Among all the renewable energy sources,the installed capacity of solar power generation is the fastest growing in recent years,so photovoltaic(PV)power generation still has great market potential.Compared with low-pow...Among all the renewable energy sources,the installed capacity of solar power generation is the fastest growing in recent years,so photovoltaic(PV)power generation still has great market potential.Compared with low-power systems,large-scale PV systems are more commercially attractive,because they can reduce the cost of the system per watt.The PV inverters with centralized and string structure have been applied in large-scale PV plant,but it is difficult to further increase the voltage and power levels for a single converter.In addition,the line-frequency isolation transformer requires a large amount of materials and has a large volume and weight.Therefore,it is a current trend for large-scale PV system to increase the voltage and power levels to directly connect to the medium-voltage power grid.Based on this,this paper investigates and compares several topologies of PV inverters without line-frequency transformer,including the MMC structure and the three-phase cascaded H-bridge(CHB)structure,which are able to directly connect to the 35kV medium-voltage power grid,and can not only make the voltage and power levels higher,but also further reduce the cost and volume of the whole system.展开更多
We consider a branching random walk in random environments, where the particles are reproduced as a branching process with a random environment (in time), and move independently as a random walk on ? with a random env...We consider a branching random walk in random environments, where the particles are reproduced as a branching process with a random environment (in time), and move independently as a random walk on ? with a random environment (in locations). We obtain the asymptotic properties on the position of the rightmost particle at time n, revealing a phase transition phenomenon of the system.展开更多
In this paper,we review the past and recent works on generating intense terahertz(THz)pulses from photoconductive antennas(PCAs).We will focus on two types of large-aperture photoconductive antenna(LAPCA)that can gene...In this paper,we review the past and recent works on generating intense terahertz(THz)pulses from photoconductive antennas(PCAs).We will focus on two types of large-aperture photoconductive antenna(LAPCA)that can generate high-intensity THz pulses(a)those with large-aperture dipoles and(b)those with interdigitated electrodes.We will first describe the principles of THz generation from PCAs.The critical parameters for improving the peak intensity of THz radiation from LAPCAs are summarized.We will then describe the saturation and limitation process of LAPCAs along with the advantages and disadvantages of working with widebandgap semiconductor substrates.Then,we will explain the evolution of LAPCA with interdigitated electrodes,which allows one to reduce the photoconductive gap size,and thus obtain higher bias fields while applying lower voltages.We will also describe recent achievements in intense THz pulses generated by interdigitated LAPCAs based on wide-bandgap semiconductors driven by ampli-fied lasers.Finally,we will discuss the future perspectives of THz pulse generation using LAPCAs.展开更多
This letter studies large-disturbance stability of the power system with a synchronous generator(SG)and a converter-interfaced generation(CIG)connected to infinite bus.The power system is multi-timescale and first sim...This letter studies large-disturbance stability of the power system with a synchronous generator(SG)and a converter-interfaced generation(CIG)connected to infinite bus.The power system is multi-timescale and first simplified.It is shown that the boundary of region of attraction(ROA)of the simplified model is composed of stable manifolds of unstable equilibrium point(UEP)or semi-singular point(SSP),named anchor points,and singular surface pieces.The type of anchor point determines the dominant instability pattern of the power system.When the anchor point is UEP or SSP,the dominant instability pattern is the instability of rotor angle of SG or the instability of phase-locked loop and outer control loop(OCL)of CIG,respectively.Transition of dominant instability pattern can be analyzed with the relative position relationship between UEP and SSP.The effect of OCL is discussed.When the OCL is activated,the ROA becomes smaller and the system is more prone to instability of CIG.It is necessary to consider the OCL when studying the large-disturbance stability of the power system.展开更多
It is essential to determine the occurrence of emerging organic contaminants(EOCs),such as pharmaceuticals and personal care products(PPCPs),in the ambient environment to address growing public concerns.However,such a...It is essential to determine the occurrence of emerging organic contaminants(EOCs),such as pharmaceuticals and personal care products(PPCPs),in the ambient environment to address growing public concerns.However,such analysis is quite challenging due to the low trace level of such contaminants in water,which therefore requires several litres of water samples.In this study,a large volume solid phase extraction(LV-SPE)device was developed and evaluated for its performance in monitoring PPCPs as an example.Relatively good recoveries and reproducibility were obtained under specific operating conditions:a water sample volume of no more than 20 L,a flow rate not exceeding 120 mL/min,and a methanol elution volume of at least 30 mL.In addition,the results from the onsite enrichment approach using LV-SPE were compared with those from the conventional approach using a standard SPE device in the laboratory for real groundwater samples.Among the eight selected PPCPs(nalidixic acid,carbamazepine,bezafibrate,clofibric acid,sulfadiazine,sulfamethoxypyridazine,sulfamethazine and sulfamonomethoxine),LV-SPE approach detected more target compounds.While the detected concentrations were generally comparable,slightly higher concentrations were observed for carbamazepine,clofibric acid,sulfadiazine and sulfamethazine using the LV-SPE method.The developed LV-SPE device provides an alternative approach for trace analysis of PPCPs and may also be applicable for other emerging organic contaminants.展开更多
The REAgSb_(2)(RE = rare earth and Y) family has drawn considerable research interest because the two-dimensional Sb net in their crystal structures hosts topological fermions and hence rich topological properties. We...The REAgSb_(2)(RE = rare earth and Y) family has drawn considerable research interest because the two-dimensional Sb net in their crystal structures hosts topological fermions and hence rich topological properties. We report herein the magnetization and magnetotransport measurements of SmAgSb_(2) single crystal, which unveil very large magnetoresistance and high carrier mobility up to 6.2 × 10^(3)% and 5.58 × 10^(3)cm^(2)·V^(-1)·s^(-1), respectively. The analysis of both Shubnikov–de Haas and de Haas–van Alphen quantum oscillations indicates nontrivial Berry phases in the paramagnetic state while trivial Berry curvature in the antiferromagnetic state, indicating a topological phase transition induced by the antiferromagnetic order. It is also supported by the first-principles calculations. The results not only provide a new interesting topological material but also offer valuable insights into the correlation between magnetism and nontrivial topological states.展开更多
This paper focuses on synchronization stability analysis of the power system,in which power electronics are synchronized by the phase-locked loop(PLL).It provides new insight into the synchronization stability of powe...This paper focuses on synchronization stability analysis of the power system,in which power electronics are synchronized by the phase-locked loop(PLL).It provides new insight into the synchronization stability of power electronics from the voltage perspective.The synchronization stability analysis based on space vector is carried out by establishing a simplified model of the grid-connected voltage source converter(VSC)system.Without complex mathematical calculation,the existence criterion of equilibrium points and the criterion of transient instability dominated by the unstable equilibrium point(UEP)are derived,respectively.With the proposed method,synchronization stability can be determined by the voltage space vectors,which are more observable in potential engineering applications.At the end of this study,the steps of the synchronization stability determination by voltage space vectors are summarized,and the effectiveness and applicability of the proposed method are demonstrated by numerical simulations performed on the PSCAD/EMTDC platform.展开更多
In order to explore the ability of airgun sources in imaging the crustal structure,we conducted an experiment along the Yangzte River at the section from Ma 'anshan to Anqing using an offshore airgun source( refer...In order to explore the ability of airgun sources in imaging the crustal structure,we conducted an experiment along the Yangzte River at the section from Ma 'anshan to Anqing using an offshore airgun source( referred as the Anhui Airgun Experiment).During the experiment,the airgun source was fired 2973 times at 20 multiple shot points,while 109 permanent stations and 700 temporary stations( 11 profiles) were used to record the signal. For 20 multiple shot points,we picked up 335 P-wave arrivals at 52 permanent stations. Then we obtained the P-wave velocity structure of the crust beneath the southern tip of the Tancheng-Lujiang( Tan-Lu) fault. The preliminary result of P-wave velocity structure confirms the possibility of 3D body wave imaging using large volume airgun sources. The imaged velocity heterogeneity at a depth of 15 km is correlated to the geological settings. High velocity anomaly beneath the Qinling-Dabie orogenic belt could be explained by the ultrahigh-pressure metamorphic rocks in the deep region,while the low velocity anomaly at the Yangtze River region may correspond to the special background of mineralization.展开更多
This paper studies the effect of phase noise and fronthaul compression on a downlink cloud radio access network(C-RAN), where several remote radio heads(RRHs) are coordinated to communicate with users by a baseband un...This paper studies the effect of phase noise and fronthaul compression on a downlink cloud radio access network(C-RAN), where several remote radio heads(RRHs) are coordinated to communicate with users by a baseband unit(BBU) on the cloud server. In the system, the baseband signals are precoded at BBU, and then compressed before being transmitted to RRHs through capacity-limited fronthaul links which results in the compressive quantization noise. We assume the regularized zero-forcing precoding is performed with an imperfect channel state information and a compression strategy is applied at BBU. The effect of phase noise arising from nonideal local oscillators both at RRHs and users is considered. We propose an approximate expression for the downlink ergodic sum-rate of considered C-RAN utilizing large dimensional random matrix theory in the large-system regime. From simulation results, the accuracy of the approximate expression is validated, and the effect of phase noise and fronthaul compression can be analyzed theoretically based on the approximate expression.展开更多
基金This work was supported by National Natural Science Foundation of China(51937003).
文摘Among all the renewable energy sources,the installed capacity of solar power generation is the fastest growing in recent years,so photovoltaic(PV)power generation still has great market potential.Compared with low-power systems,large-scale PV systems are more commercially attractive,because they can reduce the cost of the system per watt.The PV inverters with centralized and string structure have been applied in large-scale PV plant,but it is difficult to further increase the voltage and power levels for a single converter.In addition,the line-frequency isolation transformer requires a large amount of materials and has a large volume and weight.Therefore,it is a current trend for large-scale PV system to increase the voltage and power levels to directly connect to the medium-voltage power grid.Based on this,this paper investigates and compares several topologies of PV inverters without line-frequency transformer,including the MMC structure and the three-phase cascaded H-bridge(CHB)structure,which are able to directly connect to the 35kV medium-voltage power grid,and can not only make the voltage and power levels higher,but also further reduce the cost and volume of the whole system.
基金the National Natural Science Foundation of China (Grant Nos. 10271020,10471012)SRF for ROCS, SEM (Grant No. [2005]564)
文摘We consider a branching random walk in random environments, where the particles are reproduced as a branching process with a random environment (in time), and move independently as a random walk on ? with a random environment (in locations). We obtain the asymptotic properties on the position of the rightmost particle at time n, revealing a phase transition phenomenon of the system.
文摘In this paper,we review the past and recent works on generating intense terahertz(THz)pulses from photoconductive antennas(PCAs).We will focus on two types of large-aperture photoconductive antenna(LAPCA)that can generate high-intensity THz pulses(a)those with large-aperture dipoles and(b)those with interdigitated electrodes.We will first describe the principles of THz generation from PCAs.The critical parameters for improving the peak intensity of THz radiation from LAPCAs are summarized.We will then describe the saturation and limitation process of LAPCAs along with the advantages and disadvantages of working with widebandgap semiconductor substrates.Then,we will explain the evolution of LAPCA with interdigitated electrodes,which allows one to reduce the photoconductive gap size,and thus obtain higher bias fields while applying lower voltages.We will also describe recent achievements in intense THz pulses generated by interdigitated LAPCAs based on wide-bandgap semiconductors driven by ampli-fied lasers.Finally,we will discuss the future perspectives of THz pulse generation using LAPCAs.
基金supported by the National Natural Science Foundation of China(No.U2066602)。
文摘This letter studies large-disturbance stability of the power system with a synchronous generator(SG)and a converter-interfaced generation(CIG)connected to infinite bus.The power system is multi-timescale and first simplified.It is shown that the boundary of region of attraction(ROA)of the simplified model is composed of stable manifolds of unstable equilibrium point(UEP)or semi-singular point(SSP),named anchor points,and singular surface pieces.The type of anchor point determines the dominant instability pattern of the power system.When the anchor point is UEP or SSP,the dominant instability pattern is the instability of rotor angle of SG or the instability of phase-locked loop and outer control loop(OCL)of CIG,respectively.Transition of dominant instability pattern can be analyzed with the relative position relationship between UEP and SSP.The effect of OCL is discussed.When the OCL is activated,the ROA becomes smaller and the system is more prone to instability of CIG.It is necessary to consider the OCL when studying the large-disturbance stability of the power system.
基金This research was financially supported by the Major Science and Technology Program for Water Pollution Control and Treatment in China(Grant No.2017ZX07202004).
文摘It is essential to determine the occurrence of emerging organic contaminants(EOCs),such as pharmaceuticals and personal care products(PPCPs),in the ambient environment to address growing public concerns.However,such analysis is quite challenging due to the low trace level of such contaminants in water,which therefore requires several litres of water samples.In this study,a large volume solid phase extraction(LV-SPE)device was developed and evaluated for its performance in monitoring PPCPs as an example.Relatively good recoveries and reproducibility were obtained under specific operating conditions:a water sample volume of no more than 20 L,a flow rate not exceeding 120 mL/min,and a methanol elution volume of at least 30 mL.In addition,the results from the onsite enrichment approach using LV-SPE were compared with those from the conventional approach using a standard SPE device in the laboratory for real groundwater samples.Among the eight selected PPCPs(nalidixic acid,carbamazepine,bezafibrate,clofibric acid,sulfadiazine,sulfamethoxypyridazine,sulfamethazine and sulfamonomethoxine),LV-SPE approach detected more target compounds.While the detected concentrations were generally comparable,slightly higher concentrations were observed for carbamazepine,clofibric acid,sulfadiazine and sulfamethazine using the LV-SPE method.The developed LV-SPE device provides an alternative approach for trace analysis of PPCPs and may also be applicable for other emerging organic contaminants.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.12004405,12334008,and 12374148)the Double First-Class Initiative Fund of Shanghai Tech University+2 种基金the Analytical Instrumentation Center of Shanghai Tech University(Grant No.SPST-AIC10112914)the research fund from the Shanghai Sailing Program(Grant No.23YF1426900)the fund from the National Key R&D Program of China(Grant Nos.2022YFA1402702 and 2021YFA1401600)。
文摘The REAgSb_(2)(RE = rare earth and Y) family has drawn considerable research interest because the two-dimensional Sb net in their crystal structures hosts topological fermions and hence rich topological properties. We report herein the magnetization and magnetotransport measurements of SmAgSb_(2) single crystal, which unveil very large magnetoresistance and high carrier mobility up to 6.2 × 10^(3)% and 5.58 × 10^(3)cm^(2)·V^(-1)·s^(-1), respectively. The analysis of both Shubnikov–de Haas and de Haas–van Alphen quantum oscillations indicates nontrivial Berry phases in the paramagnetic state while trivial Berry curvature in the antiferromagnetic state, indicating a topological phase transition induced by the antiferromagnetic order. It is also supported by the first-principles calculations. The results not only provide a new interesting topological material but also offer valuable insights into the correlation between magnetism and nontrivial topological states.
基金supported in part by the National Natural Science Foundation of China(U2166601,51977197,51907179).
文摘This paper focuses on synchronization stability analysis of the power system,in which power electronics are synchronized by the phase-locked loop(PLL).It provides new insight into the synchronization stability of power electronics from the voltage perspective.The synchronization stability analysis based on space vector is carried out by establishing a simplified model of the grid-connected voltage source converter(VSC)system.Without complex mathematical calculation,the existence criterion of equilibrium points and the criterion of transient instability dominated by the unstable equilibrium point(UEP)are derived,respectively.With the proposed method,synchronization stability can be determined by the voltage space vectors,which are more observable in potential engineering applications.At the end of this study,the steps of the synchronization stability determination by voltage space vectors are summarized,and the effectiveness and applicability of the proposed method are demonstrated by numerical simulations performed on the PSCAD/EMTDC platform.
基金jointly sponsored by the National Natural Science Foundation of China(41374070,41574052)Academician Chen Yong Wonkstation Project of Yunnan Province(2014IC007)
文摘In order to explore the ability of airgun sources in imaging the crustal structure,we conducted an experiment along the Yangzte River at the section from Ma 'anshan to Anqing using an offshore airgun source( referred as the Anhui Airgun Experiment).During the experiment,the airgun source was fired 2973 times at 20 multiple shot points,while 109 permanent stations and 700 temporary stations( 11 profiles) were used to record the signal. For 20 multiple shot points,we picked up 335 P-wave arrivals at 52 permanent stations. Then we obtained the P-wave velocity structure of the crust beneath the southern tip of the Tancheng-Lujiang( Tan-Lu) fault. The preliminary result of P-wave velocity structure confirms the possibility of 3D body wave imaging using large volume airgun sources. The imaged velocity heterogeneity at a depth of 15 km is correlated to the geological settings. High velocity anomaly beneath the Qinling-Dabie orogenic belt could be explained by the ultrahigh-pressure metamorphic rocks in the deep region,while the low velocity anomaly at the Yangtze River region may correspond to the special background of mineralization.
基金supported in part by the Natural Science Foundation of China (NSFC) under Grant U1805262, 61871446, and 61671251supported by NSFC under Grant 61625106 and Grant 61531011
文摘This paper studies the effect of phase noise and fronthaul compression on a downlink cloud radio access network(C-RAN), where several remote radio heads(RRHs) are coordinated to communicate with users by a baseband unit(BBU) on the cloud server. In the system, the baseband signals are precoded at BBU, and then compressed before being transmitted to RRHs through capacity-limited fronthaul links which results in the compressive quantization noise. We assume the regularized zero-forcing precoding is performed with an imperfect channel state information and a compression strategy is applied at BBU. The effect of phase noise arising from nonideal local oscillators both at RRHs and users is considered. We propose an approximate expression for the downlink ergodic sum-rate of considered C-RAN utilizing large dimensional random matrix theory in the large-system regime. From simulation results, the accuracy of the approximate expression is validated, and the effect of phase noise and fronthaul compression can be analyzed theoretically based on the approximate expression.
