An experimental investigation on DC flow suppression in a single-stage G-M type pulse tube cooler is made. The influence of DC flow induced by the introduction of the double-inlet on the refrigeration performance of t...An experimental investigation on DC flow suppression in a single-stage G-M type pulse tube cooler is made. The influence of DC flow induced by the introduction of the double-inlet on the refrigeration performance of the cooler is experimentally examined. Two parallelplaced needle valves with an opposite flow direction called as double-valved configuration, instead of conventional single-valved configuration as the double-inlet is used to reduce the DC flow. With the double-valved configuration, the minimum temperatures of 18.4 K and 14.7 K, and the cooling powers of 11.5 W and 29.5 W are also obtained by RW2 and CP4000, respectively.展开更多
Conversion of hourly dispatch cases derived using DC optimal power flow(DCOPF)to AC power flow(ACPF)case is often challenging and requires arduous human analysis and intervention.This paper proposes an automated two-s...Conversion of hourly dispatch cases derived using DC optimal power flow(DCOPF)to AC power flow(ACPF)case is often challenging and requires arduous human analysis and intervention.This paper proposes an automated two-stage approach to solve ACPF formulated from DCOPF dispatch cases.The first stage involved the use of the conventional Newton Raphson method to solve the ACPF from flat start,then ACPF cases that are unsolvable in the first stage are subjected to a hotstarting incremental method,based on homotopy continuation,in the second stage.Critical tasks such as the addition of reactive power compensation and tuning of voltage setpoints that typically require human intervention were automated using a criteriabased selection method and optimal power flow respectively.Two datasets with hourly dispatches for the 243-bus reduced WECC system were used to test the proposed method.The algorithm was able to convert 100%of the first set of dispatch cases to solved ACPF cases.In the second dataset with suspect dispatch cases to represent an extreme conversion scenario,the algorithm created solved ACPF cases that satisfied a defined success criterion for 77.8%of the dispatch cases.The average run time for the hotstarting algorithm to create a solved ACPF case for a dispatch was less than 1 minute for the reduced WECC system.展开更多
There are issues with flexible DC transmission system such as a lack of control freedom over power flow.In order to tackle these issues,a DC power flow controller(DCPFC)is incorporated into a multi-terminal,flexible D...There are issues with flexible DC transmission system such as a lack of control freedom over power flow.In order to tackle these issues,a DC power flow controller(DCPFC)is incorporated into a multi-terminal,flexible DC power grid.In recent years,a multi-port DC power flow controller based on a modular multi-level converter has become a focal point of research due to its simple structure and robust scalability.This work proposes a model predictive control(MPC)strategy for multi-port interline DC power flow controllers in order to improve their steady-state dynamic performance.Initially,the mathematical model of a multi-terminal DC power grid with a multi-port interline DC power flow controller is developed,and the relationship between each regulated variable and control variable is determined;The power flow controller is then discretized,and the cost function and weight factor are built with numerous control objectives.Sub module sorting method and nearest level approximation modulation regulate the power flow controller;Lastly,theMATLAB/Simulink simulation platformis used to verify the correctness of the establishedmathematicalmodel and the control performance of the suggestedMPC strategy.Finally,it is demonstrated that the control strategy possesses the benefits of robust dynamic performance,multiobjective control,and a simple structure.展开更多
New phenomena discovered in the experimental research of the ultra-high frequency pulse tube cryocooler were presented.The cause of the new phenomena was analyzed and the third type DC flow was discovered in the pulse...New phenomena discovered in the experimental research of the ultra-high frequency pulse tube cryocooler were presented.The cause of the new phenomena was analyzed and the third type DC flow was discovered in the pulse tube cryocooler.The third type DC flow not only deteriorated cooling capacity but also led to temperature instability of the pulse tube cryocooler.From the fluid network theory and the simple regenerator model,the root of the third type DC flow was concisely investigated in theory. The asymmetric resistance of oscillating flow in pulse tube cryocooler was the key mechanism of the third type DC flow.Some suppression methods were briefly discussed.展开更多
A mathematical model describing the flow field, heat transfer and the electromagnetic phenomenon in a DC electric arc furnace has been developed. First the governing equations in the arc plasma region are solved and t...A mathematical model describing the flow field, heat transfer and the electromagnetic phenomenon in a DC electric arc furnace has been developed. First the governing equations in the arc plasma region are solved and the calculated results of heat transfer, current density and shear stresses on the anode surface are used as boundary conditions in a model of molten bath. Then a two-dimensional time-dependent model is used to describe the flow field and electromagnetic phenomenon in the molten bath. Moreover, the effect of bottom electrode diameter on the circulation of molten bath is studied.展开更多
A comprehensive mathematical model has been developed to describe the interaction of the multiple physics fields during the conventional DC casting and LFEC (low frequency electromagnetic casting) process. The model i...A comprehensive mathematical model has been developed to describe the interaction of the multiple physics fields during the conventional DC casting and LFEC (low frequency electromagnetic casting) process. The model is based on a combination of the commercial finite element package ANSYS and the commercial finite volume package FLUENT, with the former for the calculation of the electromagnetic field and the latter for the calculation of the magnetic driven fluid flow, heat transfer and solidification. Moreover, the model has been verified against the temperature measurements obtained from two 7XXX aluminum alloy billets of 200mm diameter, cast during the conventional DC casting and the LFEC casting processes. In addition, a measurement of the sump shape of the billets were carried out by using addition melting metal of Al-30%Cu alloy into the billets during casting process. There was a good agreement between the calculated results and the measured results. Further, comparison of the calculated results during the LFEC process with that during the conventional DC casting process indicated that velocity patterns, temperature profiles and the sump depth are strongly modified by the application of a low frequency electromagnetic field during the DC casting.展开更多
A direct current(DC) source excited plasma jet consisting of a hollow needle anode and a plate cathode has been developed to form a diffuse discharge plume in ambient air with flowing argon as the working gas.Using ...A direct current(DC) source excited plasma jet consisting of a hollow needle anode and a plate cathode has been developed to form a diffuse discharge plume in ambient air with flowing argon as the working gas.Using optical and electrical methods,the discharge characteristics are investigated for the diffuse plasma plume.Results indicate that the discharge has a pulse characteristic,under the excitation of a DC voltage.The discharge pulse corresponds to the propagation process of a plasma bullet travelling from the anode to the cathode.It is found that,with an increment of the gas flow rate,both the discharge plume length and the current peak value of the pulsed discharge decrease in the laminar flow mode,reach their minima at about1.5 L/min,and then slightly increase in the turbulent mode.However,the frequency of the pulsed discharge increases in the laminar mode with increasing the argon flow rate until the argon flow rate equals to about 1.5 L/min,and then slightly decreases in the turbulent mode.展开更多
多端柔性直流输电系统(voltage source converter based multi-terminal direct current system,VSC-MTDC)是在两端直流输电基础上发展而来的直流输电系统,可靠性高、适用于海上风电并网等场景,相应的控制策略更加多样,其潮流模型也更...多端柔性直流输电系统(voltage source converter based multi-terminal direct current system,VSC-MTDC)是在两端直流输电基础上发展而来的直流输电系统,可靠性高、适用于海上风电并网等场景,相应的控制策略更加多样,其潮流模型也更加复杂。该文分析VSC换流站和MTDC的稳态模型,研究可适用于VSC-MTDC的不同拓扑和连接方式的潮流模型;研究多端直流各种控制策略潮流计算模型的不同点并推导出其方程。在此基础上,提出一种兼具统一迭代法和交替迭代法优点的含VSC-MTDC交直流系统潮流算法,给出初值和边界条件求解方法,对于不同直流拓扑和交流系统都能够进行求解。最后通过2个实际算例验证该文潮流模型的有效性和收敛性,对于不同控制方式、不同运行场景和直流拓扑都能够迅速、稳定收敛。展开更多
文摘An experimental investigation on DC flow suppression in a single-stage G-M type pulse tube cooler is made. The influence of DC flow induced by the introduction of the double-inlet on the refrigeration performance of the cooler is experimentally examined. Two parallelplaced needle valves with an opposite flow direction called as double-valved configuration, instead of conventional single-valved configuration as the double-inlet is used to reduce the DC flow. With the double-valved configuration, the minimum temperatures of 18.4 K and 14.7 K, and the cooling powers of 11.5 W and 29.5 W are also obtained by RW2 and CP4000, respectively.
基金This work was supported by the ERC Program of the National Science Foundation and DOE under NSF Award Number EEC-1041877the CURENT Industry Partnership Program,and the Bredesen Centre,University of Tennessee,Knoxville.
文摘Conversion of hourly dispatch cases derived using DC optimal power flow(DCOPF)to AC power flow(ACPF)case is often challenging and requires arduous human analysis and intervention.This paper proposes an automated two-stage approach to solve ACPF formulated from DCOPF dispatch cases.The first stage involved the use of the conventional Newton Raphson method to solve the ACPF from flat start,then ACPF cases that are unsolvable in the first stage are subjected to a hotstarting incremental method,based on homotopy continuation,in the second stage.Critical tasks such as the addition of reactive power compensation and tuning of voltage setpoints that typically require human intervention were automated using a criteriabased selection method and optimal power flow respectively.Two datasets with hourly dispatches for the 243-bus reduced WECC system were used to test the proposed method.The algorithm was able to convert 100%of the first set of dispatch cases to solved ACPF cases.In the second dataset with suspect dispatch cases to represent an extreme conversion scenario,the algorithm created solved ACPF cases that satisfied a defined success criterion for 77.8%of the dispatch cases.The average run time for the hotstarting algorithm to create a solved ACPF case for a dispatch was less than 1 minute for the reduced WECC system.
基金funded by National Natural Science Foundation of China (52177074).
文摘There are issues with flexible DC transmission system such as a lack of control freedom over power flow.In order to tackle these issues,a DC power flow controller(DCPFC)is incorporated into a multi-terminal,flexible DC power grid.In recent years,a multi-port DC power flow controller based on a modular multi-level converter has become a focal point of research due to its simple structure and robust scalability.This work proposes a model predictive control(MPC)strategy for multi-port interline DC power flow controllers in order to improve their steady-state dynamic performance.Initially,the mathematical model of a multi-terminal DC power grid with a multi-port interline DC power flow controller is developed,and the relationship between each regulated variable and control variable is determined;The power flow controller is then discretized,and the cost function and weight factor are built with numerous control objectives.Sub module sorting method and nearest level approximation modulation regulate the power flow controller;Lastly,theMATLAB/Simulink simulation platformis used to verify the correctness of the establishedmathematicalmodel and the control performance of the suggestedMPC strategy.Finally,it is demonstrated that the control strategy possesses the benefits of robust dynamic performance,multiobjective control,and a simple structure.
基金Supported by the Major Project of National Natural Science Foundation of China(Grant No.50890181)National Natural Science Foundation of China(Grant No.50676100)
文摘New phenomena discovered in the experimental research of the ultra-high frequency pulse tube cryocooler were presented.The cause of the new phenomena was analyzed and the third type DC flow was discovered in the pulse tube cryocooler.The third type DC flow not only deteriorated cooling capacity but also led to temperature instability of the pulse tube cryocooler.From the fluid network theory and the simple regenerator model,the root of the third type DC flow was concisely investigated in theory. The asymmetric resistance of oscillating flow in pulse tube cryocooler was the key mechanism of the third type DC flow.Some suppression methods were briefly discussed.
文摘A mathematical model describing the flow field, heat transfer and the electromagnetic phenomenon in a DC electric arc furnace has been developed. First the governing equations in the arc plasma region are solved and the calculated results of heat transfer, current density and shear stresses on the anode surface are used as boundary conditions in a model of molten bath. Then a two-dimensional time-dependent model is used to describe the flow field and electromagnetic phenomenon in the molten bath. Moreover, the effect of bottom electrode diameter on the circulation of molten bath is studied.
文摘A comprehensive mathematical model has been developed to describe the interaction of the multiple physics fields during the conventional DC casting and LFEC (low frequency electromagnetic casting) process. The model is based on a combination of the commercial finite element package ANSYS and the commercial finite volume package FLUENT, with the former for the calculation of the electromagnetic field and the latter for the calculation of the magnetic driven fluid flow, heat transfer and solidification. Moreover, the model has been verified against the temperature measurements obtained from two 7XXX aluminum alloy billets of 200mm diameter, cast during the conventional DC casting and the LFEC casting processes. In addition, a measurement of the sump shape of the billets were carried out by using addition melting metal of Al-30%Cu alloy into the billets during casting process. There was a good agreement between the calculated results and the measured results. Further, comparison of the calculated results during the LFEC process with that during the conventional DC casting process indicated that velocity patterns, temperature profiles and the sump depth are strongly modified by the application of a low frequency electromagnetic field during the DC casting.
基金supported by National Natural Science Foundation of China(Nos.10805013,11375051)Funds for Distinguished Young Scientists of Hebei Province,China(No.A2012201045)+1 种基金Department of Education for Outstanding Youth Project of China(No.Y2011120)Youth Project of Hebei University of China(No.2011Q14)
文摘A direct current(DC) source excited plasma jet consisting of a hollow needle anode and a plate cathode has been developed to form a diffuse discharge plume in ambient air with flowing argon as the working gas.Using optical and electrical methods,the discharge characteristics are investigated for the diffuse plasma plume.Results indicate that the discharge has a pulse characteristic,under the excitation of a DC voltage.The discharge pulse corresponds to the propagation process of a plasma bullet travelling from the anode to the cathode.It is found that,with an increment of the gas flow rate,both the discharge plume length and the current peak value of the pulsed discharge decrease in the laminar flow mode,reach their minima at about1.5 L/min,and then slightly increase in the turbulent mode.However,the frequency of the pulsed discharge increases in the laminar mode with increasing the argon flow rate until the argon flow rate equals to about 1.5 L/min,and then slightly decreases in the turbulent mode.
文摘多端柔性直流输电系统(voltage source converter based multi-terminal direct current system,VSC-MTDC)是在两端直流输电基础上发展而来的直流输电系统,可靠性高、适用于海上风电并网等场景,相应的控制策略更加多样,其潮流模型也更加复杂。该文分析VSC换流站和MTDC的稳态模型,研究可适用于VSC-MTDC的不同拓扑和连接方式的潮流模型;研究多端直流各种控制策略潮流计算模型的不同点并推导出其方程。在此基础上,提出一种兼具统一迭代法和交替迭代法优点的含VSC-MTDC交直流系统潮流算法,给出初值和边界条件求解方法,对于不同直流拓扑和交流系统都能够进行求解。最后通过2个实际算例验证该文潮流模型的有效性和收敛性,对于不同控制方式、不同运行场景和直流拓扑都能够迅速、稳定收敛。