Inhibition characteristics of circulating current in parallel inverter driving system of mine hoist

This research investigated the outputting circulation current inhibition characteristic which are controlled by the instantaneous feedback voltage in inverter parallel driving of the mine hoist. We established a transfer function of the parallel inverters controlled by the close-loop adjustment of instantaneous voltage feedback. The influence of the parameters of the close-loop feedback circuit to the inhibition effects to the outputting circulation current is observed. After analyzing the circulating current inhibition characteristics, the proportion integration （PI） controller is introduced into the close-loop adjustment by instantaneous voltage feedback. The characteristics equation is gained to determine the PI parameters by drawing the Bode plots. The inhibition effects of the proposed controller are examined by the established simulation model of parallel inverter system. The harmonic distortion rate at the outputting voltage frequency value of 4, 10, 20, 41 and 50 Hz, are all lower than 2.32 % by the instantaneous outputting voltage feedback.

Hybrid Predictive Control with Simple Linear Control Based Circulating Current Suppression for Modular Multilevel Converters

The modular multilevel converter(MMC)has become a promising topology for widespread power converter applications.However,an evident circulating current flowing between the phases will increase system losses and complicate the heatsink design.This paper proposes a novel hybrid model predictive control method for MMCs.This method utilizes an indirect structure MPC and a sorting algorithm to implement current tracking and capacitor voltages balancing,considerably resulting in reduced calculation burden.In addition,different from the conventional MPC solutions,we add a simple proportional-integral(PI)controller to suppress circulating current through modifying the submodule(SM)inserted number,which is parallel to the MPC loop.This hybrid control solution combines both advantages of MPC and linear control,evidently resulting in improved performance of circulating current.Finally,the MATLAB/Simulink results of an 11-level MMC system verify the effectiveness of the proposed solution.

Zero-sequence Circulating Current Reduction for Three-phase Three-level Modular Photovoltaic Grid-connected Systems

针对共享正、负母线和交流母线的三相三电平模块化光伏并网系统的零序环流问题,建立三电平模块化系统零序环流的等效模型,并根据激励源的不同将零序环流分成3类,提出一种共享正、负母线和中线的并联方案来抑制I类零序环流,及一种基于改进型LCL滤波器的并联方案来抑制II、III类零序环流的高频分量,和采用零序环流控制器来抑制II、III类零序环流的低频分量。最后,通过10 kW的三相三电平模块化光伏并网系统的仿真和实验,证明了零序环流模型分析和抑制方法的正确性。

GENERATOR VIBRATION FAULT DIAGNOSIS METHOD BASED ON ROTOR VIBRATION AND STATOR WINDING PARALLEL BRANCHES CIRCULATING CURRENT CHARACTERISTICS

Rotor vibration characteristics are first analyzed, which are that the rotor vibration of fundamental frequency will increase due to rotor winding inter-turn short circuit fault, air-gap dynamic eccentricity fault, or imbalance fault, and the vibration of the second frequency will increase when the air-gap static eccentricity fault occurs. Next, the characteristics of the stator winding parallel branches circulating current are analyzed, which are that the second harmonics circulating current will increase when the rotor winding inter-turn short circuit fault occurs, and the fundamental circulating current will increase when the air-gap eccentricity fault occurs, neither being strongly affected by the imbalance fault. Considering the differences of the rotor vibration and circulating current characteristics caused by different rotor faults, a method of generator vibration fault diagnosis, based on rotor vibration and circulating current characteristics, is developed. Finally, the rotor vibration and circulating current of a type SDF-9 generator is measured in the laboratory to verify the theoretical analysis presented above.

Current Circulation and Water Renewal Time in the Enclosed Marine Area of Pagasitikos Basin, Greece

Greece boasts an impressive closed coastline stretching across 13,676 km, making it the largest in the Mediterranean basin and one of the largest in the world. Given the significant human activities that take place in coastal areas, understanding the behavior of the sea environment becomes crucial. In this study, we delve into the generation and movement of marine currents as well as the retention time and water age within the waters of Pagasitikos Sea inlet, Greece, through numerical simulation of hydrodynamic characteristics. The main examined points of the understudy region are the area of the port of Volos, the Trikeri channel where the ingress and egress of water from the Gulf takes place and the exchange of seawater through circulation of the Pagasitikos Gulf with the North Evian Gulf. In order to evaluate the results, they were compared with real field measurements and with simulation on a laboratory dummy of the same area. The computational simulation was performed with the ELCOM 2.2 numerical modeling tool and the AEM3D latest version and the main factors simulated are the tide, the consequence that Coriolis force, boundary conditions, the topography and bottom geometry of the bay and the actual meteorological conditions of a whole year.

The Circulating Current Analysis and Parameter Design of Interphase-Reactor of HT-7U Toroidal Power Supply

This paper analyzes the circulating current which is produced by HT-7Usuperconducting toroidal power supply-two sets of two-reverse-star converter with aninterphase-reactor in parallel running on the basis of the output voltage mathematical equation ofthree-phase semi-wave converter circuit. A new idea of omitting interphase-reactor between twoconverters is proposed, and the parameter design of interphase-reactor of HT-7 toroidal power supplyis presented. Simulated results demonstrate the validity of this new project.

Improved Ladder Wave Modulation of Circulating Current Suppressing Control Strategy of MMC

This paper partitions the arm current of MMC into uncontrollable current and controllable current. The former is determined by the load that can’t be controlled by taking any control strategy. The later caused by the unbalanced total inserted voltage of three arms can be controlled by some improved algorithms. The conclusion based on the researching the essence of circulating current is reached that change the number of the inserted sub-modules in each phase can suppress the circulating current. Combined with the improved ladder wave modulation, a novel circulating current suppression strategy particularly for the inverter station is developed. The improved strategy can adapt to load changes and reduce the circulating current and output voltage THD of MMC ac terminals greatly without increasing any peripheral circuits. Finally, the simulation model of 100 submodules in each phase is constructed in MATLAB and the simulation results verify the correctness and effectiveness of the modified control algorithm.

On the Huanghai (Yellow) Sea circulation:a review by current measurements

The Huanghai (Yellow) Sea circulations suggested in the past were constructed mostly based on hydrographic data or simple numerical models, and were not rigorously compered with direct current measurements. Ourpresent understanding of the Huanghai Sea circulation is still very poor and superficial. To evaluate our present knowledge about the circulation and to sketch a more realistic circulation pattern, the previous studies on the circulation arebriefly reviewed and directly measured current data collected in the past are revisited. The Huanghai Sea Warm Current and the northeastward extension in summer of the Changjiang diluted water are paid more attention to, which arethe major currents of the basic circulation, and then attempted to sketch the Huanghai Sea circulation by synthesizinghydrography and current data. The Huanghai Sea circulation is not considered to be persistent since the thin water column, less than 100 m, responds quickly and sensitively to external driving forces. Directly measured current data showthat saline water originating from the Kuroshio water turns clockwise around Cheju-do throughout the year and eventually joins the Tsushima Warm Current at the western channel of the Korea Strait. This clockwise current is called theCheju Warm Current. The current data do not support the persistent existence of the Huanghai Sea Warm Current,which has ho believed to supply heat and salt to the Huanghai Sea throughout the year. However, a northward flowis generated intermittently during the weakening phase of cold air outbreaks in winter. The northeast extension of theChangjiang diluted water in summer is evidenced by satellite-tracked surface drifters.

Numerical Study of the Secondary Circulations in Rip Current Systems

To investigate the mechanism of secondary circulations in rip current systems, and to explore the relationship between wave conditions and secondary circulation intensity, a series of numerical experiments is performed using coupled nearshore wave model and circulation model. In these experiments, the rip currents and secondary circulations generated above barred beaches with rip channels are simulated. A comparison experiment is conducted to investigate the formation and hydrodynamics of the secondary circulations. Model results indicate that the secondary circulations consist of alongshore flows driven by wave set-up near the shoreline, part of the feeder currents driven by the wave set-up over the bars, and onshore flows at the end of the rip channel driven by wave breaking and convection. The existence of the secondary circulation barely affects the rip current, but narrows and intensifies the feeder currents. Three groups of experiments of varying incident wave conditions are performed to investigate the relationship between wave conditions and secondary circulation intensity. The velocity of the alongshore flow of the secondary circulation is sensitive to the variation of the incident wave height and water depth. It is also found that the alongshore flow intensity is in direct proportion to the alongshore variation of the wave height gradient between the bars and the shoreline.

Optimal design of auxiliary-teeth to solve circulation current reduction of permanent magnet linear synchronous motor

A discontinuity of magnetic circuits according to the end effect is generated in the permanent magnet linear synchronous motor （PMLSM）. Due to the unbalanced back electro-motive force （EMF） and impedance produced, unbalanced current is generated. The cireulatin8 current, which is caused by a decrease in the thrust, is generated by the unbalanced current. The optimal design of auxiliary-teeth at the end of the mover was carried out to solve the unbalance of phase by using design of experiment （DOE）, and compared with the basic model through finite element analysis （FEA）. As a result, the auxiliary-teeth model compensates for the decrease of thrust caused by the unbalanced phase. Also, this model is proven to reduce the detent force by the vibration and noise of the PMLSM and copper loss caused by the circulating current.

Current-event Play and Circulation of Information in late Ming China

This essay examines current-event plays( shishiju 时事剧) and their social functions in late Ming China. Drawing on awide range or primary sources and built on secondary scholarship on information history,the essay offered a complete picture of the booming,features,social functions and the decline of the current-event plays. The essay argues that many current-event plays addressed on credibility,timeliness and easy accessibility,which enabled current-event plays to play an important role in information circulation. This essay further exposes the current-event plays not only opened a forum for public discussion but also influenced public opinions greatly from literate elites to the masses in Ming China. Through analyzing current-event plays in the context of Ming society,this essay expands the realm of studying information history.

Adaptive Droop Control for Circulating Current Suppression in Microgrid Based on Fuzzy Logic

Circulating currents in a microgrid increase the power loss of the microgrid, reduce the operational efficiency, as well as affect the power quality of the microgrid. The existing literature is seldom concerned with methods to suppress the loop currents using fuzzy logic control. In this paper, a method based on fuzzy control of droop coefficients is proposed to suppress the circulating currents inside the microgrid.The method combines fuzzy control with droop control and can achieve the effect of suppressing the circulating currents by adaptively adjusting the droop coefficients to make the power distribution between each subgrid more balanced. To verify the proposed method, simulation is carried out in Matlab/Simulink environment, and the simulation results show that the proposed method is significantly better than the traditional proportional-integral control method. The circulating currents reduce from about 10 A to several nanoamperes, the bus voltage and frequency drops are significantly improved, and the total harmonic distortion rate of the output voltage reduces from 4.66% to 1.06%. In addition, the method used in this paper can be extended to be applied in multiple inverters connected in parallel, and the simulation results show that the method has a good effect on the suppression of circulating currents among multiple inverters.

An extended variable-grid global ocean circulation model and its preliminary results of the equatorial Pacific circulation

To investigate the interaction between the tropical Pacific and China seas a variable-grid global ocean circulation model with fine grid covering the area from 20°S to 50°N and from 99° to 150°E is developed. Numerical computation of the annually cyclic circulation fields is performed. The results of the annual mean zonal currents and deep to abyssal western boundary currents in the equatorial Pacific Ocean are reported. The North Equatorial Current,the North Equatorial Countercurrent, the South Equatorial Current and the Equatorial Undercurrent are fairly well simulated. The model well reproduces the northward flowing abyssal western boundary current.From the model results a lower deep western boundary current east of the Bismarck-Solomon-New Hebrides Island chain at depths around 2 000 m has been found. The model results also show that the currents in the equatorial Pacific Ocean have multi-layer structures both in zonal currents and western boundary currents, indicating that the global ocean overturning thermohaline circulation appears of multi-layer pattern.

The summertime circulation of the Bohai Sea simulated from a high-resolution wave-tide-circulation coupled model

The Bohai Sea is a shallow semi-enclosed inner sea with an average depth of 18 m and is located at the west of the northern Yellow Sea. The climatological circulation pattern in summer of the Bohai Sea is studied by using a wave-tide-circulation coupled model. The simulated temperature and the circulation agree with the observation well. The result shows that the circulation pattern of the Bohai Sea is jointly influenced by the tidal residual current, wind and baroclinic current. There exists an obvious density current along the temperature front from the west part of the Liaodong Bay to the offshore area of the Huanghe Estuary. In the Liaodong Bay there exists a clockwise gyre in the area north to the 40°N. While in the area south to the 40°N the circulation shows a two-gyre structure, the flow from the offshore area of the Huanghe Estuary to the Liaodong Bay splits into two branches in the area between 39°N and 40°N. The west branch turns into north-west and forms an anti-clockwise gyre with the south-westward density current off the west of the Liaodong Bay. The east branch turns to the east and forms a clockwise gyre with the flow along the east coast of the Liaodong Bay. The forming mechanism of the circulation is also discussed in this paper.

Three-dimensional numerical modeling of nearshore circulation

A three-dimensional nearshore circulation model was developed by coupling CH3D, a three-dimensional hydrodynamic model and REF/DIF, a nearshore wave transformation model. The model solves the three-dimensional wave-averaged equations of motion. Wave-induced effects on circulation were introduced in the form of radiation stresses, wave-induced mass transport, wave-induced enhancement of bottom friction and wave-induced turbulent mixing. Effects of breaking waves were considered following Svendsen (1984a and 1984b) and Stive and Wind (1986). The model was successfully tested against the analytical solution of longshore currents by Longuet and Higgins (1970). The model successfully simulated the undertow as observed in a laboratory experiment by Stive and Wind (1982). In addition, the model was applied to a physical model by Mory and Hamm (1997) and successfully reproduced the eddy behind a detached breakwater as well as the longshore current on the open beach and the contiguous eddy in the open area of the wave tank. While the qualitative agreement between model results and experimental observations was very good, the quantitative agreement needs to be further improved. Albeit difficult to explain every discrepancy between the model results and observations, in general, sources of errors are attributed to the lack of understanding and comprehensive description of following processes: (1)the horizontal and vertical distribution of radiation stress, especially for breaking waves;(2)the detailed structure of turbulence;(3)Wave-current interaction (not included at this moment); and (4)the wave-current boundary layer and the resulting bottom shear stress.

Assimilation of High Frequency Radar Data into a Shelf Sea Circulation Model

High Frequency(HF) radar current data is assimilated into a shelf sea circulation model based on optimal interpolation(OI) method. The purpose of this work is to develop a real-time computationally highly efficient assimilation method to improve the forecast of shelf current. Since the true state of the ocean is not known, the specification of background error covariance is arduous. Usually, it is assumed or calculated from an ensemble of model states and is kept in constant. In our method, the spatial covariances of model forecast errors are derived from differences between the adjacent model forecast fields, which serve as the forecast tendencies. The assumption behind this is that forecast errors can resemble forecast tendencies, since variances are large when fields change quickly and small when fields change slowly. The implementation of HF radar data assimilation is found to yield good information for analyses. After assimilation, the root-mean-square error of model decreases significantly. Besides, three assimilation runs with variational observation density are implemented. The comparison of them indicates that the pattern described by observations is much more important than the amount of observations. It is more useful to expand the scope of observations than to increase the spatial interval. From our tests, the spatial interval of observation can be 5 times bigger than that of model grid.

Analysis on the Circulation of the Yangtze River Estuary Based on ADCP Measurements

According to analysis on field data obtained by ADCP （Acoustic Doppler Current Profiler）, the flow regime of the Yangtze River Estuary is studied by use of a 3-D numerical model. The flow field characteristics, under the influence of Coriolis force, saltwater intrusion and freshwater inflow and tidal current interaction, are depicted in details. The main driving forces and some important effective factors of lateral, longitudinal and horizontal circulation are also analyzed.

Effects of topography on the sub-tidal circulation in the southwestern Huanghai Sea(Yellow Sea)in summer

A nested circulation model system based on the Princeton ocean model （POM） is set up to simulate the currentmeter data from a bottom-mounted Acoustic Doppler Profiler （ADP） deployed at the 30 m depth in the Lunan（South Shandong Province, China） Trough south of the Shandong Peninsula in the summer of 2008, and to study the dynamics of the circulation in the southwestern Huanghai Sea （Yellow Sea）. The model has reproduced well the observed subtidal current at the mooring site. The results of the model simulation suggest that the bottom topography has strong steering effects on the regional circulation in summer. The model simulation shows that the Subei （North Jiangsu Province, China）coastal current flows north- ward in summer, in contrast to the southeastward current in the center of the Lunan Trough measured by the moored currentmeter. The analyses of the model results suggest that the southeastward current at the mooring site in the Lunan Trough is forced by the westward wind-driven current along the Lunan coast, which meets the northward Subei coastal current at the head of the Haizhou Bay to flow along an offshore path in the southeastward direction in the Lunan Trough. Analysis suggests that the Subei coastal current, the Lunan coastal current, and the circulation in the Lunan Trough are independent current systems con- trolled by different dynamics. Therefore, the current measurements in the Lunan Trough cannot be used to represent the Subei coastal current in general.

NUMERICAL STUDY ON THE FORMATION OF THE SOUTH CHINA SEA WARM CURRENT I. BAROTROPIC CASE

In this work, Princeton Ocean Model (POM) was used to study the formation of the South China Sea Warm Current (SCSWC) in the barotropic case. Monthly averaged wind stress and the inflow/outflow transports in January were used in the numerical simulation which reproduced the SCSWC. The effects of wind stress and inflow/outflow were studied separately. Numerical experiments showed that the Kuroshio intrusion through the Luzon Strait and the slope shelf in the northern SCS are necessary conditions for the formation of the SCSWC. In a flat bottom topography experiment, the wind stress driven northeast current in the northern SCS is a compensatory current.