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  • 1.
    Tang, Yingchun
    et al.
    Institute Materials Physics, Helmholtz-Zentrum Hereon, Max-Planck-Str. 1, 21502, Geesthacht, Germany, Max-Planck-Str. 1.
    Li, Wei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Li, Changle
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Lu, Song
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Pyczak, Florian
    Institute Materials Physics, Helmholtz-Zentrum Hereon, Max-Planck-Str. 1, 21502, Geesthacht, Germany, Max-Planck-Str. 1; Brandenburgisch Technische Universität Cottbus-Senftenberg, Konrad-Wachsmann-Allee 17, 03046, Cottbus, Germany, Konrad-Wachsmann-Allee 17.
    First-Principles Calculations of Elastic and Thermodynamic Properties for Multi-component Co-based Superalloys2023In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 54, no 5, p. 1635-1648Article in journal (Refereed)
    Abstract [en]

    First-principles calculations were performed to investigate the elastic and thermodynamic properties for multi-component Co-based superalloy systems and explored the effect of alloying on stabilizing the γ′ phase. First, the comparisons were carried out for the γ′ phase in Co3(Al,TM) (TM being transition metals) and Ni3Al systems between the present computational results using the EMTO-CPA method and other available DFT calculations as well as experimental data. The lattice parameters, elastic constants, and Debye temperatures are consistent with experimental results and other calculations. The predicted thermodynamic properties, e.g., the Gibbs free energy, excess entropy, and linear thermal expansion coefficient, agree well with CALPHAD results, experimental results, and other available first-principles calculations. A combination of EMTO-CPA method and Debye–Grüneisen model is utilized in this work to ensure that the alloying effect on the stability of the γ′ phase in a multi-component Co-based system is captured efficiently. This could open the path for designing novel multi-component Co-based alloys based on first-principles calculation. To demonstrate this, predictions for the properties of multicomponent systems were undertaken. Our results show that Ni aids in the stabilization of the (CoNi)3(Al, Mo, Nb) phase. Graphical Abstract: [Figure not available: see fulltext.]

  • 2.
    Mo, Jinyong
    et al.
    China Univ Min & Technol, Sch Chem & Chem Engn, Xuzhou 221116, Peoples R China..
    Liang, Xiubing
    Acad Mil Sci, Def Innovat Inst, Beijing 100071, Peoples R China..
    Shen, Baolong
    Southeast Univ, Sch Mat Sci & Engn, Jiangsu Key Lab Adv Metall Mat, Nanjing 211189, Peoples R China..
    Wan, Yixing
    China Univ Min & Technol, Sch Chem & Chem Engn, Xuzhou 221116, Peoples R China..
    Mao, Huahai
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Thermocalc Software AB, Rasundavagen 18A, S-16967 Solna, Sweden..
    Zhang, Zhibin
    Acad Mil Sci, Def Innovat Inst, Beijing 100071, Peoples R China..
    Li, Wei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Schönecker, Stephan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Properties.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Properties.
    Li, Xiaoqing
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Properties.
    Local lattice distortions, phase stability, and mechanical properties of NbMoTaWHfx alloys: A combined theoretical and experimental study2023In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 217, article id 111891Article in journal (Refereed)
    Abstract [en]

    Refractory high-entropy alloys (RHEAs) are of growing interest due to their potentially superior mechanical performances at elevated temperatures. Inherent lattice distortions are believed to be a major contributor to strength in solid solution phases of RHEAs. Here, we investigate the NbMoTaWHfx alloy series (x = 0, 0.27, 0.57, 0.92, 1.33, 1.82) using first-principles simulations, thermodynamic modeling, and experimental techniques. The first-principles results suggest that Hf alloying is an effective means to enhance atomic-scale lattice distortions in BCC NbMoTaWHfx solid solutions. X-ray diffraction on prepared as-cast samples shows that the alloys with Hf content x <= 0.92 are single phase BCC alloys, whereas a dual BCC phase microstructure is observed for x = 1.33 and 1.82. Elemental mappings from scanning electron microscopy for the dual-phase alloys are checked with predictions from thermodynamic modeling for equilibrium and non-equilibrium solidifications. Room-temperature compressive mechanical tests reveal that yield and ultimate strengths increase strongly with the addition of Hf and saturate for x > 0.92, whereas the compressive plasticity is slightly improved by Hf but remains limited. We predict the compositional effects on poly-crystal elastic moduli for the constituent BCC phases and the dual-phase composites and find a linear behavior between modulus-normalized yield strength and lattice distortion on average.

  • 3.
    Werner, Konstantin V.
    et al.
    Tech Univ Denmark, Dept Civil & Mech Engn, Produktionstorvet,Bldg 425, DK-2800 Lyngby, Denmark..
    Niessen, Frank
    Tech Univ Denmark, Dept Civil & Mech Engn, Produktionstorvet,Bldg 425, DK-2800 Lyngby, Denmark..
    Li, Wei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Properties. Uppsala Univ, Dept Phys & Astron, Div Mat Theory, POB 516, SE-75121 Uppsala, Sweden..
    Lu, Song
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Properties.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Properties. Uppsala Univ, Dept Phys & Astron, Div Mat Theory, POB 516, SE-75121 Uppsala, Sweden.;Wigner Res Ctr Phys, Res Inst Solid State Phys & Opt, POB 49, H-1525 Budapest, Hungary..
    Villa, Matteo
    Tech Univ Denmark, Dept Civil & Mech Engn, Produktionstorvet,Bldg 425, DK-2800 Lyngby, Denmark..
    Somers, Marcel A. J.
    Tech Univ Denmark, Dept Civil & Mech Engn, Produktionstorvet,Bldg 425, DK-2800 Lyngby, Denmark..
    Reconciling experimental and theoretical stacking fault energies in face-centered cubic materials with the experimental twinning stress2023In: Materialia, E-ISSN 2589-1529, Vol. 27, article id 101708Article in journal (Refereed)
    Abstract [en]

    Stacking fault energy and twinning stress are thought to be closely correlated. All currently available models predict a monotonous decrease in twinning stress with decreasing stacking fault energy and depart from the assumption that the intrinsic stacking fault energy has a positive value. Opposite to this prediction, for mediumand high-entropy alloys the twinning stress was shown to increase with decreasing SFE. Additionally, for metastable materials, first principles methods predict negative intrinsic stacking fault energy values, whilst experimentally determined values are always positive. In the present communication, it is postulated that the twinning stress scaled by the Burgers vector bridges the difference between intrinsic and experimentally measured stacking fault energy. The assumption is tested for Cu-Al alloys, for pure metals and for medium- and high-entropy alloys and, for the first time, provides a consistent quantitative interpretation of data for both alloys with positive and negative stacking fault energy.

  • 4.
    Sun, Xun
    et al.
    Frontier Institute of Science and Technology, State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an, 710049, China.
    Lu, Song
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Properties.
    Xie, Ruiwen
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    An, Xianhai
    School of Aerospace, Mechanical & Mechatronic Engineering, The University of Sydney, Sydney, NSW 2006, Australia.
    Li, Wei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Zhang, Tianlong
    Frontier Institute of Science and Technology, State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an, 710049, China.
    Liang, Chuanxin
    Frontier Institute of Science and Technology, State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an, 710049, China.
    Ding, Xiandong
    Frontier Institute of Science and Technology, State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an, 710049, China.
    Wang, Yunzhi
    Department of Materials Science and Engineering, The Ohio State University, 2041 College Road, Columbus, OH 43210, USA.
    Zhang, Hualei
    Frontier Institute of Science and Technology, State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an, 710049, China.
    Vitos, Levente
    Research Institute for Solid State Physics and Optics, Wigner Research Center for Physics, Budapest H-1525, P.O. Box 49, Hungary.
    Can experiment determine the stacking fault energy of metastable alloys?2021In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 199, article id 109396Article in journal (Refereed)
    Abstract [en]

    Stacking fault energy (SFE) plays an important role in deformation mechanisms and mechanical properties of face-centered cubic (fcc) metals and alloys. In many concentrated fcc alloys, the SFEs determined from density functional theory (DFT) calculations and experimental methods are found having opposite signs. Here, we show that the negative SFE by DFT reflects the thermodynamic instability of the fcc phase relative to the hexagonal close-packed one; while the experimentally determined SFEs are restricted to be positive by the models behind the indirect measurements. We argue that the common models underlying the experimental measurements of SFE fail in metastable alloys. In various concentrated solid solutions, we demonstrate that the SFEs obtained by DFT calculations correlate well with the primary deformation mechanisms observed experimentally, showing a better resolution than the experimentally measured SFEs. Furthermore, we believe that the negative SFE is important for understanding the abnormal behaviors of partial dislocations in metastable alloys under deformation. The present work advances the fundamental understanding of SFE and its relation to plastic deformations, and sheds light on future alloy design by physical metallurgy. 

  • 5.
    Yang, Yaochun
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Schönecker, Stephan
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Li, Wei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Wang, Ci
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Huang, S.
    Zhao, J.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Corrigendum to “First-principles study of the Σ3(112) grain boundary in Fe-rich Fe-Cr alloys” [Scripta Materialia 181 (2020) 140-143] (Scripta Materialia (2020) 140-143 (S1359646220301056), (10.1016/j.scriptamat.2020.02.029))2021In: Scripta Materialia, ISSN 1359-6462, E-ISSN 1872-8456, Vol. 191, p. 202-203Article in journal (Refereed)
    Abstract [en]

    The authors regret that in producing Figs. 1 and 2, and Table S1 in our article an inadvertent error was made when calculating the bulk reference energy using an incorrect lattice setting. The bulk reference energy was recalculated using the correct lattice setting. Correction of the error results in a systematic shift in the grain boundary (GB) energies towards higher energies; see the corrected figures and table below. The change weakens the effect of Cr on the GB energy of chemically homogenous Fe1-xCrx alloys as shown in Fig. 1 (squares, red line), but doesn't influence the trends for chemically inhomogeneous Fe1-xCrx alloys as shown in Fig. 2 and Table S1. The correction has no effect on the analysis of the results and the conclusions presented in the original paper. The authors would like to apologise for any inconvenience caused.

  • 6.
    Yan, Ruijuan
    et al.
    Tech Univ Darmstadt, Dept Mat Sci, D-64287 Darmstadt, Germany..
    Xie, Ruiwen
    Tech Univ Darmstadt, Dept Mat Sci, D-64287 Darmstadt, Germany..
    Xie, Wenjie
    Tech Univ Darmstadt, Dept Mat Sci, D-64287 Darmstadt, Germany..
    Shen, Chen
    Tech Univ Darmstadt, Dept Mat Sci, D-64287 Darmstadt, Germany..
    Li, Wei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Balke, Benjamin
    Fraunhofer Res Inst Mat Recycling & Resource Stra, IWKS, D-63457 Hanau, Germany..
    Yoon, Songhak
    Fraunhofer Res Inst Mat Recycling & Resource Stra, IWKS, D-63457 Hanau, Germany..
    Zhang, Hongbin
    Tech Univ Darmstadt, Dept Mat Sci, D-64287 Darmstadt, Germany..
    Weidenkaff, Anke
    Tech Univ Darmstadt, Dept Mat Sci, D-64287 Darmstadt, Germany.;Fraunhofer Res Inst Mat Recycling & Resource Stra, IWKS, D-63457 Hanau, Germany..
    Effects of Doping Ni on the Microstructures and Thermoelectric Properties of Co-Excessive NbCoSn Half-Heusler Compounds2021In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 13, no 29, p. 34533-34542Article in journal (Refereed)
    Abstract [en]

    The half-Heusler (HH) compound NbCoSn with 18 valence electrons is a promising thermoelectric (TE) material due to its appropriate electrical properties as well as its suitable thermal and chemical stability. Nowadays, doping/substitution and tailoring of microstructures are common experimental approaches to enhance the TE performance of HH compounds. However, detailed theoretical insights into the effects of doping on the microstructures and TE properties are still missing. In this work, the microstructure of NbCoSn was tailored through precipitating the full-Heusler phases in the matrix by changing the nominal ratio of Co and Ni on the Co sites, focusing on the resulting TE properties. Further, first-principles calculations were employed to understand the relationship between the microstructure and the TE properties from the thermodynamic point of view. Detailed analysis of the electronic structure reveals that the presence of excess Co/Ni contributes to the increasing carrier concentration. Through an increase in the electrical conductivity and a reduction in the thermal conductivity, the TE performance is improved. Therefore, the present work offers a new pathway and insights to enhance the TE properties by modifying the microstructure of HH compounds via tailoring the chemical compositions.

  • 7.
    Cao, Peiyu
    et al.
    Univ Sci & Technol Beijing, Beijing Adv Innovat Ctr Mat Genome Engn, State Key Lab Adv Met & Mat, Beijing 100083, Peoples R China.;Chinese Acad Sci, Inst Mech, State Key Lab Nonlinear Mech, Beijing 100190, Peoples R China..
    Tian, Fuyang
    Univ Sci & Technol Beijing, Inst Appl Phys, Beijing 100083, Peoples R China..
    Li, Wei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. KTH, School of Engineering Sciences (SCI), Applied Physics. Uppsala Univ, Div Mat Theory, Dept Phys & Astron, SE-75120 Uppsala, Sweden.;Inst Solid State Phys & Opt, Wigner Res Ctr Phys, H-1525 Budapest, Hungary..
    Wang, Yandong
    Univ Sci & Technol Beijing, Beijing Adv Innovat Ctr Mat Genome Engn, State Key Lab Adv Met & Mat, Beijing 100083, Peoples R China..
    Ideal superelasticity in Ni-based Heusler alloys2021In: Acta Materialia, ISSN 1359-6454, E-ISSN 1873-2453, Vol. 210, article id 116816Article in journal (Refereed)
    Abstract [en]

    The hysteresis that occurs during superelasticity caused by the stress-induced first-order martensite transformation is sometimes detrimental to the properties of superelastic materials. In this paper, first-principles calculations are performed to systematically investigate the effects of the chemical composition and crystal disorder on the superelasticity of Ni50-xCoxM25Ga25 (M = Mn, Fe) Heusler alloys. Calculations of the stress-strain relation in the studied alloys reproduce the recent experimental findings for nonhysteretic superelasticity within an acceptable range of composition and ordering. We evaluate the Bloch spectral function to study the Fermi surface topology in connection with nonhysteretic superelasticity. We propose the Landau-de Gennes model-dependent critical parameter P-c, which can be used to predict the composition range of nonhysteretic superelastic materials. For the ferromagnetic L2(1) Ni50-xCoxMn25Ga25 and B2 Ni50-xCoxFe25Ga25 alloys, the nonhysteretic superelasticity phenomenon theoretically occurs for Co contents over x = 16 at.% and x = 28 at.%, respectively.

  • 8.
    Dong, Zhihua
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Chongqing Univ, Coll Mat Sci & Engn, State Key Lab Mech Transmission, Chongqing 400044, Peoples R China.;Chongqing Univ, Natl Engn Res Ctr Magnesium Alloys, Chongqing 400044, Peoples R China..
    Li, Wei
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Jiang, Bin
    Chongqing Univ, Coll Mat Sci & Engn, State Key Lab Mech Transmission, Chongqing 400044, Peoples R China.;Chongqing Univ, Natl Engn Res Ctr Magnesium Alloys, Chongqing 400044, Peoples R China..
    Li, Qian
    Chongqing Univ, Coll Mat Sci & Engn, State Key Lab Mech Transmission, Chongqing 400044, Peoples R China.;Chongqing Univ, Natl Engn Res Ctr Magnesium Alloys, Chongqing 400044, Peoples R China..
    Pan, Fusheng
    Chongqing Univ, Coll Mat Sci & Engn, State Key Lab Mech Transmission, Chongqing 400044, Peoples R China.;Chongqing Univ, Natl Engn Res Ctr Magnesium Alloys, Chongqing 400044, Peoples R China..
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering. Uppsala Univ, Div Mat Theory, Dept Phys & Astron, Box 516, SE-75120 Uppsala, Sweden.;Wigner Res Ctr Phys, Res Inst Solid State Phys & Opt, POB 49, H-1525 Budapest, Hungary..
    Thermo-mechanical properties of Cr-Co-Ni alloys from longitudinal spin fluctuation theory2021In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 119, no 8, article id 081904Article in journal (Refereed)
    Abstract [en]

    Accounting for longitudinal spin fluctuations in the paramagnetic state, we calculate elastic constants and stacking fault energy as a function of temperature and chemical composition for Cr-Co-Ni alloys. The longitudinal spin fluctuations are demonstrated to be important for the quantitative description of the thermo-mechanical properties and the corresponding chemical and temperature dependences. Replacing Ni with Cr and Co is found to yield opposite influence on the mechanical properties at finite temperature. A high thermal stability in plasticity is predicted in the low Cr regime in Cr-Co-Ni alloys, while a good thermal stability in elasticity can be achieved in the high Cr and low Co regime. The present advance in thermo-chemical-magnetic-property enhances the understanding required for an intelligent design of multicomponent alloys toward high-technology applications. Published under an exclusive license by AIP Publishing.

  • 9.
    Li, Wei
    KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering, Electric Power and Energy Systems.
    PMU-based State Estimation for Hybrid AC and DC Grids2018Doctoral thesis, monograph (Other academic)
    Abstract [en]

    Power system state estimation plays key role in the energy management systems(EMS) of providing the best estimates of the electrical variables in the grid that arefurther used in functions such as contingency analysis, automatic generation control,dispatch, and others. The invention of phasor measurement units (PMUs) takes thepower system operation and control into a new era, where PMUs’ high reportingrate and synchronization characteristics allow the development of new wide-areamonitoring, protection, and control (WAMPAC) application to enhance the grid’sresiliency. In addition, the large number of PMU installation allows the PMU-onlystate estimation, which is ready to leap forward today’s approach which is based onconventional measurements.At the same time, high voltage direct current (HVDC) techniques enable totransmit electric power over long distance and between different power systems,which have become a popular choice for connecting variable renewable energy sourcesin distant locations. HVDCs together with another type of power electronic-baseddevices, flexible AC transmission system (FACTS), have proven to successfullyenhance controllability and increase power transfer capability on a long-term costeffectivebasis. With the extensive integration of FACTS and HVDC transmissiontechniques, the present AC networks will merge, resulting in large-scale hybrid ACand DC networks. Consequently, power system state estimators need to considerDC grids/components into their network models and upgrade their estimationalgorithms.This thesis aims to develop a paradigm of using PMU data to solve stateestimations for hybrid AC/DC grids. It contains two aspects: (i) formulating thestate estimation problem and selecting a suitable state estimation algorithm; (ii)developing corresponding models, particularly for HVDCs and FACTS.This work starts by developing a linear power system model and applying thelinear weighted least squares (WLS) algorithm for estimation solution. Linear networkmodels for the AC transmission network and classic HVDC links are developed. Thislinear scheme simplifies the nonlinearities of the typical power flow network modelused in the conventional state estimations and has an explicit closed-form solution.However, as the states are voltage and current phasors in rectangular coordinates,phasor angle is not an explicit state in the modeling and estimation process. Thisalso limits the linear estimators’ ability to deal with the corrupt angle measurementsresulting from timing errors or GPS spoofing. Additionally, it is cumbersome toselect state variables for an inherently nonlinear network model, e.g., classic HVDClink, when trying to fulfill its linear formulation requirement.In contrast, it is more natural to use PMU measurements in polar coordinatesbecause they can provide an explicit state measurement set to be directly used inthe modeling and estimation process without form changes, and more importantly,it allows detection and correction for angle bias which emerges due to imperfectsynchronization or incorrect time-tagging by PMUs. To this end, the state estimationproblem needs to be formulated as a nonlinear one and the nonlinear WLS is applied for solution. We propose a novel measurement model for PMU-based state estimationwhich separates the errors due to modeling uncertainty and measurement noise sothat different weights can be assigned to them separately. In addition, nonlinearnetwork models for AC transmission network, classic HVDC link, voltage sourceconverter (VSC)-HVDC, and FACTS are developed and validated via simulation.The aforementioned linear/nonlinear modeling and estimation schemes belongto static state estimator category. They perform adequately when the system isunder steady-state or quasi-steady state, but less satisfactorily when the system isunder large dynamic changes and the power electronic devices react to these changes.Testing results indicate that additional modeling details need to be included toobtain higher accuracy during system dynamics involving fast responses from powerelectronics. Therefore, we propose a pseudo-dynamic modeling approach that canimprove estimation accuracy during transients without significantly increasing theestimation’s computational burden. To illustrate this approach, the pseudo-dynamicnetwork models for the static synchronous compensator (STATCOM), as an exampleof a FACTS device, and the VSC-HVDC link are developed and tested.Throughout this thesis, WLS is the main state estimation algorithm. It requiresa proper weight quantification which has not been subject to a sufficient attentionin literature. In the last part of thesis, we propose two approaches to quantify PMUmeasurement weights: off-line simulation and hardware-in-the-loop (HIL) simulation.The findings we conclude from these two approaches will provide better guidancefor selecting proper weights for power system state estimation.

    Download full text (pdf)
    WeiLi_dissertation
  • 10.
    Li, Wei
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering, Electric Power and Energy Systems.
    Vanfretti, Luigi
    Chow, Joe H.
    Pseudo-Dynamic Network Modeling for PMU-Based State Estimation of Hybrid AC/DC Grids2018In: IEEE Access, E-ISSN 2169-3536, Vol. 6, p. 4006-4016Article in journal (Refereed)
    Abstract [en]

    This paper presents a PMU-based state estimation (SE) algorithm that uses a pseudo-dynamic network modeling approach. The pseudo-dynamic network model combines different equations with static network equations. Then it applies the weighted least squares algorithm to solve an over-determined least squares estimation problem. The proposed method can improve SE accuracy during both steady state and transient conditions without increasing the computational burden. In addition, the proposed modeling approach is applied to networks containing both a STATCOM and a voltage source converter-HVdc to demonstrate how to develop and apply a pseudo-dynamic SE model. Case studies aim to illustrate and verify the performance of the proposed method under steady state and transient conditions.

  • 11.
    Li, Wei
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems.
    Vanfretti, Luigi
    KTH, School of Electrical Engineering (EES), Electric Power and Energy Systems. Statnett SF, Norway.
    A PMU-based state estimator for networks containing FACTS devices2015In: 2015 IEEE Eindhoven PowerTech, PowerTech 2015, IEEE conference proceedings, 2015Conference paper (Refereed)
    Abstract [en]

    This paper presents a PMU-based state estimation algorithm that considers the presence of flexible AC transmission system (FACTS). The models of FACTS devices, including thyristor- and converter-based devices, are developed and then combined with an AC network model. Considering the characteristics of PMU measurements, the network model for state estimation is separated from the measurements. By applying the nonlinear weighted least squares (WLS) algorithm, a PMU-based state estimator can estimate the states of an AC system and its embedded FACTS devices simultaneously. To validate the algorithm, simulation studies for a 9-bus system are performed.

  • 12.
    Li, Wei
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Vanfretti, Luigi
    KTH, School of Electrical Engineering (EES), Electric Power Systems. Statnett SF, Research and Development, Oslo, Norway.
    A PMU-Based State Estimator For Networks Containing VSC-HVDC links2015In: 2015 IEEE POWER & ENERGY SOCIETY GENERAL MEETING, IEEE , 2015Conference paper (Refereed)
    Abstract [en]

    This paper presents a PMU-based state estimation algorithm that considers the presence of voltage source converter-based high voltage direct current (VSC-HVDC) links. The network model of a VSC-HVDC link with its control modes is developed and then combined with an AC model to accomplish a hybrid AC/DC network model. The measurement model in this algorithm considers the properties of PMU measurements, thus separating the network model with measurements. Additionally, DC link measurements are assumed to be sampled synchronously, time-stamped and reported at the same rate as PMU measurements. Then, by applying the nonlinear weighted least squares (WLS) algorithm, a PMU-based state estimator can solve for both AC and DC states simultaneously. To validate the algorithm, a simulation study for a 6-bus hybrid AC/DC test system is shown in this paper.

  • 13.
    Vanfretti, Luigi
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems. Statnett SF, R&D, Oslo, Norway.
    Li, Wei
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Egea-Alvarez, Agusti
    Gomis-Bellmunt, Oriol
    Generic VSC-Based DC Grid EMT Modeling, Simulation, and Validation on a Scaled Hardware Platform2015In: 2015 IEEE POWER & ENERGY SOCIETY GENERAL MEETING, IEEE , 2015Conference paper (Refereed)
    Abstract [en]

    This paper presents results from the KIC InnoEnergy project: Generic DC grid off-line and real-time simulation models and tools (Action 2.1, Subtask 2.1.1). The work reported here is focused on the development of a generic voltage source converter (VSC) model and its control schemes. Different test systems were designed and implemented in simulation tools and an experimental platform to verify the proposed generic model. Consistent simulation and emulation performances indicate that the generic model and its control schemes are applicable for VSC-HVDC operation, and thus, they can facilitate further research and development in DC grids.

  • 14.
    Rogersten, Robert
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Vanfretti, Luigi
    KTH, School of Electrical Engineering (EES), Electric Power Systems. Statnett SF, R and D, Oslo, Norway.
    Li, Wei
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Towards Consistent Model Exchange and Simulation of VSC-HVdc Controls for EMT Studies2015In: 2015 IEEE POWER & ENERGY SOCIETY GENERAL MEETING, IEEE , 2015Conference paper (Refereed)
    Abstract [en]

    Design of voltage source converters (VSC) for high-voltage direct current (HVdc) systems requires extensive simulation tools that are accurate and reliable. Before the installation of VSC-HVdc links, different simulation-based studies need to be performed by different parties and using different electromagnetic transient (EMT) simulation platforms. Previous work shows that even with a pedantic re-implementation of the models from the ground up, simulation results are still inconsistent between different EMT tools. This paper demonstrates a methodology, which provides a higher consistency between the simulation tools Simulink and PSCAD. This methodology is complex and exposes several limitations. Hence, the is a need for the adoption of a standardized equation-based modeling language for EMT model exchange.

  • 15.
    Rogersten, Robert
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Vanfretti, Luigi
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Li, Wei
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Zhang, Lidong
    ABB, Sweden.
    Mitra, Pinaki
    ABB, Sweden.
    A Quantitative Method for the Assessment of VSC-HVdc Controller Simulations in EMT Tools2014In: Innovative Smart Grid Technologies Conference Europe (ISGT-Europe), 2014 IEEE PES, IEEE conference proceedings, 2014, p. 1-5Conference paper (Refereed)
    Abstract [en]

    Voltage-Source Converter-Based High-voltage dc (VSC-HVdc) technology has received growing interest during recent years due to its benefits for separated active and reactive power control. Before the installation of VSC-HVdc links, different simulation-based studies need to be performed, usually by different parties and using different electromagnetic transient (EMT) simulation platforms. This paper provides a systematic method to assess discrepancies between simulation tools. Models of the VSC and its control systems are implemented from scratch in two different tools. A metric and a methodology to quantitatively assess the discrepancies between simulation results is proposed and illustrated. The paper shows that even with a pedantic re-implementation of the models from the ground up, simulation results are still inconsistent. The objective of this paper is to develop a quantification method for comparing the models developed in different EMT simulation tool and to emphasize differences that can arise with the numerical solvers in such tools.

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  • 16.
    Hasan, Md Rokibul
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Vanfretti, Luigi
    KTH, School of Electrical Engineering (EES), Electric Power Systems. SF, Norwegian Transmission System Operator, Norway .
    Li, Wei
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Khan, Naveed Ahmad
    Generic High Level VSC-HVDC Grid Controls and Test Systems for Offline and Real Time Simulation2014In: 9th International: 2014 Electric Power Quality and Supply Reliability Conference, PQ 2014 - Proceedings, IEEE Computer Society, 2014, p. 57-64Conference paper (Refereed)
    Abstract [en]

    This article describes generic high level control models for VSC-HVDC grid systems. An average value model of the voltage source converter (VSC) is used; it includes the vector current control strategy. As an improvement to traditional conventional high-level control systems, this paper includes dc voltage, negative sequence current and dc voltage droop control loops. To validate the controller's performances, two tests systems were developed: a point-to-point link and a four-terminal DC grid. Both offline and real-time simulations are carried out for several test scenarios. A methodology for calibrating controller parameters is presented and used to tune the controls of each test system considering the different scenarios presented. All simulation models were developed in SimPowerSystem/Simulink and prepared using the RT-LAB software from Opal-RT for real-time simulation; real-time simulation performance of the simulation models is also discussed.

  • 17.
    Vanfretti, Luigi
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Khan, N. A.
    Li, Wei
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Hasan, Md Rokibul
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Haider, A.
    Generic VSC and low level switching control models for offline simulation of VSC-HVDC systems2014In: 9th International: 2014 Electric Power Quality and Supply Reliability Conference, PQ 2014 - Proceedings, IEEE Computer Society, 2014, p. 265-272Conference paper (Refereed)
    Abstract [en]

    This article proposes and implements generic and detailed voltage source converter (VSC) models, and low-level switching control models for simulation of HVDC networks using Modular Multilevel Converters (MMCs) or conventional two-level converters. The low level control approaches for both MMCs and two-level converters are explained. The behavior of the test HVDC networks is analyzed with the detailed model of both types of converter. In addition, high level controls are briefly explained, focusing on their application in DC grids. The simulation results of test networks validate the proposed high level controls and low level switching controls for simulation with detailed models of MMCs and conventional two-level converters.

  • 18.
    Khan, Naveed Ahmad
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems. HVDC ABB AB, Dept Syst Studies, Ludvika, Sweden.
    Vanfretti, Luigi
    KTH, School of Electrical Engineering (EES), Electric Power Systems. Stattnet SF, Oslo, Norway.
    Li, Wei
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Haider, Arif
    KTH, School of Electrical Engineering (EES), Electric Power Systems. HVDC ABB AB, Dept Syst Studies, Ludvika, Sweden.
    Hybrid Nearest Level and open loop control of Modular Multilevel Converters with N+1 and 2N+1 levels2014In: Power Electronics and Applications (EPE'14-ECCE Europe), 2014 16th European Conference on, IEEE conference proceedings, 2014Conference paper (Refereed)
    Abstract [en]

    In this paper a Nearest Level Control with open loop approach for modular multilevel converter (MMC) is proposed. By using open loop approach it is possible to suppress circulating currents without using feedback controllers. Moreover, with this method, both N+1 and 2N+1 levels (N=number of sub-modules) can be obtained at the output voltage. In addition, the voltage quality is improved and the total harmonic distortion is reduced at higher and lower modulation indices by increasing the number of levels up to 2N+1. Simulation results of 3-phase MMC are presented for both 2N+1 and N+1 levels together with the description of proposed Nearest Level Control scheme.

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  • 19.
    Li, Wei
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Vanfretti, Luigi
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Inclusion of Classic HVDC links in a PMU-based state estimator2014In: PES General Meeting | Conference & Exposition, 2014 IEEE, IEEE conference proceedings, 2014, p. 1-5Conference paper (Refereed)
    Abstract [en]

    This article presents a PMU-based state estimator algorithm that considers the presence of classic HVDC links. A hybrid AC/DC network model is developed to combine the AC network and DC links. The algorithm assumes that DC link measurements can be sampled synchronously, are time-stamped and reported at the same rate as PMU measurements. Then, by applying the weighted least squares algorithm, a linear state estimator can solve for both AC and DC states simultaneously. To validate the algorithm, a state estimation model is built for a 9-bus and the KTH-Nordic 32 systems, for both AC-only and hybrid AC/DC networks. In addition, the influence of losing DC measurements and effects of measurement noise on the state estimator are briefly discussed.

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  • 20.
    Vanfretti, Luigi
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Li, Wei
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Bogodorova, Tetiana
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Panciatici, Patrick
    RTE, France.
    Unambiguous Power System Dynamic Modeling and Simulation using Modelica Tools2013In: 2013 IEEE Power and Energy Society General Meeting (PES), IEEE , 2013, p. 21-25Conference paper (Refereed)
    Abstract [en]

    Dynamic modeling and time-domain simulation for power systems is inconsistent across different simulation platforms, which makes it difficult for engineers to consistently exchange models and assess model quality. Therefore, there is a clear need for unambiguous dynamic model exchange. In this article, a possible solution is proposed by using open modeling equation-based Modelica tools. The nature of the Modelica modeling language supports model exchange at the 'equation-level', this allows for unambiguous model exchange between different Modelica-based simulation tools without loss of information about the model. An example of power system dynamic model exchange between two Modelica-based software Scilab/Xcos and Dymola is presented. In addition, common issues related to simulation, including the extended modeling of complex controls, the capabilities of the DAE solvers and initialization problems are discussed. In order to integrate power system Modelica models into other simulation tools (Matlab/Simulink), the utilization of the FMI Toolbox is investigated as well.

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  • 21.
    Li, Wei
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Vanfretti, Luigi
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Chompoobutrgool, Yuwa
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Development and implementation of hydro turbine and governor models in a free and open source software package2012In: Simulation (San Diego, Calif.), ISSN 1569-190X, E-ISSN 1878-1462, Vol. 24, p. 84-102Article in journal (Refereed)
    Abstract [en]

    This article studies accurate and detailed hydro turbine and governor models, and implements these models in Power System Analysis Toolbox (PSAT)-a free and open source software. The implementation of these models in PSAT is approached using a general methodology, which is described in detail. To evaluate the performances of the developed hydro turbine and governor models, simulation studies are carried out on test systems of different scales, from a single-machine infinite-bus (SMIB) system to a larger system which includes 20 generators. Further more, transient stability analysis and small signal stability analysis are carried out to assess the performance of the implemented models.

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  • 22.
    Chompoobutrgool, Yuwa
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Li, Wei
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Vanfretti, Luigi
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Development and Implementation of the Nordic Grid Model for Power System Small-Signal and Transient Stability Studies in a Free and Open Source Software2012In: Power and Energy Society General Meeting, 2012 IEEE, IEEE , 2012, p. 6344571-Conference paper (Refereed)
    Abstract [en]

    This article presents an implementation of a Nordic grid model in Power System Analysis Toolbox (PSAT) -a free and open-source software. A newly developed hydro turbine and hydro governor (HTG) model is implemented with this grid model and compared with the currently available PSAT turbine and governor models. Small-signal and transient stability analyses of the system using the two models are carried out and compared to demonstrate the difference and necessity of accurate hydro turbine and governor model utilization. The paper ends with a validation of the linearized Nordic grid model generated by PSAT including the newly implemented HTG models. This validation is done through nonlinear time-domain simulation by applying both large and small disturbances.

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  • 23.
    Li, Wei
    et al.
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Vanfretti, Luigi
    KTH, School of Electrical Engineering (EES), Electric Power Systems.
    Farrokhabadi, Mostafa
    Modeling of Custom Hydro Turbine and Governor Models for Real-Time Simulation2012In: 2012 IEEE COMPENG 2012: Workshop on Complexity in Engineering, IEEE conference proceedings, 2012, p. 83-88Conference paper (Refereed)
    Abstract [en]

    This article develops three different hydro turbineand governor (HTG) models to fulfill adequate modeling requirements for the representation of hydro power generation inthe Nordic grid. To validate the performance of the developed models, both off-line and real-time simulation studies on a single machine infinite-bus (SMIB) system are carried out. Apart from SMIB system, real-time simulations are also executed for alarger scale power system—the IEEE Reliability Test System1996 (IEEE RTS 96) in this article to determine the models’dynamic performance in larger and more complex networks. Inaddition, how to properly transfer an off-line models developedin SimPowerSystems (SPS) for real-time simulation in RT-LAB is addressed; including valuable experiences obtained from these simulation exercises.

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1 - 23 of 23
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