A IDLE CURRENTS HARMONIC COMPOSITION OF POWER TRANSFORMER
DOI:
https://doi.org/10.15588/1607-6761-2019-1-4Keywords:
field model, three phase transformer, idling, magnetic field, finite element method, harmonic anal-ysis, non-sinusoidal and asymmetrical currentsAbstract
Purpose. The aim of the work is to define the idle currents harmonic composition of the power transformer using spatial field models, that take into account the structural construction of the active part of the transformer, the hysteresis of nonlinear magnetic properties of the electrical steel, and the electrical asymmetry of the currents in phase windings.
Methodology. The research was carried out using harmonic analysis methods, the theory of magnetic fields, the theory of electrical circuits, the theory of power transformers, finite element methods, and symmetric components.
Findings. Based on the numerical implementation of 3D models of the magnetic field in a three-phase power transformer with the connection of the primary windings according to the “Y” scheme, the harmonic composition of the phase idle currents is determined. An improved approach has been applied to increase the efficiency of field simulation of the research idle mode by setting the conditions of symmetry of the magnetic field on a plane with the axes of the legs of the magnetic system. This made it possible to reduce the volume of the 3D geometric area by 2 times and to ensure a proportional decrease in the time and computational resources, required for the numerical implementation of the mathematical model by the finite element method. The features of non-sinusoidal changes in time of momentary values of idle currents are determined and their uneven distribution in the transformer phases is estimated. It is characterized by an increase in the effective currents to 113.2%, 112.9% in phases A and C and their decrease to 72.4% in phase B. It is shown that the amplitude of the main harmonic decreases for the positive phase-sequence by 5.08% of the amplitude of the transformer idling current and increases to 27.91% for the negative phase-sequence. The prevailing influence of the odd higher harmonic components of the idle phase currents, the amplitudes of which are 24, 21% and 4% of the amplitude of the idle current for its third, fifth and seventh harmonics is established. The use of a compensating winding system, which is connected to the adjusting windings in negative phase-sequence with a phase shift of 120 degrees, reduces the effective value of the idle current of the negative phase-sequence to 5%, approximates the effective value of the first harmonic of the idle current, and also reduce the deviation of the phase shift angles compared to the symmetric idle mode up to 2%.
Originality. Based on theoretical and experimental researches, the prevailing influence of the third harmonic components on the non-sinusoidal phase of the idle phase currents in the primary windings, connected in "Y" is determined. The use of additional symmetry boundary conditions provides a significant increase in the efficiency of numerical implementation due to a two-fold reduction in the volume of the 3D field modeling of a power transformer in idle mode. For the scheme of connecting the windings of the transformer “Y” without a neutral wire, taking into account the interrelated effects of hysteresis and eddy currents on specific losses and magnetization power, due to the high accuracy of describing the interrelation between magnetic flux density and magnetic field strength in ferromagnetic media, the new features of idle current harmonic composition with the advantage of the third harmonic was determined and experimentally confirmed.
Practical value. The proposed approaches and methods allow to reduce the current error and the relative error for idle losses up to 1.41% and 1.2% for a 3D model of a three-phase power transformer. The use of a system with a compensating winding allows to reduce the amplitude of the third harmonic components of idle phase currents up to 2 - 2.5 times and by balancing the current load of the phases reduce the amplitudes of phase idle currents and the effective value of idle current up to 15 - 16 %.
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