To the Problem of Protection of Medium Voltage Instrument Transformers with Fuses: Analytical Research

Introduction. In the medium voltage power grid of 6–35 kV, there is a problem of protecting voltage instrument transformers. This is due to the insufficiently effective level of their protection with fuses. Recently, there have been more and more reports of accidents associated with the failure of voltage instrument transformers not only in Ukraine, but also abroad. The issue of conducting an analytical study of the problem of protection of medium voltage instrument transformers by fuses is relevant. Goal. Investigation of designs and characteristics of fuses for medium voltage instrument transformers to improve the efficiency of their protection. Results. The article shows that protection of medium voltage instrument transformers with epoxy insulation is often provided by fuses, in which the nominal current of the fuse-link is significantly higher than the maximum permissible long-term current of the primary winding of the instrument transformer. A comparative analysis of the current values of the primary winding of medium voltage instrument transformers with the values of the rated currents of the fuse-links of fuses of various manufacturers presented on the Ukrainian market is carried out. The design features and technical characteristics of fuses for medium voltage instrument transformers have been investigated in order to increase the efficiency of their protection. The advantages and disadvantages of the designs of fuses from various manufacturers have been investigated and it has been established that the design of fuses requires further improvement in order to increase the efficiency of protection of medium voltage instrument transformers. Discussion and prospects for further development. Since in Ukraine, the need for fuses to protect medium voltage instrument transformers is provided mainly by supplies abroad, a promising direction for further development in this direction is the creation of a domestic competitive design of a fuse and its introduction into production.

Reducing Power Losses by Voltage Stabilization at the DC Rolling Stock Current Collector

The invention relates to an energy-efficient method for voltage stabilization at the electric rolling stock current collector through traction substation control means which provide a nominal voltage value during the electric train movement by an interstation section. The dependence of potential distribution in the contact wire during the electric rolling stock movement by an interstation section was investigated. Also researched and developed are the new ways of voltage stabilization at the current collector of the electric rolling stock based on synchronous (the same for two adjacent traction substations) and asynchronous paths of voltage regulation at DC buses of traction substations related to one synchronous and two asynchronous ways of voltage stabilization in the contact network with obtaining the energy performance describing them. The energy performance of the investigated methods of voltage stabilization in the contact network is compared, and the energy efficiency of each of them is determined. It is proved that the use of modern types of semiconductor converters such as an active rectifier – voltage source in the power equipment of DC traction substations will enable to implement adaptive voltage stabilization systems at the rolling stock current collector, providing nominal voltage values of traction motors on the interstation section without using additional equipment on the rolling stock and, as a consequence, justification and application of these methods is suitable for upgrading the existing and designing new traction substations.

Design and Simulation of a Servo-Drive Motor Using ANSYS Electromagnetics

The paper is devoted to determining the output parameters of a servomotor, which belongs to synchronous machines with permanent magnets, in order to further determine the characteristics of transient modes in the software package ANSYS Electromagnetics. RMxprt, part of ANSYS Electromagnetics, allows to determine the parameters of windings, losses, motor performance, but requires filling out a form with a complete set of geometric dimensions and winding data. Of course, such data are not available in the motor data sheet, so the first task solved in the paper is to determine all the necessary and sufficient parameters to perform the calculation in RMxprt. The results of the calculations were compared with the measurements on the experimental servomotor EMG-10APA22. This paper shows how to export a servomotor object from RMxprt to the Simplorer workspace, which is also part of the ANSYS Electromagnetics. According to the simulation results in ANSYS Simplorer, the characteristics of the transient modes of the servomotor powered by a stable three-phase source are obtained. Prospects for further research related to the improvement of the simulation model in ANSYS Simplorer are presented.

A Refined Mathematical Model of Physical Processes in a Conductor at a High-Current Pulse Discharge

A novel mathematical model describing physical processes during the flow of an aperiodic pulse current with amplitude of 100 kA along a conductor with a circular cross-section is proposed and investigated. It is shown how a short-term electric discharge of an aperiodic shape affects the distribution of the current density in the cross-section of the conductor, causing its nonuniform heating and the appearance of significant thermal forces as well as mechanical stresses and strains. Based on the developed mathematical model, the relation-ship between electromagnetic, thermal and mechanical phenomena is shown, allowing a deeper understanding of the multiphysics processes taking place. The maximum values of the current density are calculated, which on the surface of the conductor reach values of 47 kA/mm2, while the temperature rise of a copper conductor with a diameter of 2.44 mm is no more than 80ºC at high temperature gradients, which causes the appearance of thermal stresses that have value (40–50)% of the value of the short-term strength limit of electrical copper. Utilization of this model allows to more accurately determine the required conductor cross-section based on the characteristics of electromagnetic, thermal and mechanical pro-cesses. It is shown that the simplified model (the condition for the uniform distribution of the current over the cross-section) gives significantly underestimated values of temperatures and does not take into account temperature deformations.

A Decomposition Four-Component Model for Calculating Power Losses in Low-Voltage Power Supply Systems

A new model for decomposition of the total power losses, which includes four components is proposed. Each of the components of the proposed model has a certain physical meaning due to the nature of electromagnetic processes in a three-phase four-wire system. Definitions to describe each of the proposed components are formulated. It is shown that each of supplementary components of the total loss power is proportional to the minimum possible loss power and to the square of the RMS value of the power, which is caused by its occurrence in three-phase four-wire power supply system, and it is inversely proportional to the mean square of the net power loss. The synthesized Matlab-model for verification of the four-component structure of power losses showed a high degree of its adequacy. The proposed model allows us to rethink the description of power losses in three-phase AC circuits and can be used in specialized measuring instruments for electrical networks monitoring. Using the information obtained in the monitoring process, it is possible to plan technical measures to reduce losses of electrical energy in the power supply system, as well as to estimate the capital costs of these measures.

The Evaluation of Electric Power by Separate Analysis of Fourier Transform Components

This paper presents a substantiation of an approach for the evaluation of components of apparent power and intended to simplify the computational procedures which usually should be implemented in order to process the preliminary sampled waveform of instantaneous power. The results of carried out studies have shown that both active and reactive power can be calculated by the analysis of calculated components of sine and cosine Fourier transforms. This paper also presents the discussion of restrictions, which should be imposed on the duration of the analyzed signal and on frequencies of the auxiliary trigonometric functions, which are applied in order to calculate components of Fourier transform which are used for the evaluation of active and reactive power. The compliance with these restrictions allows us to eliminate the undesirable bias of active and reactive power estimation caused by the refusal from the decomposition of the analyzed waveform of the instantaneous power by applying the complete system of orthogonal trigonometric functions, as the evaluation of components of the apparent power is attained based on separate analysis of sine and cosine Fourier transforms calculated for the analyzed signal. The results of carried out simulations illustrate the frequency dependencies of sine Fourier transform calculated for the case of compliance with the restrictions, which allow to attain the highest accuracy of estimation and for the case when the duration of analyzed signal does not fit these restrictions.

Measures to Improve the Energy Efficiency of Street Lighting Systems in the Kharkiv City

High-quality street lighting improves visual perception of the road and provides for a significant decrease in the number of road accidents. It has been established that the total number of accidents can be reduced by 30% for national roads and by 45% for especially hazard areas (e.g., at intersections). Doubling of the average luminance of the road surface significantly reduces the number of accidents in the dark and twilight. Sufficient street lighting contributes to: reducing electricity consumption; decreasing operating costs; improving the environmental situation; promoting business, tourism and investment activity; decreasing the number of criminal offenses. The analysis of recent research and publications helped choose the best European practices which can be used to improve the outdoor lighting in Ukraine. The street lighting network is continuously growing and being modernized. Different types of light sources are being used in the network to illuminate the city. Today, high-pressure lamps (HPS) are leading. They are increasingly replaced by LED light sources. Not a small share is accounted for by metal-halide lamps. The smallest number of light sources in the street lighting system, high-pressure mercury and compact fluorescent lamps (CFLs). To increase the efficiency of the street lighting system and reduce energy consumption for its operation, measures for its reconstruction and modernization are required. For this purpose, it is necessary to address the following problems: high level of obsolescence and physical wear and tear of system elements; inefficient use and high losses of electricity at the stages of transportation and consumption, due to the use of obsolete and worn-out equipment; suboptimal distribution of the utility capacity, which leads to ineffective use of the existing lighting system; low efficiency of the street lighting control system, due to the lack of electric meters and remote monitoring and control of the street lighting. Introduction of energy-saving types of light sources is one of the ways to modernize the existing street lighting system. It is necessary to replace old lighting fixtures, which have already exhausted their life span, by new energy-saving light sources (replace CFLs by LEDs), use automatic control systems. All this in turn will help reduce energy costs by 60%. To improve energy efficiency of the street lighting system, it is also recommended to carryout the replacement of electromagnetic ballasts by electronic ones. The employment of electronic ballasts, e.g., in case of using HPS 250 lamps, allows to save approximately 255 kWh/year. Considering that the price per kW of electric energy is UAH 2.68, one luminaire helps save the amount of UAH 683.4 per year.

Visual Perception in External Lighting Conditions

LED street lighting is a topical trend in modern outdoor lighting. High light output of LEDs creates all conditions for modernization of electric lighting networks in Ukraine. Human vision is a complex process associated with retinal light perception. Vision is divided into: day vision, night vision, and twilight vision. The function of the eye is highly dependent on the distribution of brightness in the field of vision. The spectral sensitivity of photoreceptors varies for different wavelengths of the visible spectrum and different levels of light intensity. The rationing of the lighting installation is based on detailed studies of the observer’s visual performance depending on different lighting conditions. One of the main luminous parameters that can easily be measured objectively is illumination. Brightness as a function of illumination, the observer’s position and the spectral coefficient of the working surface reflection is more informative, but has some difficulty in measuring. There is a clear need to develop a system that would make it possible to uniquely assess the visual efficiency of a given spectral composition under certain observation conditions. It was decided to introduce the term equivalent brightness as the parameter of such a system. The difficulty of using the function Vek(λ,Lek) to calculate the equivalent brightness is the function’s dependence Vek(λ,Lek) on Lek. The aim of the study is to approximate the function of the relative spectral luminous efficiency in mesopathic regions by a set of standard CIE functions that do not depend on the value of equivalent luminosity. The calculation method Vek(λ,Lek) is proposed using only two normalized functions of the relative spectral radiation efficiency for day V(λ) and night V'(λ) vision. The use of such approximation function makes it possible to determine the equivalent brightness, which adequately reflects the level of visual perception under the conditions of ambient illumination, based on the photometric brightness of the light source. To calculate Vek(λ,Lek) we use the ICE recommended functions of relative spectral light efficiency for the twilight vision, which are based on the spectral composition of the blackbody radiation with a color temperature of 2042 K. The use of the developed methodology provides results that more accurately characterize the efficiency of light sources in outdoor lighting installations compared to the results of calculations obtained when using standard methods.