55 research outputs found
Modular Multilevel Converters in Hybrid Multi-Terminal HVDC Systems
Full text linkHigh-voltage direct current (HVDC) systems are becoming commonplace in modern power systems. Line commutated converters (LCCs) are suitable for bulk power and ultra-HVDC (UHVDC) transmission, while with inflexible power reversal capability and possible commutation failures. However, voltage source converters (VSCs) possess flexible power reversal capability and provide immunity to commutation failures. Modular VSC topologies offer improved performance compared to conventional 2 level/3 level VSC-based HVDC. The family of modular VSCs includes the well-established modular multilevel converter (MMC) and other emerging modular VSC topologies such as the DC-fault tolerant alternate arm converter (AAC) that share topological and operational similarities with the MMC. It is noteworthy that the integration of LCC and modular VSCs leads to unique benefits despite the challenges of different HVDC configurations. Hence, it is necessary to explore the system performance of different HVDC converter topologies, especially more complex hybrid multiterminal HVDC (MTDC) systems and DC-grids combining different converters. This thesis focuses on the combination of the LCC, MMC and AAC to constitute different hybrid HVDC transmission systems.
It is of significance to provide a common platform where the proper comparison and evaluation of different HVDC systems and control methods can be completed and independently validated. Therefore, this thesis also provides an overview of current HVDC benchmark models available in the existing literature. In addition, the detailed modeling methods of HVDC systems are discussed in this thesis. For ensuring the static security of HVDC systems especially the future DC-grids, this thesis proposes a generalized expression of DC power flow under mixed power/voltage (P/V) and current/voltage (I/V) droop control, considering the DC power flow for normal operation and after converter outage.
Detailed simulation models are established in PLECS-Blockset and Simulink to study the hybrid HVDC/MTDC systems and DC grid combining the LCC with the MMC and (or) AAC. The detailed sets of results demonstrate the functionalities of developed hybrid HVDC systems and validate the performance of systems complying with widely accepted HVDC operating standards. The developed LCC/AAC-based HVDC/MTDC systems and LCC/MMC/AAC-based DC grid in this thesis are prime steps towards the study of more complex MTDC systems and a key element in the development of future DC super grids
Modelling, System-Level Control, Fault Protection, and Power Flow Analysis in Medium-Voltage DC Systems
No full textMedium-voltage DC (MVDC) systems, typically operating between ±0.75 kV and ±50 kV, are emerging as a promising solution for modern power networks. Their potential applications span sub-transmission, distribution, and collection networks, providing enhanced support for the integration of regional renewable energy sources (RESs) and energy storage systems (ESSs). With the rapid growth of DC loads such as electrified transportation and data centers, MVDC systems are increasingly viewed as a viable alternative to traditional AC infrastructure. Furthermore, the integration of DC/DC and AC/DC converters in MVDC systems offers enhanced control flexibility compared to conventional AC architectures. This allows precise power regulation for wind farms, photovoltaic (PV) plants, ESSs, and various DC loads, improving system stability under fluctuating conditions and facilitating more efficient power dispatch.
Three critical challenges facing MVDC systems are identified in this thesis: 1) the devel opment of flexible system-level control strategies under diverse operational scenarios involving RESs, ESSs, and various types of loads; 2) the design of fault handling and fault ride-through schemes for converters used in distribution networks; and 3) the development of fast and accurate power flow (PF) computation methods for both grid-connected and islanded MVDC distribu tion systems operating under different converter control modes, with further extension to hybrid AC/MVDC systems. In addition, five specific research gaps in MVDC distribution systems are addressed. Firstly, existing droop control and adaptive droop control (ADC) strategies fail to consider the coordinated participation of ESSs, limiting their ability to regulate dc voltage and power sharing. Secondly, there is a lack of effective protection schemes specifically designed for valve-side single-phase-to-ground (SPG) faults in modular multilevel converter (MMC)-based MVDC networks. Thirdly, steady-state power distribution characteristics in hybrid AC and multi-terminal MVDC (AC/MT-MVDC) systems remain insufficiently explored, posing chal lenges for system-level planning and optimization. Fourthly, conventional PF algorithms are not well-suited for islanded MVDC and MVAC microgrids. Finally, existing data-driven PF methods often struggle to achieve both high accuracy and robust generalization across diverse MVDC operating scenarios.
Beginning from the exploration of possible network configurations for MVDC systems, this work presents typical system architectures, key converter requirements, and protection strategies at both the system and converter levels, while identifying potential applications and develop ment opportunities in Australia. It further introduces PF modelling approaches for MVDC and interfacing AC systems, alongside the operation principles and electromagnetic transient (EMT) models of symmetrical monopolar and bipolar MMCs. A Th´evenin/Norton equivalent model is developed for detailed fault analysis, and an average-value model is adopted for MT-MVDC system studies. To enhance system resilience, an ADC scheme is proposed to better utilize VSC overloading capability, coordinated through the operational status of heterogeneous ESSs. Two novel control loops are introduced to manage excess power dynamically and ensure effective post-disturbance recovery.
Two numerical (model-driven) PF algorithms are presented to solve the PF problems in hybrid AC/MT-MVDC systems and standalone MVDC & MVAC microgrids, respectively. For hybrid AC/MT-MVDC systems, a sequential Fibonacci search-based Newton-Raphson (FSNR)-based PF method with uniform bus type definitions is introduced, along with a zero-error steady state post-event calculation approach to efficiently determine power distribution following system disturbances. For standalone MVDC microgrids, a generalized accelerated iterative search (AIS)-based algorithm is developed to overcome the limitations of traditional methods under variable slack bus voltage, while an accelerated external search loop-based NR (AESL-NR) PF algorithm is proposed for the MVAC microgrids to deal with variations in both slack bus voltage and frequency.
Building on the model-driven PF algorithms, two data-driven approaches, using physics in formation guidance in fully convolutional networks (FCNs), are further proposed for the rapid PF estimation of MT-MVDC distribution systems. The first method, a physics-informed FCN (PI-FCN), employs a channel combination layer and a physics-based input reformulation strat egy to enhance prediction accuracy and generalization under varying topologies and operating conditions. The second method, a physics-embedded FCN (PEFCN), incorporates a physics operation layer and redesigned loss functions to suppress outliers and improve robustness. Both methods demonstrate strong performance across diverse scenarios, offering valuable strategies for accurate and efficient PF estimation in practical MVDC networks
Study on Spatio-Temporal Pattern Changes and Prediction of Arable Land Abandonment in Developed Area: Take Pingyang County as an Example
No full textThe problem of arable land abandonment has become increasingly prominent in China as an important hidden danger of regional and national grain security. Therefore, it is necessary to fully understand its developmental mechanism in order to improve land protection policies and maintain the sustainable use of arable land. This study took Pingyang County in the Yangtze River Delta Economic Zone as an example. Based on remote sensing image data in 2000, 2010, and 2018, the landscape pattern index was used to reveal the changes in the landscape pattern of abandoned land in the study area, and the FLUS model was used to simulate the spatial and temporal distribution changes in abandoned land in the study area in 2028. The results showed that the abandoned areas in the study area spread rapidly from 2000 to 2018, the area of abandoned land increased nearly 12 times in the past 18 years, and the areas with a high abandonment rate were concentrated in the western and northwestern mountainous areas of the study area. In the view of the landscape pattern, the areas with a high fragmentation degree of abandoned land gradually shifted to the western mountainous areas from 2000 to 2018, and the areas with high landscape complexity of abandoned land gradually shifted from the middle to the northern and western areas. The simulation results of abandoned land showed that the high-value areas of abandoned land rate in the study area would be more concentrated by 2028. Among them, the abandoned land rate of arable land in the northwest would increase to 15.76~24.89%, while the landscape fragmentation and complexity of abandoned land would be slightly lower than that in 2018. Finally, some countermeasures were proposed for the protection and sustainable utilization of cultivated land resources
The neo-endogenous development perspective of stakeholders and their synergy in rural revitalization
No full textRural revitalization amidst the backdrop of urbanization poses challenges for public policy. Adopting a neo-endogenous rural development perspective, the pivotal role lies in the amalgamation of external and internal endeavors to generate maximized synergy. However, the appropriate roles of diverse stakeholders and the intricate dynamics of their interplay remain subjects of contention. This study delves into the roles of key stakeholders in rural revitalization, drawing insights from a case study of rural practices in Shanghai, China. Employing structural equation modeling, we explore and illustrate the functioning and interplay mechanisms of these stakeholders. The findings reveal that the government significantly contributes to rural revitalization not only through direct resource investment and policy support but also indirectly by fostering extensive engagement from other stakeholders. The villagers’ organization emerges as the most noteworthy direct contributor, serving as the nexus linking the village with external entities and functioning as the core of community collective action. While villagers, as the principal inhabitants of rural areas, exhibit a significant albeit limited positive effect, underscoring that their participation levels remain suboptimal. External enterprises exert a notable yet predominantly indirect impact, interplaying with villagers’ committees and Party branches. Consequently, there is a pressing need to elucidate the roles and delineate the boundaries of various stakeholders and their interplay mechanisms to optimize their collective efforts. This clarification is essential for maximizing synergies and facilitating the generation of sustainable endogenous power essential for the success of rural revitalization initiatives. © The Author(s), under exclusive licence to Springer Nature B.V. 2025
Systemic lupus erythematosus in a patient with 22q11.2 deletion syndrome: A case report and review of the literature
No full text22q11.2 deletion syndrome (MIM: 192430/188400, ORPHA: 567) is the most common chromosomal microdeletion disorder, caused by a hemizygous microdeletion of 2.5 million base pairs on chromosome 22. There is a known association between 22q11.2 deletion syndrome (22q11.2DS), immunodeficiency and autoimmune diseases. However, the co-occurrence of 22q11.2DS and systemic lupus erythematosus (SLE) has been rarely reported. Here, we describe a case of a female teenager with distal type I 22q11.2DS who presented with alopecia, oral ulcers, fever and thrombocytopenia. Laboratory tests showed positive antinuclear antibodies (ANA) and double-stranded DNA (ds-DNA) antibodies, indicative of SLE. Treatment with prednisone, hydroxychloroquine and azathioprine resulted in improvement. We reviewed the literature on the immunological mechanisms involved in 22q11.2DS. Thymic dysplasia, T-cell lymphopenia, and B-cell abnormalities collectively contribute to the immunodeficiency and autoimmune manifestations observed in individuals with 22q11.2DS. Genetic factors such as 22q11.2DS should be considered in the diagnosis of childhood rheumatic diseases. Our case adds to the limited literature on this co-occurrence
Steady-state power distribution in VSC-based MTDC systems and dc grids under mixed P/V and I/V droop control
Full text link[EN] This paper proposes a steady-state power distribution derivation method for voltage source converter (VSC)based multi-terminal HVDC (MTDC) systems and dc grids under mixed power/voltage (P/V) and current/voltage (I/V) droop control. P/V and I/V droop control are two commonly used control schemes, which can be combined to achieve co-regulation of powers & currents in MTDC systems and dc grids. The proposed method can be used to estimate the power distributions under different scenarios for MTDC systems and dc grids based on VSCs with mixed P/V and I/V droop control. After determining the initial operating point based on an estimation-correction algorithm, redistributed power due to power disturbances, current changes or converter outages is analyzed in detail considering converter overload. An excess power reduction strategy is further proposed to avoid violation of power limits after converter outage. The accuracy of the proposed method is validated through multiple scenarios in a modular multilevel converter (MMC)-based four-terminal dc grid. The comparison between the proposed method and other approaches in the current literature further demonstrates the advantages of proposed power distribution derivation method.The third author (Muhammad Khalid) would like to acknowledge the support from Deanship of Research Oversight and Coordination (DROC) at King Fahd University of Petroleum and Minerals (KFUPM) through project No. DF201011.Sun, P.; Wang, Y.; Khalid, M.; Blasco-Gimenez, R.; Konstantinou, G. (2023). Steady-state power distribution in VSC-based MTDC systems and dc grids under mixed P/V and I/V droop control. Electric Power Systems Research. 214:1-10. https://doi.org/10.1016/j.epsr.2022.108798S11021
A data-driven identification method for impedance stability analysis of inverter-based resources
Full text linkObtaining inverter controller information may be a premise for seeking its dynamic behaviour. But accurate knowledge of such information would be unrealistic for real functioning inverter-interfaced generators (IIGs), which hinders the stability analysis of the IIG. A new data-driven impedance identification method is proposed for stability analysis, which involves an improved sparse identification algorithm as an ancillary function within the system identification framework. It contains mainly two design stages. First, the transform basis matrix (TBM) is devised systematically as a prior knowledge library to contain the possibly existing control structures. In the second stage, a sparse identification algorithm is reformulated in order to extract the relevant structures in TBM while obtaining controller parameters. The authors demonstrate that the sparse vector between the TBM and output signal is closely related to the controller structure. The effectiveness of the proposed method is verified on grid-connected inverters based on droop control and virtual synchronous machine control.<br/
Valve-side Single-phase-to-ground Fault Analysis in Bipolar MMC-HVDC Systems with Hybrid SMs
Full text linkThis letter presents a comprehensive fault analysis for a modular multilevel converter high-voltage direct current (MMC-HVDC) system with hybrid half-bridge submodules (HBSMs) and full-bridge SMs (FBSMs) under valve-side single-phase-to-ground (SPG) faults. The analysis shows protection for upper arms is necessary due to the inevitable upper arm overvoltage. It further specifies that the proportion of FBSMs utilized in each arm should be at least 86.6% of the available voltage to safely clear valve-side SPG faults without overvoltage in the lower arms. The accuracy of the theoretical analysis is validated in the positive pole of a bipolar MMC-HVDC system with different ratios of FBSMs modelled in PSCAD/EMTDC
Valve-side single-phase-to-ground fault analysis in bipolar MMC-HVDC systems with hybrid SMs
No full textThis letter presents a comprehensive fault analysis for a modular multilevel converter high-voltage direct current (MMC-HVDC) system with hybrid half-bridge submodules (HBSMs) and full-bridge SMs (FBSMs) under valve-side single-phase-to-ground (SPG) faults. The analysis shows protection for upper arms is necessary due to the inevitable upper arm overvoltage. It further specifies that the proportion of FBSMs utilized in each arm should be at least 86.6% of the available voltage to safely clear valve-side SPG faults without overvoltage in the lower arms. The accuracy of the theoretical analysis is validated in the positive pole of a bipolar MMC-HVDC system with different ratios of FBSMs modelled in PSCAD/EMTDC
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