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RC</i> circuit designed through the proposed method. The results will be valuable for improving the reliability and device utilization rate of series-connected diodes rectifiers in inverter-type AGPS.

Application studies of the modular multilevel converter topology to the acceleration grid power supply of the DEMO neutral beam injector

The present conceptual design of the Neutral Beam Injectors (NBIs) for DEMO foresees an acceleration voltage of 1 MV, subdivided in 5 steps of 200 kV each, as for the ITER NBI. This voltage is provided by the Acceleration Grid Power Supply (AGPS), which shall also guarantee the capability of handling repetitive grid breakdowns and provide the required high dynamic response, low ripple and high accuracy. These requirements addressed the ITER AGPS design towards a challenging solution based on multiple conversion stages, where the last rectification and filtering stage is gas-insulated. The research here presented aims at evaluating the possible utilization of Modular Multilevel Converters (MMCs) for the AGPS of future NBIs. A preliminary design of the DEMO AGPS has been carried out, where each 200-kV voltage step is generated by an air-insulated MMC stage operating as a rectifier. The main circuit parameters of the MMC have been determined, considering normal and anomalous working conditions. A numerical model has been developed to verify the converter performances in stationary and dynamic conditions and to simulate breakdown and beam-off events. The area occupied by the proposed MMC-based AGPS has been tentatively estimated and compared to that of the ITER solution. The results of the analysis are shown in this paper. Finally, strengths and criticalities of the MMC solution with respect to the ITER-like approach are discussed.

Design of the data acquisition system of acceleration grid power supply for CFETR N-NBI prototype

The Acceleration Grid Power Supply (AGPS) of the China Fusion Engineering Test Reactor (CFETR) negative-ion-based neutral beam injector (N-NBI) prototype is a high-voltage power supply rated at -200 kV/25 A. The output voltage measurement and state detection of AGPS are required to safely operate for the N-NBI system. Hence, a high-performance data acquisition system is essential. The performance requirements of the data acquisition system becomes remarkable when the output voltage increases and electromagnetic interference enhance. This paper proposes an effective data acquisition method based on field programmable gate array (FPGA) technology and PCI extensions for instrumentation (PXI) series hardware used in the existing control system. Compared with traditional voltage-to-frequency (VF) conversion methods, the frequency output by the proposed method can maintain a consistently high standard when a lower reference output voltage is required. The principle test and high voltage test on the pulse step modulation (PSM) power supply of the J-TEXT Tokamak indicate that the delay of the data acquisition system is no more than 20 μs, which fulfills the requirements.

Conceptual design of the power supplies for DTT Neutral Beam Injector

The Divertor Tokamak Test facility (DTT) is the new tokamak whose construction is starting in Frascati (Italy), with the main aim of testing different divertor magnetic configurations and technologies in view of DEMO. In the present design, the DTT heating mix includes one Negative Neutral Beam Injector (NBI), with a power injected in the plasma of 10 MW and a beam energy of 500 keV.The NBI conceptual design is under development and includes solutions already adopted for ITER (RF-driven ion source, gas neutralizer, electrostatic Residual Ion Dump), but with some simplifications to reduce costs and complexity deemed feasible for DTT. In particular, the beam source is air-insulated and fed by the gas-insulated Transmission Line through gas-air bushings.This paper presents the status of the conceptual design of the main power supplies of the DTT NBI. The main ratings are collected from physics requirements and NBI design. On this basis, the reference technical solutions are selected, considering the experience gained at the ITER Neutral Beam Test Facility in Padova (Italy), the specific design of the DTT NBI and the layout constraints of the DTT Site. For the Acceleration Grid Power Supply, besides the ITER-like design, an innovative approach based on the Modular Multilevel Converter technology, never adopted in this application, is considered; pros and cons of the two solutions are evaluated.

Design and test of a mitigation system against voltage ringing in MITICA Acceleration Grid Power Supply

The Acceleration Grid Power Supply of MITICA is a high power, high voltage power supply devoted to feed the acceleration grids of the full scale prototype of the ITER NBI injector. It is divided in two subsystems: the AGPS – Conversion System (CS) and the AGPS – DC Generator (DCG), procured by different domestic agencies. Numerical simulations have pointed out a voltage ringing at the interface of the two systems, which would generate repeated high overvoltage at the input of the AGPS-DCG, not in compliance with the interface requirements. The phenomenon has been investigated and a solution for the mitigation has been identified based on a suitable low pass RC filter, as a trade-off between simplicity and effectiveness. The filters have been successfully tested directly with the AGPS-CS to demonstrate the reduction of the voltage peaks in the operation condition foreseen for the AGPS.

Concept design of the control method for the NBI acceleration grid power supply-conversion system of CFETR

The China Fusion Engineering Test Reactor (CFETR) uses a negative-ion neutral beam injection (N-NBI) system that requires a powerful acceleration grid power supply (AGPS). The AGPS is rated at -1000 kV/80 A and feeds one injector. AGPS-conversion system, which uses a three-level neutral-point-clamped active front-end rectifier as well as a five-level neutral-point-clamped / cascaded H-bridge inverter, was investigated in the reference design. Because the control problem of the conversion system is crucial for the AGPS, a well-designed control method can improve the steady-state performance during operation substantially. To achieve the full potential of the proposed design and meet the challenging demands of the CFETR N-NBI system, a new control method is proposed and investigated in this paper. Considering the requirement to provide a wide range output voltage, the new control method provides an adjustable DC-link voltage for the rectifier with unity power factor and a suitable modulation factor for the inverter, while maintaining a low ripple voltage of output voltage. The steady performance of the AGPS is successfully improved using phase-shifted carrier-pulse-width modulation. Simulation results are performed that validate the new control method.

1 MV power supplies integration issues in MITICA experiment, the ITER Heating Neutral Beam Injector prototype

Abstract MITICA, the full scale prototype of ITER Heating Neutral Beam Injector required to heat up ITER plasma with 16.5 MW injected power, is under realization at the Neutral Beam Test Facility (NBTF) in Padova (Italy) with the contributions of JApanese and EUropean Domestic Agencies (JADA and EUDA, respectively). The objective of MITICA is to produce a 16.5 MW neutral beam, obtained by accelerating negative Deuterium ions up to 1 MeV for a total ion current of 40 A and then neutralized. MITICA Power Supply (PS), installed from 2016 to 2019, includes several non-standard equipment, with ratings well beyond the present industrial standard for insulation voltage level (-1 MVdc) and dimensions: • the Acceleration Grid Power Supply (AGPS), composed of five DC Generators (DCG) rated for -200 kVdc each, connected in series to produce -1 MVdc acceleration voltage; • the Ion Source and Extraction Power Supply system (ISEPS); • the large air insulated Faraday cage (High Voltage Deck1, HVD1) hosting ISEPS and connected to the Transmission Line (TL) through an air-to-SF6 Bushing (High Voltage Bushing Assembly, HVBA); • a 100 m gas (SF6) insulated TL, connecting AGPS and ISEPS to the beam source installed inside the vacuum vessel through • the SF6-to-vacuum HV Bushing (HVB). The definition of the interfaces both between components supplied by the different DA’s and towards the buildings has been studied and finalized as far as possible during the design phase. Nevertheless, during the installation phase some issues emerged and had to be solved, minimizing modifications of the components already manufactured. The paper deals with the experience gained during the installation activities, focusing on solutions to interface the aforementioned equipment with NBTF buildings according to the stringent dimensional requirements and to the electrical insulation issues of the TL from the buildings. In particular, the solutions adopted to realize the electrical and mechanical interfaces between the TL and the HVBA are described in detail.

Design and development of acceleration grid power supply-conversion system for CFETR N-NBI

The acceleration grid power supply (AGPS) is developed to provide accelerator power for China Fusion Experimental Rector (CFETR) Negative-ion Neutral Beam Injection (N-NBI) system. The rated output parameters of this AGPS are -1000kV/80A/80MW. The preliminary design of the AGPS is proposed in this paper. To improve the steady-state and dynamic performance of the AGPS, a new CFETR AGPS conversion system (AGPS-CS) based on three-level neutral point clamped (3L-NPC) rectifier and five-level neutral-point-clamped/cascaded H-bridge (5L-NPC/H) inverter is presented. The parameters of AGPS-CS are obtained theoretically, and simulations are used to verify its operational performance and show the effectiveness of the proposed topology. Results indicate that the proposed design provides good input-output features and can meet the requirements of the CFETR N-NBI system. This solution can also be used as a reference for the design of large-power high-voltage power supply.

Design and control of the accelerator grid power supply-conversion system applied to CFETR N-NBI prototype

The acceleration grid power supply (AGPS) rated 200 kV/25 A is a key component devoted to supply the acceleration grids of the China fusion engineering test reactor negative-ion-based neutral beam injector (N-NBI) prototype system. This paper focused on the design and control of the AGPS conversion system (AGPS-CS), with emphasis on the requirement of the wide range output voltage and rise time. A voltage regulation switch at the front of step-down transformer is applied to optimize the grid current and DC-link voltage. Moreover, a new feedforward control strategy with piecewise PI compensator is proposed to improve the characteristics of AGPS. The simulation results of the proposed AGPS-CS are presented, proving the performance of the power supply to achieve the desired requirements.