The DC microgrid has the advantages of high energy conversion efficiency,high energy transmission density,no reactive power flow,and grid-connected synchronization.It is an essential component of the future intelligen...The DC microgrid has the advantages of high energy conversion efficiency,high energy transmission density,no reactive power flow,and grid-connected synchronization.It is an essential component of the future intelligent power distribution system.Constant power load(CPL)will degrade the stability of the DC microgrid and cause system voltage oscillation due to its negative resistance characteristics.As a result,the stability of DC microgrids with CPL has become a problem.At present,the research on the stability of DC microgrid is mainly focused on unipolar DC microgrid,while the research on bipolar DC microgrid lacks systematic discussion.The stability of DC microgrid using CPL was studied first,and then the current stability criteria of DC microgrid were summarized,and its research trend was analyzed.On this basis,aiming at the stability problem caused by CPL,the existing control methods were summarized from the perspective of source converter output impedance and load converter input impedance,and the current control methods were outlined as active and passive control methods.Lastly,the research path of bipolar DC microgrid stability with CPL was prospected.展开更多
The study on homogeneous DBDs at atmospheric pressure has attracted much attention for their advantages in applications. Tremendous work has been conducted both experimentally and numerically at a constant applied vol...The study on homogeneous DBDs at atmospheric pressure has attracted much attention for their advantages in applications. Tremendous work has been conducted both experimentally and numerically at a constant applied voltage or driving frequency. However the investigation of dielectric barrier discharges is still scarce for a constant power or power density. In this work, a new computational approach for DBDs is developed to explore atmospheric DBDs at a constant power based on a one-dimensional fluid model. The frequency and gap spacing effects on the atmospheric plasmas are systematically analyzed based on computational data. The computational results show that at a constant power both the current density and the amplitude of the applied voltage decrease, whereas the current pulse width increases, with increasing frequency. The simulation also indicates that as the gap spacing is raised with a fixed power and frequency, the current density and electron density increase initially, then reach their peak values, and then decrease, which means that there are maximum values for both of them. These results are significant for many industrial applications, as they can be used to optimize plasma devices of DBDs with the consideration of power consumption.展开更多
The boost converter feeding a constant power load (CPL) is a non-minimum phase system that is prone to the destabilizing effects of the negative incremental resistance of the CPL and presents a major challenge in the ...The boost converter feeding a constant power load (CPL) is a non-minimum phase system that is prone to the destabilizing effects of the negative incremental resistance of the CPL and presents a major challenge in the design of stabilizing controllers. A PWM-based current-sensorless robust sliding mode controller is developed that requires only the measurement of the output voltage. An extended state observer is developed to estimate a lumped uncertainty signal that comprises the uncertain load power and the input voltage, the converter parasitics, the component uncertainties and the estimation of the derivative of the output voltage needed in the implementation of the controller. A linear sliding surface is used to derive the controller, which is simple in its design and yet exhibits excellent features in terms of robustness to external disturbances, parameter uncertainties, and parasitics despite the absence of the inductor’s current feedback. The robustness of the controller is validated by computer simulations.展开更多
The boost converter feeding a constant power load (CPL) is a non-minimum phase system that is prone to the destabilizing effects of the negative incremental resistance of the CPL and presents a major challenge in the ...The boost converter feeding a constant power load (CPL) is a non-minimum phase system that is prone to the destabilizing effects of the negative incremental resistance of the CPL and presents a major challenge in the design of stabilizing controllers. In this work, a robust nonlinear controller based on the uncertainty and disturbance estimator (UDE) scheme is successfully developed to tightly regulate the output voltage of the boost converter. A systematic procedure is developed to select the controller gains to achieve a satisfactory output response. Using simulation, the effectiveness of the proposed controller is validated and compared to a recent robust nonlinear controller.展开更多
In DC microgrid systems,interleaved boost converters(IBCs)are widely used to boost the output voltage of renewable energy sources on the source side of a DC bus owing to their high voltage gain and low current ripple....In DC microgrid systems,interleaved boost converters(IBCs)are widely used to boost the output voltage of renewable energy sources on the source side of a DC bus owing to their high voltage gain and low current ripple.However,because power electronic converters on the load side behave as constant power loads(CPLs)with negative impedance characteristics,their high penetration can degrade the system stability.Therefore,this article proposes a fractional-order nonlinear controller integrated with an extended nonlinear disturbance observer(ENDO)for N-phase IBCs.First,the reduced-order model of the IBC is transformed into a canonical form using the differential geometric method.Subsequently,with the ENDO,the dynamic performance can be enhanced by estimating the disturbances,and a fractional-order nonlinear sliding surface is established to avoid the singularity problem and increase control flexibility.In addition,the stability of the proposed controller is analyzed using Lyapunov’s theorem.In a CPL variation test,the proposed controller exhibited a faster dynamic performance and lower tracking error thanconventional controllers,with at least a 27%improvement in the integral squared error(ISE).Both simulation and experimental results demonstrated the effectivenessof the controller,which can ensure large-signal stability and improved dynamic performance in DC microgrid systems.展开更多
Modern electric power systems have increased the usage of switching power converters.These tightly regulated switching power converters behave as constant power loads(CPLs).They exhibit a negative incremental impedanc...Modern electric power systems have increased the usage of switching power converters.These tightly regulated switching power converters behave as constant power loads(CPLs).They exhibit a negative incremental impedance in small signal analysis.This negative impedance degrades the stability margin of the interaction between CPLs and their feeders,which is known as the negative impedance instability problem.The feeder can be an LC input filter or an upstream switching converter.Active damping methods are preferred for the stabilization of the system.This is due to their higher power efficiency over passive damping methods.Based on different sources of damping effect,this paper summarizes and classifies existing active damping methods into three categories.The paper further analyzes and compares the advantages and disadvantages of each active damping method.展开更多
This paper investigates the power allocation issues for joint transmission in heterogeneous network （HetNet）, which is characterized by severe cross-tier interference. The optimization problem of maximizing the HetN...This paper investigates the power allocation issues for joint transmission in heterogeneous network （HetNet）, which is characterized by severe cross-tier interference. The optimization problem of maximizing the HetNet throughput is formulated. The original problem turns out to be a non-convex problem, the global optima of which cannot be obtained by conventional optimization methods. This paper develops a novel method to achieve the global optima by tuming the original problem into quasi-convex problem. In addition, this paper considers a constant power allocation scheme, as a tradeoff between the system throughput and computational complexity. Based on duality gap theory, the bound of constant power allocation scheme is mathematically derived. Numerical results under different system parameters indicate that both the proposed schemes outperform conventional interference coordination schemes.展开更多
This paper presents a fully distributed state-of-charge balance control (DSBC) strategy for a distributed energy storage system (DESS). In this framework, each energy storage unit (ESU) processes the state-of-charge (...This paper presents a fully distributed state-of-charge balance control (DSBC) strategy for a distributed energy storage system (DESS). In this framework, each energy storage unit (ESU) processes the state-of-charge (SoC) information from its neighbors locally and adjusts the virtual impedance of the droop controller in real-time to change the current sharing. It is shown that the SoC balance of all ESUs can be achieved. Due to virtual impedance, voltage deviation of the bus occurs inevitably and increases with load power. Meanwhile, widespread of the constant power load (CPL) in the power system may cause instability. To ensure reliable operation of DESS under the proposed DSBC, the concept of the safe region is put forward. Within the safe region, DESS is stable and voltage deviation is acceptable. The boundary conditions of the safe region are derived from the equivalent model of DESS, in which stability is analyzed in terms of modified Brayton-Moser's criterion. Both simulations and hardware experiments verify the accuracy of the safe region and effectiveness of the proposed DSBC strategy.展开更多
Majority of non-Newtonian fluids are pseudoplastic with shear-thinning property, which means that the viscosity will be different in different parts of the stirred tank. In such mixing process, it is difficult to pred...Majority of non-Newtonian fluids are pseudoplastic with shear-thinning property, which means that the viscosity will be different in different parts of the stirred tank. In such mixing process, it is difficult to predict accurately the power consumption and mean shear rate for designing novel impeller. Metzner-Otto method is a widely accepted method to solve these questions in mixing non-Newtonian fluids. As a result, Metzner-Otto constant will become a key factor to achieve an optimum way of economical mixing. In this paper, taking glycerine and xanthan gum solutions as research system, the power consumption, stirred by the impeller composed of perturbed six-bent-bladed turbine （6PBT） with differently geometrical characteristics in a cylindrical vessel, is studied by means of computational fluid dynamics （CFD）. The flow is modeled as laminar and a multiple reference frame （MRF） approach is used to solve the discretized equations of motion. In order to determine the capability of CFD to forecast the flow process, the torque test experiment is used to measure the glycerine solution power consumption. The theological properties of the xanthan gum solutions are determined by a Brookfleld rheometer. It is observed that the power consumption predicted by numerical simulation agrees well with those measured using torque experiment method in stirring glycerine solution, which validate the numerical model. Metzner-Otto constant is almost not correlated with the flow behavior index of pseudoplastic fluids. This paper establishes the complete correlations of power constant and Metzner-Otto constant with impeller geometrical characteristics through linear regression analysis, which provides the valuable instructions and references for accurately predicting the power consumption and mean shear rate of pseudoplastic fluids in laminar flow, comparatively.展开更多
AIM: To evaluate the effect of different lens constant optimization methods on the accuracy of intraocular lens(IOL) power calculation formulas for highly myopic eyes.METHODS: This study comprised 108 eyes of 94 conse...AIM: To evaluate the effect of different lens constant optimization methods on the accuracy of intraocular lens(IOL) power calculation formulas for highly myopic eyes.METHODS: This study comprised 108 eyes of 94 consecutive patients with axial length(AL) over 26 mm undergoing phacoemulsification and implantation of a Rayner(Hove, UK) 920H IOL. Formulas were evaluated using the following lens constants: manufacturer’s lens constant, User Group for Laser Interference Biometry(ULIB) constant, and optimized constant for long eyes. Results were compared with Barrett Universal II formula, original Wang-Koch AL adjustment method, and modified Wang-Koch AL adjustment method. The outcomes assessed were mean absolute error(MAE) and percentage of eyes with IOL prediction errors within ±0.25, ±0.50, and ±1.0 diopter(D). The nonparametric method, Friedman test, was used to compare MAE performance among constants.RESULTS: Optimized constants could significantly reduce the MAE of SRK/T, Hoffer Q, and Holladay 1 formulas compared with manufacturer’s lens constant, whereas the percentage of eyes with IOL prediction errors within ±0.25, ±0.50, and ±1.0 D had no statistically significant differences. Optimized lens constant for long eyes alone showed non-significant refractive advantages over the ULIB constant. Barrett Universal II formula and formulas with AL adjustment showed significantly higher accuracy in highly myopic eyes(P<0.001). CONCLUSION: Lens constant optimization for the subset of long eyes reduces the refractive error only to a limited extent for highly myopic eyes.展开更多
In order to make the power level angle(PLA)have the same bijection relationship with the thrust when the environment changes before operational limit exceedance,and ensure the pilot's ability of unrestricted throt...In order to make the power level angle(PLA)have the same bijection relationship with the thrust when the environment changes before operational limit exceedance,and ensure the pilot's ability of unrestricted throttle movement as well as the safety of the engine,a method used to design a constant thrust power schedule was proposed.The research was based on a simulink turbofan engine model and the results were organized as look-up tables in different structures with limitations.Simulation was done to confirm the validity and demonstrate the effectiveness of the schedule obtained with the approach.The constant thrust takeoff schedule,constant thrust idle schedule and constant thrust part power schedule with the present constant thrust control(CTC)method were proved to be valid.The results can be applied to this specific engine but the method can be applied to any areoengine and can be even extended to the unmanned plane.展开更多
Mobile power meters allow for cyclists to monitor power output （PO） during training and competition. The Garrnin Vector power meter （VPM） measures PO at the pedal compared to the crank and has been tested in only ...Mobile power meters allow for cyclists to monitor power output （PO） during training and competition. The Garrnin Vector power meter （VPM） measures PO at the pedal compared to the crank and has been tested in only a few limited studies. The purpose of this study was to determine the validity and reproducibility of the VPM by comparing it to the SRM. The VPM validity was tested by （1） a submaximal incremental test, （2） submaximal constant power test, （3） sprint test, and （4） a field test. The reliability of the VPM was tested by repeating the laboratory tests 10 times over a 6 week span. Significant differences （P = 0.046） were found between the mean POSRM （178 ± 1.8 W） and POVPM （163.5 ± 14.7 W） for the submaximal constant-power test. No significant differences were found between the POMAX SRM and the POMAx VPM. The reproducibility of the VPM was lower than the SRM （CV = 8.52 ±4.0 vs 3.48 ± 1.9, 10.66% vs 5.50%, and 67.7% vs 55.3% for the submaximal incremental test, submaximal constant-power test, and field test respectively）. The POVPM appears to underestimate the POSRM and is less valid and reliable across various cycling efforts.展开更多
基金supported by National Natural Science Foundation of China(No.51767015)Key Project of Natural Science Foundation of Gansu Province(No.22JR5RA317)Tianyou Innovation Team Support Program of Lanzhou Jiaotong University(No.TY202009)。
文摘The DC microgrid has the advantages of high energy conversion efficiency,high energy transmission density,no reactive power flow,and grid-connected synchronization.It is an essential component of the future intelligent power distribution system.Constant power load(CPL)will degrade the stability of the DC microgrid and cause system voltage oscillation due to its negative resistance characteristics.As a result,the stability of DC microgrids with CPL has become a problem.At present,the research on the stability of DC microgrid is mainly focused on unipolar DC microgrid,while the research on bipolar DC microgrid lacks systematic discussion.The stability of DC microgrid using CPL was studied first,and then the current stability criteria of DC microgrid were summarized,and its research trend was analyzed.On this basis,aiming at the stability problem caused by CPL,the existing control methods were summarized from the perspective of source converter output impedance and load converter input impedance,and the current control methods were outlined as active and passive control methods.Lastly,the research path of bipolar DC microgrid stability with CPL was prospected.
文摘The study on homogeneous DBDs at atmospheric pressure has attracted much attention for their advantages in applications. Tremendous work has been conducted both experimentally and numerically at a constant applied voltage or driving frequency. However the investigation of dielectric barrier discharges is still scarce for a constant power or power density. In this work, a new computational approach for DBDs is developed to explore atmospheric DBDs at a constant power based on a one-dimensional fluid model. The frequency and gap spacing effects on the atmospheric plasmas are systematically analyzed based on computational data. The computational results show that at a constant power both the current density and the amplitude of the applied voltage decrease, whereas the current pulse width increases, with increasing frequency. The simulation also indicates that as the gap spacing is raised with a fixed power and frequency, the current density and electron density increase initially, then reach their peak values, and then decrease, which means that there are maximum values for both of them. These results are significant for many industrial applications, as they can be used to optimize plasma devices of DBDs with the consideration of power consumption.
文摘The boost converter feeding a constant power load (CPL) is a non-minimum phase system that is prone to the destabilizing effects of the negative incremental resistance of the CPL and presents a major challenge in the design of stabilizing controllers. A PWM-based current-sensorless robust sliding mode controller is developed that requires only the measurement of the output voltage. An extended state observer is developed to estimate a lumped uncertainty signal that comprises the uncertain load power and the input voltage, the converter parasitics, the component uncertainties and the estimation of the derivative of the output voltage needed in the implementation of the controller. A linear sliding surface is used to derive the controller, which is simple in its design and yet exhibits excellent features in terms of robustness to external disturbances, parameter uncertainties, and parasitics despite the absence of the inductor’s current feedback. The robustness of the controller is validated by computer simulations.
文摘The boost converter feeding a constant power load (CPL) is a non-minimum phase system that is prone to the destabilizing effects of the negative incremental resistance of the CPL and presents a major challenge in the design of stabilizing controllers. In this work, a robust nonlinear controller based on the uncertainty and disturbance estimator (UDE) scheme is successfully developed to tightly regulate the output voltage of the boost converter. A systematic procedure is developed to select the controller gains to achieve a satisfactory output response. Using simulation, the effectiveness of the proposed controller is validated and compared to a recent robust nonlinear controller.
基金Supported by the National Natural Science Foundation of China(52077085)Natural Science Foundation of Guangdong Province(2023A1515012273).
文摘In DC microgrid systems,interleaved boost converters(IBCs)are widely used to boost the output voltage of renewable energy sources on the source side of a DC bus owing to their high voltage gain and low current ripple.However,because power electronic converters on the load side behave as constant power loads(CPLs)with negative impedance characteristics,their high penetration can degrade the system stability.Therefore,this article proposes a fractional-order nonlinear controller integrated with an extended nonlinear disturbance observer(ENDO)for N-phase IBCs.First,the reduced-order model of the IBC is transformed into a canonical form using the differential geometric method.Subsequently,with the ENDO,the dynamic performance can be enhanced by estimating the disturbances,and a fractional-order nonlinear sliding surface is established to avoid the singularity problem and increase control flexibility.In addition,the stability of the proposed controller is analyzed using Lyapunov’s theorem.In a CPL variation test,the proposed controller exhibited a faster dynamic performance and lower tracking error thanconventional controllers,with at least a 27%improvement in the integral squared error(ISE).Both simulation and experimental results demonstrated the effectivenessof the controller,which can ensure large-signal stability and improved dynamic performance in DC microgrid systems.
文摘Modern electric power systems have increased the usage of switching power converters.These tightly regulated switching power converters behave as constant power loads(CPLs).They exhibit a negative incremental impedance in small signal analysis.This negative impedance degrades the stability margin of the interaction between CPLs and their feeders,which is known as the negative impedance instability problem.The feeder can be an LC input filter or an upstream switching converter.Active damping methods are preferred for the stabilization of the system.This is due to their higher power efficiency over passive damping methods.Based on different sources of damping effect,this paper summarizes and classifies existing active damping methods into three categories.The paper further analyzes and compares the advantages and disadvantages of each active damping method.
基金supported by the HuaWei Innovation Research Project (YJCB2011060WL)
文摘This paper investigates the power allocation issues for joint transmission in heterogeneous network （HetNet）, which is characterized by severe cross-tier interference. The optimization problem of maximizing the HetNet throughput is formulated. The original problem turns out to be a non-convex problem, the global optima of which cannot be obtained by conventional optimization methods. This paper develops a novel method to achieve the global optima by tuming the original problem into quasi-convex problem. In addition, this paper considers a constant power allocation scheme, as a tradeoff between the system throughput and computational complexity. Based on duality gap theory, the bound of constant power allocation scheme is mathematically derived. Numerical results under different system parameters indicate that both the proposed schemes outperform conventional interference coordination schemes.
基金supported by the National Natural Science Foundation of China under Grant 61933014 and Grant 62173243.
文摘This paper presents a fully distributed state-of-charge balance control (DSBC) strategy for a distributed energy storage system (DESS). In this framework, each energy storage unit (ESU) processes the state-of-charge (SoC) information from its neighbors locally and adjusts the virtual impedance of the droop controller in real-time to change the current sharing. It is shown that the SoC balance of all ESUs can be achieved. Due to virtual impedance, voltage deviation of the bus occurs inevitably and increases with load power. Meanwhile, widespread of the constant power load (CPL) in the power system may cause instability. To ensure reliable operation of DESS under the proposed DSBC, the concept of the safe region is put forward. Within the safe region, DESS is stable and voltage deviation is acceptable. The boundary conditions of the safe region are derived from the equivalent model of DESS, in which stability is analyzed in terms of modified Brayton-Moser's criterion. Both simulations and hardware experiments verify the accuracy of the safe region and effectiveness of the proposed DSBC strategy.
基金Supported by Shandong Provincial Science and Technology Development planning Program of China(Grant No.2013YD09007)Scientific Foundation of Qingdao University of Science and Technology of China
文摘Majority of non-Newtonian fluids are pseudoplastic with shear-thinning property, which means that the viscosity will be different in different parts of the stirred tank. In such mixing process, it is difficult to predict accurately the power consumption and mean shear rate for designing novel impeller. Metzner-Otto method is a widely accepted method to solve these questions in mixing non-Newtonian fluids. As a result, Metzner-Otto constant will become a key factor to achieve an optimum way of economical mixing. In this paper, taking glycerine and xanthan gum solutions as research system, the power consumption, stirred by the impeller composed of perturbed six-bent-bladed turbine （6PBT） with differently geometrical characteristics in a cylindrical vessel, is studied by means of computational fluid dynamics （CFD）. The flow is modeled as laminar and a multiple reference frame （MRF） approach is used to solve the discretized equations of motion. In order to determine the capability of CFD to forecast the flow process, the torque test experiment is used to measure the glycerine solution power consumption. The theological properties of the xanthan gum solutions are determined by a Brookfleld rheometer. It is observed that the power consumption predicted by numerical simulation agrees well with those measured using torque experiment method in stirring glycerine solution, which validate the numerical model. Metzner-Otto constant is almost not correlated with the flow behavior index of pseudoplastic fluids. This paper establishes the complete correlations of power constant and Metzner-Otto constant with impeller geometrical characteristics through linear regression analysis, which provides the valuable instructions and references for accurately predicting the power consumption and mean shear rate of pseudoplastic fluids in laminar flow, comparatively.
基金Supported by National Natural Science Foundation of China(No.81770905)
文摘AIM: To evaluate the effect of different lens constant optimization methods on the accuracy of intraocular lens(IOL) power calculation formulas for highly myopic eyes.METHODS: This study comprised 108 eyes of 94 consecutive patients with axial length(AL) over 26 mm undergoing phacoemulsification and implantation of a Rayner(Hove, UK) 920H IOL. Formulas were evaluated using the following lens constants: manufacturer’s lens constant, User Group for Laser Interference Biometry(ULIB) constant, and optimized constant for long eyes. Results were compared with Barrett Universal II formula, original Wang-Koch AL adjustment method, and modified Wang-Koch AL adjustment method. The outcomes assessed were mean absolute error(MAE) and percentage of eyes with IOL prediction errors within ±0.25, ±0.50, and ±1.0 diopter(D). The nonparametric method, Friedman test, was used to compare MAE performance among constants.RESULTS: Optimized constants could significantly reduce the MAE of SRK/T, Hoffer Q, and Holladay 1 formulas compared with manufacturer’s lens constant, whereas the percentage of eyes with IOL prediction errors within ±0.25, ±0.50, and ±1.0 D had no statistically significant differences. Optimized lens constant for long eyes alone showed non-significant refractive advantages over the ULIB constant. Barrett Universal II formula and formulas with AL adjustment showed significantly higher accuracy in highly myopic eyes(P<0.001). CONCLUSION: Lens constant optimization for the subset of long eyes reduces the refractive error only to a limited extent for highly myopic eyes.
文摘In order to make the power level angle(PLA)have the same bijection relationship with the thrust when the environment changes before operational limit exceedance,and ensure the pilot's ability of unrestricted throttle movement as well as the safety of the engine,a method used to design a constant thrust power schedule was proposed.The research was based on a simulink turbofan engine model and the results were organized as look-up tables in different structures with limitations.Simulation was done to confirm the validity and demonstrate the effectiveness of the schedule obtained with the approach.The constant thrust takeoff schedule,constant thrust idle schedule and constant thrust part power schedule with the present constant thrust control(CTC)method were proved to be valid.The results can be applied to this specific engine but the method can be applied to any areoengine and can be even extended to the unmanned plane.
文摘Mobile power meters allow for cyclists to monitor power output （PO） during training and competition. The Garrnin Vector power meter （VPM） measures PO at the pedal compared to the crank and has been tested in only a few limited studies. The purpose of this study was to determine the validity and reproducibility of the VPM by comparing it to the SRM. The VPM validity was tested by （1） a submaximal incremental test, （2） submaximal constant power test, （3） sprint test, and （4） a field test. The reliability of the VPM was tested by repeating the laboratory tests 10 times over a 6 week span. Significant differences （P = 0.046） were found between the mean POSRM （178 ± 1.8 W） and POVPM （163.5 ± 14.7 W） for the submaximal constant-power test. No significant differences were found between the POMAX SRM and the POMAx VPM. The reproducibility of the VPM was lower than the SRM （CV = 8.52 ±4.0 vs 3.48 ± 1.9, 10.66% vs 5.50%, and 67.7% vs 55.3% for the submaximal incremental test, submaximal constant-power test, and field test respectively）. The POVPM appears to underestimate the POSRM and is less valid and reliable across various cycling efforts.
