For dq control strategies in single-phase pulse width modulation(PWM)converters,the-axis current must be created by imaginary axis current estimation(IACE)methods.The estimated error of the-axis current during the tra...For dq control strategies in single-phase pulse width modulation(PWM)converters,the-axis current must be created by imaginary axis current estimation(IACE)methods.The estimated error of the-axis current during the transient process causes d-q axis current loops to be incompletely decoupled,thereby affecting the dynamic performance of the current loop.The second-order generalized integrator(SOGI)method suffers from slow dynamic response.The fictive-axis emulation(FAE)method provides fast dynamic response but it is sensitive to circuit parameters.A reference-input(RI)-based IACE method is proposed to overcome the above shortcomings.According to the characteristic that the-axis current loop has no transient process,the-axis current is estimated by the d-q axis reference inputs.This is equivalent to introducing the-axis reference input as a feedforward term into the d-q axis current loop,so the parameter sensitivity problem is solved,and the parameter tuning is not needed.The proposed method can maintain good steadystate performance and significantly improve the dynamic performance of the current loop.Furthermore,it is straightforward and can be easily implemented in digital controllers.Comprehensive hardware-in-the-loop(HIL)experimental comparisons with the SOGI and FAE methods have been conducted to verify the correctness and effectiveness of the proposed RI-based IACE method.展开更多
The emergence of distributed generators has changed the operational mode and fault characteristics of the distribu-tion network,in a way which can severely influence protection.This paper proposes a d-axis-based curre...The emergence of distributed generators has changed the operational mode and fault characteristics of the distribu-tion network,in a way which can severely influence protection.This paper proposes a d-axis-based current differential protection scheme.The d-axis current characteristics of inverter-interfaced distributed generators and synchronous generators are analyzed.The differential protection criterion using sampling values of the d-axis current component is then constructed.Compared to conventional phase-based current differential protection,the proposed protection reduces the number of required communication channels,and is suitable for distribution networks with inverter-interfaced distributed generators with complex fault characteristics.Finally,a 10 kV active distribution network model is built in the PSCAD platform and protection prototypes are developed in RTDS.Superior sensitivity and fast speed are verified by simulation and RTDS-based tests.展开更多
Based on blade element momentum theory and generator characteristic test,a dynamic simulation model of 150 kW horizontal-axis tidal current turbine was established.The matching of the dynamic characteristics between t...Based on blade element momentum theory and generator characteristic test,a dynamic simulation model of 150 kW horizontal-axis tidal current turbine was established.The matching of the dynamic characteristics between the turbine and generator under various current velocities is studied,and the influence of the pitch angle on the matching is analyzed.For the problem of maximum power output in case of low current speed and limiting power in high current speed,the relation between optimal pitch angle and output power is analyzed.On the basis of dynamic characteristic analysis,the variable pitch control strategy is developed.The performance of the turbine under various tidal conditions is simulated.The research results show that the designed controller enables the turbine to operate efficiently under the condition of low current speed,and achieve the goal of limited power at high current speed.展开更多
Using the PimpleDyMFoam solver in open-source computing software OpenFOAM,based on the SST k-ωturbulence model and PIMPLE algorithm,a numerical simulation method of vertical-axis marine current turbines(VMCTs)is prop...Using the PimpleDyMFoam solver in open-source computing software OpenFOAM,based on the SST k-ωturbulence model and PIMPLE algorithm,a numerical simulation method of vertical-axis marine current turbines(VMCTs)is proposed,and the calculated results are compared with the experimental results.The results show that the numerical simulation method is feasible.Compared with other commercial softwares,this method has the advantages of higher solution efficiency and greater flexibility.According to the needs of users,the solver can be built on the basis of original code,and the corresponding discrete method can be optimized.This method can achieve optimization algorithms,save time and cost,etc.Secondly,the effects of different parameters(mesh density,time step,the selection of sidewall boundary conditions and inlet turbulence intensity)on numerical simulation of the VMCT are studied in detail.The findings summarize an effective CFD simulation strategy based on OpenFOAM and provide a valuable reference for future CFD simulations of VMCTs.展开更多
In traction application,speed range of motor is an important index for motor drives.In AC motor control,the maximum speed of the motor is limited by the output voltage capability of the traditional three-phase inverte...In traction application,speed range of motor is an important index for motor drives.In AC motor control,the maximum speed of the motor is limited by the output voltage capability of the traditional three-phase inverter with star connenction of windings.Switching the star connection to delta connection in high-speed range can extend the speed range.In order to extend the speed operation region,star-delta switch proposed by many literatures relies on mechanical relay,which needs a dead zone of tens of milliseconds and seriously affects torque output.Besides,the traditional method will cause current overshoot during the switch transient process,decreasing the device security and reliability.Aiming at the defects existing in the star-delta hard switching,this paper proposes a star-delta soft switching method.Without adding extra power electronics devices,DC-bus capacitor is used to provide the path of zero axis current in the transient process,which helps to achieve the smooth torque output and zero current switch in the transient.Experiments have been done to validate the performance of the proposed method.The switching transient from star to delta connection in the motor drive can be much more stable than hard switching method.展开更多
The effects of trapped electrons on off-axis lower hybrid current drive （LHCD） in tokamaks are studied. The influence of the resonance regime on the current drive efficiency as well as the influence of trapped parti...The effects of trapped electrons on off-axis lower hybrid current drive （LHCD） in tokamaks are studied. The influence of the resonance regime on the current drive efficiency as well as the influence of trapped particle fraction on the current drive efficiency are emphasized.展开更多
The effects of the nuclear current in the antimagnetic rotation band of 105Cd have been investigated in a fully self-consistent and microscopic way by using the tilted axis cranking relativistic mean-field model.It wa...The effects of the nuclear current in the antimagnetic rotation band of 105Cd have been investigated in a fully self-consistent and microscopic way by using the tilted axis cranking relativistic mean-field model.It was found that the inclusion of nuclear current leads to a higher angular momentum and thus a larger kinetic moment of inertia at a given rotational frequency.As a consequence,the B(E2) values with current are always smaller than those without current.展开更多
基金supported by the National Natural Science Foundation of China under Grant 61733015,62473322High-Speed Railway Joint Funds of National Natural Science Foundation of China under Grant U1934204.
文摘For dq control strategies in single-phase pulse width modulation(PWM)converters,the-axis current must be created by imaginary axis current estimation(IACE)methods.The estimated error of the-axis current during the transient process causes d-q axis current loops to be incompletely decoupled,thereby affecting the dynamic performance of the current loop.The second-order generalized integrator(SOGI)method suffers from slow dynamic response.The fictive-axis emulation(FAE)method provides fast dynamic response but it is sensitive to circuit parameters.A reference-input(RI)-based IACE method is proposed to overcome the above shortcomings.According to the characteristic that the-axis current loop has no transient process,the-axis current is estimated by the d-q axis reference inputs.This is equivalent to introducing the-axis reference input as a feedforward term into the d-q axis current loop,so the parameter sensitivity problem is solved,and the parameter tuning is not needed.The proposed method can maintain good steadystate performance and significantly improve the dynamic performance of the current loop.Furthermore,it is straightforward and can be easily implemented in digital controllers.Comprehensive hardware-in-the-loop(HIL)experimental comparisons with the SOGI and FAE methods have been conducted to verify the correctness and effectiveness of the proposed RI-based IACE method.
基金supported by China Southern Power Grid Technology Project.
文摘The emergence of distributed generators has changed the operational mode and fault characteristics of the distribu-tion network,in a way which can severely influence protection.This paper proposes a d-axis-based current differential protection scheme.The d-axis current characteristics of inverter-interfaced distributed generators and synchronous generators are analyzed.The differential protection criterion using sampling values of the d-axis current component is then constructed.Compared to conventional phase-based current differential protection,the proposed protection reduces the number of required communication channels,and is suitable for distribution networks with inverter-interfaced distributed generators with complex fault characteristics.Finally,a 10 kV active distribution network model is built in the PSCAD platform and protection prototypes are developed in RTDS.Superior sensitivity and fast speed are verified by simulation and RTDS-based tests.
基金the Special Funds for Scientific Research in Marine Public Welfare Industry(Grant No.201205019-3).
文摘Based on blade element momentum theory and generator characteristic test,a dynamic simulation model of 150 kW horizontal-axis tidal current turbine was established.The matching of the dynamic characteristics between the turbine and generator under various current velocities is studied,and the influence of the pitch angle on the matching is analyzed.For the problem of maximum power output in case of low current speed and limiting power in high current speed,the relation between optimal pitch angle and output power is analyzed.On the basis of dynamic characteristic analysis,the variable pitch control strategy is developed.The performance of the turbine under various tidal conditions is simulated.The research results show that the designed controller enables the turbine to operate efficiently under the condition of low current speed,and achieve the goal of limited power at high current speed.
基金National Natural Science Foundation of China(11572094,5171101175,51809083)。
文摘Using the PimpleDyMFoam solver in open-source computing software OpenFOAM,based on the SST k-ωturbulence model and PIMPLE algorithm,a numerical simulation method of vertical-axis marine current turbines(VMCTs)is proposed,and the calculated results are compared with the experimental results.The results show that the numerical simulation method is feasible.Compared with other commercial softwares,this method has the advantages of higher solution efficiency and greater flexibility.According to the needs of users,the solver can be built on the basis of original code,and the corresponding discrete method can be optimized.This method can achieve optimization algorithms,save time and cost,etc.Secondly,the effects of different parameters(mesh density,time step,the selection of sidewall boundary conditions and inlet turbulence intensity)on numerical simulation of the VMCT are studied in detail.The findings summarize an effective CFD simulation strategy based on OpenFOAM and provide a valuable reference for future CFD simulations of VMCTs.
文摘In traction application,speed range of motor is an important index for motor drives.In AC motor control,the maximum speed of the motor is limited by the output voltage capability of the traditional three-phase inverter with star connenction of windings.Switching the star connection to delta connection in high-speed range can extend the speed range.In order to extend the speed operation region,star-delta switch proposed by many literatures relies on mechanical relay,which needs a dead zone of tens of milliseconds and seriously affects torque output.Besides,the traditional method will cause current overshoot during the switch transient process,decreasing the device security and reliability.Aiming at the defects existing in the star-delta hard switching,this paper proposes a star-delta soft switching method.Without adding extra power electronics devices,DC-bus capacitor is used to provide the path of zero axis current in the transient process,which helps to achieve the smooth torque output and zero current switch in the transient.Experiments have been done to validate the performance of the proposed method.The switching transient from star to delta connection in the motor drive can be much more stable than hard switching method.
基金Supported by the National Natural Science Foundation of China （10675043, 10575031 and 10675042）
文摘The effects of trapped electrons on off-axis lower hybrid current drive （LHCD） in tokamaks are studied. The influence of the resonance regime on the current drive efficiency as well as the influence of trapped particle fraction on the current drive efficiency are emphasized.
基金supported by the National Basic Research Program of China(Grant No.2007CB815000)the National Natural Science Foundation of China(Grant Nos.10975008, 11105005 and 11175002)the Research Fund for the Doctoral Program of Higher Education(Grant No.20110001110087)
文摘The effects of the nuclear current in the antimagnetic rotation band of 105Cd have been investigated in a fully self-consistent and microscopic way by using the tilted axis cranking relativistic mean-field model.It was found that the inclusion of nuclear current leads to a higher angular momentum and thus a larger kinetic moment of inertia at a given rotational frequency.As a consequence,the B(E2) values with current are always smaller than those without current.
