Analysis and Applicability of Heffron–Phillips Model

In this paper, a non-linear 7th order dynamic model and a linearized 3rd order dynamic model of a synchronous generator connected to an infinite bus are presented and compared in details. Parameters and equations of the both models are explained and summarized. They represent a useful starting point for work in areas of synchronous generators' construction, analysis, control systems design and synthesis. Novelty of this work represents a detailed research of applicability of the linearized model in an entire operating range. Established theoretical conclusions were confirmed with numerical results. Restrictions of utilizing the linearized model are presented. On a basis of the analysis for the synchronous generators of different power, their applicability with feasibility and accuracy was evaluated by certain objective criteria

SHORT TERM OPTIMIZATION OF DRAVA RIVER HYDRO POWER PLANTS OPERATION

V delu je predstavljena kratkoročna optimizacija obratovanja verige Dravskih elektrarn. Optimizacija temelji na genetskem algoritmu, ki je del programskega paketa MATLAB. Predstavljeni so postopki modeliranja hidroelektrarn in bazenov. Uporabljeni so nelinearni modeli hidroelektrarn in dinamični modeli bazenov. Parametri dinamičnih bazenov so določeni na podlagi izmerjenih vrednosti pretokov in nivojev vode v bazenih. Za model verige dravskih elektrarn so prikazane primerjave med izmerjenimi in izračunanimi časovnimi odzivi nivojev vode v bazenih. Z analizo izmerjenih vrednosti moči v obdobju enega leta je bilo zmanjšano število možnih rešitev optimizacije. Časovno obdobje optimiranja je omejeno na največ 24 ur.This work presents short term optimization of Drava river hydro power plants operation. Optimization is based on genetic algorithm, which is part of MATLAB software package. Modeling of hydro power plants and basins is presented. Nonlinear models of hydro power plants and dynamic basins are used. Parameters of dynamical basin models are obtained on measured values of flows and water levels. Calculated and measured time responses of basin water levels are compared for entire chain of Drava river hydro power plants. A mathematical analysis of measured powers within one year operation periode is used to reduce the possible solutions of optimization. Period of optimization calculations is limited to maximum of 24 hours

The impact of switching frequency of a resistance spot welding system on welding current and power losses

V doktorski disertaciji je predstavljena analiza srednjefrekvenčnega sistema za uporovno točkovno varjenje, ki se v industriji uporablja za varjenje pločevin. Sistem za uporovno točkovno varjenje sestavljajo frekvenčni pretvornik, varilni transformator z nameščenim diodnim usmernikom in varilne klešče z elektrodami. Ti sistemi se večinoma uporabljajo v avtomatizirani avtomobilski industriji za varjenje karoserij s pomočjo robotov. Varilni transformator je nameščen na premikajoči se robotski roki, zato je teža zelo pomemben dejavnik. Z manjšo težo prihranimo na energiji in povečamo dinamiko robota. Težo varilnega transformatorja lahko zmanjšamo z lažjimi in kakovostnejšimi materiali, kar pa je povezano z višjo ceno in je pogosto nesprejemljivo. Težo lahko zmanjšamo tudi s povišanjem frekvence napajalne napetosti, kar omogoča zmanjšanje preseka železnega jedra. Manjši presek železnega jedra pomeni tudi krajše ovoje navitij in s tem manjšo težo. Višanje napajalne frekvence pa negativno vpliva na velikost varilnega toka in izgube, ki sta glavni temi raziskovanja doktorske disertacije. Z višanjem frekvence se namreč zmanjšuje največji varilni tok zaradi izgube prevajalnega razmerja. Pojav izgube prevajalnega razmerja in efektivno prevajalno razmerje sta podrobno analizirana na podlagi časovnih potekov tokov in napetosti. Frekvenčna odvisnost maksimalnega varilnega toka je karakteristična lastnost varilnega transformatorja, ki jo lahko določimo z drago merilno opremo ali z zahtevnimi numeričnimi izračuni. V disertaciji je predstavljena možnost analitičnega izračuna na osnovi poznanih parametrov veznega modela. Ker gre v danem primeru za zahteven, nelinearen in časovno spremenljiv sistem, je tudi analitična rešitev temu primerno zahtevna. Analitična rešitev pa ne omogoča samo neposrednega izračuna frekvenčne odvisnosti varilnega toka, temveč tudi njegovo odvisnost od kateregakoli drugega parametra veznega modela. Analitična rešitev je potrjena tako z meritvami kot z numeričnimi izračuni z reševanjem vezij in z uporabo metode končnih elementov (MKE). Ker modeli MKE zajemajo tudi vpliv kožnega in sosedstvenega pojava, je bilo z izračuni potrjeno, da ti pojavi v opazovanem frekvenčnem področju nimajo opaznega vpliva na frekvenčno odvisnost varilnega toka. S pomočjo MKE in meritev so bili analizirani tudi vplivi frekvence na joulske izgube varilnega transformatorja. Izgube se je razdelilo na deleže stacionarnih in dinamičnih izgub, kjer slednje naraščajo z naraščanjem frekvence. Na osnovi rezultatov meritev in izračunov s predlogom šestih novih konstrukcij navitij transformatorja so podani splošni napotki za zmanjšanje neželenega zmanjšanja varilnega toka in povečanje izgub pri povečanju frekvence napajalne napetosti.This doctoral dissertation describes a medium-frequency resistance spot welding system which is used in the industry for welding metal sheets. This system consists of a frequency converter, a welding transformer with an integrated diode rectifier and a welding gun with electrodes. These welding transformers are widely used in the automated automotive industry for welding chassis. The welding transformer is usually mounted on a moving robotic arm, therefore its weight is a very important factor. The reduced weight has an influence on the robot’s energy consumption and dynamic performance. The weight of the welding transformer can be reduced with lighter and better materials, but due to the higher price this is usually not an option. The weight can also be reduced with higher frequency which allows the reduction of the iron core cross-section. A smaller cross-section means shorter windings and less weight. The higher frequency has a negative impact on the maximum welding current and on power losses, which are the main focus of this dissertation. With a higher frequency the maximum welding current decreases due to the duty cycle loss. The duty cycle loss and effective duty cycle are thoroughly described as part of the behaviour of the currents and voltages within the selected time interval. The frequency dependence of the maximum value of the welding current is a characteristic behaviour of the welding transformer, which can be determined with sophisticated and time-consuming simulations or expensive measurements. This dissertation presents the third option, which allows a direct calculation by taking into account the known parameters of the circuit model. As the analysed system is complex, nonlinear and time-variant, the analytical solution is not simple. The presented analytical solution does not allow only the direct calculation of the frequency dependent value of welding currents, but also of its dependence on any of the parameters of the circuit model. The analytical solution is confirmed by the measurements and numerical calculations of the circuit model and by a finite element method (FEM) based calculation. As the FEM takes into the account the skin effect and the proximity effect, it was confirmed that these effects do not have a considerable impact on the frequency dependent value of the welding current. The analysis of the measurements and the FEM included also the analysis of the frequency dependent transformer\u27s power losses. The power losses were separated into a static and a dynamic part, where the dynamic part increases according to the frequency. By taking into account the measurements and the results of the numerical calculations of six new transformer\u27s winding constructions the thesis provides general suggestions on how to decrease the negative impact of higher frequency on the welding current and on power losses