Fast evaluation of dynamic changes of electric power systems frequency during primary control

The paper presents the development of a simple method for the fast and sufficiently accurate assessment of dynamic changes of power systems frequency during the time period of primary control. Also, this model gives corresponding values of the total mechanical power of synchronous generators, during the some period. The development of a proposed low-order system frequency response model is based on the main assumption of uniform movement of generators, which enables representation of the power system through an equivalent controlled synchronous machine. The characteristics of the developed method were determined on an example of a real electric power system and a comparison was made with the results of a stricter approach.     

A new concept of the SCARA robot

Based on the SCARA concept, accepted worldwide, this paper considers the possibility of realizing SCARA with a full circle working area. The basis of the approach is a mechanism with two eccentrically positioned rotating discs instead several tools (hands) is also facilitated. This paper analyzes the advantages of the proposed solution and the possibilities of its realization.     

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.     

Damage accumulation in light alloys on programmed loading. Part 2. Effects of fretting corrosion

A study has been made on how fretting corrosion affects damage accumulation in programmed loading with two-step blocks for D16AT, VT14, and AMg6 alloys. The accumulation is dependent on the relation between the stresses in the steps and the yield point for the surface layer of material. The hardening in the high step is due to the residual compressive stresses, which cause plastic strain with loading beyond the yield point of the surface layer but elastic strain in the internal layers.Translated from Problemy Prochnosti, No. 12, pp. 31–37, December, 1992.     

Charge Photogeneration in Few‐Layer MoS2

The 2D semiconductor MoS₂ in its mono‐ and few‐layer form is expected to have a significant exciton binding energy of several 100 meV, suggesting excitons as the primary photoexcited species. Nevertheless, even single layers show a strong photovoltaic effect and work as the active material in high sensitivity photodetectors, thus indicating efficient charge carrier photogeneration. Here, modulation spectroscopy in the sub‐ps and ms time scales is used to study the photoexcitation dynamics in few‐layer MoS₂. The results suggest that the primary photoexcitations are excitons that efficiently dissociate into charges with a characteristic time of 700 fs. Based on these findings, simple suggestions for the design of efficient MoS₂ photovoltaic and photodetector devices are made.     

Quantitative stereological studies of a ''selection'' protocol of hepatocarcinogenesis following initiation in neonatal male and female rats

Understanding the reasons for infertility and the development of new techniques for its treatment is one of the fields within medicine which has undergone a dramatic development during the last two decades. In the past, gynecologists seemed more successful in the treatment of female problems than urologists and andrologists on the male side, while in the last couple of years this scenario has changed due to the development of both operative procedures to harvest spermatozoa from the epididymis and the testis and the ability to micro-inject single spermatozoa (ICSI) or even spermatides directly into metaphase II oocytes. It should, however, be emphasized that the successful development of in vitro fertilization (IVF) and other assisted reproduction technologies (ART), could not have taken place without the development of diagnostic innovations, such as immunoassays of hormones, high resolution ultrasound and various endoscopic techniques. Today, treatment of the infertile couple is less of a medico-technical problem and more a socio-economic dilemma. This communication is an attempt to summarize the more prominent steps in reproductive medicine during the last decades and one more general conclusion of global relevance which can be drawn, is that less money must be spent on diagnosis and more resources transferred to treatment options.