Reliability results are important for proper planning and operation of utility companies. At the base of this method of analysis is the failure rate of the system components. In the traditional method, this probabilit...Reliability results are important for proper planning and operation of utility companies. At the base of this method of analysis is the failure rate of the system components. In the traditional method, this probability of failure is determined by the components’ manufacturer and is considered to be constant. This study proposes a dynamic modeling of failure rate, taking the system operating conditions into consideration. With this new consideration, an IEEE test system has seven of its reliability indices quantified for comparison. The inclusion of the newly modeled failure rate leads to a worsening of 11.07% in the indices, on average. A second analysis is performed considering the presence of DG sources within the microgrid, namely PV and wind based. An improvement of 0.71% on the indices is noticed, once the DG sources are introduced. Finally, the effects of storage systems in the microgrid are investigated through a third scenario, in which two 2 MWh battery systems are introduced, and an improvement of 3.05% is noticed in the reliability indices.展开更多
Harmonics distortion is a crucial problem in microgrid. Harmonic sources can be categorized as two main factors: renewable energy integration and nonlinear loads. Both factors are investigated in this paper. For renew...Harmonics distortion is a crucial problem in microgrid. Harmonic sources can be categorized as two main factors: renewable energy integration and nonlinear loads. Both factors are investigated in this paper. For renewbale energy, photovoltaic (PV) power is one of the most effective solutions for energy crisis and it is showing great potential for serving customers in microgrid. A three- phase PV source model is establised and integrated at different locations in order to observe the impact of harmonics on a microgrid and power quality (PQ). A composite load is modeled using Crossed Frequency Admittance Matrix theory. A practicdal microgrid loacated at GA, USA is used as a study system. The microgrid, PV model and nonlinear load model are simulated in MATLAB/ Simulink environment. The results show the impact of installing PV sources at two types of locations considering linear and composite nonlinear loads. In addition, three PQ indices are discussed to show the numerical impacts with various perspectives.展开更多
This paper aims to find strategic locations for additional Phasor Measurement Units (PMUs) installation while considering resiliency of existing PMU measurement system. A virtual attack agent is modeled based on an op...This paper aims to find strategic locations for additional Phasor Measurement Units (PMUs) installation while considering resiliency of existing PMU measurement system. A virtual attack agent is modeled based on an optimization framework. The virtual attack agent targets to minimize observability of power system by coordinated attack on a subset of critical PMUs. A planner agent is then introduced which analyzes the attack pattern of virtual attack agent. The goal of the planner agent is to mitigate the vulnerability posed by the virtual attack agent by placing additional PMUs at strategic locations. The ensuing problem is formulated as an optimization problem. The proposed framework is applied on 14, 30, 57 and 118 bus test systems, including a large 2383 node western polish test system to demonstrate the feasibility of proposed approach for large systems.展开更多
In this paper, a practical method to establish Doubly Fed Induction Generator (DFIG) based wind farm equivalent model for switching transient analysis is demonstrated. In order to verify this method, a 3.6MW equivalen...In this paper, a practical method to establish Doubly Fed Induction Generator (DFIG) based wind farm equivalent model for switching transient analysis is demonstrated. In order to verify this method, a 3.6MW equivalent wind farm model is built. The steady state results and load switching results are verified with those of detailed models of four 0.9MW generators. Using this method, a model of 40MW wind farm, representing the capacity for a proposed South Carolina offshore wind farm is established. To study large wind farm switching transient impacts on a system, different switching operations such as cable energizing and three phase faults at different locations in wind farm are investigated and their impact on system are analysed. Finally, conclusions based on the switching cases are presented.展开更多
Water Tree is a corrosion phenomenon in cross-link polyethylene (XLPE) insulation. It is commonly found in underground cables. Water tree induced fault is difficult to detect due to its high impedance and difficult to...Water Tree is a corrosion phenomenon in cross-link polyethylene (XLPE) insulation. It is commonly found in underground cables. Water tree induced fault is difficult to detect due to its high impedance and difficult to model due to its random nature. In recent years, underground cables have become more popular in the power industry. They are resistant to environmental damage and has reduced space requirement. They are suitable to areas with high environment hazard or heavily populated areas where space is a constraining factor. As a result, studying and modeling the structure and effect of water tree become increasingly important. Since majority of the fault inducing water trees are vented trees which originated from the surface of the cable insulation, the mathematical model focuses on this particular type of water tree. To reduce the complexity of the model, the shape of the water tree afflicted region of the insulation is assumed to be ellipsoidal and the permittivity of the region is assumed to be linearly changing. Finite element analysis is used to analyze the water tree affected region. The resultant capacitance is calculated and compared with a physical model from Comsol. The result obtained using the proposed mathematical model and the result obtained using physical simulation through Comsol package agrees with each other. Hence, this method can be used to analyze the effect of water tree fault in large power systems.展开更多
Nowadays, more and more electrical power is being distributed to customers by underground cables rather than overhead transmission lines due to their advantage of providing better protection in inclement weather. They...Nowadays, more and more electrical power is being distributed to customers by underground cables rather than overhead transmission lines due to their advantage of providing better protection in inclement weather. They also have significantly reduced electromagnetic field emission because of their copper shielding. But underground cables have larger capacitance than transmission lines per unit. Thus, ferroresonance is more likely to occur in distribution systems using underground cables. Moreover, soil humidity at a depth of one meter remains 100 percent for most of the year, a factor that risks the occurrence of water tree (WT) in cables. Consequently, both ferroresonance and WT are prone to occur in underground cable systems. The objective of this paper is to determine the relationship between ferroresonance and water tree. A test system was designed to simulate and analyze ferroresonance in a cable system caused by single-phase switch and water tree. Eight scenarios of water tree were compared in the simulation. There sponses of ferroresonance are presented in this paper and two common patterns are observed from the simulation results.展开更多
The spray characteristics of a full-cone pressure swirl nozzle have been investigated in this study.The results were defined by Reynolds number,which focuses on the breakup of liquid film,droplet size,velocity,and liq...The spray characteristics of a full-cone pressure swirl nozzle have been investigated in this study.The results were defined by Reynolds number,which focuses on the breakup of liquid film,droplet size,velocity,and liquid volume flux under different Reynolds numbers at the near-field spray.The spray structure was visualized using a high-speed camera,and the characteristics of droplets were measured using a Phase Doppler Anemometer(PDA)in both the radial and axial directions.The tests were carried out at varying spray pressures(0.2 to 1.0 MPa),corresponding to different Reynolds numbers(5369 to 12006).It was found that when the Reynolds number rises,the liquid became more unstable after leaving the nozzle,causing the liquid film to break up faster.According to the measurements of PDA,the coalescence of droplets increased due to the centrifugal effect.What’s more,the velocity of the droplets fluctuates significantly in the radial direction,and the droplets with a smaller particle size had a higher average velocity.From the perspective of liquid distribution,the increase in Reynolds number caused the spray liquid to move in the radial direction gradually.In contrast,the liquid volume distribution changed in the radial direction more obviously than in the axial direction,growing to the maximum along the radial direction and gradually reducing.It can provide a reference for selecting operating parameters for actual agricultural spray operations and the design of electrostatic nozzles through the research on breakup and droplet characteristics.展开更多
Polyamide(PA)hollow fibre composite nanofiltration(NF)membranes with a coffee-ring structure and beneficial properties were prepared by adding graphene oxide(GO)into the interfacial polymerization process.The presenta...Polyamide(PA)hollow fibre composite nanofiltration(NF)membranes with a coffee-ring structure and beneficial properties were prepared by adding graphene oxide(GO)into the interfacial polymerization process.The presentation of the coffee-ring structure was attributed to the heterogeneous,finely dispersed multiphase reaction system and the“coffee-stain”effect of the GO solution.When the piperazine concentration was 0.4 wt-%,the trimesoyl chloride concentration was 0.3 wt-%,and the GO concentration was 0.025 wt-%,the prepared NF membranes showed the best separation properties.The permeate flux was 76 L·m^(−2)·h^(−1),and the rejection rate for MgSO4 was 98.6%at 0.4 MPa.Scanning electron microscopy,atomic force microscopy,and attenuated total reflectance-Fourier transform infrared spectroscopy were used to characterize the chemical structure and morphology of the PA/GO NF membrane.The results showed that GO was successfully entrapped into the PA functional layer.Under neutral operating conditions,the PA/GO membrane showed typical negatively charged NF membrane separation characteristics,and the rejection rate decreased in the order of Na2SO_(4)>MgSO_(4)>MgCl_(2)>NaCl.The PA/GO NF membrane showed better antifouling performance than the PA membrane.展开更多
The radial ultrasonic rolling electrochemical micromachining(RUR-EMM)combined rolling electrochemical micromachining(R-EMM)and ultrasonic vibration was studied in this paper.The fundamental understanding of the machin...The radial ultrasonic rolling electrochemical micromachining(RUR-EMM)combined rolling electrochemical micromachining(R-EMM)and ultrasonic vibration was studied in this paper.The fundamental understanding of the machining process especially the interaction between multiphysics in the interelectrode gap(IEG)was investigated and discussed by the finite element method.The multiphysics coupling model including flow field model,Joule heating model,material dissolution model and vibration model was built.3D multiphysics simulation based on micro dimples process in RUR-EMM and R-EMM was proposed.Simulation results showed that the electrolyte flowed into and out IEG periodically,gas bubbles were easy to squeeze out and the gas void fraction deceased about 16%to 54%,the maximum current density increased by 1.36 times in RUR-EMM than in R-EMM in one vibration period of time.And application of the ultrasonic vibration increased the electrolyte temperature about 1.3–4.4%in IEG.Verification experiments of the micro dimple process denoted better corrosion consistency of array dimples in RUR-EMM,there was no island at the micro dimple bottom which always formed in R-EMM,and an aggregated deviation of less than 8.7%for the micro dimple depth and 4%for the material removal amount between theory and experiment was obtained.展开更多
Four 1,8-naphthalimide hydrazone molecules with different electron-donating groups have been applied in the study of linear and nonlinear optical(NLO)properties.These compounds showed strong green emission in solution...Four 1,8-naphthalimide hydrazone molecules with different electron-donating groups have been applied in the study of linear and nonlinear optical(NLO)properties.These compounds showed strong green emission in solution.Their NLO properties such as two-photon absorption(TPA)behavior with femtosecond laser pulses ca.800 nm and excited-state absorption(ESA)behavior with nanosecond laser pulses at 532 nm were investigated.Compound 4 presented the largest two-photon cross section(550 GM)among them due to two factors:the conjugated length of compound 4 is the longest and the electron-donating ability of compound 4 is the strongest.Different from TPA behavior,compound 2 showed the best nonlinear absorption properties at 532 nm and its nonlinear absorption coef-ficient and third-order nonlinear optical susceptibilitiesχ^(3)were up to 1.41×10^(−10) MKS and 4.65×10^(−12) esu,re-spectively.Through the modification of the structure,the nonlinear optical properties of these compounds at differ-ent wavelengths(532 and 800 nm)were well tuned.The great broad-band nonlinear optical properties indicate hy-drazones are good candidates for organic nonlinear optical absorption materials.展开更多
文摘Reliability results are important for proper planning and operation of utility companies. At the base of this method of analysis is the failure rate of the system components. In the traditional method, this probability of failure is determined by the components’ manufacturer and is considered to be constant. This study proposes a dynamic modeling of failure rate, taking the system operating conditions into consideration. With this new consideration, an IEEE test system has seven of its reliability indices quantified for comparison. The inclusion of the newly modeled failure rate leads to a worsening of 11.07% in the indices, on average. A second analysis is performed considering the presence of DG sources within the microgrid, namely PV and wind based. An improvement of 0.71% on the indices is noticed, once the DG sources are introduced. Finally, the effects of storage systems in the microgrid are investigated through a third scenario, in which two 2 MWh battery systems are introduced, and an improvement of 3.05% is noticed in the reliability indices.
文摘Harmonics distortion is a crucial problem in microgrid. Harmonic sources can be categorized as two main factors: renewable energy integration and nonlinear loads. Both factors are investigated in this paper. For renewbale energy, photovoltaic (PV) power is one of the most effective solutions for energy crisis and it is showing great potential for serving customers in microgrid. A three- phase PV source model is establised and integrated at different locations in order to observe the impact of harmonics on a microgrid and power quality (PQ). A composite load is modeled using Crossed Frequency Admittance Matrix theory. A practicdal microgrid loacated at GA, USA is used as a study system. The microgrid, PV model and nonlinear load model are simulated in MATLAB/ Simulink environment. The results show the impact of installing PV sources at two types of locations considering linear and composite nonlinear loads. In addition, three PQ indices are discussed to show the numerical impacts with various perspectives.
文摘This paper aims to find strategic locations for additional Phasor Measurement Units (PMUs) installation while considering resiliency of existing PMU measurement system. A virtual attack agent is modeled based on an optimization framework. The virtual attack agent targets to minimize observability of power system by coordinated attack on a subset of critical PMUs. A planner agent is then introduced which analyzes the attack pattern of virtual attack agent. The goal of the planner agent is to mitigate the vulnerability posed by the virtual attack agent by placing additional PMUs at strategic locations. The ensuing problem is formulated as an optimization problem. The proposed framework is applied on 14, 30, 57 and 118 bus test systems, including a large 2383 node western polish test system to demonstrate the feasibility of proposed approach for large systems.
文摘In this paper, a practical method to establish Doubly Fed Induction Generator (DFIG) based wind farm equivalent model for switching transient analysis is demonstrated. In order to verify this method, a 3.6MW equivalent wind farm model is built. The steady state results and load switching results are verified with those of detailed models of four 0.9MW generators. Using this method, a model of 40MW wind farm, representing the capacity for a proposed South Carolina offshore wind farm is established. To study large wind farm switching transient impacts on a system, different switching operations such as cable energizing and three phase faults at different locations in wind farm are investigated and their impact on system are analysed. Finally, conclusions based on the switching cases are presented.
文摘Water Tree is a corrosion phenomenon in cross-link polyethylene (XLPE) insulation. It is commonly found in underground cables. Water tree induced fault is difficult to detect due to its high impedance and difficult to model due to its random nature. In recent years, underground cables have become more popular in the power industry. They are resistant to environmental damage and has reduced space requirement. They are suitable to areas with high environment hazard or heavily populated areas where space is a constraining factor. As a result, studying and modeling the structure and effect of water tree become increasingly important. Since majority of the fault inducing water trees are vented trees which originated from the surface of the cable insulation, the mathematical model focuses on this particular type of water tree. To reduce the complexity of the model, the shape of the water tree afflicted region of the insulation is assumed to be ellipsoidal and the permittivity of the region is assumed to be linearly changing. Finite element analysis is used to analyze the water tree affected region. The resultant capacitance is calculated and compared with a physical model from Comsol. The result obtained using the proposed mathematical model and the result obtained using physical simulation through Comsol package agrees with each other. Hence, this method can be used to analyze the effect of water tree fault in large power systems.
文摘Nowadays, more and more electrical power is being distributed to customers by underground cables rather than overhead transmission lines due to their advantage of providing better protection in inclement weather. They also have significantly reduced electromagnetic field emission because of their copper shielding. But underground cables have larger capacitance than transmission lines per unit. Thus, ferroresonance is more likely to occur in distribution systems using underground cables. Moreover, soil humidity at a depth of one meter remains 100 percent for most of the year, a factor that risks the occurrence of water tree (WT) in cables. Consequently, both ferroresonance and WT are prone to occur in underground cable systems. The objective of this paper is to determine the relationship between ferroresonance and water tree. A test system was designed to simulate and analyze ferroresonance in a cable system caused by single-phase switch and water tree. Eight scenarios of water tree were compared in the simulation. There sponses of ferroresonance are presented in this paper and two common patterns are observed from the simulation results.
基金support provided by National Natural Science Foundation of China(Grant No.31971797)National Natural Science Foundation of China(Grant No.32271997)+3 种基金China Agriculture Research System of MOF and MARA(CARS-26)General Program of Guang-dong Natural Science Foundation(2021A1515010923)Guangdong Provincial Special Fund For Modern Agriculture Industry Technology Innovation Teams(Grant No.2023KJ108)Key-Area Research and Development Program of Guangdong Province(2023B 0202090001).
文摘The spray characteristics of a full-cone pressure swirl nozzle have been investigated in this study.The results were defined by Reynolds number,which focuses on the breakup of liquid film,droplet size,velocity,and liquid volume flux under different Reynolds numbers at the near-field spray.The spray structure was visualized using a high-speed camera,and the characteristics of droplets were measured using a Phase Doppler Anemometer(PDA)in both the radial and axial directions.The tests were carried out at varying spray pressures(0.2 to 1.0 MPa),corresponding to different Reynolds numbers(5369 to 12006).It was found that when the Reynolds number rises,the liquid became more unstable after leaving the nozzle,causing the liquid film to break up faster.According to the measurements of PDA,the coalescence of droplets increased due to the centrifugal effect.What’s more,the velocity of the droplets fluctuates significantly in the radial direction,and the droplets with a smaller particle size had a higher average velocity.From the perspective of liquid distribution,the increase in Reynolds number caused the spray liquid to move in the radial direction gradually.In contrast,the liquid volume distribution changed in the radial direction more obviously than in the axial direction,growing to the maximum along the radial direction and gradually reducing.It can provide a reference for selecting operating parameters for actual agricultural spray operations and the design of electrostatic nozzles through the research on breakup and droplet characteristics.
基金The authors gratefully acknowledge financial support from the Natural Science Foundation of Zhejiang Province(Grant No.LY19E030005)MOE Key Laboratory of Macromolecular Synthesis and Functionalization,Zhejiang University(No.2017MSF 05)The authors also sincerely thank the Open Foundation from the Top Key Discipline of Environmental Science and Engineering,Zhejiang University of Technology(Grant No.20150314).
文摘Polyamide(PA)hollow fibre composite nanofiltration(NF)membranes with a coffee-ring structure and beneficial properties were prepared by adding graphene oxide(GO)into the interfacial polymerization process.The presentation of the coffee-ring structure was attributed to the heterogeneous,finely dispersed multiphase reaction system and the“coffee-stain”effect of the GO solution.When the piperazine concentration was 0.4 wt-%,the trimesoyl chloride concentration was 0.3 wt-%,and the GO concentration was 0.025 wt-%,the prepared NF membranes showed the best separation properties.The permeate flux was 76 L·m^(−2)·h^(−1),and the rejection rate for MgSO4 was 98.6%at 0.4 MPa.Scanning electron microscopy,atomic force microscopy,and attenuated total reflectance-Fourier transform infrared spectroscopy were used to characterize the chemical structure and morphology of the PA/GO NF membrane.The results showed that GO was successfully entrapped into the PA functional layer.Under neutral operating conditions,the PA/GO membrane showed typical negatively charged NF membrane separation characteristics,and the rejection rate decreased in the order of Na2SO_(4)>MgSO_(4)>MgCl_(2)>NaCl.The PA/GO NF membrane showed better antifouling performance than the PA membrane.
基金financial support of the projects from the National Natural Science Foundation of China(Nos.51975532 and 51475428)the Zhejiang Provincial Natural Science Foundation(No.LY19E050007)。
文摘The radial ultrasonic rolling electrochemical micromachining(RUR-EMM)combined rolling electrochemical micromachining(R-EMM)and ultrasonic vibration was studied in this paper.The fundamental understanding of the machining process especially the interaction between multiphysics in the interelectrode gap(IEG)was investigated and discussed by the finite element method.The multiphysics coupling model including flow field model,Joule heating model,material dissolution model and vibration model was built.3D multiphysics simulation based on micro dimples process in RUR-EMM and R-EMM was proposed.Simulation results showed that the electrolyte flowed into and out IEG periodically,gas bubbles were easy to squeeze out and the gas void fraction deceased about 16%to 54%,the maximum current density increased by 1.36 times in RUR-EMM than in R-EMM in one vibration period of time.And application of the ultrasonic vibration increased the electrolyte temperature about 1.3–4.4%in IEG.Verification experiments of the micro dimple process denoted better corrosion consistency of array dimples in RUR-EMM,there was no island at the micro dimple bottom which always formed in R-EMM,and an aggregated deviation of less than 8.7%for the micro dimple depth and 4%for the material removal amount between theory and experiment was obtained.
基金The authors thank the National Natural Science Foundation of China(Nos.21071105,20902065 and 21176164).
文摘Four 1,8-naphthalimide hydrazone molecules with different electron-donating groups have been applied in the study of linear and nonlinear optical(NLO)properties.These compounds showed strong green emission in solution.Their NLO properties such as two-photon absorption(TPA)behavior with femtosecond laser pulses ca.800 nm and excited-state absorption(ESA)behavior with nanosecond laser pulses at 532 nm were investigated.Compound 4 presented the largest two-photon cross section(550 GM)among them due to two factors:the conjugated length of compound 4 is the longest and the electron-donating ability of compound 4 is the strongest.Different from TPA behavior,compound 2 showed the best nonlinear absorption properties at 532 nm and its nonlinear absorption coef-ficient and third-order nonlinear optical susceptibilitiesχ^(3)were up to 1.41×10^(−10) MKS and 4.65×10^(−12) esu,re-spectively.Through the modification of the structure,the nonlinear optical properties of these compounds at differ-ent wavelengths(532 and 800 nm)were well tuned.The great broad-band nonlinear optical properties indicate hy-drazones are good candidates for organic nonlinear optical absorption materials.
