Franjo Kelemen

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Papers by Franjo Kelemen

Determining Natural Resonant Frequencies of Large Power Transformer Windings

Energija, Jul 11, 2022

Design, assembly process and preparation of windings for large power transformers are complex pro... more Design, assembly process and preparation of windings for large power transformers are complex procedures. It is very important to recognize resonant frequencies of the windings during the design phase in order to avoid increase of vibrations, stresses or noise levels during normal operation of the transformer. Increased vibrations and stresses can occur when the frequency of the excitation is close to the natural resonant frequency of the system. In that case amplitudes will increase, and their values will depend, among other factor, also on the amount of damping present in the system. Therefore, in order to avoid very large amplitude of vibration at resonance, the natural frequency must be known. In this work analytical calculation of natural frequencies of high voltage and low voltage windings, is described. Additionally, numerical analysis (FEA) is performed in the software package Ansys. Analysis contains descriptions of geometry simplifications, boundary conditions, mesh validation and applied loads. Finally, results obtained with numerical analysis (FEA) are compared with analytical results.

format_quoteCorrect evaluation of natural resonant frequencies is vital for transformer design to prevent overstressing and operational failures.format_quote

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Estimation of Stray Losses in Power Transformers Using 3D Fem and Statistics

Energija, Jul 19, 2022

Total losses of a power transformer are subdivided in several distinctive parts. The I 2 R losses... more Total losses of a power transformer are subdivided in several distinctive parts. The I 2 R losses are easy to calculate and can be precise to a level of measurement repeatability and tolerances of the guaranteed material properties. The additional losses inside the windings can be calculated almost with the same precision using analytic methods. The third part of the load losses that consists of stray losses is the smallest and the most difficult to estimate. However, stray losses estimation is very important in the design phase of a power transformer. Not only because the guaranteed design parameters have to be satisfied, but also the utmost care has to be taken that local losses density across the power transformer does not exceed levels that are permissible in the long term loading conditions of a transformer. The additional losses estimation process presented in this paper models the additional losses level in a transformer as a unit.

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Skin – Effect Losses in Different Loading Conditions of a Power Transformer

Energija, Jul 19, 2022

The subject of the analysis is a 300 MVA auto-connected power transformer in different loading co... more The subject of the analysis is a 300 MVA auto-connected power transformer in different loading conditions with regard to the load losses. During the electrical design time, some operating points of the transformer were analyzed in more detail using 2D electromagnetic field finite element method (FEM) software. The models included 2D magnetic stray field calculation and covered a range of transformer loading cases that covered some that are more difficult to solve with traditional analytic methods based on the static magnetic field calculations. This is due to the presence of a phase shift between the currents through the windings. The results of the static magnetic Rabins' method field calculation and the FEM method are compared and the best practice method is defined and determined accordingly.

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Estimation of zero sequence impedance in power transformers using 2D and 3D FEM

Journal of Energy - Energija

This paper describes calculation of zero sequence impedance in three phase, three leg core type p... more This paper describes calculation of zero sequence impedance in three phase, three leg core type power transformers using 2D and 3D finite element method (FEM) calculation on example of 100 MVA power transformer. Paper also describes how winding connection and arrangement affects the size and calculation of zero sequence impedance.

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Magnetizing current of a Large Power Transformer and its Harmonic Spectrum in Normal and GIC conditions

Journal of Energy - Energija

A measurement of the harmonic spectrum of magnetizing current was performed. This paper analyzes ... more A measurement of the harmonic spectrum of magnetizing current was performed. This paper analyzes the results of the measurement of harmonic spectrum of magnetizing current in standard noload test at various values of induction. Magnetizing current harmonic spectrum during single-phase noload test was measured as well as the impact of combined AC and DC magnetization on core behavior and harmonic spectrum. A mathematical model of transformer core is introduced. The calculated results are presented.

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Influence of Different Turret-Tank Connection Types on Transformer Turret Eddy Currents

Lecture Notes in Electrical Engineering, 2019

All metal parts of the transformer tank are usually electrically connected and grounded in order ... more All metal parts of the transformer tank are usually electrically connected and grounded in order to prevent the possibly hazardous floating potential from emerging. The magnetic field of transformer leads and bushings’ conductors induces eddy currents on nearby metal parts such as tank top and bushing turrets. Since the above-mentioned electrical connections can form the unwanted conducting path loops, the type of these connections can help in restricting the eddy currents and consequently the losses in metal parts and the heat generated in them. This is especially true for high-current power transformers such as generator step-up units. This research presents calculated and measured results of the influence of different bushing turret-tank connection types on the induced eddy current values. The simulation results were obtained using the finite element method based software. Comparison against the measurements shows that these eddy currents can be adequately modelled and predicted using calculations which can prove useful in the design phase of the transformer manufacturing process. By carefully choosing the connection type and the location of the shorting element, the eddy current conducting path loops can be restricted and therefore the values of these currents can be limited as well. This can, in turn, lead to lower local losses and reduced heating of the metal parts, all of which can be easily shown using Rogowski coils for current measurements and thermal imaging systems for thermography measurements, respectively.

Measurement and Calculation of Electromagnetic Field in Close Surrounding of Power Transformer

U radu se uspoređuje mjereno i racunato elektromagnetsko polje u okolini energetskog transformato... more U radu se uspoređuje mjereno i racunato elektromagnetsko polje u okolini energetskog transformatora. Cilj istraživanja je razmotriti mogucnost izbjegavanja mjerenja polja, koje bi na buducim jedinicama nadomjestio racunalni model. U racunalnom modelu napravljenom za ovaj rad također su uzeti u obzir objekti smjesteni u neposrednoj blizini transformatora te je razmotren njihov utjecaj na distribuciju elektromagnetskog polja transformatora.

Mjerenje i proračun elektromagnetskog polja u bliskom okruženju energetskog transformatora

Experimental Research of Stray Losses in Transformers

Unaprjeđenje metoda određivanja dodatnih gubitaka u transformatoru može donijeti znacajne ustede ... more Unaprjeđenje metoda određivanja dodatnih gubitaka u transformatoru može donijeti znacajne ustede proizvođacu. Zbog toga je potrebno testirati potencijalne metode određivanja gubitaka na modelima. U referatu je dan prikaz istraživanja dodatnih gubitaka u energetskim transformatorima na eksperimentalnim modelima. Na prvom modelu pokazana je mogucnost predviđanja raspodjele magnetskog toka u magnetskim zaslonima pomocu numerickih proracuna. Na drugom modelu uspoređeni su mjereni i izracunati gubici u plocama od magnetskog i nemagnetskih materijala. Na trecem modelu istražene su mogucnosti određivanja lokalnih gubitaka koristenjem termometrijske metode. Usporedbe rezultata mjerenja i proracuna na svim modelima pokazale su prednosti i nedostatke koristenih eksperimentalnih i numerickih metoda. Istaknuti su izazovi i podrucja koja treba dodatno istražiti.

Eksperimentalno istraživanje dodatnih gubitaka transformatora

Influence of capacitive no-load current component in large power transformers

Procedia Engineering, 2017

No load current of large power transformers consists of active and reactive component. Active com... more No load current of large power transformers consists of active and reactive component. Active component covers hysteresis and eddy current losses in the core. Reactive component (usually called magnetizing current) creates magnetic field / magnetic flux and lags the voltage by 90°. In most models these two components are enough to describe no-load current in large power transformer at power grid frequency (50 or 60 Hz). Due to the low frequency modeling the winding capacitance are usually neglected. For higher frequencies and/or transients, ground capacitive reactances and capacitive reactances between winding turns and windings are significant and they need to be modeled and, therefore, capacitive current component occurs [1]. If capacitances can be neglected, reactive component of no load current consists only of magnetizing part and depends only on magnetic steel properties, following its B-H curve. In this paper the presence of capacitive component of large power transformer no-load current is presented when powered by 50 Hz sinusoidal voltage. Capacitive component exist mainly as sum of the capacitive component of supplied windings and transferred currents from other windings [2], [3] Its influence on the no-load characteristic and an approach for determination of the inductive component in the no-load current based on the no-load current measurement is explained. Equivalent capacitance is compared with capacitance measurements in test field.

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Influence of the Power Transformer Tank Design on the Sound Level

Procedia Engineering, 2017

Low noise levels are today requested for power transformers in order to comply with customer spec... more Low noise levels are today requested for power transformers in order to comply with customer specifications and environmental legislations. Manufacturers have to improve the acoustic performances of their products and also reduce the costs in the same time. Therefore it is necessary to predict sound levels with a sufficient accuracy for each transformer design. Sound radiation of transformer tanks is influenced by a wide variety of parameters including placement, type, arrangement and dimensions of stiffeners, tank thickness, tank shape and type, insulating fluid or gas (mineral oil, ester or air). The vibration sources in transformers are located within an oil-immersed rib-stiffened transformer tank vessels made of thin and flexible steel plates. The vibrations generate sound pressure waves which are iteratively reflected from structural boundaries and discontinuities. They form standing patterns of vibrations, structural modes of the vessel and standing waves inside the fluidfilled acoustic cavity. If the corresponding natural frequencies of the structures are in the vicinity of the forcing frequencies, unexpectedly high sound levels are regularly observed, [1]. In order to avoid these unexpectedly high sound levels caused by the natural frequencies of the tank, optimization of the tank and tank stiffeners arrangement, dimensions and design has to be carried out.

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Analysis of short-circuit forces at the top of the low voltage U-type and I-type winding in a power transformer

2008 13th International Power Electronics and Motion Control Conference, 2008

The aim of this study was to compare the short-circuit forces in the U-type and I-type low voltag... more The aim of this study was to compare the short-circuit forces in the U-type and I-type low voltage winding of a large power transformer. The comparison of forces calculated using 2D Rabinpsilas method and 3D finite element method is conducted. The influence of the winding helix pitch on the winding forces calculated using 3D finite element method is also investigated.

Modeling and Calculation of Electromagnetic Field in the Surroundings of a Large Power Transformer

2009 20th International Zurich Symposium on Electromagnetic Compatibility, 2009

The presented study compares measured and calculated electromagnetic field quantities in the surr... more The presented study compares measured and calculated electromagnetic field quantities in the surroundings of a large power transformer with the aim to avoid the necessity of measuring the field on subsequent units and use a computer model instead. The influences of various objects located in the vicinity of the transformer during measurement are also analyzed and are taken into account in a computer model.

$Figure 14. Top view of the transformer with marked calculation points, 220 MVA, 240 kV transformer. Table 6. Peak ampere-turn values in the windings for the magnetic field model with windings, 220 MVA, 240 k\ transformer.$

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Estimation of stray losses outside of windings in power transformers using three dimensional static magnetic field solution and statistics

2008 18th International Conference on Electrical Machines, 2008

Stray losses outside of windings are a subject of numerous different analyses in the power transf... more Stray losses outside of windings are a subject of numerous different analyses in the power transformer production industry. The purpose of this paper is to illustrate an approach that combines 3D finite element static magnetic field calculation and nonlinear mathematical programming in a single method derived to increase accuracy of stray losses estimation.

Influence of a delta-connected winding on magnetizing currents of a five-limb core power transformer

The XIX International Conference on Electrical Machines - ICEM 2010, 2010

ABSTRACT Power transformers in their normal lifetime span often encounter unbalanced loads or eve... more ABSTRACT Power transformers in their normal lifetime span often encounter unbalanced loads or even DC bias currents that shift the normal operational point of the unit. The influence of a delta connected winding on the magnetizing current waveform of a power transformer will be presented here in more detail. The behavior of an active part is modeled using the system of nonlinear equations solved iteratively, and the results are compared to the ones obtained using three-dimensional nonlinear transient finite element method (FEM).

Transferred Voltages due to Single Phase Earth Fault on Power Transformers

Procedia Engineering, 2017

In the event of an earth fault in one of the two systems of a three-winding transformer, an incre... more In the event of an earth fault in one of the two systems of a three-winding transformer, an increase in the starpoint voltage occurs in the disturbed system. This voltage is transferred to the undisturbed systems due to capacitive coupling, to affected earth faulty system and to earth. As a result, three phase voltages to earth are different in magnitude and phase position compared to their undisturbed service status. In this paper the mesh analysis method is used to calculate the neutral point shift of the faulted winding necessary to obtain values of transferred voltages to the unaffected winding (the one which is not earth faulted). The results are analyzed to investigate the influence of winding and network neutral grounding (insulated, solidly grounded, grounded over impedance) on the results.

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