Offshore wind farm with a series multiterminal CSI HVDC

This paper presents an integrated design of an offshore wind farm and an interconnection circuit based on a series multiterminal HVDC link with current source inverters (CSI). The transmission converters are used to achieve variable speed operation for a group of generators, and this enables use of very simple generators. The series converter connection eliminates offshore transformers. The paper discusses the control systems for both, generator side and grid side converters. A 200 MW wind farm is simulated on PSCAD/EMTDC platform and the responses confirm satisfactory operation for a range of wind speed changes. It is shown that each generator group can operate with a different and optimal frequency and that wind variations on individual units cannot jeopardize system stability. The main challenges for the proposed topology are system insulation and management of transmission line losses, and the paper studies some possible solutions.     

Characterization of lactic acid bacteria isolated from Bukuljac, a homemade goat's milk cheese

The Bukuljac cheese is traditionally homemade cheese, produced from heat-treated goat's milk without the addition of any bacterial starter culture. The presence of lactic acid bacteria (LAB) in Bukuljac cheese has been analyzed by using a polyphasic approach including microbiological and molecular methods such as rep-PCR with (GTG)5 primer. Lactobacillus paracasei subsp. paracasei represents a dominant strain in the microflora of analyzed cheese. Out of 55 Gram-positive and catalase-negative isolates, 48 belonged to L. paracasei subsp. paracasei species. Besides lactobacilli, five Lactococcus lactis subsp. lactis and two Enterococcus faecalis were found. Results of PCR-denaturing gradient gel electrophoresis (DGGE) of DNA extracted directly from the fresh cheese revealed the presence of Leuconostoc mesenteroides. Only lactobacilli showed a high proteolytic activity and hydrolyzed alpha(s1)- and beta-caseins. They are also producers of diacetyl. In addition, 34 out of 55 isolates, all determined as lactobacilli, showed the ability of auto-aggregation. Among 55 isolates, 50 also exhibited antimicrobial activity.     

Surface properties of Lactobacillus and Leuconostoc isolates from homemade cheeses showing auto-aggregation ability

Ten lactobacilli and one leuconostoc showing auto-aggregation ability were isolated from artisanal cheeses. Furthermore, non-aggregation strains were isolated from the same cheese sample, if existed. The analysis of factor(s) possibly involved in auto-aggregation was performed. The pretreatment of cells with proteinase K resulted in the disappearance of auto-aggregation ability. Moreover, cells also lost aggregation ability after three-times, successive washing in distilled water. Testing the ability of strain Lactobacillus paracasei subsp. paracasei BGSJ2-8 and its aggregation-deficient derivative BGSJ2-81 to co-aggregate with Listeria innocua ATCC33090, Escherichia coli ATCC25922 or Salmonella typhimurium TR251 showed that strain BGSJ2-8 co-aggregated with these strains, but derivative BGSJ2-81 was not. However, the treatment of L. paracasei subsp. paracasei BGSJ2-8 with proteinase K prior to co-aggregation tests resulted in losing co-aggregation ability. Surface properties of selected strains were analyzed by MATS (microbial adhesion to solvents) method. It was noticed that the strains with auto-aggregation ability were highly hydrophobic in comparison with aggregation-deficient ones. Comparative analyses of the surface features of strain L. paracasei subsp. paracasei BGSJ2-8 and its derivative BGSJ2-81 revealed notable difference.     

The in vivo effect of liposomes on hematopoiesis

The influence of liposome structure on hematopoiesis in vivo was assessed in relation to the different contents and origins of phospholipids that make up their membrane structures. Changes within different hematopoietic cells and serum tumor necrosis factor alpha (TNF-alpha) levels were estimated up to 14 days following intravenous administration of liposomes made of either pure egg yolk phosphatidylcholine (LEY) or a soybean phospholipid preparation (LSB) into normal CBA mice. In peripheral blood, only transient changes within white blood cells were observed. In bone marrow, a persistent decline in the number of mature granulocytes, monocytes, and lymphocytes was found. The changes within femoral granulocytic proliferative compartments in various stages of differentiation and a maturation compartment pointed out that, parallel with the depletion of the granulocyte-storage pool, stimulation of de novo production of granulocytic cells occurred. Although both types of tested liposomes induced similar cellular changes, only liposomes made of pure egg yolk phosphatidylcholine induced a transient increase in serum TNF-alpha levels.     

Control system design for VSC transmission

This paper investigates a suitable control system for a DC transmission system based on voltage source converters (VSC). Each of the two VSC converters has two control inputs and the four control channels on a VSC transmission system offer potential for a versatile control. However, the main challenge is the dynamic interaction among the control loops. It is proposed in this study that the overall system stability and good robustness should be achieved with two high-gain feedback loops, one at each converter. Eigenvalue and robustness analysis with MATLAB software shows that the best fast feedback signals are inverter DC voltage and rectifier AC voltage q component. The slow controller consists of three PI controllers that regulate: rectifier AC voltage, inverter AC voltage and the DC power. The VSC transmission control under fault conditions is achieved with a separate controller that takes over system control for fault-level currents. The fault controller regulates the local DC currents at each converter. The proposed controller is tested using PSCAD/EMTDC for a wide range of small-signal step inputs and the design performance is confirmed. The design is also tested for typical fault scenarios on AC and DC side to verify the fault controller operation.     

Offshore wind farm with centralised power conversion and DC interconnection

The study of a hypothetical large offshore wind farm based on centralised power conversion and interconnected to the grid using a multiterminal parallel high voltage direct current (HVDC) link is presented. The 300 MW wind farm consists of 60 squirrel-cage based 5 MW generators connected to a common DC bus using ten voltage source converters (VSCs). The transmission system converters provide variable speed generator control, and therefore individual converters are not required for each wind generator, implying savings in wind farm costs. The paper studies the technical and economical benefits of the proposed topology, as well as the selection of the main components. A detailed analysis of the control circuits for both generator and grid facing converters, with respect to primary control functions, is also given. PSCAD/EMTDC simulation of the proposed concept is presented for realistic wind signals. The results confirm operation at an average optimum coefficient of performance at each respective generator group, as well as satisfactory stability even for severe wind speed changes. The proposed concept reduces the costs associated with DC interconnection and may simplify integration of large offshore wind farms at substantial distances.     

A survey of the lactic acid bacteria isolated from Serbian artisanal dairy product kajmak

Kajmak is an artisanal Serbian dairy product made by fermentation of milk fat. Overall, 374 bacterial isolates were collected from six kajmak samples of different ages produced in the households located in distinct regions of Serbia. In order to identify lactic acid bacteria present in chosen samples of kajmak, total 349 Gram-positive and catalase-negative isolates were analyzed. The recognition of isolates was performed by phenotypic characterization followed by molecular identification using (GTG)(5)-PCR and sequence analysis of 16S rRNA gene. Leuconostoc mesenteroides and Enterococcus faecium were the most frequently isolated species from kajmak samples. In contrast, leuconostocs and enterococci were found in BGMK3 and BGMK1 kajmak respectively, only after using enrichment technique for isolation suggesting they are present in low numbers in these kajmaks. Lactococcus lactis, Lactococcus raffinolactis and Lactococcus garvieae were also found in those samples but in lower proportion. Results showed that Lactobacillus plantarum, Lb. paracasei and Lb. kefiri were the most frequently isolated Lactobacillus species in analyzed kajmaks.     

Phase locked loop system for FACTS

This research addresses the special requirements of phase locked loops (PLLs) for a typical application with FACTS elements. A new PLL system that uses adaptation algorithms is developed with the aim of improving speed of responses, robustness to AC voltage depressions, and harmonic rejection. The adaptive PLL consists of the three control units that individually control frequency, phase angle, and voltage magnitude. The voltage controller output is used to compensate for reduced gain caused by the AC voltage magnitude depressions. The output phase angle and its derivative, the frequency signal, are controlled in two independent control systems in order to enable elimination of frequency and phase error without compromising transient responses. The simulation results are compared with a PLL available with the PSB MATLAB block-set and noticeable improvements are demonstrated. In particular, settling time and overshooting are significantly lower with conditions of reduced AC voltage magnitude.