Influence of HVDC System on the Shaft of Hydro-generator Unit

Widely concerns of the influence of the HVDC system on the Three Gorges power station have been shown so far. Based on the understanding of the mechanism of the non-characteristic harmonics introduced by HVDC system, the influence of interharmonics on the shaft of hydro-generator unit is studied. A MATLAB simulation model for the Three Gorges hybrid AC and DC transmission system with considering the effect of generator-unit shaft is established. Through time-domain simulation, the influence of the minor-associated component of interharmonics on the generator-unit shaft is analyzed, which arise from the asynchronous operation of the HVDC system. Also, the complex torque coefficient method in frequency-domain is used to analyze this problem. Same conclusions can be drawn from the results of timedomain and frequency-domain analyses.     

STATE VARIABLE METHOD FOR DYNAMIC SIMULATION OF HVDC SYSTEMS

A flexible mathematical model used for dynamic simulation studies of HVDC systems which is based on the stale variable method is presented, and the correspongding computer program has been implemented. Generality, easy to use, and suitable to run on microcomputers are the main features of the model and the computer program. Therefore the model is easy to be extended for application. An example is given in the paper to verily the coneclness of the model and the program.     

Recommendations on Shrinkage and Temperature Reinforcements in Concrete Slabs

The mechanism of shrinkage cracking is discussed with a simple member, which is fully restrained and subjected only to an axial restraining force caused by shrinkage, while a simple procedure is presented for the determination of the number and spacing of the cracks and the average crack width. With the simple model, some examples are shown to compare the provisions for minimum shrinkage and temperature reinforcement in GB 50010-2002 Code, ACI 318-02 Code and AS 3600-1988 Code. The results show that the provisions in GB 50010-2002 Code are too weak in comparison with the others. Finally, some recommendations are proposed.     

Comparison of DC Pollution Flashover Performance of Various Types of Long String Insulators

With the development of EHV/UHV DC transmission projects, it is inevitable to study DC pollution flashover performance of long insulator strings. The DC artificial pollution tests of five types of porcelain and glass insulators have been carried out in the artificial climate chamber. The paper analyzes the relationship between 50% flashover voltage U50 and string length, the relationship between U50 and salt deposit density, flashover voltage gradients and valid creepage distances of DC polluted insulators. Based on the test results of insulator strings of 5 - 23 units, there are linear relationship between the 50% DC flashover voltages and string length. The experimental results show that the pollution flashover performances of various types of insulators are different, and that the pollution degree exponents of polluted insulators are affected ainsulator materials and shapes, whose values are 0.3 to 0.36. Under the same pollution degree, the pollution performances of glass insulators have advantage of those of porcelain insulators with the same configuration and the flashover voltage gradient along creepage distance and valid creepage distances of glass insulators are higher than those of porcelain insulators.     

早稻种子包衣直播育秧研究

早稻不催芽直播育秧，抗寒力强，成秧率高，可早播、早熟和省工、省种。是传统育秧方法的创新。种衣剂有防治病、虫、鼠害和营养壮苗作用，它与直播育秧技术结合，可以不侵种直播干谷，又使种衣剂免于在浸种中溶解淋失，两者优势互补，合成一项具有高科技结晶，操作性强。新型实效，简单易行的育秧技术。     

北方粳稻“植生带”育秧设备定向排种装置的研制

为满足超级稻每穴(2±1)粒种子精量播种的特殊农艺要求,在纸夹定位水稻精密育秧设备研究基础上,研发出一种新型超级稻定向排种装置。初步排种试验表明,该装置可将种箱中杂乱无章的稻种分离出来后,在组合滑道上逐渐沿其长轴方向排列成连续的种子流,为超级稻植生带育秧设备的研制提供理论与实践依据。     

播种检测技术的研究进展与趋势

播种作业过程中,一旦出现播种异常现象应能及时报警,并检测出故障的性质和位置,以便机手排除故障,提高播种机的播种质量。要实现这一目的,就必须对播种性能施全程监控。为此,介绍了播种检测技术发展概况,指出了提高精密播种机田间播种质量的关键问题,分析了播种检测技术的发展趋势。     

我国谷物干燥机械的发展现状及对策

谷物干燥是粮食生产的重要步骤。我国是粮食生产和消费大国,因地制宜,大力发展谷物干燥机械,对于解决粮食生产过程中的损失和提高谷物品质有着极其重要的意义。为此,针对我国谷物干燥机械发展过程中存在的问题,提出了对策及建议,为我国谷物干燥机械的发展研究提供了参考依据。     

用于直流传动控制系统的串行通讯

介绍了用Visual Basic提供的串行通讯控件，通过编程实现了PC机与应用单片机触发电路的直流传动系统的通信。