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Yunxiao Yang,Ke Ma,Weiyao Wang 전력전자학회 2019 ICPE(ISPE)논문집 Vol.2019 No.5
Modular Multilevel Converter (MMC) has been widely used in medium/high voltage direct current transmission, motor drive, and renewable energy power generation, etc. The reliability requirements of MMC system which is composed of sub-modules(SM) as building blocks, are getting crucial. However, the testing for reliability of SMs in a full MMC system is time consuming and costly. In this paper, a mission profile emulator for SMs in MMC system is therefore focused, by using this mission profile emulator, the SMs can be tested individually without building a full MMC system. Simulation results show that the mission profiles of the SM under test is well agrees with the SM in a complete MMC system.
Molecular Dynamics Simulation of supercritical spray at micro & nano scale
( Zhang Yunxiao ),( Fu Qingfei ),( Mo Chaojie ),( Yang Lijun ) 한국액체미립화학회 2017 한국액체미립화학회 학술강연회 논문집 Vol.2017 No.-
The supercritical jets have many applications in many fields ranging from aerospace propulsion to chemical engineering. However, there is still lack of relevant theoretical research on the characteristics of supercritical fluids, and also lack of deep understanding of the destabilization mechanism of supercritical jets. Molecular dynamics method, which has been quite mature, can simulate extreme harsh physical conditions and exceptionally complex geometric model. The simulation process spontaneously contains all the physical effects. Therefore, the purpose of this paper is to study the characteristics and destabilization mechanism of supercritical jets from the microcosmic perspective using molecular dynamics method. Firstly, the molecular dynamics simulation of EOS (Equation of State) of liquid nitrogen was carried out. The influence of different simulation parameters on the simulation results was analyzed and then compared with the theoretical results. Four conditions were selected to study the influences of different time step, total simulation steps and truncation distance on the simulation results. The results show that the truncation distance has a great influence on the simulation precision, the time step and the total simulation steps have no significant impact on the EOS of liquid nitrogen. The results are in good agreement with the SRK (Soave-Redlich-Kwong) equation, which show the correctness of the simulation of supercritical fluids using molecular dynamics method. Secondly, the molecular dynamics simulation of supercritical nitrogen jet was carried out at different conditions. The supercritical jets ejected into a supercritical environment and a subcritical environment, respectively. Through the analysis of the results, the distribution of jet velocity, density, temperature and other jet characteristics was obtained. The interface was determined where concentration of supercritical fluid component reached 50% because the supercritical jet has no obvious vapor-liquid interface. Stable analysis of the results was performed and the disturbance growth rate of the supercritical shear layer at different conditions was obtained. Finally, the growth rate achieved by processing the results of molecular simulation was compared to the results by the linear stability analysis.
Weiyao Wang,Ke Ma,Yunxiao Yang,Xu Cai 전력전자학회 2019 ICPE(ISPE)논문집 Vol.2019 No.5
The modular multilevel converter (MMC) is one of the most promising converter topologies for medium/high voltage energy conversion. This paper focuses on the balancing method of the submodule capacitor voltages for MMC based on Carrier-Phase-Shift Pulse Width Modulation (CPS-PWM). Firstly, the unbalance mechanism of the submodule capacitor voltages is theoretically revealed. Then the capacitor voltage disturbances are extracted by sampling over a non-synchronized switching period. Based on the characteristics of the extracted disturbances in nonsynchronized switching period, a method for capacitor voltage balancing is thereby proposed which is efficient for various working conditions and easy to realize. Finally, the effect of the proposed method is verified by comprehensive simulation in PLECS.
Houyin Xi,Dong Zhang,Tao Zhou,Yunxiao Yang,Qiang Wei 제어·로봇·시스템학회 2021 International Journal of Control, Automation, and Vol.19 No.3
Wind disturbance may significantly reduce the flight control performance of quadrotors when flying. In order to meet high performance flight control requirements, this paper presents a quadrotor anti-wind model (QAWM) and a flight control scheme for quadrotors. The QAWM takes the influence of aerodynamic effects on the propeller, gyroscopic effect and wind disturbance into account. The model can represent more flight states including not only hovering but also flight in windy conditions even maneuver flight. A cascade control schemewith an improved extended state observer (IESO) is adopted to decompose the quadrotor flight control problem into position control loop and attitude control loop. In the attitude control loop, the IESO is used to estimate the unmodeled dynamics, parameter uncertainties and external disturbances, as well as compensate the angular velocity control. In addition, the stability of IESO and the closed loop system are proved. Simulation and experimental results show the effectiveness of the nonlinear quadrotor model and control scheme, and the control scheme can effectively improve the flight performance of quadrotors under wind disturbance even maneuver flight.