Impacts of control on the power losses of a resistance spot welding system

V doktorski disertaciji je predstavljen srednje-frekvenčni sistem za uporovno točkovno varjenje (UTV) s katerim se v industriji varijo pločevine. Tak sistem je sestavljen iz frekvenčnega pretvornika, transformatorja, diodnega usmernika in varilnih klešč ter varjenih pločevin. S frekvenčnim pretvornikom lahko generiramo poljubne dolžine pulzov napetosti, s katero nato napajamo transformator. Posledično pa lahko uporabimo različne metode generiranja pulzov napajalne napetosti. V industriji se v ta namen najpogosteje uporablja pulzno-širinska modulacija, ki ji moramo definirati modulacijsko frekvenco, ki je pogosto konstantna. Ker lahko generiramo poljubne dolžine pulzov napajalne napetosti, pa lahko te prožimo tudi glede na potrebe procesa UTV. V disertaciji je tako predstavljen algoritem histereznega vodenja sistema za UTV, ki transformator napaja z minimalno frekvenco napajalne napetosti, ki jo sistem potrebuje, da lahko zagotovi želen bremenski tok. Pri tem pa se lahko frekvenca napajalne napetosti med obratovanjem tudi spremeni, kar ni značilno za pulzno-širinsko modulacijo. Zaradi spreminjanja frekvence napajalne napetosti v disertaciji obravnavamo število pulzov napajalne napetosti v enako dolgih varilnih ciklih. Spreminjanje frekvence pa vpliva na tako imenovane dinamične izgube sistema za UTV, ki so odvisne od frekvence napajalne napetosti. Med te izgube uvrščamo stikalne izgube frekvenčnega pretvornika, izgube povezane s kožnim pojavom v navitjih transformatorja in histerezne izgube železnega jedra transformatorja za UTV. Z zmanjšanjem frekvence se te običajno zmanjšajo, kar smo potrdili tudi v primeru sistema za UTV. Pri uporabi algoritma histereznega vodenja, ki transformator napaja z minimalno frekvenco napajalne napetosti pa naraste valovitost bremenskega toka. Ta je lahko še posebej velika pri varjenju pločevin z nizko vrednostjo nadomestne varilne upornosti. Za tak primer smo pripravili tudi algoritem vodenja, ki zmanjša valovitost bremenskega toka na polovico tako, da maksimalno dolžino pulza napajalne napetosti prepolovi. Pri tem algoritem vodenja samodejno določi dolžino pulza napajalne napetosti na podlagi prvega pulza napajalne napetosti, ki vrednost gostote magnetnega pretoka spremeni od ene točke nasičenja do druge. Oba razvita algoritma pa lahko uporabljamo tudi pri DC-DC pretvornikih, ki vsebujejo transformator. V disertaciji pa smo analizirali tudi vplive bremenskega toka, nadomestne upornosti bremena in napetosti enosmernega vodila na obremenitev sistema. Od obremenitve je namreč odvisno koliko bo lahko algoritem vodenja znižal frekvenco oziroma zmanjšal število pulzov napajalne napetosti. Na podlagi te analize lahko izberemo tak nabor varilnih parametrov, pri katerih bo izkoristek sistema za UTV največji. Vrednosti varilnih parametrov lahko namreč tudi nekoliko spremenimo, pri tem pa se kvaliteta nastalega spoja ne spremeni.This doctoral dissertation describes a medium-frequency resistance spot welding system which is used for welding metal sheets. This system consists of a frequency converter, a transformer, a diode rectifier, a welding gun and metal sheets. Voltage pulses of different lengths can be generated by the frequency converter. This voltage is than used to supply the welding transformer. Consequently, different methods of generating these voltage pulses can be used. The most common method used in the industry is the pulse-width modulation, which generates voltage pulses with the selected modulation frequency. This frequency is usually constant and equal to the rated frequency of the transformer. However, the frequency of the generated voltage can be changed even during the welding process based on the needs of the welding process. The frequency can be changed automatically using the developed algorithm named Minimum switching cycle hysteresis control. This algorithm generates the supply voltage of the transformer with the lowest possible number of switching cycles of the frequency converter. Because the frequency of the supply voltage changes during the welding cycle, the number of switching cycles in a welding cycle is analysed. The change of the frequency affects the so-called dynamic losses, such as the switching losses of the frequency converter, the losses connected to the skin effect in the copper conductors of the transformer windings and the hysteresis losses of the iron core of the transformer. This is also confirmed in the case of the resistance spot welding system where the dynamic losses decrease at low load values where the supply voltage frequency can be reduced. The use of the presented Minimum switching cycle hysteresis control increases the load current ripple, which is the highest at the low load resistance values and low load current values. This can occur when welding metal sheets with low welding resistance values. For this case, the new algorithm is developed which halves the load current ripple by determining the maximum duration of the voltage pulse and dividing it into two pulses with equal length. The determination of the supply voltage pulse is done automatically by the algorithm at the first change of the magnetic flux density in the iron core from one saturation point to another. Both developed algorithms can be used on DC-DC converters which consists of a transformer. Additionally, the impact of the load current value, the load resistance value and the supply voltage level on the working point of the resistance spot welding system is analysed. Consequently, an optimal working point with the highest efficiency value can be selected from the welding range as the metal sheets can be welded at different welding parameters which still provide the same quality weld

LABORATORY PROTOTYPE OF A STATIC VAR COMPENSATOR

V diplomskem delu so predstavljene možnosti izvedbe trifaznega statičnega kompenzatorja. Predstavljena je zgradba trifaznega kompenzatorja skupaj z dimenzioniranjem izhodnega filtra. Regulacija je izvedena v sinhrono vrtečem koordinatnem sistemu d-q, pri čemer so uporabljeni PI regulatorji. Podan je tudi način določitve ojačanj regulatorja. Delovanje kompenzatorja je najprej predstavljeno s simulacijo v programskem paketu Matlab/Simulink. Nato smo delovanje preskusili še na laboratorijskem prototipu, ki smo ga s pomočjo digitalnega signalnega procesorja sinhronizirali z omrežjem.The possibilities of designing a static var compensator are presented in diploma thesis. Design of a three-phase static var compensator is described along with process of choosing components for prototype. The process of designing a passive LCL filter is described. Voltage oriented control with PI controllers is also described along with process of determining gains for controllers. The system is first tested in simulation in Matlab/Simulink. After confirmation the experimental system is presented. For the control of the prototype a digital signal processor was used

Power-Based Concept for Current Injection by Inverter-Interfaced Distributed Generations during Transmission-Network Faults

This paper analyzes the influence of inverter-interfaced distributed generations’ (IIDGs) response during transmission network faults. The simplest and safest solution is to switch IIDGs off during network faults without impacting the network voltages. A more elaborate and efficient concept, required by national grid codes, is based on controlling the IIDGs’ currents, involving positive- and negative-sequence voltage measured at the connection point. In this way the magnitude and phase of the injected currents can be adjusted, although the generated power will depend on the actual line voltages at the connection point. Therefore, an improved concept is proposed to adjust IIDGs’ fault current injection through the required active and reactive power, employing the same voltage characteristics. The proposed, i.e., power-based concept, is more definite than the current-based one, since the required power will always be generated. The discussed concepts for the fault current injection by IIDGs were tested in different 110-kV networks with loop and radial topologies, and for different short-circuit capabilities of the aggregated network supply. Based on extensive numerical calculations, the power-based concept during transmission networks faults generates more reactive power compared to the current-based concept. However, the voltage support by IIDGs during transmission networks faults, regardless of the concept being used, is influenced mainly by the short-circuit capability of the aggregated network supply. As regards distance protection operation, it is influenced additionally by the network topology, i.e., in radial network topology, the remote relay’s operation can be delayed due to a largely seen impedance