Zijad Haznadar

Stray Losses Computation in Power Transformer

Stray losses of three-phase power transformer are predicted using 3D-FEM. The computation is made under harmonic steady state load condition treating the transformer core magnetizing characteristic as non-linear. Various kinds of methods for estimating stray losses due to leakage magnetic field are discussed. In this paper, the eddy current losses in the clamp plates and unshielded transformer tank are obtained modeling them by non-linear surface impedance method. Some results of calculation and measurement are shown

Macro elements in the finite element method-application to the high-voltage insulating system design

This paper describes the use of the macro elements in the Finite Element Method. Macro elements correspond to the parts of the region and are formed using the triangular isoparametric finite elements (up to fourth order). Some internal nodes are eliminated by the so-called condensation process. Many times, the configuration being analyzed has a part of region which is being repeated (i.e., insulator chains are composed of units of the same shape, etc.). In those cases, the use of macro elements can greatly reduce the total number of unknowns, the amount of input data and the computation time. The complex potential distribution caused by resistive films on the insulator surface is calculated by decoupling the problem in two parts.

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.

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.