Zhejiang University Press

Voltage source converter high-voltage direct current (VSC-HVDC) is a new power transmission technology preferable in small or medium power transmission. In this paper we discuss a new control system based on space vector modulation (SVM) without any voltage line sensors. Using direct power control (DPC) SVM and a new double synchronous reference frame phase-locked loop (DSRF-PLL) approach, the control system is resistant to the majority of line voltage disturbances. Also, the system response has accelerated by using a feed forward power decoupled loop. The operation of this control strategy was verified in a SIMULINK/MATLAB simulation environment. To validate this control system, a 5 kV·A prototype system was constructed. Compared to the original controllers, the current total harmonic distortion (THD), the active and reactive deviations and the DC voltage overshoot were lowered by 2.5%, 6.2% and 8%, respectively. The rectifier power factor in the worst condition was 0.93 and the DC voltage settling time was 0.2 s.

Air pollution has become the predominant environmental problem caused by rapid industrialization and urbanization in China. In this study, measurements of the concentrations of PM2.5, PM10, SO2, NO2, CO, and O3 at six monitoring stations in Baotou, China were used to investigate the characteristics of heavy haze pollution in Dec. 12–25, 2013. Source locations of PM2.5 in Baotou were identified using satellite remote sensing data, an air mass trajectory model, and a conditional probability function (CPF). The results showed that the average concentrations of PM2.5 and PM10 were (113.8±84.0) μg/m3 and (211.1±149.2) μg/m3, respectively. The similar trends in temporal variation of the air pollutants PM2.5, PM10, SO2, NO2, and CO suggested they may share common sources. The results of satellite observations and backward trajectories supported the hypothesis that the pollutants causing the haze event originated mainly from local anthropogenic sources. According to the CPF analysis, low-speed winds from the south and southwest, upwind industrial emissions, and the northern mountains were mainly responsible for the formation of haze in Baotou. The study provides some insights to help governments optimize industrial layouts for improving air quality in the future.

Suspension-cultured apple fruit cells (Malus pumila Mill. cv. Braeburn) were exposed to a low oxygen atmosphere to test whether programmed cell death (PCD) has a role in cell dysfunction and death under hypoxic conditions. Protoplasts were prepared at various times after low oxygen conditions were established, and viability tested by triple staining with fluorescein diacetate (FDA), propidium iodide (PI) and Hoechst33342 (HO342). DNA breakdown and phosphatidylserine exposure on the plasma membrane were observed using terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL), and annexin V binding. About 30% of protoplasts from cells after 48 h under low oxygen showed an increased accumulation of HO342, indicating increased membrane permeability. Positive TUNEL and annexin V results were also only obtained with protoplasts from cells under low oxygen. The results suggest that apple cell death under low oxygen is at least partially PCD mediated, and may explain tissue breakdown under controlled atmosphere (low oxygen) conditions in apple fruit.

The buffeting of long-span cable-supported bridges under strong winds is one of the key issues in bridge wind engineering. In order to study the effectiveness of the multiple tuned mass dampers (MTMDs) in buffeting control of long-span bridges, the Sutong Cable-stayed Bridge (SCB) with a main span of 1088 m in China is taken as an example in this paper. The spatial finite element model of the SCB is established and the modal analysis is conducted based on ANSYS. After the 3D turbulence wind field of the SCB is simulated using the measured wind parameters, the time-domain buffeting analysis on the SCB is conducted with the aerodynamic self-excited forces included. According to the dynamic characteristics and the time-domain buffeting analysis results of the SCB, the parameter sensitivity analysis on buffeting vibration control with MTMD is conducted in ANSYS. The optimum parameters are then obtained with the construction difficulty and economic factors considered. Results show that the control efficiency is sensitive to the number of the TMD, mass ratio, frequency band-width ratio, and damping ratio. Both the vertical and the lateral vibrations can be effectively controlled when proper design parameters of a MTMD system are used. In addition, the control effect on lateral vibration is better than that on vertical vibration. Results obtained in this study can provide references for anti-wind design and buffeting control of long-span cable-stayed bridges.

This paper aims to address the problem of geometric state control of large-segment steel box girders in offshore hoisting during the construction of large-span bridges. First, the geometric state control indexes of a large-segment steel box girder are determined, such as the manufacturing parameters of the top and bottom slabs, the width of the annular joint, and the support position. Second, the geometric state equations and state transfer matrixes of large-segment steel box girders under different conditions are deduced by taking the mileage and elevation of control points as basic state variables. In application of the geometric state transfer method in the construction control of the Hong Kong-Zhuhai-Macao Bridge, the width of the annular joint and the position parameters for the support of the large-segment steel box girder are predicted precisely. Moreover, the manufacturing parameters of the top and bottom slabs of the steel box girders are calculated reliably. The measured values show that the width of the annular joint is basically the same with the difference of less than 2 mm, the eccentricity of bridge support is less than 20 mm, and the elevation error of the bridge deck is within −10 mm to +15 mm, which meets the construction accuracy. Using the geometric state transfer method, the rapid and accurate installation of the Hong Kong-Zhuhai-Macao Bridge has been realized, demonstrating that the precise control of the geometric state of a steel box girder with ectopic installation and multi-state transition can be realized by using the geometric state transfer method.

The process of hydration and solidification of young concrete has significant effect on the long term strength and durability of concrete. The electrical property of concrete provides a direct and practical method for monitoring and investigating the hydration process of young concrete. This study developed an advanced system for measurement of electrical parameters, used to study the electrical properties of young concrete. The test results provided the electric parameters for concretes with different water binder ratios and different mineral admixture incorporations. The variations and characteristics of the measured electrical parameters were closely related to the physical and chemical properties of young concrete. These parameters were used to analyze and study the hydration process of young concrete.

The linear stability of fiber suspensions between two concentric cylinders rotating independently is studied. The modified stability equation is obtained based on the fiber orientation model and Hinch-Leal closure approximation. The primary instabilities and bicritical curves have been calculated numerically. The critical Reynolds number, wavenumber and wave speeds of fiber suspensions as functions of the aspect ratio, volume concentration of the fibers and the gap width of cylinders are obtained.