Bulletin of the National Technical University "KhPI". Series: Energy: Reliability and Energy Efficiency
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Published By National Technical University Kharkiv Polytechnic Institute


Iryna Shcherbak ◽  
Yuliia Kovalova ◽  
Volodymyr Korobka

It is proposed on the electrical load graphs of transformer substations 10/0.4 kV in residential areas to allocate the stationarity areas for further modelling of load schedules and the implementation of controlling influences on the modes of consumers-regulators in order to align the overall graph of the electrical load. The relevance and complexity of the problem under consideration is caused by the fact that the load variation of transformer substations 10/0.4 kV in residential areas occurs randomly. This is due to the significant number, nomenclature and diversity of types of connected consumers, as well as the lack of deterministic connections between consumers of electricity, in addition, the random load function in the daily interval is non-stationary. In this regard, there was a need to develop the stages of selecting the areas of stationarity on the electrical load graphs of transformer substations 10/0.4 kV of residential areas. A measurement of the load graphs of 10/0,4 kV transformer substations is carried out, according to the results of which the distribution law of active and reactive power measurements is investigated. After confirming the hypothesis of normal distribution law, parametric tests are performed. Fisher's F-criterion is used to confirm the hypothesis of a constant variance, and Student's t-criterion is used to confirm the hypothesis of a constant mathematical expectation. The next stage, based on constancy of the variance and mathematical expectation, is the determination of autocorrelation coefficients of the studied random function and plotting of the autocorrelation function. To approximate the function the autocorrelation coefficients are determined by the least squares method and the autocorrelation function attenuation analysis is performed. The implementation of the defined stages allows to identify the areas of stationarity on the load graphs of transformer substations 10/0.4 kV. For a reliable description of the process of changing the load of transformer substations 10/0.4 kV the use of probabilistic-statistical method of modelling is justified that takes into account the stochastic nature of the load changes on the selected areas of stationarity.

Mykhailo Syvenko ◽  
Oleksandr Miroshnyk

A detailed substantiation of the use of electric energy storage devices in the presence of generators on renewable energy sources in the power supply system is given. The dependence of the storage parameters on the composition and priority of generation in the system is investigated. The solution of the problem of determining the parameters of electricity storage devices by means of purposeful simulation of generation parameters is considered. The results of the choice of power and capacity of the energy storage using technical and economic indicators are shown. Optimal parameters of electric energy storage devices as one of the most important means of ensuring the activity of isolated power supply systems together with selection of generating devices are determined. The results of calculations of capacity of renewable energy sources in isolated power supply systems in combination with classical energy sources are given. The necessity of using the principle of activity of the distribution electric network and the possibility of its realization is demonstrated. The optimal storage capacity as a function of the share of renewable generation, the non-integrated energy produced by renewable sources and the total storage capacity are plotted for several isolated systems. The main points of the used model of the power supply system of isolated networks are given. In the studied isolated power supply systems, wind power plants and solar power plants, which have significant unpredictability of generation, are used as generation based on renewable energy sources. The problem of undersupply of electricity to the grid by stepwise increase of generation is analysed. The results of the multi-step selection of power and energy consumption of electricity storage are determined by technical criteria.

Oleksii Hanus ◽  
Kostiantyn Starkov

A non-linear dynamic mathematical model of voltage transformer has been considered and overvoltages arising on the elements of voltage transformer equivalent circuit during transient processes have been investigated. The influence of voltage transformer secondary circuit capacitance on overvoltage multiplicity in the primary circuits and the duration of transients has been determined. The advantages of approximation of nonlinearity of voltage transformers by hyperbolic sine are used. Mathematical expressions determining the nature of changes in the forced and free components of the transient process in an electrical network with a voltage transformer have been obtained. It is shown that with the increase of the electric network capacitance the duration of the transition process damping increases and the frequency of the forced oscillations and the level of overvoltage decrease. It is proved that even small, in comparison with the primary nominal sinusoidal voltage, aperiodic components of the voltage transient process can lead to significant overvoltages during voltage transformer outages. It has been substantiated that both the secondary resistance and the switching torque influence the overvoltage multiplicity arising in the primary winding of voltage transformers. It is shown that the closed secondary winding worsens the disconnection process of non-linear inductance of voltage transformers. The values to which overvoltages increase in this case are determined. According to the results of calculations it is determined that with open secondary winding of voltage transformers the duration of transient process significantly increases. It has been found that the decrease of frequency of forced oscillations, which occurs in this case, is accompanied by an increase of currents in the primary winding of the voltage transformer, which is dangerous in terms of thermal stability of the winding insulation. It is shown, that closing the secondary winding of voltage transformers leads to significant reduction of transient damping time. It is suggested that this algorithm can be used to provide a rapid breakdown (suppression) of ferroresonant processes. The effectiveness of such a measure of stopping of ferroresonance processes as short-term shunting of secondary winding of voltage transformers has been investigated. The correlation of parameters of electric networks (capacity of busbar sections, nonlinearity of characteristics of voltage transformers, disconnection torque, etc.) at which ferroresonance process may occur and consideration of which may allow, in terms of prevention of ferroresonance processes, to identify substations (electric networks) that require more detailed research has been determined. The results of analytical studies were tested in the electric networks of JSC "Kharkivoblenergo" and used in the electricity distribution system for the selection of specific voltage transformers for certain configurations of electrical networks.

Vyacheslav Balalaiev ◽  
Olena Fedoseenko

The analysis of methods for calculating the electrical characteristics of earth electrodes in case of emergency currents of industrial frequency flowing through the elements of earthing arrangements is carried out. A method for improving complex earthing arrangements of electrical installations by optimization of their design parameters is proposed. The proposed method consists in installing an artificial earth electrode with an increased contact area of its surface with the ground, which makes it possible to increase the conductivity of earthing spreading. The use of these electrodes on the territory of projected or operating electrical installations ensures that the values of the normalized parameters of earthing arrangements are brought to permissible values. The use of the method given in the calculations of earthing arrangements requires the replacement of volumetric earth electrodes with a set of linear vertical electrodes. The design model is substantiated by equivalent electrical characteristics relative to the two-layer model of the electrical structure of the earth. The equivalent model was obtained by a given approximation of the electrical characteristics of a set of straight electrodes in the process of increasing their number to the equivalent characteristics of the reference model. In turn, the determination of the characteristics of the reference model was carried out directly by solving the boundary value problem for the potential satisfying Laplace equation using finite difference method. Theoretical investigations using induced potential method and methods of calculation of branched electric circuits with distributed parameters for calculation of electric field and resistance of the complex non-equipotential earth electrode in the ground with two-layer structure have been carried out. The developed electrodes of increased spread conductivity are mounted as experimental samples and are involved in the formation of the electrical characteristics of the earthing arrangements.

Mykola Volodymyrovych Bazylevych

The modes of the electrical network have not been calculated for the photovoltaic power plants if they are used for small and medium-sized applications. It is known that the voltage at the point of installation of a photovoltaics power plant may exceed the critical limit voltage and as a result it may cause the emergency with the electrical equipment. According to the existing methods, it is possible to find the voltage only by full electrical network modes calculating. In practice such calculation is not convenient, especially for 0.4 kV distribution networks. The suggestion is to find the most probable limit voltage at the point of installation of a photovoltaic power plant without calculating the mode of the electrical network using instead the information about the voltage value in the power supply centre. The voltage at the installation point of the photovoltaic power plant mainly depends on the generated power of the photovoltaics power plant, line resistance, load power connected to a voltage of 0.4 kV and bus voltages of 0.4 kV of the power supply. The voltage hardly depends on the power of the power transformer, the parameters of 10 kV lines and the parameters of 0.4 kV lines, except for the line "power centre–photovoltaic power plant". The highest voltage at the point of installation of the photovoltaics power plant will be in the case when the load on the line is absent. The suggested methods allow finding analytically the maximum value of voltage at the point of installation of a photovoltaic power plant without using of the successive approximations methods. As a rule the photovoltaics power plant operates in the mode of delivery of the maximum active power. By the range of various analytical formulas, the value of voltage at the point of installation of a photovoltaic power plant has been determined. The accuracy of finding the voltage by different formulas has also been determined. It is established that in order to achieve the required accuracy, the dependence of the current at the output of the photovoltaics power plant on the voltage at the connection point should be taken into account. Modification of formulas for the purpose of simplification of their application is considered. The conditions for finding the maximum possible voltage value at the point of installation of a photovoltaics power plant are considered. This requires that the argument of the voltage drop vector on the line be equal to the argument of the voltage vector on the busbars of the power supply. This is achieved by the fact that the photovoltaic power plant generates active and reactive power in a certain ratio. It is shown the inexpediency of finding the maximum possible voltage value at the point of installation of a photovoltaic power plant due to a small difference with the most possible voltage value with a significant complication of calculations.

Serhii Ponomarenko

The results of the analysis of influence of operational factors and peculiarities of transformer design on intensity of oxidative reactions of transformer oils in the tanks of 110 kV high-voltage power transformers are presented. As a result of two-way analysis of variance it was found that with increasing operating time there is a statistically significant increase in the value of the organic acid content in the oil. At the same time the rate of increase in the organic acids content in the transformer oil, operated under different conditions, significantly differs, which indicates a significant influence of operating conditions on the intensity of oxidative reactions. The results of two-way analysis of variance also show that the effects of changes in factor levels are not additive, that is, the effect of a change in the level of influence of one factor leads to a change in the effect of the level of influence of another. In other words, the process of oil oxidation is cumulative and a certain level of organic acids in the oil can be achieved either over a longer period of operation, but with relatively 'light' operating conditions, or over a shorter period of time, but with more 'heavy' operating conditions. In order to determine the factors most affecting the intensity of oxidative reactions, an analysis of the quality of filled oil, operating time, the influence of the region, the influence of the type and nominal characteristics of transformers on 6 data sets with identical rates of oxidative reactions was carried out. The results of the analysis show that the intensity of oxidation reactions is strongly influenced by the operating time, the transformer loading factors, the consumer composition (region of Ukraine) as well as the type and quality of oils. At the same time, factors such as rated capacity, type of transformer, number of windings, and the value of rated voltage on the medium and low voltage windings do not influence the intensity of oxidation of oils. The results obtained allow the correction of the maximum permissible values of oil acidity, taking into account the factors affecting the intensity of oxidation of oils.

Yevhen Kolenchenko ◽  
Viacheslav Bezruchko ◽  
Roman Buinyi ◽  
Igor Dihtyaruk

The analysis of the state of 35–110 kV circuit breakers in the Chernihiv region, which are on the balance sheet of the operator of the distribution system JSC «Chernigivoblenergo», has been carried out. It was shown that the number of circuit breakers that satisfy regulatory documents is only 18.9 % in 35 kV and 2 % in 110 kV power networks. Unfortunately, most of the 35–110 kV circuit breakers in the Chernihiv region are low-oil and bulk-oil type, which required heating during cold meteorological conditions. The article assess the duration of the circuit breakers heating and their drives, depending on the selected set point of the temperature and the averaging interval of the ambient temperature, and the value of technological energy losses. The article considers following ways to reduce technological costs for heating: optimization of the heating control system, replacement of oil circuit breakers with modern vacuum and SF6 circuit breakers, which require lower costs on heating, and applied both above ways. The effectiveness of the above ways was done with apply Net Present Value, which includes not only investments and Net Profit Margin, but also operating costs and Discount Rates. It has been shown that all of the above ways are effective. In particular, the least costly is the modernization of the heating control system, which has payback period is low three years. The payback period for second way to replace all 35–110 kV oil circuit breakers with modern ones at JSC «Chernigivoblenrego» is about 6.5 years, but this way requires large investments. However, this way cannot be done in one year. Therefore effectiveness will be less due to energy losses to operated oil circuit breakers.

Ihor Nazarenko ◽  
Oleksandr Kovalov

In Ukraine, the number of farms, private land users and protected soil structures has increased significantly in recent years. To increase the efficiency of vegetable production, small-sized mobile units in the form of motoblocks with internal combustion engines of domestic and foreign production have been widely used. However, during the operation of mobile units with internal combustion engines in greenhouses and hotbeds, the noise level and air pollution increase, which negatively affect not only people but also plants. The disadvantages of motoblocks with internal combustion engines should also include a fairly high specific consumption of liquid fuel, which has a high cost, difficulties in starting and stopping the unit, as well as the relatively low reliability of the internal combustion engine. More promising for work in protected soil structures are mobile units with traction motors, called electric motors, as environmentally friendly units that do not have these shortcomings. The main technical characteristics of the experimental sample of a small-sized electrified soil-cultivating motoblock are given in the work. The structural scheme of the power energy channel of the electric motor unit is substantiated, which clearly demonstrates the processes of energy conversion in it. The equation of energy balance of the electric motor unit and the main energy ratios that determine the properties of the traction motor in the drive of the electric motor unit are obtained. The algorithm of optimal control of a direct current traction motor by the maximum efficiency for direct current motors of serial and mixed excitation is substantiated. The results of field tests of the prototype of the electric motor unit are presented. The analysis of the obtained results of experimental researches of the electrified tillage motor-drive driven by the direct current motor of sequential excitation testifies to the reduction of specific energy consumption for the main types of tillage by 12-15%.

Mykola Kuznietsov ◽  
Olha Lysenko ◽  
Oleksandr Melnyk

The paper is devoted to solving the balancing problem in local power systems with renewable energy sources. For a power system optimization problem, whose operation depends on random weather factors, a convex parameter optimization or optimal control problem was solved using controlled generation, for each individual realization of a random process as a deterministic function, and then statistical processing of results over a set of random realizations was performed and distribution density functions of the desired target function were constructed, followed by estimation of expected values and their confidence intervals. The process describing current deviations of generated power from mean value is modelled as discrete stray model and has properties of Ornstein-Uhlenbeck process, which allowed varying the duration of unit interval, in particular to select data bases of operating objects with inherent temporal discreteness of their monitoring systems. Random components are investigated and modelled, while the average values are considered to be deterministic and are provided within a predictable schedule using also traditional energy sources (centralised power grid). A mathematical model of the combined operation of renewable energy sources in a system with variable load, electric storage device and auxiliary regulating generator is implemented as a scheme of sequential generation and consumption models and random processes describing the current state of the power system. The operation of the electricity accumulators is dependent on the processes mentioned, but in the full balance, it appears together with generation or load losses, which are cumulative sums of unbalanced power and may have a different distribution from the normal one. However, these processes are internal, relating to the redistribution of energy within a generation system whose capacity is generally described satisfactorily, given the relevant criteria, by a normal law. Under this condition, it is possible to estimate the probability of different circumstances - over- or under-generation, that is, to give a numerical estimate of the reliability of energy supply.

Vadim Bodunov

One way to stimulate the development of renewable energy is preferential electricity tariffs for businesses, electricity consumers, including energy cooperatives and private households, whose generators produce electricity from alternative energy sources. Such a state policy exists both in Ukraine (the "green tariff") and in other countries (Feed-in tariffs). The simplified connection mechanism of generating power of private households and the fast payback period of solar power plants have led to a rapid increase in the number of such facilities. The peculiarity of network photovoltaic installations of private households is that the investment is proportional, and sometimes even less than the cost of additional reconstruction, for example, to increase the capacity of the electrical network, so, as a rule, their connection to low voltage networks is carried out without any additional changes of intersections of power lines. At the same time, according to the Law of Ukraine on the Electricity Market, the connection of generating installations to consumers, including private households, should not lead to deterioration of regulatory parameters of electricity quality. Another feature of these objects is the almost complete lack of information about the parameters of the electrical network. In the absence of reliable information, it becomes necessary to develop approximate methods for estimating the allowable capacity of solar power plants of private households depending on the place of connection, the parameters of the modern low-quality distribution network and modes of its operation. The article proposes the use of the distribution network model in the form of a line with evenly distributed load from the solar power plants in the form of a concentrated load at the corresponding point of the line. The voltage distribution along the line is simulated when the power of the solar power plant and the place of its connection change. Analytical relations are obtained for the calculation of the solar power plants power limit values to ensure the admissibility of the mode parameters according to the voltage level when varying the solar power plant connection point. A method is proposed to maintain the allowable minimum voltage levels at the terminals of consumers in short-term congested sections of the network.

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