LABORATORY STUDIES OF PROPERTIES OF SOFT MAGNETIC POWDER COMPOSITE MATERIALS

Изучение эффективности использования электротехнических материалов является актуальной проблемой в области изготовления электрических машин. Одним из важнейших аспектов изготовления электрических машин является проектирование магнитной системы машины. В качестве магнитной системы используют магнитопроводы из различных магнитомягких материалов. Эти материалы отличаются магнитной проницаемостью и удельными магнитными потерями. Данные параметры материалов влияют на нагрев, размер, стоимость и эффективность электрической машины. В целях экономии экспериментальная оценка параметров магнитомягких материалов производится на заготовках различных форм и размеров, на специальных измерительных стендах, согласно международным энергетическим стандартам. В данной статье предлагается экспериментальная установка для лабораторных исследований магнитных свойств магнитомягких материалов, методом кольцевых заготовок, в соответствие со стандартом МЭК-60404-6. В составе установки используется только стандартное недорогое оборудование. Необходимый коэффициент формы магнитной индукциидостигается последовательной коррекциейнапряжения вторичной обмотки с помощью цифрового регулятора. Подход к программной реализации алгоритма последовательной коррекции напряжения вторичной обмотки изложен в статье. С помощью предлагаемой установки проведено исследование свойств образца из магнитомягкого композиционного порошкового материала Somaloy 700-3p (800 MPa) и сравнение результатов с каталожными данными производителя. По итогам работы выявлено, что с помощью предлагаемойустановки могут производитьсяизмерения свойств магнитомягких материалов, в соответствие со стандартом МЭК-60404-6 с необходимой точностью. Предлагаемая установка может быть использована как в качестве учебного стенда, так и в качестве измерительной установки для идентификации свойств магнитомягких материалов при проектировании электрических машин. The study of the efficiency of using electrical materials is of great interest in the field of manufacturing electrical machines. One of the most important aspects of the manufacture of electrical machines is the design of the magnetic cores of the machine. Magnetic cores made of various magnetically soft materials are used as a magnetic system. These materials differ in magnetic permeability and specific magnetic losses. These material parameters affect the heating, size, cost and efficiency of electric machines. In order to reduce expenses, the experimental evaluation of the parameters of soft magnetic materials is carried out on samples of various shapes and sizes, on special experimental setups, in accordance with international electrotechnical standards. This article proposes an experimental setup for laboratory studies of the magnetic properties of soft magnetic materials by the method of ring specimens, in accordance with the IEC-60404-6 standard. The setup uses only standard inexpensive equipment. The required shape factor of the magnetic flux density is achieved by sequential correction of the secondary winding voltage using a digital regulator. The approach to the software implementation of the algorithm for sequential correction of the secondary winding voltage is described in the article. The proposed experimental setup was used to study the properties of a sample made of a soft magnetic composite powder material Somaloy 700-3p (800 MPa) and compare the results with the manufacturer's catalog data. Based on the results of the work, it was revealed that the proposed setup can be used to measure the properties of soft magnetic materials in accordance with the IEC-60404-6 standard with the required accuracy. The proposed experimental setup can be used both as a training stand and as a measuring installation for identifying the properties of soft magnetic materials in the design of electrical machines.

The Method to Determine the Turns Ratio Correction of the Inductive Current Transformer

This paper presents the method for evaluation of the turns ratio correction of the inductive current transformer using the magnetization curves determined at the non-load state and in the load conditions. The presented method may be applied to determine even a fractional winding correction factor. The standard IEC 61869-2 provides the method to determine the turns ratio correction of the tested CT from the measured rms values of voltages on its primary and secondary winding in the non-load state. However, this approach is limited in determining the significant changes in the number of turns of the secondary winding. Moreover, the paper presents the influence of the applied turns ratio correction on the frequency characteristics of the current error and phase displacement of the inductive current transformers evaluated for the transformation of the distorted current.

STUDY OF THE NATURE OF OVERVOLTAGES IN THE ELECTRICAL NETWORK ARISING FROM VOLTAGE TRANSFORMERS

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.

Influence of geoinduced currents on impedance bonds with secondary windings used in railway automation circuits

Objective: To consider the possibility of DC magnetization of the core of the impedance bond with secondary winding when using AC electric traction. To determine the possible influence of solar activity, and in particular the geoinduced currents, on the signaling arrangements (SAs). Methods: Analysis of the conditions and causes of magnetization of the core of the impedance bond with a secondary winding, which is the reason for a decrease in its inductance and a change in the track circuit coefficient of transmission. Description of the influence of geoinduced currents on the operation of traction substation power transformers and the using this scenario for modeling the operation of impedance bonds with secondary windings. Results: The conditions and reasons for the incorrect operation of the impedance bonds with secondary windings associated with the magnetization of the core by direct current, when using electric traction with alternating currents, have been stated; the possibility of saturation of the magnetic system of the impedance bond with geoinduced current has been analyzed; the possible consequences of the transition of the impedance bond to the saturation mode have been determined. Practical importance: The study findings broaden the knowledge about the possible influence of solar activity on the railway infrastructure facilities, which in its turn is necessary for the development of methods and means aimed at ensuring the uninterrupted operation of railway transport.

ANALIZA UNUI ECRAN MAGNETIC POZIțIONAT ÎN SPAȚIUL DINTRE ÎNFĂȘURĂRILE UNUI TRANSFORMATOR MONOFAZAT, DE MICĂ PUTERE

"This paper aims to analyze the impact of using a thin magnetic shield placed in the space between the primary and secondary winding of a simplified, low power, single-phase transformer used in energy harvesting applications that demand power transformers not only in the energy conditioning stage but also in the energy harvesting stage. By using magnetic shields, the saturation of the ferromagnetic core and, in some particular cases, the destruction of electronic devices is avoided. For this purpose two scenarios are studied: one which doesn't take into account the magnetic shield, as it considers only the air space between the primary and secondary windings, respectively, and the second case study which considers a magnetic screen placed in the centre of the air space domain. The size of the air space domain, d, is varied as the secondary winding distance itself from the primary one until it reaches the core. The number of turns in the primary and secondary winding is equal, N1 = N2 = 300 turns. By moving the secondary winding away from the primary winding, the variation of the distance d between the coils is achieved, thus keeping the same cross-section of the secondary winding. The thickness of the magnetic shield is chosen arbitrarily, as thin as possible, with a dimension of 400 µm. The idealy, 1:1, simplified, low-power, single-phase transformer powered by a harmonic voltage supply at V1 = 20 V and at a frequency, f = 50 Hz, with load resistance of Rs = 100 Ω, is analyzed in a time-dependent study and its computational domain is taken from literature [4]. Different materials can be used for realizing this magnetic shieling, even copper and aluminum, but in this paper a magnetic sheet metal material is considered because of its small, almost nonexistent electrical conductivity. Our goal is to analyze the effect of magnetic shielding on the saturation of the ferromagnetic core, and the reactance and resistance values of the primary and secondary winding, respectively, for different dimensions of the air space, d. For comparison purposes, the second model, the one in which we have the magnetic sheet metal, an analysis is performed in the permanent harmonic regime, in addition to the one performed in the dynamic one."

Computational and experimental study of air-core HTS transformer electrothermal behaviour at current limiting mode

The paper provides the results of the experimental and computational study of the processes occurring in high temperature superconducting transformer windings while secondary winding is short-circuited. The obtained mathematical simulation matches closely with the experimental results. The temperature variation curves for superconducting windings were analysed, and conclusions were made on the necessity of changes in HTS transformer design, namely the necessity of windings heat-insulation from each other and adding a high-resistance coating material for HTS wire in HTS transformer primary winding.

Solution of Topical Issues in Theory of Transformers

In the article, the authors summarized their experience in solving some controversial issues in the theory of transformers, and briefly outlined the essence of their developments. For power transformers of the TM series, the presence of an excess of the active resistance of the primary winding relative to the reduced active re-sistance of the secondary winding was established. The obtained correction factors, taking into account the electrical skin effect in the windings and their heating, can be used to calculate short-circuit currents and heat distribution in other types of transformers. An expression is given that makes it possible to correct the transfor-mation ratio of one of the transformers connected to parallel operation in such a way that it becomes possible to fully load both devices, despite their different short-circuit voltages. Expressions are presented that make it possible to reveal the ratio of hysteresis and eddy-current losses in the magnetic circuit of a particular trans-former, as well as the exponent with which the losses in the magnetic circuit depend on the frequency of mag-netization reversal. A method is proposed for determining the three components of losses in a magnetic circuit, based on an idle experiment at three frequencies. A method is described for determining the thickness of sheets of a magnetic circuit, at which the total losses in it are minimal, and a method for determining the heating time constant of dry transformers is presented, which makes it possible to accelerate their thermal tests.