Modular multilevel converter losses model for HVdc applications

Multi-terminal high voltage dc (HVdc) grids can eventually became a feasible solution to transport energy to remote and/ or distant areas and its exploitation depend, among other things, on the performance of the converter terminals. Therefore, to optimize the power transmission strategy along such a grid, it is necessary to recognize the efficiency of all the converters in all points of operation, namely with the different load conditions. In this vision, the aim of this work is to provide the methodology to model the modular multilevel converter (MMC) efficiency by means of a mathematical expression that can describe, over a broad range of active and reactive power flow combinations, the power losses generated by the semiconductors. According to the presented methodology, a polynomial-based model with a reduced number of coefficients is deducted, in such a way that can be directly used for optimal power flow (OPF) studies. The accuracy of the proposed model is characterized by an absolute relative error, at the worst scenario, approximately equal to 3%.Postprint (author's final draft

Design, Modeling, Identification and Control of Mechatronic Systems

Les societats modernes plantegen nous reptes que demanden noves maneres de tractar els projectes d'enginyeria. Els enginyers han d'afrontar aquests reptes i desenvolupar solucions òptimes i eficients pels problemes clàssics i nous. Els diferents avenços produïts en la tecnologia hi poden ajudar, però una nova manera de tractar els problemes enginyerils és també necessària, no considerant únicament les diferent especialitats de l'enginyeria aïlladament. En aquest context, podem parlar de la creació d'una nova filosofia de fer enginyeria: la Mecatrònica. La Mecatrònica s'ha definit com l'aplicació de decisions complexes a l'operació de sistemes físics. La Mecatrònica ha estat també definida com la integració o sinergia de diferents disciplines de l'enginyeria. Aquestes disciplines inclouen l'enginyeria mecànica, l'enginyeria elèctrica, l'enginyeria electrònica, l'enginyeria de control, les comunicacions industrials i l'enginyeria de software. No obstant, la importància del concepte no està únicament en la definició sinó a la filosofia que hi ha al fons. És important de veure, que la Mecatrònica no és només la suma dels resultats de diferents disciplines, sinó la filosofia enginyeril per afrontar els problemes com un de sol, fent servir les eines que subministren les diferents disciplines. La present tesi es divideix en dos parts que tracten problemes de diferent natura. La primera part es titula Regles de disseny i modelat d'actuadors per l'optimització de sistemes mecatrònics i es centra en proporcionar un anàlisis detallat de diferents actuadors utilitzant un procediment general, orientat a millorar el disseny de l'actuador. S'introdueix una nova metodologia per analitzar actuadors lineals electromagnètics i hidràulics modelitzant la seves magnituds mecàniques (força, treball i desplaçament) com a funcions de la geometria i les propietats del material, estudiant l'escalabilitat (en el sentit de produir la mateixa distribució de tensions i deformacions per diferents mides). La motivació de realitzar aquest treball neix de la necessitat d'estructures i sistemes lleugers i de volum reduït, que han de ser integrats en el procediment de disseny el més aviat possible. Per tant, s'estudien les relacions geomètriques, les proporcions i les propietats dels materials que maximitzen les magnituds mecàniques de sortida dels actuadors per un volum i pes limitat, així com l'escabilitat per la seva posterior integració en estructures. Els resultats són validats mitjançant anàlisi dimensional de les expressions obtingudes i comparant els resultats numèrics amb dades d'actuadors industrials. Es considera el comportament estàtic de diferents classes d'actuadors, incloent actuadors hidràulics i electromagnètics lineals. La segona part es titula Identificació i Control d'Actuadors Piezoelèctrics. Els actuadors piezoelèctrics estan demostrant ser una solució fiable per moltes aplicacions, des del microposicionament (màquines eina, dispositius òptics o microscopis moderns) al control actiu d'estructures. La principal motivació de la segona part de la tesis és aprofundir en models per representar el comportament histerètic dels actuadors piezoelèctrics per posteriorment aplicar els models al desenvolupament de controladors pels sistemes histerètics. Es desenvolupa inicialment un estudi general dels actuadors piezoelèctrics per després tractar la seva modelització. Degut a l'elevat comportament no-lineal observat es necessita un model d'histèresis. El model de Bouc-Wen ha estat escollit i s'ha treballat en la identificació dels paràmetres del model i la seva validació. Un cop el sistema ha estat modelat, s'olinebreak ha dissenyat un nou controlador lineal i s'ha implementat en una plataforma experimental utilitzant un DSP (Processador digital de senyal). Encara que les dues parts estan clarament diferenciades, la unitat de la tesis pot ser trobada a les arrels de la Mecatrònica. La tesis tracta la resposta a una sola pregunta: Com podem millorar un sistema mecatrònic? No obstant, la resposta és doble: Dissenyant i modelant actuadors òptims considerant el sistema o estructura complet i controlant el sistema adequadament fent servir els models desenvolupats.Fast changing societies come up with new challenges that require new engineering approaches. Engineers have to face such challenges and provide novel and more efficient solutions to classical and new problems. This can be done by using different relevant advances produced in technology. Furthermore, a new way of addressing the engineering problems has to be applied, not considering only isolated engineering specialties. In this frame, we can talk about the creation of a new engineering philosophy: Mechatronics. Mechatronics has been defined as the application of complex decision making to the operation of physical systems. Mechatronics has been also defined as the integration or synergy of different engineering disciplines. Such disciplines may include Mechanical Engineering, Electrical Engineering, Electronics Engineering, Control Engineering, Industrial Communications and Software Engineering. Nevertheless, the importance of the concept does not lie only in the definition but in the philosophy behind it. It is very important to note that Mechatronics is not only the sum of the results of the different disciplines, but the engineering philosophy to face engineering problems as a whole, employing the tools provided by the different disciplines. The present thesis has been divided in two parts that deal with problems of different nature. The first part is titled Design Rules and Actuator Modeling for the Optimization of Mechatronic Systems and focuses on providing the detailed analysis of different actuators using a general procedure and oriented towards improving the actuator design. It introduces a new methodology to analyze linear electromagnetical and hydraulic actuators by modeling their maximum output mechanical quantities (force, work and stroke) as functions of the geometry and material properties and discusses the scalability (in the sense of producing the same stress and strain distribution for different sizes). The motivation to undertake such a work stems from the need for light and volume reduced structures and systems, which are to be integrated in the design procedure as early as possible. Hence, the geometric relationships, aspect ratios and material properties that maximize the actuator output quantities with a certain limited volume or weight, along with their scalability for the integration in structures are studied. A validation of the results is done by performing dimensional analysis of the expressions obtained and comparing numerical results with industrial actuator data. The static behavior of different classes of actuators is considered. Such actuators include linear hydraulic and electromagnetic actuators. The second part is titled Identification and Control of Piezoelectric Actuators}. Piezoelectric actuators are proving to be a reliable solution for many engineering applications, ranging from micro-positioning (machine tools, optic devices or modern microscopes) to active control of structures. The main motivation of this thesis part is to delve into models to represent the hystereticbehavior of piezoelectric actuators in order to apply them to the conception of controllers for such hysteretic systems. A general study of piezoelectric actuators is performed to later deal with the modeling of such actuators. Due to the high non-linear behavior observed, the problem of identifying and modeling the actuator requires a hysteresis model. The Bouc-Wen model has been chosen and investigations to identify its parameters and to validate the model have been undertaken. Once the system has been identified, a linear controller has been designed and implemented in a real platform, employing a DSP (Digital Signal Processor). Although the two parts are clearly differentiated, the thesis unity can be found in the roots of Mechatronics. The thesis deals with the response to one single question: How can we improve a mechatronic system? However, the answer is twofold: By designing and modeling optimum actuators taking into account the whole system or structure and by controlling the system appropriately using developed models.Postprint (published version

Laboratoris remots i virtuals per assignatures de l'àmbit de l''enginyeria elèctrica

Peer Reviewe

Control of multi-terminal HVDC networks towards wind power integration: A review

© 2015 Elsevier Ltd. More interconnections among countries and synchronous areas are foreseen in order to fulfil the EU 2050 target on the renewable generation share. One proposal to accomplish this challenging objective is the development of the so-called European SuperGrid. Multi-terminal HVDC networks are emerging as the most promising technologies to develop such a concept. Moreover, multi-terminal HVDC grids are based on highly controllable devices, which may allow not only transmitting power, but also supporting the AC grids to ensure a secure and stable operation. This paper aims to present an overview of different control schemes for multi-terminal HVDC grids, including the control of the power converters and the controls for power sharing and the provision of ancillary services. This paper also analyses the proposed modifications of the existing control schemes to manage high participation shares of wind power generation in multi-terminal grids.Postprint (author's final draft

Control of doubly fed induction generators under balanced and unbalanced voltage conditions

The present chapter presents a control technique to deal with the control of doubly fed induction generators under different voltage disturbances. Certain current reference values are chosen in the positive and negative sequences so that the torque and theDCvoltage are kept stable during balanced and unbalanced conditions. Both rotor-side and grid-side converters are considered, detailing the control scheme of each converter while considering the effect of the crow-bar protection. The control strategy is validated by means of simulations.Postprint (published version

Short circuit analysis of an offshore AC network having multiple grid forming VSC-HVDC links

This article presents the short circuit analysis of an offshore AC network which consists of wind power plants interconnected using HVAC cables. The power generated in the offshore AC network is transmitted to several onshore grids using VSC-HVDC system. The offshore AC network is formed by the VSC-HVDC systems using frequency and voltage droop control. A coordinated control scheme is proposed for wind turbines and offshore VSCs during short circuit conditions in the offshore grid to ensure fault ride through (FRT) without compromising the system stability. The theoretical analysis used for developing this control scheme allows to calculate the system limits taking into consideration the active and reactive power capability. In order to verify the proposed control scheme, three phase symmetric faults have been applied on a wind turbine busbar, HVAC busbar, and at the AC cable that interconnects the VSC-HVDC system. Additionally, a frequency coordination control scheme without communication between wind power generation and VSC-HVDC system has been proposed. The methodology and control system have been validated by performing a nonlinear simulation.Postprint (author's final draft

Analysis of reactive power strategies in HVDC-connected wind power plant clusters

© 2017 John Wiley & Sons, Ltd. Offshore wind power plants (WPPs) built near each other but far from shore usually connect to the main grid by a common high-voltage DC (HVDC) transmission system. In the resulting decoupled offshore grid, the wind turbine converters and the high-voltage DC voltage-source converter share the ability to inject or absorb reactive power. The overall reactive power control dispatch influences the power flows in the grid and hence the associated power losses. This paper evaluates the respective power losses in HVDC-connected WPP clusters when applying 5 different reactive power control strategies. The case study is made for a 1.2-GW-rated cluster comprising 3 WPP and is implemented in a combined load flow and converter loss model. A large set of feasible operating points for the system is analyzed for each strategy. The results show that a selection of simulations with equal wind speeds is sufficient for the annual energy production comparison. It is found that the continuous operation of the WPPs with unity power factor has a superior performance with low communication requirements compared with the other conventional strategies. The optimization-based strategy, which is developed in this article, allows a further reduction of losses mainly because of the higher offshore grid voltage level imposed by the high-voltage DC voltage-source converter. Reactive power control in HVDC-connected WPP clusters change significantly the overall power losses of the system, which depend rather on the total sum of the injected active power than on the variance of wind speeds inside the cluster.Postprint (author's final draft

Reactive power management in Wind Farms using PSO technique

This paper presents a particle swarm optimization (PSO) technique for a reactive power wind farm (WF)dispatch function, in order to calculate the reactive power reference for each wind turbine (WT). The dispatch can be formulated as the problem of minimize the di erence in power interchange at interconnection point (PCC). Incorporation of PSO as a optimization technique for the WF dispatch make possible consider di erent parameters to improve it performance and give it more capabilities.Postprint (published version

Optimal control of VSC for STATCOM applications

Postprint (published version

Operation and control of a current source converter series tapping of an LCC-HVDC link for integration of offshore wind power plants

This work presents a series tapping station for integrating Offshore Wind Power Plants (OWPP) into a (Line Commutated Converter High Voltage Direct Current) LCC-HVDC transmission system. The tapping station allows to integrate wind power resources without building a new HVDC link and it is based on a Current Source Converter (CSC). However, the CSC requires a minimum DC current to extract the power coming from the OWPP which may not be guaranteed depending on the power conditions of the HVDC corridor. For this reason, this paper proposes a coordinated operation and control of the CSC and the OWPP. A steady-state analysis is performed to determine the appropriate AC voltage level of the CSC. A power reduction algorithm is presented to limit power extraction during a reduction in the current of the HVDC transmission system and under loss of communications between the CSC and the OWPP. The proposed algorithm and the performance of the system are validated through simulation results.Peer ReviewedPostprint (author's final draft