Academic literature on the topic 'Hybrid AC/DC Grid'

The proposals for large-scale deployment of renewable energy sources is leading to the need for more cross-border interconnections creating a pan-European power system, at the EU level. Such interconnections may take advantage of existing AC systems, but also of the DC links or DC grids under development; hence, hybrid AC/DC grids. Although hybrid AC/DC transmission grids are far from being truly large-scale, they are beginning to evolve as key components of future transmission infrastructures. By hybrid AC/DC grids, power electronic interfaces are invariably implied. As a consequence, several challenges are beginning to emerge at the network level due to the increasing adoption of these devices. The dynamics and interactions that may appear in such integrated systems are relatively unknown since they will interconnect at least two subnetworks of AC and DC characteristics. Understanding how several classes of interaction could occur is key to proper design of controllers to mitigate them. Further, changes to how subnetworks should be securely operated in unison is required. Therefore, a rethought is necessary in the presence of these devices. Nevertheless, the answer to the question of how to detect detrimental behaviours? Is central to any proposed solution. Currently, traditional methods for modelling and analysis are showing inadequacies. Lack of consistent methods to model and analyse the phenomena often result to complicated solutions as existing literature suggests. Even more important are models that are tractable, flexible, and technology agnostic to allow abstraction of the underlying challenges. Hence, methods to better understand and assess the mechanism of interactions at system level, that may impact secure operation are required. In addition, methods that are intuitive and efficient to detect sources of interactions, and isolate them as rapidly as possible are preferred. This goes to the heart of flexibility and tractability. Therefore, this thesis presents methodologies and strategies for modelling and control of large-scale hybrid AC/DC transmission grids from a systematic perspective, with the consideration of the controllable devices. This thesis employs several potent high-level methodologies that possess physical connotation, are technology agnostic, and provide tractability for control. Subsequently, recommended control strategies are easy to adopt as their physical significance can be established. The principal findings of this thesis are that, system interactions between subnetworks are dependent on the broad characteristics of the each subnetwork. Thus, manipulating any of these characteristics subject to considerations, improves the overall behaviour. Within each subnetwork, interactions depend mainly on the dynamics of existing controllers, and the interconnection between several devices. Then, questions on how to detect and mitigate interactions as efficiently as possible, while incorporating the most relevant behaviour is answered. Furthermore, the conflict of control requirements of each subnetwork of the hybrid AC/DC network is highlighted. Thus, information about these requirements are leveraged to achieve overall compromises without jeopardizing minimum performance. To conclude, following the assessment of detrimental interactions and their corresponding mechanism, control strategies that take these into consideration are proposed and demonstrated.<br>Per complir amb l'objectiu del gran desplegament de les energies renovables cal ampliar les interconnexions entre diferents països creant una xarxa elèctrica de transmissió a nivell Europeu. Aquestes interconexions poden aprofitar els sistemes en corrent altern (CA) ja existents però també de Links en corrent continu (CC) or xarxes en CC. Tot i que les xarxes de transmissió hibrides CA i CC son encara lliuny de ser una realitat, s'estan començant a posicionar como una component principal de les infrastructures de transmissió del futur. A les xarxes CA i CC, electronica de potencia n'és part fortament implicada. Com a conseqüència de l'ús d'aquests elements, varis reptes comencen a emergir degut a la seva complexitat innerent i la seva gran utilització. Les dinàmiques i les interaccions que poden apareixer en aquest sistema integrat són desconegudes doncs, aquest interconnectarà com a minim dos sub xarxes amb diferents caracteristiques com son la CA i la CC. Entendre com diferents interaccions poden succeir es clau per a poder dissenyar de manera adequada els control·ladors i mitigar-les. A més a més, canvis en la operació coordinada de les subxarxes és requerit. Aleshores, repensar com resoldre el problema és necessari quan ens trovem en presencia d'aquestes components. Però, la responsta a la pregunta; Com detectem comportaments negatius? es rellevant per a qualsevol soluciió que es vulgui propossar. Els mètodes tradicionals de modelització i anàlisi mostren insuficiències. La manca de mètodes coherents per modelar i analitzar els fenòmens sovint es tradueix en solucions complicades com suggereix la literatura existent. Encara són més importants els models tractables, flexibles i agnòstics que permetin la abstracció dels reptes subjacents. Per tant, es requereixen mètodes per comprendre i valorar millor el mecanisme dinteraccions a nivell del sistema que puguin afectar un funcionament segur. A més, es prefereixen mètodes intuïtius i eficients per detectar fonts dinteraccions i aïllar-les el més ràpidament possible. Això arriba al cor de la flexibilitat i la traçabilitat. Per tant, aquesta tesi discuteix les estratègies de modelatge i control de les xarxes de transmissió híbrides CA / CC a gran escala des d'una perspectiva sistemàtica, tenint en compte els dispositius controlables. Aquesta tesi utilitza una potent metodologia dalt nivell que té una connotació física, és tecnologia agnòstica i tractable. Posteriorment, les estratègies de control recomanades són fàcils dadoptar ja que es pot establir la seva significació física. Els principals resultats d'aquesta tesi són que, les interaccions del sistema entre subxarxes depenen de les àmplies característiques de cada subxarxa. Per tant, la manipulació daquestes característiques subjectes a consideracions millora el comportament general. Dins de cada subxarxa, les interaccions depenen principalment de la dinàmica dels controladors existents en dispositius controlables i de la interconnexió entre diversos dispositius. A continuació, es responen preguntes sobre com detectar i mitigar les interaccions de la manera més eficaç possible, alhora que incorporar el comportament més rellevant. A més, es posa de manifest el conflicte de requisits de control de cada subxarxa de la xarxa híbrida CA / CC. Així, es pot aprofitar la informació sobre aquests requisits per aconseguir compromisos generals sense posar en perill el rendiment mínim. Validar i demostrar aquests models i estratègies de control ha estat una contribució clau en aquesta tesi.

Modern power networks incorporate communications and information technology infrastructure into the electrical power system to create a smart grid in terms of control and operation. The smart grid enables real-time communication and control between consumers and utility companies allowing suppliers to optimize energy usage based on price preference and system technical issues. The smart grid design aims to provide overall power system monitoring, create protection and control strategies to maintain system performance, stability and security. This dissertation contributed to the development of a unique and novel smart grid test-bed laboratory with integrated monitoring, protection and control systems. This test-bed was used as a platform to test the smart grid operational ideas developed here. The implementation of this system in the real-time software creates an environment for studying, implementing and verifying novel control and protection schemes developed in this dissertation. Phasor measurement techniques were developed using the available Data Acquisition (DAQ) devices in order to monitor all points in the power system in real time. This provides a practical view of system parameter changes, system abnormal conditions and its stability and security information system. These developments provide valuable measurements for technical power system operators in the energy control centers. Phasor Measurement technology is an excellent solution for improving system planning, operation and energy trading in addition to enabling advanced applications in Wide Area Monitoring, Protection and Control (WAMPAC). Moreover, a virtual protection system was developed and implemented in the smart grid laboratory with integrated functionality for wide area applications. Experiments and procedures were developed in the system in order to detect the system abnormal conditions and apply proper remedies to heal the system. A design for DC microgrid was developed to integrate it to the AC system with appropriate control capability. This system represents realistic hybrid AC/DC microgrids connectivity to the AC side to study the use of such architecture in system operation to help remedy system abnormal conditions. In addition, this dissertation explored the challenges and feasibility of the implementation of real-time system analysis features in order to monitor the system security and stability measures. These indices are measured experimentally during the operation of the developed hybrid AC/DC microgrids. Furthermore, a real-time optimal power flow system was implemented to optimally manage the power sharing between AC generators and DC side resources. A study relating to real-time energy management algorithm in hybrid microgrids was performed to evaluate the effects of using energy storage resources and their use in mitigating heavy load impacts on system stability and operational security.

L'intégration croissante des ressources énergétiques natives en courant continu à l'échelle de réseaux électriques, telles que les systèmes photovoltaïques, les véhicules électriques, les systèmes de stockage et les centres de données, remet en question le choix conventionnel des systèmes de distribution en courant alternatif (AC) au niveau de la moyenne tension (MV). Cette recherche vise à évaluer la viabilité technique et économique des réseaux de distribution MV AC/DC en développant des outils de planification pour les réseaux hybrides.L'étude propose des modèles de planification optimale des systèmes de distribution pour traiter les connexions MVDC point à point et les topologies hybrides, qui incorporent des ressources DC connectées au système par le biais de convertisseurs DC/DC, en utilisant des formulations telles que la programmation linéaire en nombres entiers mixtes, quadratique et conique de second ordre. Parmi les principales contributions de cette recherche, une analyse documentaire approfondie des hypothèses de planification a été réalisée, la proposition d'un modèle linéaire de pertes pour les stations de conversion AC/DC et DC/DC tenant compte de l'efficacité variable en fonction de la charge, l'incorporation de contraintes topologiques pour tenir compte des topologies radiales par morceaux, et l'évaluation des avantages économiques pour un large ensemble de paramètres dans le cadre des marchés de l'électricité dérégulés.Les perspectives futures comprennent l'étude de l'scalabilité des modèles proposés à des réseaux de distribution à plus grande échelle, l'exploration des tendances émergentes dans la résolution de l'optimisation et l'inclusion de choix liés à la conception dans les outils de planification<br>The increasing integration of utility-scale DC-native energy resources, such as photovoltaic systems, electric vehicles, storage systems, and data centers, challenges the conventional choice of AC distribution systems at the Medium Voltage (MV) level. This research aims to evaluate the technical and economic viability of AC/DC MV distribution networks by developing planning tools for hybrid networks.The study proposes optimal distribution system planning models to address point-to-point MVDC connections and hybrid topologies, which incorporate DC resources connected to the system through DC/DC converters, using formulations such as Mixed Integer Linear, Quadratic and Second Order Conic Programming. Some of the key contributions of this research include an extensive literature review of planning hypotheses, the proposal of a linear model of losses for AC/DC and DC/DC conversion stations accounting for part-load efficiency, the incorporation of topological constraints to accommodate piece-wise radial topologies, and the assessment of economic benefits across a wide range of parameters within the framework of unbundled electricity markets.Future perspectives include investigating the scalability of the proposed models to larger-scale distribution networks, exploring emerging trends in the optimization solving and including design-related choices in the planning tools

In un impianto fotovoltaico connesso alla rete elettrica, l’ integrazione di un sistema di accumulo permette di raccogliere l’ energia dal solare nelle ore di minor richiesta di rete (di giorno), ed erogarla nei momenti di bassa produzione e di maggiore richiesta di rete (la sera). In collaborazione con ENGIE Eps, è sorta l’ esigenza di confrontare tre diverse tipologie di accoppiamento delle batterie in un impianto ibrido PV+Batteria connesso alla rete elettrica. La prima architettura è chiamata AC coupling poiché il BESS (Battery Energy Storage System) è connesso tramite opportuni convertitori, direttamente alla rete elettrica. La terza e la seconda architettura sono denominate DC Coupling poiché il BESS è collegato tramite un convertitore o senza, al lato DC dell’ impianto. Il confronto è stato realizzato analizzando i flussi di potenza dell’ impianto facendo riferimento a dati di produzione reali forniti da ENGIE Eps. Più in particolare, sono stati forniti i dati di produzione e di irraggiamento di un impianto reale di potenza massima pari a 285 MW, con storage di capacità pari a 275 MWh. La valutazione della potenza richiesta all’ impianto è stata ottenuta dall’analisi del segnale AGC relativo alla rete nella quale l’ impianto è inserito. Tale segnale `e stato generato a partire da dati di frequenza di rete forniti dall’ azienda. Dall’ analisi precedentemente descritta si è individuata l’ architettura migliore in termini di rendimento, che risulta essere la DC coupling con DC/DC sulla batteria. Nell’ ultima parte della tesi si è inoltre svolto su richiesta di ENGIE Eps, lo studio del controllo dei convertitori relativi all’ architettura in esame. La strategia di controllo individuata è descritta nel dettaglio in questo documento.

Transient stability studies are primarily concerned with the generator response of ac power systems and use only steady state type equations to model HVdc converter terminals. These equations are adequate for small disturbances at the converter terminals but cannot accurately represent a converters behaviour during, and through its recovery of, a significant transient disturbance. A detailed three phase electromagnetic analysis is necessary to describe the converters correct behaviour. This thesis describes an accurate and effective hybrid method combining these two types of studies, for analyzing dynamically fast devices such as HVdc converters within ac power systems. Firstly, conventional techniques are reviewed for both a transient stability analysis of power systems and for an electromagnetic transient analysis of HVdc converters. This review deals in particular with the two programs that constitute the hybrid developed in this thesis. Various techniques are then examined to efficiently and accurately pass the dynamic effects of an HVdc link to an ac system stability study, and the dynamic effects of an ac system to a detailed HVdc link study. An optimal solution is derived to maximise the inherent advantages of a hybrid. Finally, the hybrid is applied to a test system and its effectiveness in performing its task is shown.

In today’s increasingly energy conscious society, the methods of providing thermal comfort to humans are constantly under scrutiny. Depending on the climate, and the comfort requirements of the occupants, buildings can be designed to heat and cool occupants with passive methods, as well as mechanical methods. In the subtropics, where buildings often need to be heated in the winter and cooled in the summer, a synthesis of these two methods would be ideal. However, there is a disconnect between the integration of passive cooling and mechanical air conditioning, in subtropical architecture. A study of user attitudes, based out of Australia, found that, “Central control of temperatures has been used to cut demand by preventing users from altering thermostats and other parts of the building for microclimate control. In particular, windows are sealed to prevent tampering.”1 Reliance on air conditioning has the everyday person convinced that if we save energy in the right places, we can use air conditioning as much as we like. The same study goes on to state, “Air-conditioning has been assumed to replace the need for climate design features in buildings creating poor thermal design and high energy use.”2 This can be most clearly seen in our public buildings. Fully conditioned buildings pump cool air into sealed envelopes, adjusting the thermostat to regulate thermal comfort year-round, often in a climate in which mechanical air conditioning is needed only four months of the year, and during the warmest hours of the day. Inversely, ventilated buildings provide passive cooling in a climate in which the temperature and humidity are often too high for thermal comfort during the same four months of the year. In his book Natural Ventilation in Buildings, Francis Allard points out that the global energy efficiency movement, begun in the early 1990s, has now emerged as a concept that incorporates active air conditioning and sitespecific climate design of buildings into one holistic approach.3 However, these buildings exist in more dry and temperate climates, and do not fully apply to the subtropics as cooling models. A model is needed for subtropical architecture allowing a building to reach both ends of the spectrum; from natural ventilation, through mechanical ventilation, to mechanical air conditioning. The goal of this thesis is to design a hybrid model for subtropical architecture which maximizes the use of natural and mechanical ventilation, and minimizes the use of mechanical air conditioning. The vehicle for this explanation is the design of an educational facility. Research of thermal comfort needs for occupants in the subtropics was accompanied with observation studies. This research was compared with case study, site and program analysis. The analysis was supplemented by a handbook of passive and mechanical cooling which was compiled to aid in establishing cooling strategies for the design process. The implementation of the research and analysis was brought to a conclusion that successfully achieved the goals of this thesis. By using passive methods to lower the temperature of the air surrounding the classroom buildings, the incoming air used to cool the occupants reached temperatures low enough to be considered comfortable inside the classrooms.

A hybrid AC/DC distribution network can be formed after integration of increasing numbers of electronically interfaced distributed generators (DGs) and microgrids (MGs). Voltage/Var control (VVC) in this evolved system requires to control not only the traditional VVC resources such as transformer tap changer and shunt capacitor but also the newly emerged ones such as DGs and MGs. In addition, various stakeholders such as MG's operator and distribution network operator coexist in this evolved system. They usually represent distinct operational interests and make the VVC even more difficult. Therefore, VVC in hybrid AC/DC distribution networks expects to addresses not only the coordination among various voltage/var resources but also the conflicting interests of different stakeholders. In this thesis, various VVC schemes are developed to address above mentioned concerns, which provide promising solutions for system operators in future hybrid AC/DC distribution networks. Driven by advancements in power conversion and communication technologies, the dispersed MGs can be participated in VVC scheme and coordinated with other VVC resources. This thesis firstly proposes a VVC scheme to leverage the flexible power injections from grid-tied MGs. The tuning range of MGs' reactive power injections are widen by controlling local energy storage systems. Then, a two-layer VVC control scheme is developed. The upper layer is a multi-objective voltage/var optimisation model that schedules all the VVC resources including grid-tied MGs. The lower layer consists of various controllers in MGs, which aims to maintain the local power quality and to execute the setpoints sending from the upper layer. Although VVC performance can be enhanced by controlling MGs' reactive power injections, a lack of consideration for their active power control could result in deterioration on MG's local power supply. To this end, a coordinated VVC scheme is proposed to link MG's power management with VVC. The control of MG's local devices and control of voltage/var resources in distribution network are allocated into two different but interlinked optimisation models. Furthermore, the mechanically controlled OLTC and SC and electronically controlled DGs and MGs are scheduled in different timescales. Finally, a multi-objective VVC scheme is proposed to maximise the VVC contributions from various stakeholders such as MGs and distribution network operator. Since trade-off solutions are utilised in this scheme, it is promising to be used in multi-lateral cases where MGs are not unilaterally controlled by distribution network operators. The scheme is supported by an identification approach which fairly quantifies the VVC contributions from various stakeholders. Each stakeholder could be rewarded in proportion to its VVC contribution, so their participations are motivated without conflicts to satisfy voltage control requirements.