碳纤维混凝土电热效应的试验研究

导电发热水泥基材料是近年来在国内外发展较快的一种新型功能水泥基复合材料,它是将具有导电发热功能的组分掺加到水泥基材料中,通过外加电场的作用使混凝土这种传统材料既保持了原有的物理化学性能又增加了其导电发热功能的复合材料。使用这种导电发热功能的混凝土,室内可用作采暖材料,室外可用作人行道、车道及机场跑道等的冬季除冰材料。从干湿环境和直流电交流电两个方面入手研究了碳纤维混凝土电热效应。

Development and Prospect of China Power Grids

In this paper the growing process of China power grid from formation of local power grids to nationwide interconnection is reviewed. The scale and structure of power grid construction in the near future, especially the planning on sending power from west to east, North-South supplementation and nationwide interconnection are introduced. In addition, the technologies to be extended in future grid development are briefed, such as HVDC, FACTS and compact transmission line, etc.

A new dimmer for alternating-current directly driven light-emitting-diode lamp

A new dimmer using a mental-oxide-semiconductor field-effect transistor (MOSFET) for alternating-current (AC) directly driven light-emitting-diode (LED) lamp was presented. The control method of proposed dimmer is pulse width control (PWM) method. Compared with the conventional phase-controlled dimmer, the proposed PWM dimmer can produce sine wave and did not cause harmonics problem. Furthermore, the proposed control method did not amplify the light flicker due to the independence of input voltage. Therefore, the PWM dimmer can be used as the dimmer of the AC LED lamp instead of the conventional phase-controlled dimmer. The experimental result shows that the proposed PWM dimmer has good performances.

Effect of Surface Charge on Flashover for Oil-Pressboard Insulation under AC-DC Combined Electric Field

For converter transformer, AC-DC combined electric field can trigger space charge accumulation on oil-impregnated pressboard interface. The accumulation of space charge on oil-pressboard interface can result in electric field distortion, trend to trigger surface discharge of barriers. This paper studied the influence of surface charge on flashover voltage of oil-impregnated pressboard under AC-DC combined electric field. The study finds that the flashover voltage of oil-pressboard interface under negative polarity DC superimposed AC electric field is higher than that.of positive DC superimposed AC voltage to form composite electric field. It was found that homopolar surface charge has been accumulated on the interface of oil-pressboard with positive or negative DC voltage through measuring surface potential by the electrostatic capacitive probe. The surface charge produced electric field in the opposite direction, which weakening the synthetic electric field strength. What＇s more, under the same conditions, the negative surface charge density oil-pressboard is much larger than the positive.

Weibull Statistics in Dielectric Strength of Oil-impregnated Pressboard under Ramped AC and DC Voltages

AC and DC breakdown tests were performed on large populations of oil-impregnated pressboard samples. The effect of voltage ramp rate on dielectric strength has been investigated. A statistical analysis of breakdown data was carried out using the Weibull distribution. The 90% confidence intervals of Weibull graphs were calculated. The study shows that dielectric strength and shape parameter change versus ramp rate. The variations are attributed to the size and number of defects. Discharges occur from the oil to the oil-pressboard interface and lead to breakdown. DC dielectric strength is larger than that corresponding to AC voltage. This is ascribed to the dissipated energy difference under the two types of field and the fatigue produced by the alternating voltage. This phenomenon is related to space charge. Under DC stress, dielectric strength is higher under negative polarity. It is assigned to the different quantities of space charge accumulated under the two polarities.

A novel design of power management integrated circuit with 90 plus efficiency used in AC/DC converter

Recently, resonant AC/DC converter has been accepted by the industry. However, the efficiency will be decreased at light load. So, a novel topology with critical controlling mode combined with resonant ones is proposed in this paper. The new topology can correspond to a 90 plus percent of power converting. So,a novel topology of an state of art integrated circuit, which can be used as power management circuit, has been designed based on the above new topology. A simulator which is specifically suitable for the power controller has been founded in this work and it has been used for the simulation of the novel architecture and the proposed integrated circuit.

Comparative Analysis of Sonar Heads Drive Systems

This paper is dealing with a comparative analysis, from technical point of view of the solutions with the highest potentiality utilized in sonar heads drives. Even though the use of DC servomotors is a convenient solution for most customers, from some modem analysis criteria points of view, this type of drive system has a low reliability and a greater impact on the environment, compared to AC servomotors. From this class of AC servomotors, high behaviors, in such an application, have stepper motors and electronically commutated motor （brushless DC）. That is why, analysis in this paper, balances these two classes of AC servomotors. The systems performed are analyzed in Matlab/Simulink and PowerSim environments.

A novel control strategy for load converter of DC isolated distribution system under unbalanced loading conditions

A novel control strategy for the load converter supplying the unbalanced AC load in a DC isolated distribution system is presented. The control algorithm results in balanced and sinusoidal load voltages under unbalanced AC loading. The unbalanced load is characterized in the d-q-0 rotating coordinate based on symmetrical sequence components. Also, the mathematical model of the load converter in both a-b-c and d-q-0 coordinates is derived by using the average large signal model. Then, two control strategies for the load converter are presented. The first one uses the conventional d-q-0 controller to ensure the voltage and current regulation. The second one is a newly proposed control strategy based on the decomposition of the voltage and current into in-stantaneous positive, negative, and zero sequences. These three sequences are controlled independently in their own reference frames as DC signals. The performance of the load converter using these two control strategies is compared. Simulation results show the validity and capability of the newly proposed control strategy.

Influence of Qinghai-Tibet HVDC Transmission System on AC Transmission Line Protections in Qinghai Power System

To analyze the influence of _＋400 kV Q^nghai-Tibet HVDC transmission system on transmission- line protections in Qjnghai AG power system, a closed-loop simulation system was constructed by combing HyperSim system with HVDG control protection devices. Various faults on double-circuit 750 kV and multi- circuit 330 kV AC transmission lines in Qjnghai power system were simulated. The impedance characteristics and harmonic components at Qjnghai side of Qjng-Tibet DG transmission line were analyzed. The harmonic proportion in voltages and currents were studied for faults that took place at different locations near the DG system. The inflence of Qing-Tibet DG system on the directional components of protections, differential protections and distance protections of AC transmission lines was discussed and drew the conclusions that the DC sytem had little influence on differential protections, while had great inflence on directional components and distance protection. The conclusions can provide reference for studying the interaction between AC and DG systems.