Suppression of DC line current oscillation of HVDC transmission system using voltage source forced commutation converter

The conventional converter used in an HVDC system requires line voltage to turn off the current flowing through a thyristor or a mercury-arc valve. Hence, the HVDC system cannot supply electric power to the system which has no ac source, such as a generator or a synchronous condenser. Thus, the HVDC system has some difficulties in operating with the weak system. The conventional solution to avoid these problems is to install large-capacity synchronous condensers which require considerable construction costs and maintenance. Another solution is the application of a forced commutation converter. The recent development of high-power and high-frequency semiconductor devices with fast gate turn-off ability (e.g., GTOs and SI thyristors) suggests the application of a voltage-source-type forced commutation converter (VSFC) to an HVDC system. To ensure the promising characteristics of the overall system, further investigation is of course necessary. The performance of dc transmission into a remote load-only system without ac source has been investigated by means of computer simulation. In many runs of a simulation program of the HVDC system using VSFC, we noticed that long sustaining oscillation of dc line current and dc voltage occurs when a large disturbance is applied. The performance is worse when series dc reactance (DCL) is installed to suppress higher harmonics of dc line current. To improve the performance of the HVDC system using VSFC, an attempt was made to install the L-R parallel damper circuit in series of dc transmission line. Simulation results when DCL and L-R damper are installed in series show that suppression of higher harmonics is as effective as the installation of DCL only, and the damping of oscillation also is substantial to improve the dynamic performance of the whole system.     

Inhibition of hemorrhagic activities of various snake venoms by purified antihemorrhagic factor obtained from Japanese Habu snake

The ability of purified antihemorrhagic factor isolated from the serum of Japanese Habu (Trimeresurus flavoviridis) was tested on 17 snake venoms to inhibit their hemorrhagic activity. The factor strongly inhibited that of the venoms of Crotalus horridus horridus and Vipera latastei gaditana in addition to that of the homologous (T. flavoviridis) venom. Hemorrhagic activity of Agkistrodon halys blomhoffi, Agkistrodon contortrix contortrix, Bothrops atrox asper and Crotalus atrox venoms was also remarkably inhibited, but that of Vipera mauretanica to less extent. The hemorrhagic activities of nine other snake venoms were not inhibited.     

Huffman coding with an infinite alphabet

A new type of sufficient condition is provided for a probability distribution on the nonnegative integers to be given an optimal D-ary prefix code by a Huffman-type algorithm. In the justification of our algorithm, we introduce two new (essentially one) concepts as the definition of the “optimality” of a prefix D-ary code, which are shown to be equivalent to that defined in the traditional way. These new concepts of the optimality are meaningful even for the case where the Shannon entropy H(P) diverges     

Decentralized voltage control in multiarea power system

This paper discusses the feasibility of a decentralized voltage control scheme for large-scale power systems. The algorithm is intended to apply to a reactive normal operating state from an emergency state caused by illcondition and also to keep the operating state away from approaching unstable boundaries in preventive control. The procedure for releasing voltage deviation (over/under voltage) is formulated as a multistage decision process over a certain time interval. The optimization problem is transformed into a two-point boundary value problem using the discrete maximum principle and is solved easily by using the discrete Riccati equation. For a large-scale power system, control values must be computed from a large number of state variables, and this inevitably prolongs the slow dynamics with controllers. The centralized control system in a large-scale system cannot be justified from the economical and technical viewpoints. To resolve the foregoing problems, a decentralized voltage control system incorporating slow voltage dynamics is presented.     

Synthesis of graft copolymers having hydrophobic backbone and hydrophilic branches by phase transfer catalysis

Polystyrene-oligoacrylamide graft copolymer was synthesized by the esterification between chloromethylated polystyrene and carboxyl group terminated acrylamide oligomers using phase transfer catalyst such as tetrabutylammonium chloride in toluene- and/or DMF/water. The graft copolymers having hydrophobic backbone and hydrophilic branches obtained were found to have a water dispersibility.     

A convenient method for the preparation of asialo-GM1

A convenient and efficient procedure has been devised for the large-scale preparation of asialo-G_M1 from bovine brain gangliosides. The procedure relies on the complete desialylation of brain ganglio-sides, consisting primarily of G_M1, G_D1a, G_D1b and G_T1b, by mild formic acid hydrolysis (0.1 N, 100 C; 2 hr). Following the hydrolysis step, asialo-G_M1 can be isolated and purified by Folch partitioning and Iatrobeads column chromatography, with an overall yield of more than 50%. The ganglioside nomenclature used is that of Svennerholm (1).     

Deposition and microhardness of SiC from the Si2Cl6-C3H8-H2-Ar system

We obtained SiC coating layers on a graphite substrate using hexachlorodisilane (Si₂Cl₆, boiling point 144° C) as a silicon source and propane as a carbon source. We examined the deposition conditions, contents of carbon, silicon and chlorine in the deposits, and the microhardness. Mirror-like amorphous silicon layers were deposited in the reaction temperature range 500 to 630° C. well-formed silicon carbide layers with good adherency to the substrate were obtained above 850° C. The lowest deposition temperature of SiC was estimated to be 750 to 800° C. The Vickers microhardness of the SiC layer was about 3800 kg mm^–2 at room temperature and 2150 kg mm^–2 at 1000° C.     

Synthesis of Mesoporous, Nanocrystalline Lanthanum Phosphate in the Presence of Citric Acid and Stearic Acid

Lanthanum phosphate was prepared in the presence of citric acid and stearic acid under methanolic conditions at pH 4.5 and pH 7, respectively. The samples obtained were intensively characterized using X-ray diffraction, nitrogen adsorption-desorption isotherm study, transmission electron microscopy （TEM）, thermal gravimetric and differential thermal analysis, and Fourier transform infrared （FTIR） analysis . The as-synthesized samples prepared at pH 4.5 showed lamellar mesostruroned form with high crystallinity. Results showed that the pore size and pore volume changed when the materials were prepared under different pH conditions. Morphology of the samples was observed by using TEM, which showed that the samples possessed relatively small particles closely packed together. The as-synthesized samples were investigated using FTIR, and the mesopore formation mechanism was discussed.