<p> Continuous Casting (CC) is an essential process in the steel industry to transform molten steel into solid product. This process begins with primary cooling (PC) where the molten steel is cooled, and the initial solidification begins. It is important to monitor the process of PC as defects such as thinning of the shell in the mold can lead to breakouts. Key parameters in PC are the mold design, casting condition, and steel composition. In the research conducted, key parameters for PC are investigated to analyze the impact on flow formation and solidification. To optimize mold design, angular taper to the narrow face can be employed to accommodate for any shell shrinkage. Utilizing computational fluid dynamics, a range of mold taper is simulated per the developed solidification model with defined temperature-dependent material properties. When simulated without a taper, significant air gap formation in the corners of the mold is visible due to thermal shrinkage of the shell. This air gap decreases the cooling rate due to the shell’s lack of contact with the cooling mold wall. A parametric study of mold taper ranging from no taper to 3° as well as change in casting conditions, superheat and casting speed, are conducted to analyze the impact of taper with respect to the casting conditions. Per the conditions applied, angular taper between 1° and 2° resulted into reduction of undercooling and overcooling in the corner of the mold which is subjected to cooling from the broad face and narrow face of the cool mold wall. The turbulent flow in the mold region was found to drastically influences the quality of steel produced during continuous casting. The flow itself can lead to surface defects or slag entrainment based on the formation. A high surface wave due to turbulence of the injected melt lead to fluctuations and the instability compromised the quality of the steel produced as well as entrained the slag. To regulate the flow, electromagnetic forces can be applied in the mold, dampening the local turbulent flow. As the electrically conductive molten steel interacts with the induced magnetic field, it reduced the velocity of the steel jet released from the ports of the submerged entry nozzle. Per the simulation-based study conducted increasing the EMBr strength from 2975G to 4350G reduced the peak surface wave height by 59.47% and volume of flux rate of decrease by 4.25%. Additionally, increasing the SEN depth from 110 mm to 350 mm increased the average wave height by 19% and volume of flux rate of decrease by 2.6%. Lastly, increasing the mold width from 1.067 m to 1.50m increased average wave height by 8.71% and volume of flux rate of decrease by 0.9%. </p>

Thesis (MScEng)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: The slip synchronous-permanent magnet generator (SS-PMG) is a direct-driven, direct-to-grid generator for wind turbine applications. This investigation focuses on achieving automated grid connection and low voltage ride-through for a small-scale SS-PMG. To reduce cost and complexity, components such as blade pitch controllers and frequency converters are avoided. Instead, electromagnetic braking is employed to control turbine speed prior to grid synchronisation and compensation resistances are used to facilitate grid fault ride-through. The conditions under which the SS-PMG can be successfully synchronised with the grid are determined, indicating a need for speed control. An evaluation of electromagnetic braking strategies reveals that satisfactory speed control performance can be achieved when employing back-to-back thyristors to switch in the braking load. Simulations show that controlled synchronisation can be executed successfully under turbulent wind conditions. All controllable parameters are held within safe limits, but the SS-PMG terminal voltage drop is higher than desired. Compensation is developed to allow the SS-PMG to ride through the voltage dip profile specified by the Irish distribution code. It is found that a combination of series and shunt resistances is necessary to shield the SS-PMG from the voltage dip, while balancing active power transfer. The flexibility offered by thyristor switching of the shunt braking load is instrumental in coping with turbulent wind conditions and unbalanced dips. The South African voltage dip profile is also managed with conditional success. Following on from the theoretical design, the grid connection controller is implemented for practical testing purposes. Protection functions are developed to ensure safe operation under various contingencies. Before testing, problems with the operation of the thyristors are overcome. Practical testing shows that grid synchronisation can be undertaken safely by obeying the theoretically determined conditions. The speed control mechanism is also shown to achieve acceptable dynamic performance. Finally, the SS-PMG is incorporated into a functioning wind turbine system and automated grid connection is demonstrated under turbulent wind conditions. Future investigations may be focused on optimal control strategies, alternative solid-state switching schemes, and reactive power control. Low voltage ride-through should also be optimised for the South African dip profile and validated experimentally. / AFRIKAANSE OPSOMMING: Die glip-sinchroon permanente magneet generator (GS-PMG) is ‘n direkte dryf, direkte netwerkgekoppelde generator vir windturbine toepassings. Hierdie ondersoek fokus op die bereiking van ’n ge-outomatiseerde netwerkkoppeling en lae spanning deurry vir ‘n kleinskaalse GS-PMG. Om kostes en kompleksiteit te verminder, word komponente soos lemsteekbeheerders en frekwensie-omsetters vermy. In plaas daarvan word elektromagnetiese remwerking gebruik om die turbine spoed, voorgaande net-werksinchronisasie, te beheer, en word kompensasieweerstande gebruik om netwerkfoutdeurry te handhaaf. Die omstandighede waaronder die GS-PMG suksesvol met die netwerk gesinchroniseer kan word, is vasgestel en dit het die behoefte aan spoedbeheer uitgewys. ‘n Evaluering van elektromagnetiese remstrategië wys uit dat ’n bevredigende spoedbeheervermoë verkry kan word as anti-parallelle tiristors gebruik word om die remlas te skakel. Simulasies wys dat beheerde netwerksinchronisasie suksesvol uitgevoer kan word, selfs onder turbulente windtoestande. Alle beheerbare parameters is binne veilige perke gehou, maar die GS-PMG se klemspanningsval is gevind as hoë as verwag. Kompensasie is ontwikkel om die GS-PMG toe te laat om deur die spanningsvalprofiel, soos gespesifieer deur die Ierse distribusiekode, te ry. Dit is gevind dat ‘n kombinasie van serie- en parallelle weerstande nodig is om die GS-PMG teen die spanningsval te beskerm, terwyl aktiewe drywingsoordrag gebalanseer word. Die buigbaarheid wat verkry word met die tiristorskakeling van die parallele weerstand is noodsaaklik in die hanteering van turbulente windtoestande en ongebalanseerde spanningsvalle. Die Suid-Afrikaanse spanningsvalprofiel is ook met voorwaardelike sukses hanteer. In opvolg van die teoretiese ontwerp is die netwerkkoppelingsbeheerder vir praktiese toetsdoeleindes in werking gestel. Beskermingsfunksies is ontwikkel om veilige werking onder verskeie gebeurlikhede te verseker. Die probleme met die werking van die tiristors is oorkom voor die aanvang van die toetse. Die praktiese toetse bewys dat netwerksinchronisasie veilig gedoen kan word deur die teoretiese bepaalde voorwaardes te volg. Dit is ook getoon dat met die spoedbeheermeganisme aanvaarbare dinamiese gedrag verkry kan word. Ten laaste is die GS-PMG in ‘n werkende windturbinestelsel geïnkorporeer en outomatiese netwerkkoppeling is onder turbulente windtoestande gedemonstreer. Toekomstige ondersoeke kan toegespits word op optimale beheerstrategië, alternatiewe vaste toestand skakelingskemas en reaktiewe drywingsbeheer. Lae spanning deurry moet nog vir die Suid- Afrikaanse spanningsprofiel ge-optimeer en eksperimenteel bevestig word.