With the desire for conventional coal-fired power plants to perform flexible operations, the impact of this operation has become important to the field of steam turbine modelling. This study sought to develop a computationally inexpensive turbine model with minimal OEM intervention in order to predict the internal clearances of high-pressure and intermediate-pressure turbines from Eskom's current turbine fleet. The study saw the utilisation of the Nozzle Analogy theory to develop a 1D multistage turbine thermofluid model as well as the development of a representative 1D turbine process model in order to predict the internal temperature gradients promoted within a steam turbine during transient operation. From this model a further 3D FEA turbine model of both the HP and IP turbine units were developed from simple turbine diagrams to apply the predicted temperature boundaries and predict the thermal and structural response of turbine components during transient loading during a full Cold Start procedure. The result of this study was the successful validation of the 1D and 3D Turbine models against plant data from the candidate unit. This was in the form of known process data of unit performance, as well as thermocouple and differential expansion data taken from sensors housed on the turbine unit itself. Through the validation of these parameters, various calibrations techniques were developed over the course of the study with these techniques allowing investigators to gain insight into turbine aging, operator intervention as well as brought turbine component response. The successful establishment of the paired turbine model allowed investigators to evaluate the cold clearances defined during construction and maintenance of these turbine units in industry, which contributes greatly to the availability and efficiency of the unit during these transient operations. Additionally, the establishment of this model allowed for the investigation of the role that start up speed has on turbine component response. This study demonstrated that the development of such a modelling methodology was possible and yielded results with were accurate and insightful in understanding turbine component responses which are otherwise impossible to measure during real-world operation.

The trend of future wind farms moving further offshore requires much higher reliability for each wind turbine in order to reduce maintenance cost. The drive-train system and power electronic converter system have been identified as critical sub-assemblies that are subject to higher failure rates than the other sub-assemblies in a wind turbine. Modern condition monitoring techniques may help schedule the maintenance and reduce downtime. However, when it comes to offshore wind turbines, it is more crucial to reduce the failure rates (or reduce the stresses) for the wind turbines during operation since the harsh weather and a frequently inaccessible environment will dramatically reduce their availability once a failure happens. This research examines the mechanical, electrical and thermal stresses in the sub-assemblies of a doubly-fed induction generator (DFIG) wind turbine and how to reduce them by improved control strategies. The DFIG control system (the rotor-side and the grid-side converter control) as well as the wind turbine control system are well established. The interactions of these control systems have been investigated. This research examines several further strategies to reduce the mechanical and electrical stresses. The control system's coordination with the protection schemes (crowbar and dc-chopper) during a grid fault is presented as well. An electro-thermal model of the power converter has been developed to integrate with the DFIG wind turbine model, for the evaluation of the thermal stresses under different operating states and control schemes. The main contributions of this thesis are twofold. A first contribution is made by providing all the control loops with well-tuned controllers in a more integrated methodology. The dynamics of these controllers are determined from their mathematical models to minimize the interference between different control-loops and also to reduce the electrical transients. This thesis proposes a coordination strategy for the damping control, pitch control and crowbar protection which significantly reduces the mechanical oscillations. On the other hand, an integrated model of the wind turbine and converter electro-thermal system is established that can illustrate the performance integration with different control strategies.

Thesis (PhD (Mechanical and Mechatronic Engineering))--Stellenbosch University, 2008. / ENGLISH ABSTRACT: The power conversion unit of a large solar chimney power plant converts the fluid power, first into mechanical power, and then into electrical power. In this dissertation a tool is developed to determine the layout and the number of turbines of the solar chimney power conversion unit providing the lowest cost of electricity. First, the history of the solar chimney concept and the related fields of research are presented. Potential features and configurations of the power conversion unit are introduced, and it is shown how the solar chimney power conversion unit compares to those of other applications. An outline of the dissertation is given, and its potential impact is discussed. An analytical turbine model is developed. Several modelling approaches and the performance of single rotor and counter rotating turbine layouts are compared. Preliminary turbine designs are investigated, experimentally and numerically. The main aim of the experimental investigation is to verify the applicability of the loss model used in the analytical turbine model. The aim of the numerical investigation is to evaluate a commercial software package as a tool in context with solar chimney turbines. For each component of the power conversion unit an analytical performance model is introduced. Using these models, the single vertical axis, multiple vertical axis and multiple horizontal axis turbine configurations are compared from an efficiency and energy yield point of view, and the impact of the various losses on the overall performance is highlighted. A detailed cost model for the power conversion unit is also presented. To optimize for cost of electricity this cost model is then linked to the performance models, and the resulting optimization scheme is applied to several plant configurations. It is shown that for a large solar chimney power plant the power conversion unit providing minimal cost of electricity consists of multiple horizontal axis turbines using a single rotor layout including inlet guide vanes. / AFRIKAANSE OPSOMMING: Die drywingsomsettingseenheid van ’n groot sonskoorsteenaanleg sit die vloeidrywing om, eers in meganiese drywing en dan in elektriese drywing. In hierdie proefskrif word ’n gereedskapstuk ontwikkel om die uitleg en aantal turbines van die sonskoorsteen-drywingsomsettingseenheid te bepaal wat die laagste koste van elektrisiteit lewer. Eerstens word die geskiedenis van die sonskoorsteen en verwante navorsingsvelde behandel. Moontlike eienskappe en konfigurasies vir die drywingsomsettingseenheid word voorgestel, en daar word aangetoon hoe die sonskoorsteendrywingsomsettings- eenheid vergelyk met ander toepassings. ’n Raamwerk van die proefskrif word gegee, en die potensiële trefkrag daarvan word bespreek. ’n Analitiese turbine-model word ontwikkel. Verskeie nabootsingsbenaderings en die vertoning van ’n enkelrotor en teenroterende turbine-uitlegte word vergelyk. Voorlopige turbine-ontwerpe word ondersoek, eksperimenteel en numeries. Die hoofdoel van die eksperimentele ondersoek is om die toepaslikheid van die verliesmodel in die analitiese turbine-model te bevestig. Die doel van die numeriese ondersoek is om kommersiële sagteware op te weeg as ’n gereedskapstuk in die konteks van sonskoorsteenturbines. Vir elke onderdeel van die drywingsomsettingseenheid word ’n analitiese model voorgestel. Met gebruik van hierdie modelle word die enkele vertikale-as, die veelvoudige vertikale-as an die veelvoudige horisontale-as turbinekonfigurasies vergelyk vanuit ’n benuttingsgraad- en energie-opbrengsoogpunt,en die uitwerking van die verskillende verliese op die algehele gedrag word uitgewys. ’n Kostemodel in besonderhede word vir die drywingsomsettingseenheid aangebied. Om vir die koste van elektrisiteit te optimeer word hierdie kostemodel dan gekoppel aan die vertoningsmodelle, en die gevolglike optimeringskema word toegepas op verskeie aanlegkonfigurasies. Daar word aangetoon dat vir ’n groot sonskoorsteenaanleg die drywingsomsettingseenheid wat die minimumkoste van elektrisiteit gee, bestaan uit veelvoudige horisontale-as turbines met enkelrotoruitleg en inlaatleilemme. / Centre for Renewable and Sustainable Energy Studies

The increasing use of turbochargers is leading to an outstanding research to understand the internal flow in turbomachines. In this frame, computational fluid dynamics (CFD) is one of the tools that can be applied to contribute to the analysis of the fluid-dynamic processes occurring in a turbine. The objective of this thesis is the development of a methodology for performing simulations of radial turbomachinery optimizing the available computational resources. This methodology is used for the characterization of a vaned-nozzle turbine under steady and pulsating flow conditions. An important effort has been devoted in adjusting the case configuration to maximize the accuracy achievable with a certain computational cost. Concerning the cell size, a local mesh independence analysis is proposed as a procedure to optimize the distribution of cells in the domain, thus allowing to use a finer mesh in the most suitable places. Particularly important in turbomachinery simulations is the influence of the approach for simulating rotor motion. In this thesis two models have been compared: multiple reference frame and sliding mesh. The differences obtained using both methods were found to be significant in off-design regions. Steady flow CFD results have been validated against global measurements taken on a gas-stand. The modeling of a turbine, installed either on a turbocharger test rig or an engine, requires the calculation of the flow in the ducts composing the system. Those ducts could be simulated assuming a one-dimensional (1D) approximation, and thus reducing the computational cost. In this frame of ideas, two CFD boundary conditions have been developed. The first one allows performing coupled 1D-3D simulations, communicating the flow variables from each domain through the boundary. The second boundary condition is based in a new formulation for a stand-alone anechoic end, which intends to represent the flow behavior of an infinite duct. Finally, the turbine was simulat / Fajardo Peña, P. (2012). Methodology for the Numerical Characterization of a Radial Turbine under Steady and Pulsating Flow [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/16878 / Palancia

[ES] Este trabajo está centrado en analizar el flujo y la eficiencia de turbinas de doble entrada, así como desarrollar modelos de capacidad de flujo y eficiencia que sean capaces de predecir su comportamiento en condiciones de admisión desiguales. Dichas condiciones son las más comunes en funcionamiento real, por lo que deben ser evaluadas adecuadamente. Se ha realizado un análisis profundo de los patrones de flujo y las principales fuentes de pérdidas mediante simulaciones CFD y campañas experimentales, identificando y cuantificando los fenómenos más importantes en distintas condiciones de admisión. El análisis CFD y la campaña experimental con la técnica LDA han mostrado que el flujo de cada rama no se mezcla completamente con el otro dentro del rotor. Esto significa que las turbinas de doble entrada podrían estudiarse como dos turbinas de entrada simple trabajando en paralelo en modelos unidimensionales. Además, las áreas de entrada y salida del rotor correspondientes a cada rama dependen linealmente de la relación de gastos másicos (MFR). Los principales fenómenos de pérdidas han sido identificados. Fenómenos ya conocidos como las pérdidas por fricción en las volutas, interespacio y rotor, las pérdidas por incidencia o las pérdidas en punta de álabe se han cuantificado. Sin embargo, se han encontrado fuentes de pérdidas adicionales que ayudan a explicar el comportamiento en condiciones de admisión desiguales. Se ha encontrado una expansión brusca aguas abajo de la unión de las volutas que produce pérdidas en la rama con más presión. Aunque el flujo de cada rama no se mezcla completamente dentro del rotor, hay un intercambio de momento entre ramas producido en la región de contacto entre ramas. La rama con mayor momento transmite parte de este a la rama con menor momento. Este fenómeno produce pérdidas en la rama con mayor momento en el interespacio y el rotor, pero también produce ganancias en la rama con menor momento. Este intercambio de momento es un fenómeno esencial para entender correctamente el funcionamiento de las turbinas de doble entrada en condiciones de admisión desiguales. Finalmente, como la mezcla completa de los flujos de cada rama se produce en la región de salida, es aquí donde se computan las pérdidas por mezcla. Toda esta información se ha usado para desarrollar modelos de área efectiva y eficiencia. El modelo de área efectiva se utiliza para extrapolar en el mapa de capacidad flujo. Este modelo se ha validado con medidas experimentales. Su capacidad de extrapolación hacia otros MFR se ha demostrado fidedigna, obteniendo un error menor del 3% en cada rama cuando solo se proporcionan al modelo los mapas de condiciones de admisión completa y parcial. El modelo de eficiencia se utiliza para extrapolar en el mapa de eficiencia. Este modelo también se ha validado con medidas experimentales. Su capacidad de extrapolación hacia otros valores de MFR también se ha demostrado fidedigna, obteniendo un error combinado de las dos ramas menor del 7%. Además, las predicciones que ofrece se han comparado con modelos empíricos y comerciales, obteniendo predicciones más precisas en condiciones de admisión desiguales. Como estas condiciones son las más comunes en funcionamiento real, el comportamiento estará mejor predicho la mayor parte del tiempo de operación. Esta mejora en las predicciones de las prestaciones puede ayudar a trabajar en condiciones de operación óptimas, lo que puede significar una eficiencia del motor de combustión interna mayor y su correspondiente reducción en consumo de combustible y emisión de gases contaminantes. Adicionalmente, otra turbina de doble entrada con una geometría distinta se ha analizado, encontrado un comportamiento muy similar. Los modelos desarrollados se han aplicado a esta geometría con buenos resultados, corroborando que dichos modelos proporcionan una descripción física razonable del comportamiento de las turbinas de doble entrada bajo condiciones de admisión desiguales. / [CA] El present treball està centrat en analitzar el flux i l'eficiència de turbines de doble entrada, així com desenvolupar models de capacitat de flux i eficiència que siguen capaços de predir el seu comportament en condicions d'admissió desiguals. Aquestes condicions són les més comunes en funcionament real, per la qual cosa s'han d'avaluar adequadament. S'ha realitzat una anàlisi profunda dels patrons de flux i de les principals fonts de pèrdues mitjançant simulacions CFD i campanyes experimentals, identificant i quantificant els fenòmens més importants en distintes condicions d'admissió. L'ànalisi CFD i la campanya experimental amb la tècnica LDA han mostrat que el flux de cada rama no es mescla completament amb l'altre dins del rotor. Açò significa que les turbines de doble entrada podrien estudiar-se com dues turbines d'entrada simple treballant en paral·lel en models unidimensionals. A més, les àrees d'entrada i eixida del rotor corresponents a cada rama depenen linealment de la relació de gastos màssics (MFR). Els principals fenòmens de pèrdues han estat identificats. Fenòmens ja coneguts com les pèrdues per fricció en les volutes, interespai i rotor, les pèrdues per incidència o les pèrdues en punta de pala s'han quantificat. Tanmateix, s'han trobat fonts de pèrdues addicionals que ajuden a explicar el comportament en condicions d'admissió desiguals. S'ha trobat una expansió brusca aigües avall de la unió de les volutes que produeixen pèrdues en la rama amb més pressió. Encara que el flux de cada rama no es mescla completament dins del rotor, hi ha un intercanvi de moment entre rames produit en la regió de contacte entre rames. La rama amb més moment transmet part d'aquest a la rama amb menor moment. Aquest fenomen produeix pèrdues en la rama amb major moment en l'interespai i el rotor, però també produeix guanys en la rama amb menor moment. Aquest intercanvi de moment entre rames es un fenomen essencial per a entendre correctament el funcionament de les turbines radials de doble entrada en condicions d'admissió desiguals. Finalment, com la mescla completa dels fluxos de cada rama es produeix en la regió d'eixida, és en aquesta regió on es computen les pèrdues per mescla. Tota aquesta informació s'ha utilitzat per desenvolupar models d'àrea efectiva i eficiencia. El model d'àrea efectiva s'utilitza per a extrapolar en el mapa de capacitat de flux. Aquest model s'ha validat amb mesures experimentals. La seua capacitat d'extrapolació cap a altres condicions d'admissió s'ha demostrat fidedigna, obtenint un error menor del 3% en cada rama quan sols es proporciona al model els mapes de condicions d'admissió completa i parcial. El model d'eficiència s'utilitza per a extrapolar en el mapa d'eficiència. Aquest model també s'ha validat amb mesures experimentals. La seua capacitat d'extrapolació cap a altres valors d'MFR també s'ha demostrat fidedigna, obtenint un error combinat de les dues rames menor del 7%. A més, les prediccions que ofereixen els nous models basats en pèrdues han estat comparades amb models empírics i comercials, aconseguint prediccions més precises en condicions d'admissió desiguals. Com les condicions d'admissió desiguals són les més comunes en funcionament real, el comportament de les turbines de doble entrada estaran millor predites la major part del temps d'operació. Aquesta millora en les prediccions de les prestacions pot ajudar a treballar en condicions d'operació òptimes la major part del temps, el qual pot significar una major eficiència del motor de combustió interna i la seua corresponent reducció en consum de combustible i emissió de gasos contaminants. Addicionalment, una altra turbina de doble entrada amb una geometria considerablement diferent s'ha analitzat, trobant un comportament molt similar. Els models proporcionen una descripció física raonable del comportament de les turbines de doble entrada baix condicions d'admissió desiguals. / [EN] The current work focuses on the flow capacity and efficiency analysis and modelling of twin-entry radial turbines under unequal admission conditions. These conditions are the most common in real operation, so they must be properly assessed. A thorough analysis of the flow patterns within twin-entry turbines and the main sources of losses have been carried out by means of computational fluid dynamics (CFD) simulations and experimental campaigns, identifying and quantifying the most important phenomena under different admission conditions. The CFD analysis and the laser Doppler anemometry experimental campaign have shown that the flow from each branch does not fully mix within the rotor. It means that twin-entry turbines could be studied as two single-entry turbines working in parallel in one-dimensional models. Moreover, the rotor inlet and outlet areas corresponding to each branch depend linearly on the mass flow ratio (MFR). The main phenomena producing losses in twin-entry turbines have been identified. Well-known sources of losses have been quantified, such as passage losses in volutes, interspace and rotor, incidence losses or tip leakage losses. However, additional sources of losses have been found that explain the behaviour of twin-entry turbines under unequal admission conditions. There is a sudden expansion downstream of the junction of the volutes that produces losses in the branch with higher pressure. Although the flow from each branch does not fully mix within the rotor, there is a momentum exchange between branches produced in the contact region between branches. The branch with higher momentum transmits some of it to the branch with lower momentum. This phenomenon produces losses in the branch with higher momentum within the interspace and the rotor, but it also produces gains in the branch with lower momentum. This momentum exchange between branches is an essential phenomenon to properly understand the behaviour of twin-entry turbines under unequal admission conditions. Finally, since the full mixing of both flows is produced in the outlet region, the mixing losses are only computed in the outlet region. The flow behaviour information extracted from the CFD simulations and experimental campaigns has been used to develop effective area and efficiency models. The effective area model is used to extrapolate the flow capacity map. The model has been validated with experimental data. Its capability of extrapolating towards other MFR values has been proven, obtaining an error lower than 3% in each branch when only partial and full admission maps are provided to feed the model. The efficiency model is used to extrapolate the efficiency map. This model has also been validated with experimental data. Its capability of extrapolating towards other MFR values is also reliable, obtaining a combined error between both branches lower than 7%. Moreover, the predictions of this loss-based efficiency model have been compared to empirical and commercial models, obtaining more accurate predictions under unequal admission conditions. Since unequal admission conditions are the most common in real operation, the performance of twin-entry turbines could be better predicted most of the time. This improvement in the performance prediction could help to work in optimum operational points most of the time, which could lead to higher internal combustion engine efficiency and a reduction in fuel consumption and pollutant emissions. Additionally, a twin-entry turbine with a considerably different geometry has been analysed, finding the same flow behaviour. The models developed have been applied to this geometry, giving good results. These results corroborate that these models provide a reasonable physical description of the behaviour of the twin-entry turbines under unequal admission conditions. / The author would like to acknowledge the financial support received through contract FPU17/02803 of the Programa de Formación de Profesorado Universitario of Spanish Ministerio de Ciencia, Innovación y Universidades. / Medina Tomás, N. (2022). Flow Capacity and Efficiency Modelling of Twin-Entry Radial Turbines under Unequal Admission Conditions through CFD Analysis and Experiments [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/185820 / TESIS

The popularity of distributed generating (DG) sources have been increasing over the past few years. With the increasing penetration of these DGs, the concept of micro grid is becoming popular. A micro grid is a small power system network with distributed generating sources which can operate seamlessly irrespective of the presence of the utility grid. Operating the micro grid in this manner increases system reliability and reduces power interruptions. However, it introduces several control challenges. This thesis aims at analysing the behaviour of a micro grid system during the transition between grid connected mode and islanded mode of operation and address the control challenges through novel schemes. With the presence of grid, the micro grid system variables, such as voltage and frequency, are strictly regulated by the grid. The local sources follow the voltage and frequency reference set by the grid and supply constant power. With the loss of grid, that is when the system is islanded, the network variables need to be regulated by the local sources. The control structures for the inverter-based sources during the two operating modes are detailed in the present work. With the loss of grid, the system should be able to transfer seamlessly to islanded mode without any transients. Similarly, when the grid supply is restored, the micro grid should seamlessly resynchronize to the grid without any transients. This thesis proposes two novel controller schemes for achieving seamless transfer between grid-connected and islanded mode in micro grids. The rst scheme uses an output feedback topology to reduce the transitions during mode transfer. The second scheme uses a Linear Quadratic Regulator (LQR) theory based compensator to achieve seamless transfer. The performance of the proposed schemes have been validated through simulations on a benchmark micro grid network for various operating conditions. An experimental micro grid set-up is developed with a single inverter based DG source. The droop control scheme for islanded mode of operation has been validated on hardware.