Impedance Modeling and Stability Mechanism Analysis for Grid Forming and Grid Following Converters

Zhu Zuobin,Sun Shumin,Li Zhenxing,Huang Shaoping 대한전기학회 2024 Journal of Electrical Engineering & Technology Vol.19 No.5

When diff erent types of converters exist in a local area, the use of diff erent control methods between converters may bring about system instability, which will lead to the inability of distributed power supply to operate normally. it is important to propose a stability analysis method for diff erent types of shunt converters. Existing impedance models are less studied for modeling and stability mechanism analysis of hybrid converter operation modes of diff erent control Hence, a generalized virtual synchronous generator (VSG) impedance model and stability mechanism analysis method is proposed. Firstly, a generalized VSG impedance model is established and a stability mechanism analysis is performed. Secondly, external stability analysis and internal stability analysis on the system are conducted. Finally, a simulation model and an experimental platform for diff erent control converters is respectively established for verifi cation stability analysis theory. The stability mechanism analysis of converter grid connection lays a theoretical foundation for providing stable and reliable interfaces for large-scale new energy integration into the power grid. To suppress the resonance problem caused by the increase of the number of grid-following VSGs, a virtual conductor is introduced into parallel system to improve system’s stability. After introduced virtual conductor, output current waveform is smooth, and the harmonic content is reduced from 12.3 to 2.4%, compared with unimproved method.

Individual Channel Design-based Modeling and Analysis of Three Phase Grid-tied Inverter with Unbalanced Grid Impedance

Weimin Wu,Yun Li,Jiahao Liu,Frede Blaabjerg 전력전자학회 2019 ICPE(ISPE)논문집 Vol.2019 No.5

Three-phase grid-tied inverters are usually designed to operate in symmetric grid impedance. In actual operations, due to the imbalanced loads, three-phase systems with significant single-phase loads, imbalanced power line impedance or the imbalanced short-circuit impedance of the three grid power transformer, the equivalent three-phase grid impedance is unbalanced. It is, consequently, important to develop an effective model and stability analysis method to address three-phase grid-tied inverter with unbalanced grid impedance. Traditionally, the stability of unbalanced system can be analyzed by harmonic linearization technique, however, the model derivation is complex and the controller parameters need second designing. This paper develops the system model in a multivariable frequency domain, by using the individual channel analysis and design (ICAD), simplifying the stability analysis of the unbalanced system. The detailed study on the three-phase LCL filter-based grid-tied inverter under the unbalanced grid impedance condition is presented. Simulation and experiment analysis are carried out on a 380 V/50 Hz/6 kW grid-tied inverter prototype.

노후 콘크리트포장 위에 덧씌운 섬유그리드

이주명,백승범,이강훈,정진훈 한국도로학회 2017 한국도로학회논문집 Vol.19 No.3

PURPOSES: The objective of this study is to verify the effect of fiber grid reinforcement on the long-term performance of asphalt pavement overlaid on old concrete pavement by performing field investigation, laboratory test, and finite element analysis. METHODS : The reflection cracking, roughness, and rutting of fiber grid reinforced overlay sections and ordinary overlay sections were compared. Cores were obtained from both the fiber grid reinforced and ordinary sections to measure bonding shear strength between the asphalt intermediate and asphalt overlay layers. Fracture energy, displacement after yield, shear stiffnesses of the cores were also obtained by analyzing the test results. Finite element analysis was performed using the test results to validate the effect of the fiber grid reinforcement on long-term performance of asphalt pavement overlaid on the old concrete pavement. The fatigue cracking and reflection-cracking were predicted for three cases: 1) fiber grid was not used; 2) glass fiber grid was used; 3) carbon fiber grid was used. RESULTS : The reflection-cracking ratio of fiber grid reinforced sections was much smaller than that of ordinary sections. The fiber grid reinforcement also showed reduction effect on rutting while that on roughness was not clear. The reflection-cracking was not affected by traffic volume but by slab deformation and joint movement caused by temperature variation. The bonding shear strength of the fiber grid reinforced sections was larger than that of the ordinary sections. The fracture energy, displacement after yield, and shear stiffnesses of the cores of the fiber grid reinforced sections were also larger than those of the ordinary sections. Finite element analysis results showed that fatigue cracking of glass or carbon fiber grid reinforced pavement was much smaller than that of ordinary pavement. Carbon fiber grid reinforcement showed larger effect in elongating the fatigue life of the ordinary overlay pavement compared to glass fiber grid reinforcement. The binder type of the overlay layer also affected the fatigue life. The fiber grid reinforcement resisted reflection-cracking and the carbon fiber grid showed the greater effect. CONCLUSIONS: The results of field investigation, laboratory test, and finite element analysis showed that the fiber grid reinforcement had a better effect on improving long-term performance of asphalt pavement overlaid on the old concrete pavement.

Stability and Adaptability Analysis for PLL-Synchronized VSC-HVDC with Frequency Regulation Scheme under Islanded Grid

Meiqing Zhang,Xiaoming Yuan,Jiabing Hu 전력전자학회 2019 ICPE(ISPE)논문집 Vol.2019 No.5

Voltage-source converter (VSC) high voltage direct current (HVDC) connected to islanded grid is becoming a normal system operation mode. However, there is little research on the stability analysis of VSC-HVDC connected to islanded grid. On the basis of previous work, this paper analyzes the stability and adaptability of PLL-synchronized VSC-HVDC adopted with the additional frequency regulation scheme (FRS) under islanded grid. By defining the instantaneous frequency and regarding it as a state variable, the small-signal model of PLL-synchronized VSC under both grid-islanded mode and grid-connected mode is synthetically established. On this basis, the impact of FRS on the stability of PLL-synchronized VSC under islanded grid is analyzed, and then the adaptability of FRS to different grid strengths is investigated via considering different short current ratios (SCRs), including islanded grid (SCR=0), weak grid (SCR<2) and strong grid (SCR>5). The analysis results show that the FRS has important role on enhancing the small signal stability of the islanded grid, and also has excellent adaptability to the wide change of grid conditions, so as to further improve the possibility of the FRS applied in practical projects. These results finally are verified by time-domain simulations.

Near-tip grid refinement for the effective and reliable natural element crack analysis

J.R. Cho 국제구조공학회 2019 Structural Engineering and Mechanics, An Int'l Jou Vol.70 No.3

This paper intends to introduce a near-tip grid refinement and to explore its usefulness in the crack analysis by the natural element method (NEM). As a sort of local h-refinement in FEM, a NEM grid is locally refined around the crack tip showing the high stress singularity. This local grid refinement is completed in two steps in which grid points are added and Delaunay triangles sharing the crack tip node are divided. A plane-state plate with symmetric edge cracks is simulated to validate the proposed local grid refinement and to examine its usefulness in the crack analysis. The crack analysis is also simulated using a uniform NEM grid for the sake of comparison. The near-tip stress distributions and SIFs that are obtained using a near-tip refined NEM grid are compared with the exact values and those obtained using uniform NEM grid. The convergence rates of global relative error to the total number of grid points between the refined and non-refined NEM grids are also compared.

Effect of Spacer Grids on CHF at PWR Operating Conditions

Ahn, Seung-Hoon,Jeun, Gyoo-Dong Korean Nuclear Society 2001 Nuclear Engineering and Technology Vol.33 No.3

The CHF in PWR rod bundles is usually predicted by the local flow correlation approach based on subchannel analysis while difficulty exists due to the existence of spacer grids especially with mixing vanes. In order to evaluate the effect of spacer grids on CHF, the experimental rod bundle data with various types of spacer grids were analyzed using the subchannel code, COBRA-IV-i. For the Plain grid data, a CHF correlation was described as a function of local flow conditions and heated length, and then the residuals of the CHF in mixing vaned grids predicted by the correlation were examined in various kinds of grids. In order to compensate for the residual, three parameters, distances between grids and from the last grids to the CHF site, and equivalent hydraulic diameter were introduced into a grid parameter function representing the remaining effect of spacer grids predicted most of the CHF data points in plaing grids within $\pm$20 percent error band. Good agreement with the CHF data was also shown when the grid parameter function for mixing vaned grids of a specific design was used to compensate for the residuals of the CHF data predicted by the correlation.

핵연료집합체 내진해석모델 입력변수에 따른 민감도 분석

남현석(Hyun-Suk Nam),김태순(Tae-Soon Kim),김갑순(Kap-Sun Kim),이상섭(Sang-Seob Lee),이미연(Mi-Yeon Lee) 대한기계학회 2021 대한기계학회 춘추학술대회 Vol.2021 No.4

In this paper, the influence of major input variable used in the fuel assembly analysis model on the seismic analysis evaluation was analyzed. The seismic evaluation of fuel assembly is performed by comparing the grid impact load calculated by FE model with the grid impact strength. Also, stress analysis results calculated by detailed stress analysis model compare allowable criteria according to ASME Section III NB. In general, the margin of the gird impact load evaluation result is smaller than that of the detailed stress evaluation results. Therefore, sensitivity of the grid impact load according to the change of major input variables was analyzed in this study. The impact load generated on the grid is determined through the horizontal reactor internal structure and detailed core seismic analysis. The sensitivity analysis was conducted by main property values used as input variables. The sensitivity of the impact load according to the fuel assembly damping ratio, assembly stiffness, through grid stiffness and core gap was confirmed. As a results, it was analyzed that the through-grid stiffness had the most significant effect on the increase/decrease of the grid impact load. These results are expected to be utilized in the development of seismic analysis method for fuel assembly under beyond design basis earthquake loading condition.

효율적이고 신뢰성있는 자연요소 균열해석을 위한 균열선단 그리드 세분화기법

조진래 한국전산구조공학회 2019 한국전산구조공학회논문집 Vol.32 No.3

본 논문은 균열선단 그리드 세분화기법을 소개하고 자연요소법을 이용한 균열해석에 이 기법을 적용함으로서 그 유효성을 고찰하였다. 유한요소법에 있어서의 국부적 h-세분화와 같이 높은 응력 특이성을 보이는 균열선단 주위를 따라 자연요소법 그리드를 국부적으로 세분화하였다. 본 논문에서 소개되는 그리드 세분화기법은 2단계로 구성되며, 1단계에서는 그리드 점들이 추가되고 2단계에서는 균열선단 절점을 공유하는 델라우니 삼각형들이 나뉘게 된다. 제안하는 그리드 세분화기법의 타당성과 균열해석에서의 유효성을 입증하기 위해 대칭 엣지 균열을 갖는 평면 변형률 상태의 사각 평판을 해석하였다. 수치해석 결과의 상대비교를 위해 균일한 자연요소 그리드를 이용한 균열해석도 수행하였으며, 균열선단이 세분화된 그리드는 균일한 그리드와는 달리 이론해와 조밀한 그리드와 유사한 균열선단 응력분포를 나타내었다. 또한, 총 그리드 절점수에 대한 해석결과의 전역 상대오차에서도 세분화된 그리드가 균일한 그리드에 비해 높은 수렴율 나타내었다. This paper introduces a near-tip grid refinement and explores its usefulness in the crack analysis by the natural element method(NEM). As a sort of local h-refinement in finite element method(FEM), a NEM grid is locally refined around the crack tip showing high stress singularity. This local grid refinement is completed in two steps in which grid points are added and Delaunay triangles sharing the crack tip node are divided. A plane strain rectangular plate with symmetric edge cracks is simulated to validate the proposed local grid refinement and to examine its usefulness in the crack analysis. The crack analysis is also simulated using a uniform NEM grid for comparison. Unlike the uniform grid, the refined grid provides near-tip stress distributions similar to the analytic solutions and the fine grid. In addition, the refined grid shows higher convergence than the uniform grid, the global relative error to the total number of grid points.

계통연계형 마을단위 마이크로그리드의 보호협조 운용방안에 관한 연구

이나경(Na-Kyung Lee),김지명(Ji-Myung Kim),김경화(Kyung-Hwa Kim),김윤호(Yun-Ho Kim),이일무(Il-Moo Lee),노대석(Dae-Seok Rho) 한국산학기술학회 2024 한국산학기술학회논문지 Vol.25 No.1

Micro-grid research is being performed due to installation of renewable energy sources and ESSs, and demonstration projects on a community micro-grid for the self-energy supply rate are being carried out in Jeollanam-do province in South Korea. A community micro-grid may be operated as a type of grid-connected micro-grid when the self-energy supply rate is 50%. An operation method for protection devices is required because the magnitude and direction of the fault current may be changed depending on the location and type of the fault. Therefore, this paper proposes an operation algorithm for protection devices that considers the location and type of the fault in order to properly perform protection coordination in grid-connected micro-grid. We modeled a grid-connected micro-grid, which was composed of an ESS, PV system, and geothermal plant, and we used PSCAD/EMTDC S/W in order to analyze the fault current characteristics in the case of a short circuit and single-line ground faults. The simulation results confirmed that the proposed algorithm is useful to operate protection devices in a grid-connected micro-grid because the coordination time intervals were properly obtained.

Resonance Characteristics Analysis of Grid-connected Inverter Systems based on Sensitivity Theory

Jian Wu,Wanqin Han,Tao Chen,Jiaqi Zhao,Binbin Li,Dianguo Xu 전력전자학회 2018 JOURNAL OF POWER ELECTRONICS Vol.18 No.3

Harmonic resonance exists in grid-connected inverter systems. In order to determine the network components that contribute to harmonic resonance and the composition of the resonant circuit, sensitivity theory is applied to the resonance characteristic analysis. Based on the modal analysis, the theory of sensitivity is applied to derive a formula for determining the sensitivities of each network component parameter under a resonance circumstance that reflects the participation of the network component. The solving formula is derived for both parallel harmonic resonance and series harmonic resonance. This formula is adopted to a 4-node grid-connected test system. The analysis results reveal that for a certain frequency, the participation of parallel resonance and series resonance are not the same. Finally, experimental results demonstrate that the solving formula for sensitivity is feasible for grid-connected systems.