Hitachi (Sweden)

Power systems are undergoing a significant transformation around the globe. Renewable energy sources (RES) are replacing their conventional counterparts, leading to a variable, unpredictable, and distributed energy supply mix. The predominant forms of RES, wind, and solar photovoltaic (PV) require inverter-based resources (IBRs) that lack inherent synchronous inertia desired for the grid and thereby warrant additional interventions for maintaining grid stability by organizing various contingency planning. Such scenarios become more pertinent in the wake of rapid decarbonization objectives adopted by different countries, stringent grid code compliance, and improved grid resilience milestones. Energy storage technologies can potentially address these concerns viably at different levels. This paper reviews different forms of storage technology available for grid application and classifies them on a series of merits relevant to a particular category. The varied maturity level of these solutions is discussed, depending on their adaptability and their notion towards pragmatic implementations. Some specific technologies that require particular mention are - hydrogen ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$H_{2}$ </tex-math></inline-formula> ) storage with fuel cells (FC) as the reconversion medium, molten metal, and gravity batteries due to their highly scalable and siteable characteristics participating in load shifting; batteries and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$ H_{2}$ </tex-math></inline-formula> FC due to their high flexibility for peak shaving; and flywheels and supercapacitors for quick response applications, such as frequency regulation and voltage support. Various performance metrics are critically evaluated by comparing them on their usability scale, thus helping readers make a subjective judgment on a particular technology while being aware of the forthcoming limitations. Finally, the paper delves into some emerging trends that decide the selection of a particular technology based on life cycle assessment, economic viability, and commercial and environmental considerations that are presented under the given circumstances. The paper is believed to offer a broad overview of possible directions for the electric grid business, eventually emphasizing the need for more hybrid solutions with opportunities for short and long-term storage options.

The grid-forming converter is an important unit in the future power system with more inverter-interfaced generators. However, improving its performance is still a key challenge. This paper proposes a generalized architecture of the grid-forming converter from the view of multivariable feedback control. As a result, many of the existing popular control strategies, i.e., droop control, power synchronization control, virtual synchronous generator control, matching control, dispatchable virtual oscillator control, and their improved forms are unified into a multivariable feedback control transfer matrix working on several linear and nonlinear error signals. Meanwhile, unlike the traditional assumptions of decoupling between AC and DC control, active power and reactive power control, the proposed configuration simultaneously takes all of them into consideration, which therefore can provide better performance. As an example, a new multi-input-multi-output-based grid-forming (MIMO-GFM) control is proposed based on the generalized configuration. To cope with the multivariable feedback, an optimal and structured <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mathcal {H}_{\infty }$ </tex-math></inline-formula> synthesis is used to design the control parameters. At last, simulation and experimental results show superior performance and robustness of the proposed configuration and control.

With the increasing deployment of offshore wind power plants (WPPs), the grid-forming (GFM) battery energy storage system (BESS) has recently emerged as an attractive solution to improve the dynamic performances of WPPs. However, the control interactions of the GFM-BESS and offshore WPP, under different grid strengths, tend to complicate the controller-parameter tuning. This paper presents a modeling method for analyzing control interactions of offshore WPP and GFM-BESS, which sheds clear insights into the critical controller parameters to the system dynamics. Differing from conventional methods, a frequency-domain model of GFM-BESS, obtained by taking the Laplace transform of the corresponding state-space model, is developed first. This allows the impedance model of offshore WPP, including a black-box model of long transmission cable, to be flexibly integrated. Based on the model, both closed-loop transfer-function and pole-based dynamic analyses are then performed. Electromagnetic transient simulations corroborate the effectiveness of the model and analysis.

Renewable power-to-hydrogen (P2H) technology is one of the most promising solutions for fulfilling the increasing global demand for hydrogen and to buffer large-scale, fluctuating renewable energies. The high-power, high-current ac/dc converter plays a crucial role in P2H facilities, transforming medium-voltage (MV) ac power to a large dc current to supply hydrogen electrolyzers. This work introduces the general requirements, and overviews several power converter topologies for P2H systems. The performances of different topologies are evaluated and compared from multiple perspectives. Moreover, the future trend of eliminating the line frequency transformer (LFT) is discussed. This work can provide guidance for future designing and implementing of power-electronics-based P2H systems.

Abstract Inverter‐based generators (IBGs) are becoming popular in modern power systems. When the penetration of IBGs is increasing in power systems, new stability, protection, and monitoring challenges are introduced in the grid. Grid‐forming (GFM) control of converters is seen as a promising solution for future power grids to overcome particular stability challenges. Here, the technical challenges of the GFM‐based IBGs are reviewed from the point of view of TSOs and academic research. The properties of different GFM methods are studied for different GFM‐based IBGs for a single grid‐tied IBG and using the IEEE 9‐bus test system. Simulation results are provided by using the PSCAD‐EMT simulation software.

This review focuses on the use of polyolefins in high-voltage direct-current (HVDC) cables and capacitors. A short description of the latest evolution and current use of HVDC cables and capacitors is first provided, followed by the basics of electric insulation and capacitor functions. Methods to determine dielectric properties are described, including charge transport, space charges, resistivity, dielectric loss, and breakdown strength. The semicrystalline structure of polyethylene and isotactic polypropylene is described, and the way it relates to the dielectric properties is discussed. A significant part of the review is devoted to describing the state of art of the modeling and prediction of electric or dielectric properties of polyolefins with consideration of both atomistic and continuum approaches. Furthermore, the effects of the purity of the materials and the presence of nanoparticles are presented, and the review ends with the sustainability aspects of these materials. In summary, the effective use of modeling in combination with experimental work is described as an important route toward understanding and designing the next generations of materials for electrical insulation in high-voltage transmission.

BACKGROUND: Primary biliary cholangitis (PBC) is a rare liver disease with significant unmet need for second-line/add-on treatments. Setanaxib, a NOX1/4 inhibitor, has shown anti-fibrotic effects in in vitro and animal studies. This phase 2, randomized, multicentre study investigated the efficacy and safety of setanaxib in patients with PBC. METHODS: Patients with ≥6 months of ursodeoxycholic acid (UDCA) treatment were randomized 1:1:1 to oral setanaxib 400 mg once daily (OD), twice daily (BID), or placebo, in addition to UDCA for 24 weeks. Other inclusion criteria included alkaline phosphatase (ALP) ≥1.5 × ULN and gamma-glutamyl transferase (GGT) ≥1.5 × ULN. The primary endpoint was percentage change from baseline in GGT at Week 24; secondary endpoints included change from baseline in ALP, liver stiffness (LS; via transient elastography), fatigue at Week 24, and safety outcomes. p values compare setanaxib 400 mg BID and placebo groups. RESULTS: Of patients randomized (setanaxib 400 mg OD and BID: 38, and 36; placebo: 37), 104/111 completed Week 24. Mean (standard deviation [SD]) change in GGT to Week 24 was -4.9% (59.6%) for setanaxib 400 mg OD, -19.0% (28.9%) for setanaxib 400 mg BID, and -8.4% (21.5%) for placebo; p = .31. Patients treated with setanaxib 400 mg OD and BID showed decreased serum ALP levels from baseline to Week 24 (p = .002: setanaxib BID versus placebo). Patients treated with setanaxib 400 mg OD and BID showed mean (SD) percentage increases in LS to Week 24 of 3.3% (35.0%) and 7.9% (43.7%), versus 10.1% (33.1%) for placebo (p = .65). Changes in mean (SD) PBC-40 fatigue domain scores to Week 24 were +0.3% (24.9%) for setanaxib 400 mg OD, -9.9% (19.8%) for setanaxib 400 mg BID and +2.4% (23.1%) for placebo, p = .027. Two patients (one placebo, one setanaxib 400 mg BID) experienced serious treatment-emergent adverse events, deemed unrelated to study drug. CONCLUSIONS: The primary endpoint was not met. However, the secondary endpoints provide preliminary evidence for potential anti-cholestatic and anti-fibrotic effects in PBC, supporting the further evaluation of setanaxib in a future phase 2b/3 trial.

In recent years, the integration of the high-power static synchronous compensator (STATCOM) and energy storage in the same device has gained interest. Such a system is referred to as ES-STATCOM. Modular multilevel converter (MMC) topologies constitute a promising converter family for ES-STATCOM realization, providing a modular and scalable solution with a high efficiency that handles high-power and high-voltage ratings in grid applications. There is a gap in technical literature discussing the design and the comparison of MMC-based ES-STATCOMs while utilizing batteries to find the most suitable MMC topology for ES-STATCOMs. Therefore, this paper benchmarks MMC family members for ES-STATCOM realization. Both centralized and distributed energy storage approaches are investigated. The proposed design flowcharts can be employed for comparison and optimization purposes. In total, seven topologies are compared in terms of number of cells, required silicon area and total battery volume. Different semiconductor devices and battery types are analyzed. The result indicates that centralized energy storage systems are the most suitable due to their design flexibility, low volume and small silicon area. Moreover, the possibility of using over-modulation in MMC using bridge cells has an important role in the optimization of ES-STATCOM. The results for the adopted case study shows that the decentralized approach can lead to 55% higher silicon area and 30% higher volume than the centralized approach. The double-star bridge cell MMC with centralized energy storage is determined as the most suitable solution for ES-STATCOM systems.

This article presents an online health monitoring scheme for dc capacitors in modular multilevel converters (MMCs). The health monitoring algorithm is based on detecting changes in the dc capacitance value over time. The proposed algorithm only utilizes measurements that are typically available in flexible alternating current transmission systems and high-voltage direct current applications. Hence, in the proposed estimation method, no additional sensors are used. The estimation scheme considers the presence of noise in voltage and current measurements, and utilizes a recursive least square estimator in conjunction with a special low-pass filter to minimize the estimation errors. Simulation results of a hardware replica, as well as experimental results on a low-power MMC prototype show that the proposed scheme can identify the dc-link capacitance value with a maximum error of 1%.

Dense coding is the seminal example of how entanglement can boost qubit communication, from sending one bit to sending two bits. This is made possible by projecting separate particles onto a maximally entangled basis. We investigate more general communication tasks, in both theory and experiment, and show that simpler measurements enable strong and sometimes even optimal entanglement-assisted qubit communication protocols. Using only partial Bell state analysers for two qubits, we demonstrate quantum correlations that cannot be simulated with two bits of classical communication. Then, we show that there exists an established and operationally meaningful task for which product measurements are sufficient for the strongest possible quantum predictions based on a maximally entangled two-qubit state. Our results reveal that there are scenarios in which the power of entanglement in enhancing quantum communication can be harvested in simple and scalable optical experiments.

Recent developments in medium-voltage (MV) silicon and silicon carbide (SiC) power semiconductor devices are challenging state-of-the-art converter and auxiliary power supply (APS) designs. The APS is an important converter component, which energizes the gate-drive units and, therefore, has an influence on the overall reliability and efficiency of the converter system. There has, however, been comparably little research on how the APS of high-power converter submodules can be realized, in particular, for high-voltage applications. New, or improved, solutions may build on state-of-the-art topologies in the near future, but utilize MV SiC technology in the APS circuit itself to enable improved efficiency, reliability, simplicity, and compactness. Externally-fed APS concepts could provide several further advantages. Their various benefits on converter and system level may enable them to be a competitive solution for future APS concepts. Especially, light-based power supply systems are considered most useful since they offer extreme voltage isolation capability and immunity to electromagnetic interference. This article presents a review of the wide range of solutions for APSs, possible implementation options, and the most important design considerations. The different solutions are evaluated in a qualitative fashion, providing an overview of available APS concepts with regard to the requirements for high-power converter applications.

This paper focuses on the thermal modelling of power transformers using physics-informed neural networks (PINNs). PINNs are neural networks trained to consider the physical laws provided by the general nonlinear partial differential equations (PDEs). The PDE considered for the study of power transformer’s thermal behaviour is the heat diffusion equation provided with boundary conditions given by the ambient temperature at the bottom and the top-oil temperature at the top. The model is one dimensional along the transformer height. The top-oil temperature and the transformer’s temperature distribution are estimated using field measurements of ambient temperature, top-oil temperature and the load factor. The measurements from a real transformer provide more realistic solution, but also an additional challenge. The Finite Volume Method (FVM) is used to calculate the solution of the equation and further to benchmark the predictions obtained by PINNs. The results obtained by PINNs for estimating the top-oil temperature and the transformer’s thermal distribution show high accuracy and almost exactly mimic FVM solution.

Grid-forming (GFM) converters are a promising solution to enable large scale integration of renewable energy sources into the power system. However, due to the intrinsic voltage-source behaviour of GFM converters, current limitation during large grid disturbances is challenging. This paper presents a novel limitation strategy that preserves the GFM properties of the converter and at the same time effectively limits the converter current to the desired value. Through the limitation of the converter's internal voltage, stable operation even during faults and in case of large frequency disturbances in the grid is achieved. Experimental results show the effectiveness of the proposed current limitation strategy in case of various grid disturbances.

Fast dc chargers are the key enablers for the massive rollout of electric vehicles due to the reduced charging time. On the other hand, the rapid growth in battery technology with different voltages and charging requirements has imposed additional hurdles on the charger design to meet the efficiency requirements. Multiphase interleaved converters with conventional phase-shedding control improve the efficiency for a wide range of operations. However, they tend to operate certain phases, resulting in uneven thermal stress among the converter phases. This article proposes a rotating phase-shedding control to distribute the switching activities among all phases, enhancing the system's reliability while retaining the efficiency improvement. The proposed technique selects the proper number of active phases based on the required charging profile and periodically swaps them with other phases to even out the stress. The thermal profile is extracted to assess the thermal damage of the power switches. The performance of the proposed approach is evaluated and compared with the conventional phase shedding. The simulation and experimentally validated results confirm that the proposed technique achieves a better even distribution of the thermal damage between the phases compared with the conventional one. This will ultimately extend the lifetime of the system.

Thick GaN layers with a low concentration of defects are the key to enable next-generation vertical power electronic devices. Here, we explore hot-wall metalorganic chemical vapor deposition (MOCVD) for the development of GaN homoepitaxy. We propose a new approach to grow high quality homoepitaxial GaN in N2-rich carrier gas and at a higher supersaturation as compared to heteroepitaxy. We develop a low temperature GaN as an optimum nucleation scheme based on the evolution and thermal stability of the GaN surface under different gas compositions and temperatures. Analysis in the framework of nucleation theory of homoepitaxial layers simultaneously grown on GaN templates on SiC and on hydride vapor phase epitaxy GaN substrates is presented. We show that residual strain and screw dislocation densities affect GaN nucleation and subsequent growth leading to distinctively different morphologies of GaN homoepitaxial layers grown on GaN templates and native substrates, respectively. The established comprehensive picture provides a guidance for designing strategies for growth conditions optimization in GaN homoepitaxy. GaN with atomically flat and smooth epilayer surfaces with a root-mean-square roughness value as low as 0.049 nm and low background carbon concentration of 5.3 x 1015 cm-3 has been achieved. It is also shown that there is no generation of additional dislocations during homoepitaxial growth. Thus, our results demonstrate the potential of the hot-wall MOCVD technique to deliver high-quality GaN material for vertical power devices.

Thermal conductivity of AlxGa1−xN layers with 0≤x≤0.96 and variable thicknesses is systematically studied by combined thermoreflectance measurements and a modified Callaway model. We find a reduction in the thermal conductivity of AlxGa1−xN by more than one order of magnitude compared to that of GaN, which indicates a strong effect of phonon-alloy scattering. It is shown that the short-mean free path phonons are strongly scattered, which leads to a major contribution of the long-mean free path phonons to the thermal conductivity. In thin layers, the long-mean free path phonons become efficiently scattered by the boundaries, resulting in a further decrease in the thermal conductivity. Also, an asymmetry of thermal conductivity as a function of Al content is experimentally observed and attributed to the mass difference between Ga and Al host atoms.

This paper presents a control of the parallel operation of diode rectifier based high-voltage direct current (DR-HVDC) and voltage source converter based HVDC (VSC-HVDC) links. The two links are connected to offshore wind farms (OWFs). The VSC-HVDC controls the voltage and frequency of the offshore AC-grid needed for the OWFs start-up. The proposed grid forming control is based on controlling the OWFs voltage by controlling the active power balance at the VSC-HVDC AC-bus. When the OWFs inject more power than a certain active power reference established for the VSC-HVDC, the OWFs voltage increases and the DR-HVDC starts conducting. Then, the offshore voltage automatically accommodates the required value for transmitting the OWFs power through the DR-HVDC link while the system frequency is controlled by the VSC-HVDC through satisfying the reactive power balance. The power transmitted through the VSC-HVDC can be set by adjusting the corresponding power command while the DR-HVDC will automatically close the power balance. The main advantage of the proposed control system is that the VSC-HVDC can be sized just to support the system energization; while once the OWFs are in operation, the DR-HVDC will be loaded automatically. Simulation results verify the proposed control system.

The thermal boundary conditions in a numerical simulation for heat transfer are often imprecise. This leads to poorly defined boundary conditions for the energy equation. The lack of accurate thermal boundary conditions in real-world applications makes it impossible to effectively solve the problem, regardless of the advancement of conventional numerical methods. This study utilises a physics-informed neural network to tackle ill-posed problems for unknown thermal boundaries with limited sensor data. The network approximates velocity and temperature fields while adhering to the Navier-Stokes and energy equations, revealing unknown thermal boundaries and reconstructing the flow field around a square cylinder. Optimal sensor placement, determined by the QR pivoting technique, enhances the capture of dynamics, improving model accuracy. An ensemble PINN approach is implemented to increase robustness and generalisability, mitigating overfitting and underfitting risks, and providing a measure of model confidence. This enables the identification of reliable prediction regions and highlights potential inaccuracies, broadening applicability in complex heat transfer problems with unknown boundary conditions. Key findings include the ensemble physics-informed neural networks' superior predictive accuracy over single models and its ability to quantify uncertainty, offering insights into model validity. The study further highlights the importance of sensor placement, boundary condition enforcement, and activation function choice, with a notable shift from tanh to sin functions improving vortex shedding depiction.

The urgency of climate change forces many countries and regions to accelerate toward the goal of a carbon-neutral economy. The consequence of replacing fossil-fuel synchronous generation with inverter-based renewables, such as wind and solar, leads to a paradigm shift from power systems dominated by synchronous machines to inverter-dominated power systems. When the share of inverter-based generation increases, the behavioral differences in terms of the grid services provided to the power network by generation become more salient. Services for grid stability that were historically inherently provided by synchronous generators now must be replaced by services from these inverter-based generation resources, other grid devices, or the combination of the two to ensure stable grid operation in the future as well.

The article presents a detailed analysis and comparison of proton exchange membrane fuel cell (PEMFC) models with different levels of complexity. Since the full PEMFC model is highly nonlinear and depends on numerous technical parameters that are not usually readily available, this article investigates the adequate complexity level of the PEMFC model for power system dynamic studies. PEMFC models with associated control system are presented and developed in DIgSILENT PowerFactory. The performance of observed models in grid frequency control and low-voltage ride through is demonstrated on a simple single machine infinite bus system as well as on the IEEE 14-bus test system. Results show that the PEMFC can be represented with a simplified model in power system dynamic studies, providing identical power response and satisfying the PEMFC stack boundaries.