Ing. Libor Straka, Ph.D.

Commercially available LaFeO3 powder was processed using the spark plasma sintering (SPS) technique. The results of the dielectric measurement showed high permittivity, but this was strongly frequency-dependent and was also accompanied by a high loss tangent. The chemical purity of the powder and changes induced by the SPS process influenced the stability of the dielectric parameters of the bulk compacts. A microstructure with a homogeneous grain size and a certain porosity was produced. The microhardness of the sintered LaFeO3 was rather high, about 8.3 GPa. All the results are in reasonable agreement with the literature related to the production of LaFeO3 using different techniques. At frequencies as low as 100 Hz, the material behaved like a colossal permittivity ceramic, but this character was lost with the increasing frequency. On the other hand, it exhibited persistent DC photoconductivity after illumination with a standard bulb.

The high pressure forming at 300 MPa and room temperature was applied before firing of pure lithium fluoride powder. This material has very low sintering temperature, only about one half of typical oxide-ceramic dielectrics. Despite this fact, the resulting fired bulk was tested as dielectrics and showed very interesting characteristics. The best sample, fired at 750 °C and for 8 hours, had relative permittivity 12.1 and loss tangent 0.0006, frequency-and temperature-independent up to at least 150 °C. These parameters are comparable with oxide ceramics, fired at temperatures over 1500 °C, as for example alumina or YAG.

Various kinds of natural garnets belonging to the almandine type (3FeO·Al2O3·3SiO2) were sprayed by plasma spray technique to build coatings on metallic substrates. The experimental garnet powders came from different mines in the Czech Republic and Mongolia. After coating and cooling the substrates were removed. In this way, self-supporting plates were obtained and further studied with microscopy, X-ray diffraction, and dielectric spectroscopy. Mechanical properties were in our focus as well. Microhardness was measured on cross sections dedicated to microstructure observation. Wear resistance in wet conditions was tested in a slurry. Reflectance was measured applying visible and infrared (VIS-NIR) radiation. Dielectric properties of coatings were studied at low voltage capacitance, loss tangent and also under direct current (DC) resistance. The results show that garnet minerals are interesting candidates for various optical and electronic applications; they have similar dielectric behavior as, for example, aluminum oxide or similar high-purity synthetic oxides, and, simultaneously, they have extraordinarily low reflectance in VIS-NIR radiation. The differences between natural powders and resulting coatings are discussed in connection with their chemical and phase compositions.

LaFeO 3 commercial powder was fired by spark plasma sintering. Microstructure with homogeneous grain size and certain porosity was produced. Microhardness of the compact was rather high. The dielectric measurements showed high permittivity, but very frequency dependent, associated also with a high loss tangent. Analysis of the data showed high contribution of grain boundaries to the total impedance. The results roughly agree with the literature.

Barium titanate coatings were, for the first time, sprayed by a high feed-rate plasma torch with water stabilization of the plasma. Two power levels of the torch were applied for spraying to cover steel substrates. Various substrate preheating levels from 125 C to 377 C were used to modify cooling conditions. Microstructure and phase composition including crystallinity quantification were observed. Dielectric measurements proved that the relative permittivity between 300 and 400 coatings is too temperature sensitive over 170 C but fits the requirements of the EIA temperature coefficient between room temperature and 170 C. Simultaneously, the loss tangent remains rather low, between 0.02 and 0.07, in a broad range of temperatures and frequencies. Annealing was performed in air to heal the oxygen deficiency, but only modified the microstructure insignificantly. The dielectric properties of as-sprayed and annealed samples were discussed, with the main finding that the temperature coefficient of permittivity was improved by annealing. This study contributes to the search for the suitability of plasma-sprayed BaTiO3 coatings for application in the electrical industry, namely by the optimization of conditions for high feed-rate spraying.

This paper deals with the possibility of energy consumption measurement using numerical integration. In this paper is described method based on Simpson integration and its comparison with standard analytical method of evaluating of energy consumption. The result of this article is a comparison of the error of energy of the analytical and numerical methods, depending on the number of samples required.

This paper presents a mathematical model that is developed to economically optimize a small and a medium-sized local cogeneration system. This model was developed to find the economically optimal configuration of the cogeneration system with respect to state aid, which forms a significant part of the revenue for small and medium-sized systems. The model respects the production of heat, so it won't be exceeded the economically justifiable demand, which means that the same amount of heat will be produced as in the supply of heat through conventional sources, which is essential for obtaining the aid. Therefore, the model is suitable for use in the design of cogeneration systems with average seasonal heat demand up to 5 MW. These are typical hospitals, hotels, office buildings and small factories. The model can design an accumulation tanks and a bivalent heat source to cover the maximum and minimum peaks in heat requirements, it also respects the thermal losses of buildings and climatic data in the given location. It is also possible to design a system for the preparation of hot water during the summer months.

This paper deals with possibility of energy efficiency increase of annealing parts of rotors of gas turbines using induction heating. The efficiency is evaluated using mathematical model in SW Agros2D which allows 2D modeling of thermal and electromagnetic fields. The results are compared with the measurements. The influence of particular parameters (design the inductor, speed of rotation, supplying power and frequency) on the efficiency of heating is discussed.

Rostoucí podíl obnovitelných zdrojů energie (OZE) v energetické bilanci států, tedy i v bilanci České republiky. Prognózy rozvoje energetiky k roku 2050 prokazují opětovný růst těchto zdrojů energie, a to na úkor klasických, fosilních paliv. V článku se budou autoři zabývat dopadem těchto změn na řízení elektrizační soustavy, podmínkami jejich připojování do distribučních soustav a vývojem technologií, které umožní sladit okamžitou potřebu elektrické energie s možnostmi zdrojů.

This article is focused on using of optical voltages sensors for high-voltage measuring. The measuring method of optical sensors is based on electro-optical effect of some inorganic crystals which have a linear electro-optical effect, called Pockels effect. The article deals with the design of electro-optic voltage measurement systems and shows basics of principle.

In this paper we present a new approach to determine the presence of fault in near-real time conditions and to determine the fault existence in the given line length based on the implementation of Artificial Neural Networks (ANN's) in the transmission lines, of an electrical power system.

Since there are still higher efforts for transmission systems equipment utilization, more control mechanisms and devices are integrated to the systems and their dispatch centers and these will be also in future. FACTS devices purpose is preferably to maintain voltage levels and to control active and reactive power flows. Their implementation in the grid can result from the necessity to improve the system capabilities in a longterm period or to solve some extraordinary states. The paper deals with the implementation of shunt and series FACTS devices into electrical transmission systems and their influence on active power losses. There are provided comparison analyses of how different FACTS types have a potential to changes power losses in the system either in a negative or in a positive way. There are explained basic principles and the results are presented on the IEEE/CIGRE transmission system model.

V příspěvku je popisována nová metoda určování fázorů a frekvence. Přesné určení fázorů a frekvence v elektrických sítích je velmi důležitou oblastí v elektroenergetice. Tato oblast je důležitá pro řízení výkonových toků v soustavě s využitím systémů FACTS, přesnou lokalizaci poruch, určení parametrů prvků v soustavě apod. Popisovaná nová metoda je založená na statistickém určení frekvence sítě z navzorkovaného vstupního signálu.

V tomto článku je popsán moderní způsob lokalizace poruch v sítích velmi vysokého napětí, který využívá H- matice přenosové linky. Následně je tato metoda porovnávána s jiným typem hledání poruchy, v tomto případě s lokalizací poruchy pomocí distanční ochrany. Veškeré simulace jsou vypočteny v software Wolfram Mathematica.

Static and dynamic characteristics of a PPMT actuator are modeled. The magnetic fluxes in the system are controlled by generally time varying external currents in the field coils. The movement of the right ferromagnetic part of the PPMT actuator is controlled by a mechanical spring. The paper describes the arrangement of the system, its mathematical model and the most important results.

Path Magnetic Technology (PPMT) belongs to novel promising possibilities of controlling magnetic fluxes and forces in numerous devices such as electric motors, generators and actuators. It is based on placing two or more permanent magnets into the magnetic circuit, where the distribution of magnetic fluxes is controlled by field coils. It was shown that the devices working with this technology exhibit higher efficiency, power density and smaller physical sizes compared to the devices of classical arrangement. The principal problem in the area is how and where the control field coils should be located in order to reach the best results. We modelled an actuator consisting of two ferromagnetic bars of rectangular cross sections connected with two prismatic permanent magnets of the same orientation. There are two more prismatic ferromagnetic parts in the system attracted to the magnetic circuit by the permanent magnets. Each ferromagnetic bar carries one field coil. Magnetic fluxes in the system are controlled by generally time varying external currents in the coils. The aim of this work was to find the dependence of magnetic forces attracting the additional parts of the circuit on these currents. The paper describes the arrangement of the system, its mathematical model and the most important results.

The paper deals with the possibilities of utilizing series and shunt active filters compensation in order to mitigate voltage fluctuation and flicker caused by Electrical Arc Furnace (EAF) in industrial distribution systems. A voltage fluctuation that results in flicker is caused by EAF power fluctuation. The Unified Power Quality Conditioner (UPQC) belongs to the group of FACTS devices based on power electronics converters, particularly equipped with component with turn-off capability (e.g. IGBT). They offer many compensation functions for power transmission systems and represent the most progressive way to increase power quality in the systems. The UPQC can be an effective means to suppress voltage fluctuation connected with the function of the EAF. Flicker has been compensated for mainly by using Static Var Compensators (SVC) whose capability to eliminate voltage subharmonics is limited by the frequency up to approximately 10 Hz at maximum. Recently, some studies and tests have been done to prove advantages of active filters (AF) over SVC in this field. The UPQC consists of the parallel and series AF. Therefore it offers more possibilities to solve instantaneous active and reactive power fluctuation than either the parallel or series AF alone. A model of a power system with an electric arc furnace was developed for a typical metallurgical plant. There is an important aspect of the suitability to keep the current SVC compensation. The advanced solution is to add SVC by less dimensioned UPQC for sparing investments and improving the system conditions together. The paper would like to emphasize that FACTS devices should have their position not only in large transmission system but for specific tasks also in local distribution systems.

The paper describes the use of a solar concentrator for producing electrical energy. It deals with production of electrical energy using solar radiation. The principle of the device is based on concentration of solar energy into a given area and the use of photoelectric effect for transforming this solar energy into electrical energy. The first goal was to find the amounts of electrical power and waste heat from the solar concentrator. For the chosen design a mathematical model was proposed and solved by Wolfram Mathematica. The following physical quantities and processes were modelled: the total electric power (described by photoelectric phenomenon equations), the flow of oil used as coolant (Bernoulli equation), and heat transfer between the concentrator, the cooling medium and ambient environment (laws of heat transfer by convection, conduction and radiation). The device was built in the laboratory and the numerical model was consequently tested and improved using the measured data. To achieve a relatively optimal level of concentration, an aluminium sheet in the shape of the letter "U" was used. Focused solar rays were used for the electrical energy production in a solar cell located in the lowermost part of the device. Oil flowing through the pipes removes the heat from the solar cell and cooling of the semiconductor layer improves the efficiency of solar-electrical energy conversion. The influence of device positioning (azimuth and elevation angles) was also studied. The authors hope that future experiments will lead to better understanding of the roles of direct and diffusive radiation components.