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  • 1.
    Carrander, Claes
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Engdahl, Göran
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    A semi-empirical approximation of static hysteresis for high flux densities in highly grain-oriented silicon iron2017In: 8th Joint European Magnetic Symposia (JEMS2016), Institute of Physics (IOP), 2017, article id UNSP 012039Conference paper (Refereed)
    Abstract [en]

    In calculations and simulations regarding magnetic materials, it is important to have a have an accurate model of the hysteresis loop. The major loop, in particular, is used in many simulations. However, it is generally not possible to measure the true major loop, and it must therefore be approximated using a minor loop. There are several methods available for approximating magnetization curves, but they are primarily designed for paramagnetic materials, and are poorly suited to the highly grain-oriented steels used in modern transformers. Therefore, we propose two expressions for approximating the magnetization curves of grain-oriented silicon-iron steels. Both methods give close agreement with measurements and can be extrapolated to in order to describe the major loop.

  • 2.
    Carrander, Claes
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Mousavi, Seyedali
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Engdahl, Göran
    KTH, School of Electrical Engineering (EES).
    An application of the time-step topological model for three-phase transformer no-load current calculation considering hysteresis2017In: Journal of Magnetism and Magnetic Materials, ISSN 0304-8853, E-ISSN 1873-4766, Vol. 423, p. 241-244Article in journal (Refereed)
    Abstract [en]

    In many transformer applications, it is necessary to have a core magnetization model that takes into account both magnetic and electrical effects. This becomes particularly important in three-phase transformers, where the zero-sequence impedance is generally high, and therefore affects the magnetization very strongly. In this paper, we demonstrate a time-step topological simulation method that uses a lumped-element approach to accurately model both the electrical and magnetic circuits. The simulation method is independent of the used hysteresis model. In this paper, a hysteresis model based on the first-order reversal-curve has been used.

  • 3.
    Carrander, Claes
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Magnetizing Currents in Power Transformers: Measurements, Simulations, and Diagnostic Methods2017Doctoral thesis, monograph (Other academic)
    Abstract [en]

    This thesis demonstrates a method for transformer core diagnostics. The method uses the no-load current of the transformer as an indicator, and gives different characteristic signatures for different types of faults or defects. Using the no-load current for the diagnostic gives high sensitivity. The method is therefore able to detect defects that are too small to have an impact on the losses. In addition to different types of fault, the method can in some cases also distinguish between faults in different locations within the core.

    Both single-phase and three-phase transformers can be diagnosed using this method, and the measurements can be easily performed at any facility capable of measuring the no-load loss. There are, however, some phenomena that occur in large transformers, and in transformers with high rated voltages. Examples include capacitive resonance and magnetic remanence. This thesis proposes and demonstrates techniques for compensating for these phenomena. With these compensating techniques, the repeatability of the measurements is high. It is shown that units with the same core steel tend to have very similar no-load behavior.

    The diagnostics can then be performed either by comparing the transformer to another unit, or to simulations. The thesis presents one possible simulation method, and demonstrates the agreement with measurements.

    This topological simulation method includes both the electric circuit and an accurate model of the magnetic hysteresis. It is therefore also suitable for other, related, studies in addition to core diagnostics. Possible subjects include ferroresonance, inrush, DC magnetization of transformers, and transformer core optimization.

    The thesis also demonstrates that, for three-phase transformers, it is possible to compare the phases to each other. This technique makes it possible to diagnose a transformer even without a previous measurement to compare to, and without the data required to make a simulation.

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  • 4.
    Carrander, Claes
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Engdahl, Göran
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Magnetizing current measurements on full scale power transformers2015In: International journal of applied electromagnetics and mechanics, ISSN 1383-5416, E-ISSN 1875-8800, Vol. 48, no 2-3, p. 159-162Article in journal (Refereed)
    Abstract [en]

    The magnetic properties of a power transformer core are generally held to be quite similar to those of the core steel itself. Due mainly to it being rare and practically difficult to acquire a transformer for testing and verification, testing of large units is usually only performed concerning no-load losses. However, other parts of the magnetic hysteresis loop are more sensitive to variations in material and geometry and could be used for more detailed diagnostics. This paper shows that measurements of magnetic hysteresis can be performed with good results on large power transformers. Methods to compensate for capacitive currents and to calculate the effective magnetic length of the core are shown and the results are compared to standard material measurements. The results show good agreement with Epstein frame measurements on annealed samples.

  • 5.
    Carrander, Claes
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    On methods of measuring magnetic properties of power transformers2015Licentiate thesis, monograph (Other academic)
    Abstract [en]

    The core of a power transformer can weigh hundreds of tons, and represents a significant part of the total ownership cost of the transformer, in terms of both the construction cost and of the monetary cost of losses. It is therefore desirable to have a method of diagnosing the core to ensure optimal performance, and to enable the operator to plan repairs of damaged or defective cores. This study shows that measuring the magnetizing current gives an amplification effect, compared to measuring just the no-load loss, that allows even small variations in the state of the core to be detected. The measurement is performed on several large power transformers, demonstrating that the method is easy to use and that it gives reliable results that are comparable between different transformer designs, though further experiments are necessary in order to fully validate it. Experiments performed on small-scale transformer models show that different types of core faults give different characteristic changes in the magnetizing current, thus making it possible to diagnose the core.

    The large scale does, however, introduce other phenomena that must be taken into account. The most important of these is that, at high voltages, the capacitances in the transformer become noticeable and must be compensated for. This study describes a method of approximating the capacitive part of the no-load current using naturally occurring harmonics in the supply voltage.

    Additionally, there is, at present, no satisfactory method of describing the magnetizing current of a three-phase transformer. Because of the non-linearity of the core steel, there is a large difference between the measured no-load current and the actual magnetizing current. Three methods for measuring the magnetizing current in a three-phase transformer are described. However, these methods are only applicable under very special circumstances, and are generally not useful under real conditions. As an alternative, a simulation method is outlined. Measurement results from a transformer can then be compared to the simulation result, and any differences between the two can be interpreted to make a diagnosis of the transformer core. The method is used to simulate a large three-phase transformer, and produces a qualitative agreement. An improvement to the hysteresis model used in the simulation, which could potentially make it possible to make quantitative predictions as well, is discussed.

    Much work remains before the methods described in this thesis can become fully functional diagnostic tools, but the work shows the feasibility of the project, and the potential benefits. By measuring the magnetizing current it is possible to obtain valuable information about the status of the core. This makes it easier for the transformer operator to plan maintenance, or for the manufacturer to assess the quality of the core, and increases operational reliability and efficiency.

  • 6.
    Mousavi, Seyedali
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Engdahl, Göran
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Carrander, Claes
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Comprehensive Study on Magnetization Current Harmonics of Power Transformers due to GICs2013Conference paper (Refereed)
    Abstract [en]

    This paper studies the effect of DC magnetization of power transformers on injected harmonics to power systems. DC magnetization due to geomagnetically induced currents can saturate the core of a power transformer during a half cycle. It causes a very asymmetric, high value magnetization current that contains large harmonic components.

    In this work, by using a three-dimensional finite element model, the effect of core type and DC current level on generated harmonics is investigated.

    The results could help power system engineers to choose the proper types of power transformers and improve the protection aspects of the network.

     

  • 7.
    Mousavi, Seyedali
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Engdahl, Göran
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Carrander, Claes
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Electromagnetic transients due to interaction between power transformers and network during a GIC attack2013Conference paper (Refereed)
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