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Kai Shi,Peng Lu,Yuxin Sun,Peifeng Xu 전력전자학회 2022 JOURNAL OF POWER ELECTRONICS Vol.22 No.8
In recent years, a series of studies concerning the oscillation of interconnected systems has shown that frequency coupling has important effects on the stability of the system. In this paper, a more accurate theoretical model considering the frequency coupling effects is adopted to derive the transmission process in vector control, phase lock loop, and proportional–resonant controller of the harmonic perturbation in the frequency domain. This model is used to further deepen the application of harmonic linearization modeling to impedance modeling. An integrated three-phase modular multilevel converter simulation model based on the PSCAD platform is built to verify the accuracy of the theoretical modeling through the frequency sweep method. The influence of related system parameters on sequence and coupling impedance is analyzed in detail. The generalized Nyquist criterion is used to study the importance of coupling impedance to the stability analysis of the system.
Impact of Stirring Speed on β-Lactoglobulin Fibril Formation
Shy Kai Ng,Kar Lin Nyam,Imededdine Arbi Nehdi,Oi Ming Lai,Gun Hean Chong,Chin Ping Tan 한국식품과학회 2016 Food Science and Biotechnology Vol.25 No.suppl1
β-Lactoglobulin (β-lg) can produce fibrils that have multi-functional properties. Impacts of different stirring speeds on characteristics of β-lg fibrils as a stable form in β-lg fibril solutions were investigated. Fibril concentration, fibril morphology, turbidity, particle size distribution, zeta potential, and rheological behavior of solutions were studied. Stirring enhanced fibril formation and stability of a fibril solution, in comparison with unstirred solutions. Increasing the stirring speed produced more turbidity and a greater distribution of particle sizes, higher viscosity values, but no differences in zeta potential values of β-lg fibril solutions. However, a high stirring speed is not feasible due to reduction of the fibril yield and changes in fibril morphology.
Shi, Kai,Li, Tong,Ren, Mingwei,Xu, Peifeng The Korean Institute of Power Electronics 2021 JOURNAL OF POWER ELECTRONICS Vol.21 No.5
Virtual synchronous generators (VSGs), with the operational characteristics of synchronous generators (SGs), have been employed in renewable energy generation grid-connected systems to solve the problem of insufficient equivalent inertia, which is caused by the high permeability of distributed generation systems in the grid. However, a VSG does not possess low voltage ride-through (LVRT) capability. A novel LVRT control strategy for a VSG based on virtual self-inductive flux linkage is proposed in this paper. First, the electromagnetic transient response mechanism of a SG under grid faults is analyzed in detail. Then, a memory and retention strategy are proposed to simulate the effect of the switch law. Furthermore, the virtual q-axis armature self-inductance is introduced into the VSG and a virtual self-inductive magnetic chain is utilized to block the change of the transient fault current. This helps the inverter adjust the output voltage quickly. In addition, under the premise of meeting the reactive power margin, the reactive power compensation strategy is optimized to achieve a quick response in terms of the reactive power compensation of grid faults, which is helpful for realizing the LVRT of a VSG. Finally, the feasibility and effectiveness of the proposed LVRT method are verified by thorough simulation results.
Grid-Connected Dual Stator-Winding Induction Generator Wind Power System for Wide Wind Speed Ranges
Shi, Kai,Xu, Peifeng,Wan, Zengqiang,Bu, Feifei,Fang, Zhiming,Liu, Rongke,Zhao, Dean The Korean Institute of Power Electronics 2016 JOURNAL OF POWER ELECTRONICS Vol.16 No.4
This paper presents a grid-connected dual stator-winding induction generator (DWIG) wind power system suitable for wide wind speed ranges. The parallel connection via a unidirectional diode between dc buses of both stator-winding sides is employed in this DWIG system, which can output a high dc voltage over wide wind speed ranges. Grid-connected inverters (GCIs) do not require booster converters; hence, the efficiency of wind energy utilization increases, and the hardware topology and control strategy of GCIs are simplified. In view of the particularities of the parallel topology and the adopted generator control strategy, we propose a novel excitation-capacitor optimization solution to reduce the volume and weight of the static excitation controller. When this excitation-capacitor optimization is carried out, the maximum power tracking problem is also considered. All the problems are resolved with the combined control of the DWIG and GCI. Experimental results on the platform of a 37 kW/600 V prototype show that the proposed DWIG wind power system can output a constant dc voltage over wide rotor speed ranges for grid-connected operations and that the proposed excitation optimization scheme is effective.
Grid-Connected Dual Stator-Winding Induction Generator Wind Power System for Wide Wind Speed Ranges
Kai Shi,Peifeng Xu,Zengqiang Wan,Feifei Bu,Zhiming Fang,Rongke Liu,Dean Zhao 전력전자학회 2016 JOURNAL OF POWER ELECTRONICS Vol.16 No.4
This paper presents a grid-connected dual stator-winding induction generator (DWIG) wind power system suitable for wide wind speed ranges. The parallel connection via a unidirectional diode between dc buses of both stator-winding sides is employed in this DWIG system, which can output a high dc voltage over wide wind speed ranges. Grid-connected inverters (GCIs) do not require booster converters; hence, the efficiency of wind energy utilization increases, and the hardware topology and control strategy of GCIs are simplified. In view of the particularities of the parallel topology and the adopted generator control strategy, we propose a novel excitation–capacitor optimization solution to reduce the volume and weight of the static excitation controller. When this excitation–capacitor optimization is carried out, the maximum power tracking problem is also considered. All the problems are resolved with the combined control of the DWIG and GCI. Experimental results on the platform of a 37 kW/600 V prototype show that the proposed DWIG wind power system can output a constant dc voltage over wide rotor speed ranges for grid-connected operations and that the proposed excitation optimization scheme is effective.
Effect of Tensile Deformation of Austenite on the Morphology and Strength of Lath Martensite
Zengmin Shi,Kai liu,Maoqiu Wang,Jie Shi,Han Dong,Jian Pu,Bo Chi,Yisheng Zhang,Li Jian 대한금속·재료학회 2012 METALS AND MATERIALS International Vol.18 No.2
A hot-rolled steel, 22SiMn2TiB, was employed to study the effect of austenite deformation on the micro-structure and strength of the subsequently formed lath martensite. It was revealed that the sizes of the mar-tensite packet, block and lath were refined by the tensile deformation of austenite at temperatures above 850 °C. With the increase of the deformation temperature, the packet size increased, whereas the block size decreased. The width of the lath was independent of the prior austenite grain size and the deformation temperature. The refinement of martensite blocks was considered to strengthen the ausformed martensite.
A new cavitation model considering inter-bubble action
Shi Yazhen,Luo Kai,Chen Xiaopeng,Li Daijin,Jia Laibing 대한조선학회 2021 International Journal of Naval Architecture and Oc Vol.13 No.1
The process of cavitation involves generation, growth, coalescence, and collapse of small bubbles and is tremendously influenced by bubbleebubble interactions. To understand these interactions, a new cavitation model based on the transport equation is proposed herein. The modified RayleighePlesset equation is analyzed to determine the bubble growth rate by assuming equal-sized spherical bubble clouds. The source term in the transport equation is then derived according to the bubble growth rate with the bubble-bubble interaction. The proposed model is validated by various test simulations, including microscopic bubble cloud evolution as well as macroscopical two- and three-dimensional cavitating flows. Compared with previous models, namely the Kunz and Zwart cavitation models, the newly proposed model does not require adjustable parameters and generally results in better predictions both microscopic and macroscopical cases. This model is more physical.
Design Methodology for Optimal Phase-Shift Modulation of Non-Inverting Buck-Boost Converters
Shi, Bingqing,Zhao, Zhengming,Li, Kai,Feng, Gaohui,Ji, Shiqi,Zhou, Jiayue The Korean Institute of Power Electronics 2019 JOURNAL OF POWER ELECTRONICS Vol.19 No.5
The non-inverting buck-boost converter (NIBB) is a step-up and step-down DC-DC converter suitable for wide-input-voltage-range applications. However, when the input voltage is close to the output voltage, the NIBB needs to operate in the buck-boost mode, causing a significant efficiency reduction since all four switches operates in the PWM mode. Considering both the current stress limitation and the efficiency optimization, a novel design methodology for the optimal phase-shift modulation of a NIBB in the buck-boost mode is proposed in this paper. Since the four switches in the NIBB form two bridges, the shifted phase between the two bridges can serve as an extra degree of freedom for performance optimization. With general phase-shift modulation, the analytic current expressions for every duty ratio, shifted phase and input voltage are derived. Then with the two key factors in the NIBB, the converter efficiency and the switch current stress, taken into account, an objective function with constraints is derived. By optimizing the derived objective function over the full input voltage range, an offline design methodology for the optimal modulation scheme is proposed for efficiency optimization on the premise of current stress limitation. Finally, the designed optimal modulation scheme is implemented on a DSPs and the design methodology is verified with experimental results on a 300V-1.5kW NIBB prototype.