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  Problemele energeticii regionale

E-Journal 1(45) 2020



1 Electromagnetic Processes Simulation of Power Transformers in Operation and in No-load Modes
Authors: Yarymbash D.S., Kotsur M.I., Yarymbash S.T., Kylymnyk I.M. Zaporizhzhia National Technical University Zaporizhzhia, Ukraine
  Abstract: In the designing preproduction of power transformers for calculating design data, a circuit models are widely used, which doesn’t fully reflect the structural features of the transformer and spatial energy conversion. This leads to a significant increase of the error of design calculations and calculations of the magnetization parameters in load operation of transformers. This is especially the case in networks with alternative generation of electrical energy. Therefore, the aim of the work is to estimate the influence of design features and non-linear characteristics of electrical steel on the magnetization parameters in loading variations mode of the transformer. To achieve this aim, an improved model of electromagnetic processes in loaded power transformers is proposed. Its scientific novelty makes sense in the conjugation circuit models of external network and user’s network with a spatial model of the magnetic field in the transformer. When the transformer working in operation mode, the magnetizing currents and idle currents are characterized by asymmetry and non-sinusoidality. Reducing the load of the transformer from operation to idle modes, these effects are enhanced. Therefore, the effective values of the magnetization currents increase in different intervals: (1.066%; 1.161%) for phase A; (1.275%; 1.303%) for phase B; and (1.187%, 1.308%) for phase C. At using of the data of the coupled circuit models and the magnetic field model for the ratios and coefficients of parabolic and polynomial regression provides high accuracy in calculating of the magnetization parameters under various transformer load conditions.
  Keywords: three-phase transformer, rated mode, circuit models, magnetic field model, finite element method, magnetic field energy, magnetization currents, harmonic components, regression equations.
DOI: 10.5281/zenodo.3713396
2 Magnetic Field Active Shielding of Overhead Power Lines with Triangular Phase Conductors Arrangements
Authors: Kuznetsov B., Nikitina T., Bovdui I. State Institution «Institute of Technical Problems of Magnetism of the National Academy of Sciences of Ukraine» Kharkiv, Ukra
  Abstract: The purpose of the work is synthesis of two degree of freedom robust two circuits system of active shielding of magnetic field, generated by overhead power lines with triangular phase conductor’s arrangements for reducing the initial magnetic flux density to the sanitary standards level and reducing the system sensitivity to the plant parameters variations. Two circuits system of active shielding includes two shielding coils. System of active shielding synthesis reduced to the multiobjective stochastic game solution, in which the vector payoff is calculated on the basis of the Maxwell equations solutions in the quasi-stationary approximation. The calculation of multiobjective game solution is based on the multi swarm particles optimization algorithms. The initial parameters for the synthesis are geometry and number of phase conductors, operating currents of overhead power lines, as well as the size of the shielding space As a result of the synthesis the number, configuration, spatial arrangement, and currents of shielding coil, algorithm parameters of the two degree of freedom robust two circuit system of active shielding of magnetic field as well as the resulting value of the magnetic flux density in the shielding space are determined. New scientific results are theoretical and field experimental research of the shielding factor of two circuit system of active shielding with two shielding coils carried out. The possibility of reduction of the initial magnetic flux density level is given. It is shown that the experimental and calculated magnetic field induction values differ by not more than 20 %.
  Keywords: overhead power lines, triangular phase conductor’s arrangements, magnetic field, active shielding system, theoretical and field experimental research.
DOI: 10.5281/zenodo.3713400
3 Characteristics of the Static Zigzag-Triangle Frequency Converter
Authors: Kalinin L.P., Zaitsev D.A., Tirsu M.S., Golub I.V. Institute of Power Engineering Chisinau, Republic of Moldova Pogorletsky V.M., Kaloshin D.N. Transnistrian State University named after Shevchenko Tiraspol, Republic of Moldova
  Abstract: The object of research is a static transformer frequency converter, which can be used to combine parallel-operating power systems having different operating frequencies. The aim of the work is to study operational characteristics and evaluate strategies and laws of the proposed device from the point of view of developing methods and means of power transmission improving quality via electrical communication, containing a frequency converter based on phase-shifting transformer made according to zigzag-triangle scheme and controlled by power keys. In order to achieve the above object, it is proposed to sectionalize control windings providing a 48-position with a discreteness of 2.5 °, and a 24-position with a discreteness of 5 ° switching in each 120 ° sector of "coarse" regulation. Structural-simulation models were built and computational experiments were carried out for electrical communication combining two power systems with frequencies of 60 and 50 Hz and 50 and 49.6 Hz, respectively. The most significant results are: a new circuit version of the frequency converter, various strategies for controlling the conversion process. The two-channel frequency converter allows to reduce by 2 times the number of "coarse" control keys, which switch not after 60°, but after 120°. Also succeeded to reduce the switching step from 5 degrees to 2.5 degrees, which made it possible to improve the conversion quality. The significance of the obtained results is that during regulation the number of keys connected in series in operation remains equal to 4, which can significantly improve the reliability indicators of the control system.
  Keywords: intersystem power connection, static frequency converter, phase-shifting transformer, active power deviation, current non-linear distortion coefficient.
DOI: 10.5281/zenodo.3713403
4 Calculation of the Electrostatic Field in Non-Homogeneous Structures
Authors: 1Berzan V., 2Patsyuk V. 2Rybacova G., 3Porumb R., 3Postolache P. 1Institute of Power Engineering, 2Moldova State University, 3University POLITEHNICA of Bucharest 1,2Chisinau, Republic of Moldova; 3Bucharest, Romania
  Abstract: The paper examines a new approach of the finite volume method in determining the distribution of the electric field in non-homogeneous structures. The numerical calculation grid of the three-dimensional electric field is constructed using of the Delaunay triangulation concept and Voronoi cells. The purpose of the investigations is to simplify and order the algorithm for calculating the electric field distribution in non-homogeneous structures. The proposed algorithm is robust for calculating the potential and intensity of the electric field in non-homogeneous structures at the arbitrary distribution of voltage potential. The developed algorithm, during the computation process, ensures storage only the nonzero values of matrixes elements in the computer memory. The finite volume method keeps all the advantages of the finite difference method. Compared to the finite element method, the algorithm for constructing the finite difference relations is to simpler. In this case, there is no need to build a local and global rigidity matrix to form an equation solving system for the determination of capacitive coefficients of the electrical field. To solve the built system, we use the iterative method of conjugate gradients that converge very quickly for problems of the type examined. Numerical calculation of the electric field in the hollow cylinder with limited height and in the voltage divider from the glass-insulated micro conductors has been developed. The calculated the distribution of the electric field in the expanded voltage divider in the electrostatic screen. It is presented dates, which estimates the screen influence on capacitance of the voltage divider.
  Keywords: potential, intensity vector, capacitive coefficients, method, finite volume, conjugate gradients, voltage divider.
DOI: 10.5281/zenodo.3723641
5 Methodology for Assessment the Possibility of Transfer Six-Phase Power Line into the Mode of Operation with Incomplete Number of Phases
Authors: Kiorsak M.V., Turturica N.N. Technical University of Moldova Chisinau, Moldova
  Abstract: In electric power systems due to various reasons: short circuits, phase breaks, ice melting, phase-by-stage repair of a high-voltage power line (transmission line), interruptions in the power sup-ply to the consumer occur. On single-sided power lines, this leads to a prolonged shutdown of consumers, and to a significant emergency lack of electricity supply. Switching the line to opera-tion with incomplete number of phases significantly increases the reliability of consumes power supply. For double-circuit power lines it is always possible to keep in work at least two phases out of six. Among the various options for double-circuit power lines, the so-called self-compensating and controlled self-compensating high-capacity power transmission lines (SCL and CSCL) were proposed. For unidirectional power supply lines, these cases are accompanied by a prolonged shutdown of consumers from the electrical energy and a long time with a lack of supply. The article is devoted to the methodology for calculation the throughput of SCL and CSCL, which operate with incomplete number of phases. The methodology is based on the use of six symmetric components of currents and voltage of the SСL (СSС) and can be extended and applied in principle to any six-phase power transmission lines, in particular, two-circuit con-ventional power lines. The equivalents substitution complex schemes in the coordinates of the six symmetrical components in the MATLAB program are modeled. This allows us to find the currents and transmitted power through the line and voltages in the nodes of the network and evaluate the possibility of its operation in this mode.
  Keywords: electrical lines with self-compensation and controlled self-compensation; incomplete number of phases; six symmetrical components
DOI: 10.5281/zenodo.3723643
7 Numerical Analysis of Parameters of the Radiant Heating System with Radiating Pa
Authors: Redko A.A., Kulikova N.V., Burda Yu.A., Pavlovsky S.V., Redko O.F., Pivnenko Yu.A. Kharkiv National University of Construction and Architecture Kharkiv, Ukraine
  Abstract: The main goal of the work is to study radiation and convection heat-exchange in a closed volume of a panel radiator to assess the possibility of transferring short-wave radiation in the working area to a comfortable long-wave one. The goals have been achieved through the use of a mathematical model of three-dimensional unsteady flow and complex heat exchange in a radiant heating system, in which the system of Reynolds-averaged Navier – Stokes equations is added, supplemented by the radiative transfer equation. The effect of the shape and size of the panel, surface material and the presence of thermal insulation has been numerically investigated. It has been found that the arrangement of radiating panels reduces the surface temperature to 450–500ºC, but the increase in the surface area of the radiator provides a more uniform radiation intensity over the area being heated, not exceeding the permissible sanitary and hygienic requirements. The most important results are the creation of a more uniform radiation flux density when placing additional panels around a high-temperature radiator and the conversion of short-wave radiation in the working area to a comfortable long-wave one. The significance of the obtained results consists in the fact that the results of these studies can be put into practice in the design and installation of radiant heating systems for various buildings. The results obtained in the work show the promise of further theoretical and experimental studies on subjects related to the optimization of radiant heating system parameters for industrial and civil buildings.
  Keywords: radiant heating, radiating panels, heating system, infrared burner
DOI: 10.5281/zenodo.3713405
8 Study of Prospects of Two-Phase Gravity Thermosiphons Used in Waste Heat Boilers of Cogeneration Units
Authors: Dolganov Yu.A., Yepifanov А.A., Patsurkovskyi P.A., Sorokina T.N., Lychko B.M. Admiral Makarov National University of Shipbuilding Mykolaiv, Ukraine
  Abstract: The significance of this work is justified by the lack of experimental data on the operation of thermosiphons as part of the waste heat boilers (WHB) with gas turbine engines (GTE), whose capacity is from 2 to 10 MW. The aim of the work was achieved by physical modeling of the heat transfer process in the thermosiphon cavity in the range of heat loads from 0.5 to 17 kW/m2. The study of the internal temperature difference of two-phase gravity thermosiphons at thermal loads up to 17 kW/ m2 was performed experimentally. The paper shows a scheme of the experimental research stand. The graphical dependence of the temperature difference in the thermosiphon cavity on the heat flux density is presented. The root-mean-square error of experimental results was calculated, being up to 5.7%. The significance of the obtained results lies in that the existing calculation method was improved due to the mathematical dependences obtained for calculations of the internal temperature difference, and became applicable in the calculation of heat exchangers based on two-phase thermosiphons operating in the heat load range of up to 17 kW/m2. The experiments performed confirm the competitiveness and high thermal efficiency of the two-phase gravitational thermosiphons under the regime conditions typical for the WHB plants. This makes promising the use of the two-phase gravitational thermosiphons compared to traditional coil heating surfaces.
  Keywords: two-phase gravitational thermosiphon, waste heat boiler, heat flow density, thermal resistance.
DOI: 10.5281/zenodo.3713407
9 Combined District Heating System CHP- Local HP
Authors: Sit M.L., Juravliov A.A. Institute of Power Engineering of Moldova, Chișinău, Republic of Moldova Suvorov D.M., Sushchikh V.M. Vyatka State University, Kirov, Russian Federation
  Abstract: The work relates to the problems of using heat pumps in district heating systems. The aim of the study is to increase the energy economic efficiency of combined heat supply systems with heat pumps using natural refrigerants. This goal is achieved by developing a scheme of the system in which part of the building is heated centrally from the CHP, and the other part is from a heat pump installed in the building, and heat pump which receives low potential heat from the return network water leaving the part of the building, - centralized, and from outside air. The most sig-nificant results of the work are the developed scheme of a carbon dioxide heat pump, which combines both the possibilities of working in a quantitative heat supply system and when pre-paring hot water for buildings in a non-heating period. The significance of the obtained results lies in the fact that the proposed technical solution allows to reduce significantly fuel consump-tion for thermal power plants and the costs of consumers when they pay for consumed energy. Heat pump circuits have also been established for the quantitative law of regulating the opera-tion mode of the heat supply system. With the proposed scheme of operation, the heat pump for domestic hot water works only in the non-heating period, and domestic hot water is provided during the heating period from the CHP.
  Keywords: district heating system, heat pump, carbon dioxide, control law, CHP.
DOI: 10.5281/zenodo.3713430
11 Long-Term Solar Irradiance Forecasting Braga D.a, Chicco G.b, Golovanov N.c, Poru
Authors: Braga D.a, Chicco G.b, Golovanov N.c, Porumb R.c Technical University of Moldovaa, University Politecnico di Torinob, University Politehnica of Bucharestc Chisinau, Republic of Moldovaa, Torino, Italyb, Bucharest, Roman
  Abstract: The past decade has been characterized by considerable increase of the penetration level of so-lar photovoltaic systems in energy systems throughout the world. At the same time, solar irradi-ance has an intermittent nature. Thus, the efficient management of existing and new solar pho-tovoltaic systems requires an accurate forecasting system of solar irradiance. The purpose of the paper is to develop and validate a long-term forecasting model for solar irradiance. This purpose is achieved by applying of clustering method and standard mathematical statistics. The modeling includes pre-processing of historical data used for forecasting and post-processing of the types of days resulted from the clustering analysis. Historical data include solar irradiance and sky coverage by clouds. Pre-processing supposes bi-normalization of the solar irradiance in time and amplitude, as well as clustering, and post-processing supposes denormalization to get the actual values. Error metrics and confusion matrix indices have been used to assess the accuracy of the proposed forecasting method. Four different model variants have been considered, which differ by pre-processing approach of initial data. The comparison of these model variants shows that for better accuracy it is required to use seasonality aspects of solar irradiance. The main result of paper is the created model, which can be used for the solar irradiance forecast with acceptable accuracy for this type of forecasting and for generating of the types of days for different annual scenarios. The importance of paper results consists in the possibility of using of these scenarios for feasibility assessment of the solar photovoltaic system and identifying of the best solutions for their integration in the energy system.
  Keywords: prediction, solar irradiance, forecasting, clustering, statistical error parameters, predictive model, confusion matrix, scenario analysis.
DOI: 10.5281/zenodo.3713424
13 Improvement of Methods for Evaluating Electrical Engineering Enterprises Functioning and Their Structural Components Based on Identical Indicators
Authors: Yakovlev A.I., Vasiltsova S.A., Larka L.S. National Technical University "Kharkov Polytechnic Institute" Kharkov, Ukraine
  Abstract: To improve the performance evaluation of electrical engineering enterprises, firms should more accurately, in comparison with the existing methods, determine the contribution of each of their structural units to the results of the activity of the business entity as a whole. The purpose of the article is to identify the reserves for improving the efficiency of electrical engineering enterprises, to establish the results of specific units in the sale of elements of the enterprise's product, produced directly in these structures and sold in the markets. The task is achieved through the use of conditional prices for workshop products and expanding the scope of the break-even point method. Methods for establishing break-even points, which, in contrast to classical dependencies, when the entire volume of goods produced is realized, consider the option of incomplete sale and, accordingly, unproductive use of part of the resources are suggested as well. The value of this indicator for various values of prices and sales by options, their limit values, including when the volumes of production (sales) exceed their predetermined design values are presented. The most significant results are the development of methods for identifying the performance indicators of electrical enterprises as a whole and their units, the development of methods for calculating the breakeven point in various production situations. Their significance lies in the further development of the theory and methods of evaluating the activities of electrical enterprises and the possibilities of their practical application.
  Keywords: workshops, electrical enterprise, work results, indicators, efficiency, influence options, quality, volume of production, improvement of the breakeven point method.
DOI: 10.5281/zenodo.3713426
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