Power Transformer Core Research Articles

On-Site Low Voltage Determination of Zero-Sequence Impedances for Power Transformers

Accurate zero-sequence transformer data is important for correct fault analysis and protection settings. Utilities have been forced to approximate zero-sequence impedances for many three-legged core power transformers in service today that did not undergo testing for zero-sequence data at the factory before site installation. Where approximation of the zero-sequence impedances from positive sequence data is of insufficient accuracy, on-site tests have been proposed. However, these usually involve measurements from the high-voltage windings, which are either expensive with high test voltages or do not offer sufficient current resolution with low test voltages. This paper proposes new methods for carrying out on-site zero-sequence testing entirely from the lowest voltage winding. Affordable equipment can then be used for the test with minimal switchyard test preparations. Two test configurations are described; a One-Open-Phase test and a Two-Open-Phase test. To validate the method, three typical transformer types are considered; a station auxiliary transformer (SAT), a generation step-up transformer (GSU), and a transmission transformer. The proposed method was tested on actual SAT and GSU units, and simulated for a transmission transformer. All test results validated the method and offered high accuracy.

An integrated approach for power transformer modeling and manufacturing

Essential characteristics of smart factories, such as flexibility and resource efficiency, can be leveraged and improved by the power of machine learning and optimization techniques. For instance, the manufacturing process of a power transformer core constitutes a highly complex optimization problem. It involves creating a cost optimal slitting plan that meets all customer requirements and at the same time takes into account flexible and short-term constraints from production (e.g. current available metal bands in stock). As many of these constraints rely on forecasts, a learning system may provide the necessary predictions for these constraints. In addition, companies apply and maintain engineering software for a variety of tasks in construction, simulation, and interpretation of data. For instance, electrical engineers use a variety of tools to design an initial model of a power transformer according to customer requirements and constraints. Such tools often incorporate knowledge that serves as input for optimization and forecast models as described before. If these models are improved over time using external machine learning libraries, the newly developed models must find their way back into the implementation of engineering tools. Knowledge scattered across multiple software systems bears risk of being inconsistent. Furthermore, keeping knowledge consistent without a systematic approach is time-consuming and error-prone. In this paper, we describe an approach that leverages software engineering methods and tools and that supports knowledge transfer between software systems for optimization and modelling tasks. The approach follows the idea of a single source of knowledge together with transformation into different representations, as required by different engineering tasks. The proposed approach was introduced at an industrial site to improve the manufacturing process of power transformer cores.

Novel Remanence Determination for Power Transformers Based on Magnetizing Inductance Measurements

The remanence (residual flux) in the core of power transformers needs to be determined in advance to eliminate the inrush current during the process of re-energization. In this paper, a novel method is proposed to determine the residual flux based on the relationship between residual flux and the measured magnetizing inductance. The paper shows physical, numerical, and analytical explanations on the phenomenon that the magnetizing inductance decreases with the increase of residual flux under low excitation. Numerical simulations are performed by EMTP (Electro-Magnetic Transient Program) on a 1 kVA power transformer under different amounts of residual flux. The inductance–remanence curves are nearly the same when testing current changes. Laboratory experiments conducted on the same transformer are in line with the numerical simulations. Furthermore, numerical simulation results on a 240 MVA are reported to demonstrate the effectiveness of the proposed method.

Experimental Evaluation of Transformer Internal Fault Detection Based on V–I Characteristics

Winding deformation is a common problem that takes place within operating power transformers due to various internal and external conditions. Due to their progressive nature, winding deformations should be detected and rectified as soon as they emerge. Unfortunately, the only reliable technique to detect such faults at this stage is the frequency response analysis which is conducted offline. This has motivated researchers to propose alternative techniques to detect power transformer incipient winding and core deformations in real time. One of the promising online techniques proposed in the literature is the V – I Lissajous pattern method. While this technique was validated through simulation analysis, its feasibility for practical applications has not been explored yet. This paper takes a further step toward the field implementation of this technique through practical measurements on a custom-made transformer to assess its sensitivity, repeatability, and antinoise capability. Also, a new compensation approach is proposed to eliminate the effect of the transformer loading variation on the V – I Lissajous pattern. For the practical application of this technique, a hardware setup has been developed and installed to monitor the V – I characteristics of an operating power transformer within a 220-kV substation in real time.

Hydrothermal Synthesis of Zinc Doped Nickel Ferrites: Evaluation of Structural, Magnetic and Dielectric Properties

Abstract Nickel–zinc ferrites are soft magnetic materials that have different technological applications due to their low magnetic coercivity, high electrical resistivity and low eddy current loss. The soft ferrites are the best for manufacturing core of power transformers, recording heads, data storage devices, telecommunication equipment, media devices, microwaves and electronic devices. In current investigation, a series of zinc doped nickel ferrites (ZnxNi1−xFe2O4, x = 0.1, 0.2, 0.3, 0.4 and 0.5) were prepared by hydrothermal method (at 220 °C, 16 h). The structural, magnetic and dielectric properties were measured. X-ray diffractometer and fourier transform infrared spectroscopy (FTIR) were used to analyze the structural properties and structure of zinc doped nickel ferrites was spinel. As the concentration of Zn was increased, the structural properties (lattice constant, density, dislocation density, radii of tetra and octahedral sites and oxygen positional parameters) were also changed. FTIR results showed the strongest absorption band at 500–600 cm−1, which indicate the intrinsic stretching vibration of Zn+2 ↔ O bond (Mtetra ↔ O) at tetrahedral site. Vibrating sample magnetometer (VSM) was used to measure magnetic properties and it was observed that the value of saturation magnetization increased with Zn concentration. The investigation of frequency dependent dielectric property was measured by Impedance Analyzer. Dielectric constant and tangent loss decreased at higher frequency, whereas AC conductivity increased with frequency.

Features of Measuring the Windings Resistance of Power Transformers to Direct Current

Analysis is provided of the processes of change of magnetic induction in the magnetic core of power transformers during measurements of windings resistance to direct current. A methodology was developed to calculate the time needed for exit to measurement mode. It is shown that this time is proportional to the ratio of the rated winding voltage to the voltage from the power supply of the measuring device. It is noted also that the time needed for exit to the mode of measurement of windings resistance to direct current on the side cores is less than the analogous time for the winding on the central core. The methodology has been confirmed with practical measurements on the power transformers of the Donbass electrical power system.

Experimental Determination of Specific Power Losses and Magnetostriction Strain of Grain-Oriented Electrical Steel Coils

This paper presents a new method for measuring the specific losses and magnetostriction of grain-oriented electrical steel coils of up to 6000 kg. Since large power transformer (LPT) core manufacturers are mainly supplied with entire steel coils, a fast incoming goods inspection is desirable. The proposed method identifies the specific losses of the entire steel coil. In addition, we measured the magnetostriction of the coiled material, which in turn enabled us to classify the main property for LPT no-load noise. Furthermore, we show the structure and setup of the developed coil measurement system. We provide an explanation of the measurement technique and identify its requirements. Exemplary results of specific power losses and magnetostriction of steel coils are provided, and the reproducibility of the results is shown.

Lamination effects on a 3D model of the magnetic core of power transformers

AbstractIn this paper the lamination effect on the model of a power transformer’s core with stacked E-I structure is analyzed. The distribution of the magnetic flux in the laminations depends on the stacking method. In this work it is shown, using a 3D FEM model and an experimental prototype, that the non-uniform distribution of the flux in a laminated E-I core with alternate-lap joint stack increases substantially the average value of the magnetic flux density in the core, compared with a butt joint stack. Both the simulated model and the experimental tests show that the presence of constructive air-gaps in the E-I junctions gives rise to a zig-zag flux in the depth direction. This inter-lamination flux reduces the magnetic flux density in the I-pieces and increases substantially the magnetic flux density in the E-pieces, with highly saturated points that traditional 2D analysis cannot reproduce. The relation between the number of laminations included in the model, and the computational resourses needed to build it, is also evaluated in this work.

Modeling of grain-oriented Si-steel and amorphous alloy iron core under ferroresonance using Jiles–Atherton hysteresis method

Amorphous alloy is increasingly widely used in the iron core of power transformer due to its excellent low loss performance. However, its potential harm to the power system is not fully studied during the electromagnetic transients of the transformer. This study develops a simulation model to analyze the effect of transformer iron core materials on ferroresonance. The model is based on the transformer π equivalent circuit. The flux linkage–current (ψ–i) Jiles–Atherton reactor is developed in an Electromagnetic Transients Program-Alternative Transients Program and is used to represent the magnetizing branches of the transformer model. Two ferroresonance cases are studied to compare the performance of grain-oriented Si-steel and amorphous alloy cores. The ferroresonance overvoltage and overcurrent are discussed under different system parameters. Results show that amorphous alloy transformer generates higher voltage and current than those of grain-oriented Si-steel transformer and significantly harms the power system safety.

Reduction of Power Transformer Core Noise Generation Due to Magnetostriction-Induced Deformations Using Fully Coupled Finite-Element Modeling Optimization Procedures

This paper focuses on the development of an algorithm for the prediction of transformer core deformation, using a fully coupled magneto-mechanical approach. An imposed magnetic flux method coupled with finite-element modeling is employed for the magnetic resolution. The constitutive law of the material is considered with a simplified multi-scale model describing both magnetic and magnetostrictive anisotropies. Magnetostriction is introduced as an input free strain of a mechanical problem to get the deformation and displacement fields. A 3-D estimation of acoustic power is carried out to evaluate the global core deformation and acoustic noise level. The numerical process is applied to a three-phase transformer made of grain-oriented FeSi core, leading to a set of noise estimation for different flux excitations and core geometries.

A review of condition monitoring techniques and diagnostic tests for lifetime estimation of power transformers

Power transformers are a key component of electrical networks, and they are both expensive and difficult to upgrade in a live network. Many utilities monitor the condition of the components that make up a power transformer and use this information to minimize the outage and extend the service life. Routine and diagnostic tests are currently used for condition monitoring and appraising the ageing and defects of the core, windings, bushings and tap changers of power transformers. To accurately assess the remaining life and failure probability, methods have been developed to correlate results from different routine and diagnostic tests. This paper reviews established tests such as dissolved gas analysis, oil characteristic tests, dielectric response, frequency response analysis, partial discharge, infrared thermograph test, turns ratio, power factor, transformer contact resistance, and insulation resistance measurements. It also considers the methods widely used for health index, lifetime estimation, and probability of failure. The authors also highlight the strengths and limitations of currently available methods. This paper summarizes a wide range of techniques drawn from industry and academic sources and contrasts them in a unified frame work.

Modeling the saturation processes of a power-transformer core under simultaneous direct and alternating current passing through the winding

A mathematical model of a three-phase two-winding step-up power transformer with the winding circuit Δ/Yo with simultaneous ac and dc currents passed through the grounded winding has been developed. The proposed model is represented by a system of differential equations, which are nonlinear because of the nonlinearity of the inductive resistances of the magnetization curve. The calculation results for a TDTs-400000/242/20 shell–core transformer are given. The model takes into account the effect of dc on the saturation of the power-transformer core, which is especially important under asymmetrical modes that can occur during thunderstorms, seismic activities, or space-weather disturbances.

Measurement research on magnetic properties of electrical sheet steel under different temperature, harmonic and dc bias

The iron core of large power transformer is mainly composed of electrical sheet steel, which is easily affected by temperature, harmonic, and DC bias. Therefore, it is necessary to measure the magnetic properties of electrical sheet steel under different temperature, harmonic and DC Bias. This paper presents the experiment measurement system for the 30ZH120 electrical steel sheet. The B-H magnetization curve, permeability, and loss curve under different temperature, different harmonic, and different DC bias are given, respectively. The simulation of transformer is carried out by using measuring result under DC bias. The presented research provides a reference for optimizing the design of power transformer.

Effect of Microsecond Pulse Laser Modification on Electromagnetic Properties of Grain Oriented Silicon Steel

A microsecond pulsed laser beam was used to local magnetic domain modification of electrical grain oriented silicon steel. It was carried out using three different laser pulse regimes: a single pulse laser regime, a multipulse laser regime and a multipulse laser regime with modulation of laser pulses. The laser processing variables were pulse energy and and number of pulses. The samples were tested for nanohardness and coercivity before and after laser treatment. Light optical microscopy, scanning electron microscopy and magnetic force microscopy were used to observe the cross-sectional profile, surface of the samples, and magnetic domain visualization, respectively. The local laser treatment of grain oriented silicon steel surface has been studied in terms of its influence on the magnetic domains and coercivity. It was found that laser-modified samples showed coercivity improvement in comparison to the non-treated samples. The most significant improvement in coercivity was obtained in the modulated multipulse regime and negligible improvement in the single pulse laser regime. Three main effects responsible for the observed improvement were identified, namely: magnetic domain refinement, influence of number of laser pulses and shape of laser HAZ profile. The present work highlights on differences in the magnetic domain structure, microstructure of the laser modified material and basic electromagnetic and mechanical properties. In present study, the pulse laser surface processing was presented as a useful energy efficient alternative to other techniques e.g. mechanical scribing, electrical discharge scribing, plasma jet scribing, etc. The refined magnetic domains in electrosteels are responsible for the observed low coercivity, which indicates perspective application of the investigated laser modified steels in the power transformer cores with lower core losses.

Application of digital image processing to detect transformer bushing faults and oil degradation using FRA polar plot signature

Frequency Response Analysis (FRA) technique is commonly used to assess the mechanical integrity of power transformer core and windings. A few papers investigating the ability of FRA technique to detect bushing faults and transformer oil degradation can be found in the literatures. However, in all of these papers, only magnitude of the FRA signature along with visual analysis was used for fault identification and quantification which may lead to inconsistent conclusions for the same FRA signature when interpreted by various personnel. As such, there is an essential need to standardize and automate the FRA interpretation process to improve its reliability and accuracy. This paper is taking a step forward toward this goal through presenting a new FRA interpretation approach by incorporating the magnitude and phase angle plots of the measured FRA signature into one polar plot that comprises more features than the conventional magnitude plot currently used for fault diagnosis. Moreover, the proposed polar plot facilitates the application of digital image processing techniques (DIP) to automate and standardize the whole process. The new proposed technique is assessed through its application to detect minor transformer bushing faults and insulating oil degradations. In this regard, two transformers of different ratings, winding structure and physical dimensions are simulated using 3D finite element analysis to implement various levels of transformer bushing faults and oil degradation. The obtained FRA polar plots of the two investigated transformers under various bushings and oil health conditions are manipulated using the developed DIP codes to investigate the impact of each fault type / level on the proposed polar plot signature. Also, Practical measurements are performed to validate simulation results and evaluate the feasibility of the proposed technique. Results show the ability of the proposed technique to automate the detection of minor levels of bushing faults and insulating oil degradation with a high degree of accuracy.

Influence of Sn4+-substituted on the magnetic properties and power loss of Ni-Zn soft magnetic ferrites

Sn4+-substituted Ni0.65Zn0.35SnxFe2-xO4 (x=0–0.08) soft ferrites were prepared by solid-state reaction method. The structural characteristics and electromagnetic properties were investigated by X-ray diffraction (XRD), LCR Tester, B-H analyzer, respectively. It was found that lattice constant (a) increased gradually, saturation magnetic flux density (Bs) continuously decreased while initial permeability (μi) increased initially and then decreased with increasing of Sn4+ content. Power loss (Pcv) decreased when the temperature increased. However, initial permeability had an opposite change with Pcv. Our results showed that the material as a power transformer core had a lower power loss over room temperature.

Identification of Acoustic Emission Signals Originating from the Core Magnetization of Power Oil Transformer

Abstract In this paper, the properties of AE signals originating from phenomena occurring during magnetization of ferromagnetic materials which are used to construct power transformer cores are presented. The AE signals in a selected power oil transformer were recorded and analyzed. The analysis included, i.e., time, frequency, and time-frequency analyses, calculations of amplitude distributions of the signals and defined AE descriptors, determination of the descriptor map on the side walls of transformers, as well as a detailed analysis of selected part of the signals. The maps of descriptors were analyzed in the frequency bands of 20–70 kHz, 70–100 kHz, and 100–200 kHz. The analysis of the properties of the signals was performed in time and frequency domains. Based on the analysis, there were identified the AE signals originating from the phenomena occurring during the core magnetization of a power oil transformer. To identify those phenomena, the maps of the ADC descriptor calculated in the band of 20–70 kHz when selecting the measurement points in which there were no AE sources from partial discharges were used. An analysis of magnetoacoustic emission signals in the bands of 70–100 kHz and 100–200 kHz was also performed. The analysis of the signal properties in such an extended frequency range allowed determining the properties of the magnetoacoustic signals coming from core sheets of power oil transformers.

Development of a Novel Three Phase Grid-Tied Multilevel Inverter Topology

The conventional line-commutated ac-to-dc converters/ inverters have square-shaped line current. It contains higher-order harmonics which generates EMI and it causes more heating of the core of distribution or power transformers. PWM based inverters using MOSFET/IGBT have higher switching losses, and the power handling capability and reliability are quite low in comparison to thyristors/ SCR. A thyristor based forced commutated inverters are not suitable for PWM applications due to the problems of commutation circuits. A pure sinusoidal voltage output or waveform with low harmonic contents is most desirable for ac load using dc to ac conversion. This paper presents a new multilevel inverter topology in which three phase ac- to-dc converter circuits are used in inversion mode by controlling the switching angle. Due to natural commutation, no separate circuit is required for synchronization. In this paper simulation and analysis are done for grid-tied three-phase 6-pulse, Two three-phase, 3-pulse and 12-pulse converter. These converters are analysed for different battery voltage and different switching angle combinations in order to reduce the total harmonic distortion (THD). Three-phase harmonic filters are further added to the grid side to reduce the harmonic content in the line current. A comparative study of these converters is also presented in this paper.

Modeling and Suppression of Chaotic Ferroresonance in a Power System by Using Memristor-based System

Power systems contain capacitances and inductances that can initiate chaotic ferroresonance due to the non-linear operating characteristics of ferromagnetic materials, such as power transformer cores. Chaotic ferroresonance is a highly disturbing complex, non-linear phenomenon that may cause over-voltages and over-currents; hence, it can compromise regular system operation. The memristor is a non-linear passive two-terminal electricalcomponent, foreseen and introduced as the fourth ideal circuit element. In this study's novel approach, a memristive system is proposed for modeling chaotic ferroresonance in power systems. Employing the memristive circuit as the source of ferroresonance, flexible and adjustable solutions of the dynamic system can be obtained easily. The chaotic characteristic of the proposed system is verified by using a bifurcation diagram and Lyapunov exponent analysis. To reduce the effects of chaotic ferroresonance, a memristor-based system is proposed for damping chaotic oscillations of the system as a protection element. simulation program with integrated circuit emphasis (SPICE) simulations have been carried out for the performance analysis of the proposed system during ferroresonance, and suppression modes are presented.

Nonlinear Resonance Suppression as Distorting and Saturating Source in Voltage and Current of Power Transformer: UPFC and STATCOM Performance Evaluation

Non linear resonance usually resonates in network which is consisting of ferromagneticcore. When distributed capacitance calculated form circuit breaker; mainly circuit breaker and cablecapacitance; after switching and opening the network feed apparatus, magnetization current on ferromagnetic core jump to saturation. Duration of ferroresonance is deeply relying on capacitance, and itwill be decaying by disembarking total store energy. Although, ferroresonance commence by switching circuit breaker off; protecting relay and other protecting schemes have no reaction, because thelast protection step for saving a device is a circuit breaker. There is no reported method to mitigatedisruptive phenomenon; which elder publication focus on the explosion of capacitive transformer,melting of power transformer core lamination and arresters problem. This work is tried to dampingferroresonance and reducing devastation effect on the apparatus. While, protecting devices is useless,fault point topology mutating by FACTS to control and decrease this phenomenon. FACTS deviceis settled at the upstream of apparatus to improve power quality, is switched on at initiating momentresonating. Modeling and simulating of ferroresonance are done by the actual value of transformerand other power system related devices extracted by UTM-TNB.