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With the development of VSC HVDC transmission, the realization of the first VSC MTDC grid is coming within reach. An outstanding issue is DC voltage control in MTDC Systems. The easiest way to maintain stable operation is to assign the task of DC voltage regulation to one converter. This paper discusses a control strategy for an extended VSC MTDC grid using a typical DC voltage control on one DC bus, combined with a DC voltage droop characteristic on the other DC buses. The impact of the proposed control structure on the stability of the AC and DC grid is investigated by numeric simulations using the MatDyn software package.

SUMMARYVoltage source converter high voltage direct current (VSC HVDC) is a relatively new technology which allows the connection of islanded power systems. As such, it can form a connection between large-scale future offshore wind farms and the onshore transmission grid. In this paper, the VSC HVDC solution is proposed as a promising alternative for high voltage alternating current (HVAC). The choice for a transmission technology is primarily based on economic considerations. Therefore, the economic value of VSC HVDC is investigated in this paper. VSC HVDC has important technical advantages for submarine installation compared to HVAC. The aim is to monetize these advantages in an economic comparison. Possible wind farm topology simplifications and optimizations due to the use of VSC HVDC are taken into account. Depending on the ownership of the transmission cable, the break-even cable length, above which the HVDC option is chosen, varies. A sensitivity analysis is included in this paper.

Chaetomium globosum Kunze, has been identified as a potential antagonist of Cochliobolus sativus (S. Ito & Kurib.) Deschler ex Dastur. (Syn = Drechslera sorokiniana). Production of antifungal compounds by Chaetomium globosum (Cg) and their role in suppression of spot blotch of wheat caused by this fungus under in vitro and in vivo has been evaluated. Interaction between Chaetomium globosum isolates and C. sativus showed mycoparasitism by isolates Cg 1 and Cg 6 whereas isolates Cg 2, Cg 3, Cg 4 and Cg 5 showed antibiosis. Syringe filtered culture extracts of Cg 2 completely inhibited mycelial growth of C. sativus in liquid broth. In vitro bioassays were undertaken by amending the medium with crude extracts and agar diffusion method in order to assess the fungistatic activity of crude extracts from culture filtrates of different isolates of Chaetomium globosum. Significant differences in antagonism between isolates were observed. Antifungal metabolite profiling, on TLC (Thin Layer Chromatography) plates identified 13 compounds in isolate Cg 2, 11 compounds in Cg 3 and 7 compounds in Cg 6. Isolate Cg 1 produced only two faint bands and Cg 5 produced two bands of the same Rf value but of higher intensity. The production of antifungal compounds by isolates was positively correlated with antagonism to C. sativus on seedlings in glasshouse studies. The results showed high antifungal metabolite production by isolate Cg 2, which also gave maximum bioefficacy under laboratory and glasshouse conditions.

The effects of the highly aggressive isolate KB-2 of the Karnal bunt pathogen (Neovossia indica) on phenol metabolism, peroxidase (POX) and its isoenzymes were studied in wheat. Two resistant genotypes, HD 29 of bread wheat (Triticum aestivum) and DWL 5023 of durum wheat (T. durum), and one susceptible bread wheat, WL 711, were used. In the bread wheats, total phenols reached a maximum 2 days after inoculation (d.a.i.). In the resistant durum line, total phenols did not change significantly for 6 d.a.i., but declined significantly at 10 d.a.i. Three phenolic compounds, caffeic acid, l-tyrosine and hydroquinone, were detected using thin-layer chromatography. The first two were detected at all times at and after inoculation, but hydroquinone was detected only in the resistant wheats at 6 d.a.i. The activity of POX was highest at 2 d.a.i. in the two resistant wheats, but increased more slowly to a peak at 6 d.a.i. in the susceptible wheat. The number of isoenzymes of POX detected by polyacrylamide gel electrophoresis (PAGE) changed after inoculation with KB-2. The maximum number of isoenzymes occurred at 2 d.a.i. in the two resistant wheats and at 6 d.a.i. in the susceptible wheat. Although the isoenzymes detected in seedlings were not identical to those detected in seeds, the PAGE banding patterns of seeds and seedlings were the same for the two resistant wheats. The potential use of the additional band at a relative mobility of 0´42 in seeds and 0´28 in seedlings as markers for Karnal bunt resistance is discussed.

The scene of electric power systems has changed drastically and will continue to do so in the years to come. Causes of this evolution are the liberalization of the electricity markets and the increased use of intermittent renewable energy. Uncertainty in the transmitted power and congestion of the lines raise the need for new technologies. High Voltage Direct Current (HVDC) is one of these emerging technologies that may have a major impact, and more specifically Voltage Source Converter (VSC) HVDC. The main part of this paper is to verify the feasibility, technical as well as economic, of VSC HVDC on loss minimization in meshed networks. The losses of the different components of a VSC HVDC link are studied and the impact of the installation on overall system losses in meshed networks is verified through simulations. Simulations are performed on two networks: the small IEEE 14 bus network and the larger IEEE 118 bus network. The networks are investigated in their original state and in a state of increased loading. Finally, an economic analysis is performed to investigate the economic feasibility of VSC HVDC on loss reduction in meshed networks.

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Increased bulk power transactions in competitive energy markets together with large scale integration of renewable energy sources are posing challenges to high-voltage transmission systems. Environmental constraints and energy efficiency requirements also have significant effects on future transmission infrastructure development. This paper reviews the recent development in HVDC technologies and discusses the needs of the hybrid AC/DC grid structure for future power systems with focus on VSC-HVDC applications in meshed ac grid. It has also been recognized that hybrid AC/DC transmission system together with the wide area measurement system (WAMS) could effectively manage the overall power grid operation security and efficiency under uncertain supply and demand conditions. Index Terms-Transmission expansion, security and efficiency of power delivery system, HVDC transmission, VSC-HVDC, wide area measurement system Jiuping Pan received his B.S.