Sead Berberović

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

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).

Electric and magnetic field calculations of vacuum circuit breaker

This article describes mathematical models and shows a numerical calculation of the electric and magnetic fields, as well as the numerical calculation of the charged particle trajectory in the space between the electrodes of the vacuum circuit breaker. The accurate knowledge of the electric, and particularly of the magnetic field and their effects upon the charged particle trajectory, enables designing of such electrode shapes in order to get a spiral trajectory of the charged particles and a uniform distribution of the arc current density on these electrodes.

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

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.

Turbogenerator refurbishment with up-rating based on embedding stator damper winding and modern electromagnetic materials

40 MVA two-pole air cooled turbogenerator has been up-graded with stator refurbishment only. Generator active power has been increased from 32 MW to 42 MW with reference to 31% and apparent power has been increased for 23.5%, from 40 MVA to 49.4 MVA. It was necessary to redesign stator core and winding, to embed the stator damper winding and implement new magnetic and insulating materials in order to reach new higher power. Redesigned stator core needs to be of the same outside dimensions as the original one, and to be placed in the old stator casing. Refurbishment requirements with remaining the original machine parameters (but higher output) such as synchronous reactance, audible noise level, voltage and frequency regulation, reactive power and temperature rise in insulation class B have been achieved. Impact of embedding the stator damper winding has been determined by finite-element analysis since the other parameters of refurbished generator have been calculated using analytical and finite element modeling.

Numerical Field Calculation of Earthing Systems

This article describes results of the long-standing research on computer aided design, analysis and calculation of earthing systems. Numerical calculations of the quasistatic current field have been carried out by the boundary element method (current simulation method). Analyses of the complex earthing systems of the large industrial plants have been based on these calculations. The results obtained by measuring models and real objects have proved of satisfactory accuracy for the numerical procedure.