Dissertations / Theses on the topic 'HVAC control systems'

It is of interest for companies to keep the annual operating cost of their buildings as low as possible. A substantial share of the annual operating costs are due to the large amount of energy needed for heating of the ventilated air and heating of the rooms inside the buildings. Also much of the electrical energy in the world today is created using fossil fuel or charcoal. This has an environmental aspect and the consumers of energy becomes more and more aware of this. Thus reducing the energy used by a buildings HVAC system can save the users for considerable expenditures and also has an environmental aspect.To find an estimate of the energy consumption a mathematical model representing a building and its HVAC system have been made. This model has been made up of several smaller models representing each component present in the building. These models have then been implemented in verb|Simulink| and the response of the system has been simulated for different scenarios. From these simulations the energy consumption has been extracted and compared to each other. Thus the amount of energy saved for each scenario has been found. The models include two type of controllers to see whether or not the choice of controller design affects the energy efficiency of the system. These two controller designs are the PID controller and the MPC control scheme. Also a discretized and simplified model of the building to be used together with the MPC controller has been found using system identification. In addition to this a Kalman filter that estimates unknown states and filter out disturbances are included in the MPC control scheme.The results from the simulations using a PID controller indicates a possible annual saving of substantial amounts. Thus this report shows that the annual energy consumption in a building can be greatly reduced by introducing simple and relatively cheap modifications to the HVAC system. The results from the simulations using the MPC scheme indicates that even more energy can be saved using this advanced control scheme. However, in order to verify this the MPC controller needs to be fine tuned and several more experiments needs to be reviewed.

Advances in Information and Communication Technology (ICT) paved the way for decentralized Heating, Ventilation, and Air-Conditioning (HVAC) HVAC operations. It has been envisioned that development of personal thermal comfort profiles leads to accurate predictions of each occupant's thermal comfort state and such information is employed in context-aware HVAC operations for energy efficiency. This dissertation has three key contributions in realizing this envisioned HVAC operation. First, it presents a systematic review of research trends and developments in context-aware HVAC operations. Second, it contributes to expanding the feasibility of the envisioned HVAC operation by introducing novel sensing technologies. Third, it contributes to shedding light on viability and potentials of comfort-aware operations (i.e., integrating personal thermal comfort models into HVAC control logic) through a comprehensive assessment of energy efficiency implications. In the first contribution, by developing a taxonomy, two major modalities – occupancy-driven and comfort-aware operations – in Human-In-The-Loop (HITL) HVAC operations were identified and reviewed quantitatively and qualitatively. The synthesis of previous studies has indicated that field evaluations of occupancy-driven operations showed lower potentials in energy saving, compared to the ones with comfort-aware operations. However, the results in comfort-aware operations could be biased given the small number of explorations. Moreover, required data representation schema have been presented to foster constructive performance assessments across different research efforts. In the end, the current state of research and future directions of HITL HVAC operations were discussed to shed light on future research need. As the second contribution, moving toward expanding the feasibility of comfort-aware operations, novel and smart sensing solutions have been introduced. It has been noted that, in order to have high accuracy in predicting individual's thermal comfort state (≥90%), user physiological response data play a key part. However, the limited number of applicable sensing technologies (e.g., infrared cameras) has impeded the potentials of implementation. After defining required characteristics in physiological sensing solutions in context of comfort-aware operations (applicability, sensitivity, ubiquity, and non-intrusiveness), the potentials of RGB cameras, Doppler radar sensors, and heat flux sensors were evaluated. RGB cameras, available in many smart computing devices, could be a ubiquitous solution in quantifying thermoregulation states. Leveraging the mechanism of skin blood perfusion, two thermoregulation state quantification methods have been developed. Then, applicability and sensitivity were checked with two experimental studies. In the first experimental study aiming to see applicability (distinguishing between 20 and 30C with fully acclimated human bodies), for 16 out of 18 human subjects, an increase in their blood perfusion was observed. In the second experimental study aiming to evaluate sensitivity (distinguishing responses to a continuous variation of air temperature from 20 to 30C), 10 out of 15 subjects showed a positive correlation between blood perfusion and thermal sensations. Also, the superiority of heat flux data, compared to skin temperature data, has been demonstrated in predicting personal thermal comfort states through the developments of machine-learning-based prediction models with feature engineering. Specifically, with random forest classifier, the median value of prediction accuracy was improved by 3.8%. Lastly, Doppler radar sensors were evaluated for their capability of quantifying user thermoregulation states leveraging the periodic movement of the chest/abdomen area induced by respiration. In an experimental study, the results showed that, with sufficient acclimation time, the DRS-based approach could show distinction between respiration states for two distinct air temperatures (20 and 30C). On the other hand, in a transient temperature without acclimation time, it was shown that, some of the human subjects (38.9%) used respiration as an active means of heat exchange for thermoregulation. Lastly, a comprehensive evaluation of comfort-aware operations' performance was carried out with a diverse set of contextual and operational factors. First, a novel comfort-aware operation strategy was introduced to leverage personal sensitivity to thermal comfort (i.e., different responses to temperature changes; e.g., sensitive to being cold) in optimization. By developing an agent-based simulation framework and thorough diverse scenarios with different numbers and combinations of occupants (i.e., human agents in the simulation), it was shown that this approach is superior in generating collectively satisfying environments against other approaches focusing on individual preferred temperatures in selection of optimized setpoints. The energy implications of comfort-aware operations were also evaluated to understand the impact from a wide range of factors (e.g., human and building factors) and their combinatorial effect given the uncertainty of multioccupancy scenarios. The results demonstrated that characteristics of occupants' thermal comfort profiles are dominant in impacting the energy use patterns, followed by the number of occupants, and the operational strategies. In addition, when it comes to energy efficiency, more occupants in a thermal zone/building result in reducing the efficacy of comfort-driven operation (i.e., the integration of personal thermal comfort profiles). Hence, this study provided a better understanding of true viability of comfort-driven HVAC operations and provided the probabilistic bounds of energy saving potentials. These series of studies have been presented as seven journal articles and they are included in this dissertation.
Doctor of Philosophy
With vision of a smart built environment, capable of understanding the contextual dynamics of built environment and adaptively adjusting its operation, this dissertation contributes to context-aware/decentralized HVAC operations. Three key contributions in realization of this goal include: (1) a systematic review of research trends and developments in the last decade, (2) enhancing the feasibility of quantifying personal thermal comfort by presenting novel sensing solutions, and (3) a comprehensive assessment of energy efficiency implications from comfort-aware HVAC operations with the use of personal comfort models. Starting from identifying two major modalities of context-aware HVAC operations, occupancy-driven and comfort-aware, the first part of this dissertation presents a quantitative and qualitative review and synthesis of the developments, trends, and remaining research questions in each modality. Field evaluation studies using occupancy-driven operations have shown median energy savings between 6% and 15% depending on the control approach. On the other hand, the comfort-aware HVAC operations have shown 20% energy savings, which were mainly derived from small-scale test beds in similar climate regions. From a qualitative technology development standpoint, the maturity of occupancy-driven technologies for field deployment could be interpreted to be higher than comfort-aware technologies while the latter has shown higher potentials. Moreover, by learning from the need for comparing different methods of operations, required data schemas have been proposed to foster better benchmarking and effective performance assessment across studies. The second part of this dissertation contributes to the cornerstone of comfort-aware operations by introducing novel physiological sensing solutions. Previous studies demonstrated that, in predicting individual's thermal comfort states, using physiological data in model development plays a key role in increasing accuracy (>90%). However, available sensing technologies in this context have been limited. Hence, after identifying essential characteristics for sensing solutions (applicability, sensitivity, ubiquity, and non-intrusiveness), the potentials of RGB cameras, heat flux sensors, and Doppler radar sensors were evaluated. RGB cameras, available in many smart devices, could be programmed to measure the level of blood flow to skin, regulated by the human thermoregulation mechanism. Accordingly, two thermoregulation states' quantification methods by using RGB video images have been developed and assessed under two experimental studies: (i) capturing subjects' facial videos in two opposite temperatures with sufficient acclimation time (20 and 30C), and (ii) capturing facial videos when subjects changed their thermal sensations in a continuous variation of air temperature from 20 to 30C. Promising results were observed in both situations. The first study had subjects and 16 of them showed an increasing trend in blood flow to skin. In the second study, posing more challenges due to insufficient acclimation time, 10 subjects had a positive correlation between the level of blood flow to skin with thermal sensation. With the assumption that heat flux sensing will be a better reflection of thermoregulation sates, a machine learning framework was developed and tested. The use of heat flux sensing showed an accuracy of 97% with an almost 4% improvement compared to skin temperature. Lastly, Doppler radar sensors were evaluated for their capability of quantifying thermoregulation states by detecting changes in breathing patterns. In an experimental study, the results showed that, with sufficient acclimation time, the DRS-based approach could show distinction between respiration states for two distinct air temperatures (20 and 30C). However, using a transient temperature was proven to be more challenging. It was noted that for some of the human subjects (38.9%), respiration was detected as an active means of heat exchange. It was concluded that specialized artifact removal algorithms might help improve the detection rate. The third component of the dissertation contributed by studying the performance of comfort-driven operations (i.e., using personal comfort preferences for HVAC operations) under a diverse set of contextual and operational factors. Diverse scenarios for interaction between occupants and building systems were evaluated by using different numbers and combinations of occupants, and it was demonstrated that an approach of addressing individual's thermal comfort sensitivity (personal thermal-comfort-related responses to temperature changes) outperforms other approaches solely focusing on individual preferred temperatures. The energy efficiency implications of comfort-driven operations were then evaluated by accounting for the impact of human and building factors (e.g., number of thermal zones) and their combinations. The results showed that characteristics of occupants' thermal comfort profiles are dominant in driving the energy use patterns, followed by the number of occupants, and operational strategies. As one of the main outcomes of this study, the energy saving and efficiency (energy use for comfort improvement) potentials and probabilistic bounds of comfort-driven operations were identified. It was shown that keeping the number of occupants low (under 6) in a thermal zone/building, boosts the energy saving potentials of comfort-driven operations. These series of studies have been presented as seven journal articles, included in this dissertation.

Practicing engineers need an integrated building, HVAC and control simulation tool for optimum HVAC design and retrofit. Various tools are available to the researchers, but these are not appropriate for the consulting engineer. To provide the engineer with a tool which can be used for typical HVAC projects, new models for building, HVAC and control simulation are introduced and integrated in a user-friendly, quick-to-use tool. The new thermal model for buildings is based on a transfer matrix description of the heat transfer through the building shell. It makes provision for the various heat flow paths that make up the overall heat flow through the building structure. The model has been extensively verified with one hundred and three case studies. These case studies were conducted on a variety of buildings, ranging from a 4m2 bathroom, to a 7755 m2 factory building. Eight of the case studies were conducted independently in the Negev Desert in Israel. The thermal model is also used in a program that was custom-made for the AGREMENT Board (certification board for the thermal performance of new low-cost housing projects). Extensions to the standard tool were introduced to predict the potential for condensation on the various surfaces. Standard user patterns were incorporated in the program so that all the buildings are evaluated on the same basis. In the second part of this study the implementation of integrated simulation is discussed. A solution algorithm, based on the Tarjan depth first-search algorithm, was implemented. This ensures that the minimum number of variables are identified. A quasi-Newton solution algorithm is used to solve the resultant simultaneous equations. Various extensions to the HVAC and control models and simulation originally suggested by Rousseau [1] were implemented. Firstly, the steady-state models were extended by using a simplified time-constant approach to emulate the dynamic response of the equipment. Secondly, a C02 model for the building zone was implemented. Thirdly, the partload performance of particular equipment was implemented. Further extensions to the simulation tool were implemented so that energy management strategies could be simulated. A detailed discussion of the implications of the energy management systems was given and the benefits of using these strategies were clearly illustrated, in this study. Finally, the simulation tool was verified by three case studies. The buildings used for the verification ranged from a five-storeyed office and laboratory building, to a domestic dwelling. The energy consumption and the dynamics of the HVAC systems could be predicted sufficiently accurately to warrant the use of the tool for future building retrofit studies
Thesis (PhD)--University of Pretoria, 1997.
gm2014
Mechanical and Aeronautical Engineering
unrestricted

During the last 10 years solar cooling systems attracted more and more interest not only in the research area but also on a private and commercial level. Several demonstration plants have been installed in different European countries and first companies started to commercialise also small scale absorption cooling machines. However, not all of the installed systems operate efficiently and some are, from the primary energy point of view, even worse than conventional systems with a compression chiller. The main reason for this is a poor system design combined with suboptimal control. Often several non optimised components, each separately controlled, are put together to form a ‘cooling system’. To overcome these drawbacks several attempts are made within IEA task 38 (International Energy Agency Solar Heating and Cooling Programme) to improve the system design through optimised design guidelines which are supported by simulation based design tools. Furthermore, guidelines for an optimised control of different systems are developed. In parallel several companies like the SolarNext AG in Rimsting, Germany started the development of solar cooling kits with optimised components and optimised system controllers. To support this process the following contributions are made within the present work: - For the design and dimensioning of solar driven absorption cooling systems a detailed and structured simulation based analysis highlights the main influencing factors on the required solar system size to reach a defined solar fraction on the overall heating energy demand of the chiller. These results offer useful guidelines for an energy and cost efficient system design. - Detailed system simulations of an installed solar cooling system focus on the influence of the system configuration, control strategy and system component control on the overall primary energy efficiency. From the results found a detailed set of clear recommendations for highly energy efficient system configurations and control of solar driven absorption cooling systems is provided. - For optimised control of open desiccant evaporative cooling systems (DEC) an innovative model based system controller is developed and presented. This controller consists of an electricity optimised sequence controller which is assisted by a primary energy optimisation tool. The optimisation tool is based on simplified simulation models and is intended to be operated as an online tool which evaluates continuously the optimum operation mode of the DEC system to ensure high primary energy efficiency of the system. Tests of the controller in the simulation environment showed that compared to a system with energy optimised standard control the innovative model based system controller can further improve the primary energy efficiency by 19 %.

Room models, currently used for controller tests, assume the room air to be perfectly mixed. A new room model is developed, assuming non-homogeneous room conditions and distinguishing between different sensor positions. From measurement in real test rooms and detailed CFD simulations, a list of convective phenomena is obtained that has to be considered in the development of a model for a room equipped with different HVAC systems. The zonal modelling approach that divides the room air into several sub-volumes is chosen, since it is able to represent the important convective phenomena imposed on the HVAC system. The convective room model is divided into two parts: a zonal model, representing the air at the occupant zone and a second model, providing the conditions at typical sensor positions. Using this approach, the comfort conditions at the occupant zone can be evaluated as well as the impact of different sensor positions. The model is validated for a test room equipped with different HVAC systems. Sensitivity analysis is carried out on the main parameters of the model. Performance assessment and energy consumption are then compared for different sensor positions in a room equipped with different HVAC systems. The results are also compared with those obtained when a well-mixed model is used. A main conclusion of these tests is, that the differences obtained, when changing the position of the controller's sensor, is a function of the HVAC system and controller type. The differences are generally small in terms of thermal comfort but significant in terms of overall energy consumption. For different HVAC systems the cases are listed, in which the use of a simplified model is not recommended. This PhD has been submitted in accordance to the conditions for attaining both the French and the German degree of a PhD, on a co-national basis, in the frame of a statement of the French government from January 18th, 1994. The research has been carried out in the Automation and Energy Management Group (AGE), Department of Sustainable Development (DDD), at the "Centre Scientifique et Technique du Bâtiment" (CSTB) in Marne la Vallée, France, in collaboration with the "Centre Energétique" (CENERG) at the "Ecole Nationale Supérieure des Mines de Paris" (ENSMP), Paris, France and the Technical University of Dresden (TUD), Germany.

Buildings are among the largest consumers of energy in the world. A significant part of this energy can be attributed to Heating, Ventilation and Air Conditioning (HVAC) systems, which play an important role in maintaining acceptable thermal and air quality conditions in common building. For this reason, improving energy eciency in buildings is today a primary objective for the building industry, as well as for the society in general. However, in order to successfully control buildings, control systems must continuously adapt the operation of the building to various uncertainties (external air temperature, occupants' activities, etc.) while making sure that energy eciency does not compromise occupant's comfort and well-being. Several promising approaches have been proposed; among them, Model Predictive Control has received particular attention, since it can naturally achieve systematic integration of several factors, such as weather forecasts, occupancy predictions, comfort ranges and actuation constraints. This advanced technique has been shown to bring signicant improvements in energy savings. Model Predictive Control employs a model of the system and solves an on-line optimization problem to obtain optimal control inputs. The on-line computation, as well as the modelling eort, can lead to diculties in the practical integration into a building management system. To cope with this problem, another possibility is to obtain o-line the optimal control prole as a piecewise ane and continuous function of the initial state. By doing so, the computation associated with Model Predictive Control becomes a simple function evaluation, which can be performed eciently on a simple and cheap hardware. In this thesis, an implicit and an explicit formulation of Model Predictive Control for HVAC systems are developed and compared, showing the practical advantages of the explicit formulation.

Udržovaní vnitřních klimatických podmínek tak, aby byly v souladu s tepelným komfortem lidí, je klíčovou otázkou pro řízení systémů vytápění, větrání a klimatizace (HVAC systémy). Počítačové modelování nabízí virtuální prostředí pro simulaci vnitřních i vnějších podmínek a s jeho pomocí je možné navrhnout řešení pro řízení technických zařízení budov. Tento proces vyžaduje pochopení těchto prostředí z fyzikálního a matematického hlediska tak, aby bylo možné fyzikální procesy daných prostředí prezentovat pomocí vztahů a rovnic odrážejících jejích různé parametry. Simulační proces dále nabízí možnost popsat interakci mezi těmito modely a jejich chování v čase, dává výchozí reprezentace těchto prostředí, a umožňuje pochopení jejich chování před přenosem těchto modelů do reálných aplikací. Simulace umožnuje respektovat, a ovlivňovat jejích chování přes kontrolu navržených modelů. MATLAB/SIMULINK software má pokročilé schopnosti pro simulace systémů HVAC, a to vytvořením širokého pracovního prostředí pro designéry v závislosti na vývoji matematických modelů a jejích simulace pomocí SIMULINK, aby výsledky mohly být slučitelné s požadovanými výstupy. Tato dizertační práce se zaměřuje na proces modelování vnitřního prostředí v budovách, aby bylo možné pochopit chování klíčových parametrů, které mají vliv na tepelnou pohodu obyvatel či uživatelů, matematické modely vnitřního prostředí posluchárny byly navržené speciálně pro tři základní parametry: koncentrace oxidu uhličitého, teplota vzduchu a relativní vlhkost. Změny chování těchto parametrů v průběhu času jsou simulovány a poté strategie kontroly návrhu těchto parametrů může je udržet ve vhodných rozmezích komfortních pro obyvatele či uživatele, i když změny venkovního klimatu, tepelné a hmotnostní zatíží interiér. Pomocí matematických metod, některé optimalizační metody byly navrženy za účelem snížení spotřeby energie bez vlivu na mezní hodnoty těchto parametrů. Proces validace modelu se provádí porovnáním výsledků s reálnými výstupy monitoringu Honeywell Enterprise Buildings Integrator systémem (EBI) nainstalován v areálu univerzity.

The efficacy of the energy management systems at dealing with energy consumption in buildings has been a topic with a growing interest in recent years due to the ever-increasing global energy demand and the large percentage of energy being currently used by buildings. The scale of this sector has attracted research effort with the objective of uncovering potential improvement avenues and materializing them with the help of recent technological advances that could be exploited to lower the energetic footprint of buildings. Specifically, in the area of heating, ventilating and air conditioning installations, the availability of large amounts of historical data in building management software suites makes possible the study of how resource-efficient these systems really are when entrusted with ensuring occupant comfort. Actually, recent reports have shown that there is a gap between the ideal operating performance and the performance achieved in practice. Accordingly, this thesis considers the research of novel energy management strategies for heating, ventilating and air conditioning installations in buildings, aimed at narrowing the performance gap by employing data-driven methods to increase their context awareness, allowing management systems to steer the operation towards higher efficiency. This includes the advancement of modeling methodologies capable of extracting actionable knowledge from historical building behavior databases, through load forecasting and equipment operational performance estimation supporting the identification of a building’s context and energetic needs, and the development of a generalizable multi-objective optimization strategy aimed at meeting these needs while minimizing the consumption of energy. The experimental results obtained from the implementation of the developed methodologies show a significant potential for increasing energy efficiency of heating, ventilating and air conditioning systems while being sufficiently generic to support their usage in different installations having diverse equipment. In conclusion, a complete analysis and actuation framework was developed, implemented and validated by means of an experimental database acquired from a pilot plant during the research period of this thesis. The obtained results demonstrate the efficacy of the proposed standalone contributions, and as a whole represent a suitable solution for helping to increase the performance of heating, ventilating and air conditioning installations without affecting the comfort of their occupants.
L’eficàcia dels sistemes de gestió d’energia per afrontar el consum d’energia en edificis és un tema que ha rebut un interès en augment durant els darrers anys a causa de la creixent demanda global d’energia i del gran percentatge d’energia que n’utilitzen actualment els edificis. L’escala d’aquest sector ha atret l'atenció de nombrosa investigació amb l’objectiu de descobrir possibles vies de millora i materialitzar-les amb l’ajuda de recents avenços tecnològics que es podrien aprofitar per disminuir les necessitats energètiques dels edificis. Concretament, en l’àrea d’instal·lacions de calefacció, ventilació i climatització, la disponibilitat de grans bases de dades històriques als sistemes de gestió d’edificis fa possible l’estudi de com d'eficients són realment aquests sistemes quan s’encarreguen d'assegurar el confort dels seus ocupants. En realitat, informes recents indiquen que hi ha una diferència entre el rendiment operatiu ideal i el rendiment generalment assolit a la pràctica. En conseqüència, aquesta tesi considera la investigació de noves estratègies de gestió de l’energia per a instal·lacions de calefacció, ventilació i climatització en edificis, destinades a reduir la diferència de rendiment mitjançant l’ús de mètodes basats en dades per tal d'augmentar el seu coneixement contextual, permetent als sistemes de gestió dirigir l’operació cap a zones de treball amb un rendiment superior. Això inclou tant l’avanç de metodologies de modelat capaces d’extreure coneixement de bases de dades de comportaments històrics d’edificis a través de la previsió de càrregues de consum i l’estimació del rendiment operatiu dels equips que recolzin la identificació del context operatiu i de les necessitats energètiques d’un edifici, tant com del desenvolupament d’una estratègia d’optimització multi-objectiu generalitzable per tal de minimitzar el consum d’energia mentre es satisfan aquestes necessitats energètiques. Els resultats experimentals obtinguts a partir de la implementació de les metodologies desenvolupades mostren un potencial important per augmentar l'eficiència energètica dels sistemes de climatització, mentre que són prou genèrics com per permetre el seu ús en diferents instal·lacions i suportant equips diversos. En conclusió, durant aquesta tesi es va desenvolupar, implementar i validar un marc d’anàlisi i actuació complet mitjançant una base de dades experimental adquirida en una planta pilot durant el període d’investigació de la tesi. Els resultats obtinguts demostren l’eficàcia de les contribucions de manera individual i, en conjunt, representen una solució idònia per ajudar a augmentar el rendiment de les instal·lacions de climatització sense afectar el confort dels seus ocupants

Dynamically determining input signals to a complex system, to increase performance and/or reduce cost, is a difficult task unless users are provided with feedback on the consequences of different input decisions. For example, users self-determine the set point schedule (i.e. temperature thresholds) of their HVAC system, without an ability to predict cost--they select only comfort. Users are unable to optimize the set point schedule with respect to cost because the cost feedback is provided at billing-cycle intervals. To provide rapid feedback (such as expected monthly/daily cost), mechanisms for system monitoring, data-driven modeling, simulation, and optimization are needed. Techniques from the literature require in-depth knowledge in the domain, and/or significant investment in infrastructure or equipment to measure state variables, making these solutions difficult to implement or to scale down in cost. This work introduces methods to approximate complex system behavior prediction and optimization, based on dynamic data obtained from inexpensive sensors. Unlike many existing approaches, we do not extract an exact model to capture every detail of the system; rather, we develop an approximated model with key predictive characteristics. Such a model makes estimation and prediction available to users who can then make informed decisions; alternatively, these estimates are made available as an input to an optimization tool to automatically provide pareto-optimized set points. Moreover, the approximation nature of this model makes the determination of the prediction and optimization parameters computationally inexpensive, adaptive to system or environment change, and suitable for embedded system implementation. Effectiveness of these methods is first demonstrated on an HVAC system methodology, and then extended to a variety of complex system applications.

This study investigates the software automation and control framework for the MMT thermal system. Thermal-related effects on observing and telescope behavior have been considered during the entire software development process. Regression analysis of telescope and observatory subsystem data is used to characterize and model these thermal-related effects. The regression models help predict expected changes in focus and overall astronomical seeing that result from temperature variations within the telescope structure, within the primary mirror glass, and between the primary mirror glass and adjacent air (i.e., mirror seeing). This discussion is followed by a description of ongoing upgrades to the heating, ventilation and air conditioning (HVAC) system and the associated software controls. The improvements of the MMT thermal system have two objectives: 1) to provide air conditioning capabilities for the MMT facilities, and 2) to modernize and enhance the primary mirror (M1) ventilation system. The HVAC upgrade necessitates changes to the automation and control of the M1 ventilation system. The revised control system must factor in the additional requirements of the HVAC system, while still optimizing performance of the M1 ventilation system and the M1's optical behavior. An industry-standard HVAC communication and networking protocol, BACnet (Building Automation and Control network), has been adopted. Integration of the BACnet protocol into the existing software framework at the MMT is discussed. Performance of the existing automated system is evaluated and a preliminary upgraded automated control system is presented. Finally, user interfaces to the new HVAC system are discussed.

This research is related to the control of energy consumption and efficiency in building Heating Ventilation and Air Conditioning (HVAC) systems and is primarily concerned with controlling the function of heating. The main goal of this thesis is to develop a control system that can achieve the following two main control functions: a) detection of unexpected indoor conditions that may result in unnecessary power consumption and b) energy efficiency control regarding optimal balancing of two parameters: the required energy consumption for heating, versus thermal comfort of the occupants. Methods of both orientations were developed in a multi-zone space composed of nine zones where each zone is equipped with a wireless node consisting of temperature and occupancy sensors while all the scattered nodes together form a wireless sensor network (WSN). The main methods of both control functions utilize the potential of the deterministic subspace identification (SID) predictive model which provides the predicted temperature of the zones. In the main method for detecting unexpected situations that can directly affect the thermal condition of the indoor space and cause energy consumption (abnormal situations), the predictive temperature from the SID model is compared with the real temperature and thus possible temperature deviations that indicate unexpected situations are detected. The method successfully detects two situations: the high infiltration gain due to unexpected cold air intake from the external surroundings through potential unforeseen openings (windows, exterior doors, opened ceilings etc) as well as the high heat gain due to onset of fire. With the support of the statistical algorithm for abrupt change detection, Cumulative Sum (CUSUM), the detection of temperature deviations is accomplished with accuracy in a very short time. The CUSUM algorithm is first evaluated at an initial approach to detect power diversions due to the above situations caused by the aforementioned exogenous factors. The predicted temperature of the zone from the SID model utilized appropriately also by the main method of the second control function for energy efficiency control. The time needed for the temperature of a zone to reach the thermal comfort zone threshold from a low initial value is measured by the predicted temperature evolution, and this measurement bases the logic of a control criterion for applying proactive heating to the unoccupied zones or not. Additional key points for the control criterion of the method is the occupation time of the zones as well as the remaining time of the occupants in the occupied zones. Two scenarios are examined: the first scenario with two adjacent zones where the one is occupied and the other is not, and the second scenario with a multi-zone space where the occupants are moving through the zones in a cascade mode. Gama and Pareto probability distributions modeled the occupation times of the two-zone scenario while exponential distribution modeled the cascade scenario as the least favorable case. The mobility of the occupants modeled with a semi-Markov process and the method provides satisfactory and reasonable results. At an initial approach the proactive heating of the zones is evaluated with specific algorithms that handle appropriately the occupation time into the zones.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Architecture, 1995.
Includes bibliographical references.
The parameters for a dynamic, computer simulation model were developed. The parameters characterize the physical and geometric properties of a building shell, the internal and external building loads, the buildings secondary systems, and the plant or primary energy source. The purpose of the model is to provide a standard testbed for the evaluation of control algorithms and strategies related to variable air volume HVAC systems. This work was conducted in collaboration with, and under subcontract to Loughborough University of Technology, Loughborough England. The prototype building is a four level commercial, multi-use building and activities in the building include classroom / educational space professorial and student offices, and office / administrative. The building contains three air-handling units; one unit and the volume it serves provides the basis for the testbed. The model volume is divided into thirty four zones, each with its own single duct, pressure independent V A V terminal box with hot water reheat. A perimeter heating system, composed of hot water convectors, radiators and baseboard heaters, augments the room comfort control system. Local loop control in the mechanical room and for all but one of the zones is micro-processor based pneumatic actuated. One prototype direct digital control terminal box system was in use for a classroom zone. DDC control systems and motor driven actuators were substituted in the testbed for the pneumatic equipment. Zoning in the volume was redistributed into six zones; the supply and return duct system was redesigned to accommodate the simplified zone configuration. A survey was conducted to determine the availability of sub-one-hour solar and collateral weather data. Historically, data in this frequency has been collected, but, not reported. A relatively new program called the Automated Surface Observing System (AS OS) and operated by the National Oceanic and Atmospheric Administration will eventually provide weather data at varying intervals down to one minute, depending on the type of information required. Daily and monthly summaries are available, however, resolution is reduced and averaged to one hour intervals. The SOLMET program, under the auspices of DOE, provides archived solar data at one hour intervals on CD Rom. Data is collected from twenty-six stations distributed around the United States. Collateral weather data is also provided with the solar data and for simulation purposes the SOLMET data provides the best resource.
by Mark DeSimone.
M.S.

Background. One of the challenges about smart control valves system is processing and analyzing sensors data to extract useful information. These types of information can be used to detect the deviating behaviors which can be an indication of faults and issues in the system. Outlier detection is a process in which we try to find these deviating behaviors that occur in the system.Objectives. First, perform a literature review to get an insight about the machine learning (ML) and data mining (DM) techniques that can be applied to extract patternfrom time-series data. Next, model the operating behaviors of the control valve system using appropriate machine learning and data mining techniques. Finally,evaluate the proposed behavioral models on real world data.Methods. To have a better understanding of the different ML and DM techniques for extracting patterns from time-series data and fault detection and diagnosis of building systems, literature review is conducted. Later on, an unsupervised learning approach is proposed for modeling the typical operating behaviors and detecting the deviating operating behaviors of the control valve system. Additionally, the proposed method provides supplementary information for domain experts to help them in their analysis.Results. The outcome from modeling and monitoring the operating behaviors ofthe control valve system are analyzed. The evaluation of the results by the domain experts indicates that the method is capable of detecting deviating or unseen operating behaviors of the system. Moreover, the proposed method provides additional useful information to have a better understanding of the obtained results.Conclusions. The main goal in this study was achieved by proposing a method that can model the typical operating behaviors of the control valve system. The generated model can be used to monitor the newly arrived daily measurements and detect the deviating or unseen operating behaviors of the control valve system. Also, it provides supplementary information that can help domain experts to facilitate and reduce the time of analysis.

Au cours des deux dernières décennies, la prise de conscience du changement climatique et des conséquences du réchauffement climatique a incité diverses institutions à prendre de nouvelles directives. Ces directives portent principalement sur le contrôle des émissions des gaz à effet de serre, sur l'utilisation des ressources énergétiques non conventionnelles et l'optimisation de la consommation d'énergie dans les systèmes existants. L'Union européenne a proposé de nombreux projets dans le cadre du 7e PCRD pour réaliser jusqu'à 20% d’économies d'énergie d’ici 2020. En particulier, selon la directive sur l'efficacité énergétique, les bâtiments sont majoritairement responsables de 40% des dépenses énergétiques en Europe et de 36% des émissions de CO2 ; c’est la raison pour laquelle un ensemble d’initiatives européennes dans le cadre du 7ième PCRD favorise l'utilisation de technologie intelligente dans les bâtiments et rationalise les règles existantes. Energy IN TIME est l'un des projets axés sur l'élaboration d'une méthode de contrôle basée sur la simulation intelligente de l'énergie qui permettra de réduire la consommation des bâtiments non résidentiels. Ce mémoire de thèse propose plusieurs solutions novatrices pour réaliser les objectifs du projet mandaté à l'Université de Lorraine. Les solutions développées dans le cadre de ce projet devraient être validées sur différents sites européens de démonstration. Une première partie présente l'analyse détaillée de ces sites de démonstration et leurs contraintes respectives. Un cadre général correspondant à la construction type de ces sites a été élaboré pour simuler leur comportement. Ce cadre de construction de référence sert de banc d'essai pour la validation des solutions proposées dans ce travail de thèse. Sur la base de la conception de la structure de construction de référence, nous présentons une formulation de contrôle économique utilisant un modèle de contrôle prédictif minimisant la consommation d'énergie. Ce contrôle optimal possède des propriétés de contrôle conscientes de la maintenance. En outre, comme les bâtiments sont des systèmes complexes, les occurrences de pannes peuvent entraîner une détérioration de l'efficacité énergétique ainsi que du confort thermique pour les occupants à l'intérieur des bâtiments. Pour résoudre ce problème, nous avons élaboré une stratégie de diagnostic des dysfonctionnements et une stratégie de contrôle adaptatif des défauts basé sur le modèle économique ; les résultats en simulation ont été obtenus sur le bâtiment de référence. En outre, l'application des solutions proposées peut permettre de relever des défis ambitieux en particulier dans le cas de bâtiments à grande échelle. Dans la partie finale de cette thèse, nous nous concentrons sur le contrôle économique des bâtiments à grande échelle en formulant une approche novatrice du contrôle prédictif de mode réparti. Cette formule de contrôle distribué présente de nombreux avantages tels que l'atténuation de la propagation des défauts, la flexibilité dans la maintenance du bâtiment et les stratégies simplifiées de contrôle du plug-and-play. Enfin, une attention particulière est accordée au problème d'estimation des mesures dont le nombre est limité sur des bâtiments à grande échelle. Les techniques d'estimation avancées proposées sont basées sur les méthodologies de l'horizon mobile. Leur efficacité est démontrée sur les systèmes de construction de référence
Since the last two decades, there has been a growing awareness about the climate change and global warming that has instigated several Directorate initiatives from various administrations. These initiatives mainly deal with controlling greenhouse gas emissions, use of non-conventional energy resources and optimization of energy consumption in the existing systems. The European Union has proposed numerous projects under FP7 framework to achieve the energy savings up to 20% by the year 2020. Especially, stated by the Energy Efficiency Directive, buildings are majorly responsible for 40% of energy resources in Europe and 36% of CO2 emission. Hence a class of projects in the FP7 framework promotes the use of smart technology in the buildings and the streamline existing rules. Energy IN TIME is one of the projects focused on developing a Smart Energy Simulation Based Control method which will reduce the energy consumption in the operational stage of existing non-residential buildings. Essentially, this thesis proposes several novel solutions to fulfill the project objectives assigned to the University of Lorraine. The developed solutions under this project should be validated on the demonstration sites from various European locations. We design a general benchmark building framework to emulate the behavior of demonstration sites. This benchmark building framework serves as a test bench for the validation of proposed solutions given in this thesis work. Based on the design of benchmark building layout, we present an economic control formulation using model predictive control minimizing the energy consumption. This optimal control has maintenance-aware control properties. Furthermore, as in buildings, fault occurrences may result in deteriorating the energy efficiency as well as the thermal comfort for the occupants inside the buildings. To address this issue, we design a fault diagnosis and fault adaptive control techniques based on the model predictive control and demonstrate the simulation results on the benchmark building. Moreover, the application of these proposed solutions may face great challenges in case of large-scale buildings. Therefore, in the final part of this thesis, we concentrate on the economic control of large-scale buildings by formulating a novel approach of distributed model predictive control. This distributed control formulation holds numerous advantages such as fault propagation mitigation, flexibility in the building maintenance and simplified plug-and-play control strategies, etc... Finally, a particular attention is paid to the estimation problem under limited measurements in large-scale buildings. The suggested advanced estimation techniques are based on the moving horizon methodologies and are demonstrated on the benchmark building systems

This thesis considers the development of a Hammerstein-bilinear approach to non-linear systems modelling, analysis and control systems design, which builds on and extends the applicability of an existing bilinear approach. The underlying idea of the Hammerstein-bilinear approach is to use the Hammerstein-bilinear system models to capture various physical phenomena of interest and subsequently use these for model based control system designs with the premise being that of achieving enhanced control performance. The advantage of the Hammerstein-bilinear approach is that the well-structured system models allow techniques that have been originally developed for linear systems to be extended and applied, while retaining moderate complexity of the corresponding system identification schemes and nonlinear model based control designs. In recognition of the need to be able to identify the Hammerstein-bilinear models a unified suite of algorithms, being the extensions to the simplified refined instrumental variable method for parameter estimation of linear transfer function models is proposed. These algorithms are able to operate in both the continuous-time and discrete-time domains to reflect the requirements of the intended purposes of the identified models with the emphasis being placed on straightforward applicability of the developed algorithms and recognising the need to be able to operate under realistic practical system identification scenarios. Moreover, the proposed algorithms are also applicable to parameter estimation of Hammerstein and bilinear models, which are special cases of the wider Hammerstein-bilinear model class. The Hammerstein-bilinear approach has been applied to an industrial heating, ventilation and air conditioning (HVAC) system, which has also been the underlying application addressed in this thesis. A unique set of dynamic control design purpose oriented air temperature and humidity Hammerstein-bilinear models of an environmentally controlled clear room manufacturing zone has been identified. The greater insights afforded by the knowledge of the system nonlinearities then allow for enhanced control tuning of the associated commercial HVAC control system leading to an improved overall control performance.

The present work deals with a study related to the analysis of weather data for heat pump system control improvement based on both instantaneous and forecasted measurements. In particular, the analysis is firstly focused on the comparison of multiple weather sources for the assessment of weather forecast uncertainties, based on the evaluation of errors in prediction with respect to measured values. Afterwards, the results are compared with the ones related to persistent predictions methods that assumes the state of the atmosphere to be stationary over the considered time interval. The development and testing of a new preliminary “predictive” control logic is also performed, thanks to TRNSYS numerical simulations, considering a typical Swedish single-family house located in Stockholm, with the aim of optimizing the operation of a heat pump heating system based on solar radiation prediction to yield energy and cost savings. With the crucial points of accuracy and precision by which the local weather processes can be predicted, the same TRNSYS model is run accounting for perfect predictions and solar radiation forecasted values. From this perspective, given the fact that forecast of solar radiation are usually absent within most of the weather forecast datasets, a deep analysis is also performed on hourly measurements of solar radiation to define a simple and effective methods to calculate hourly solar radiation predictions. The results show that, when a short-time horizon is considered, persistent predictions allow to provide forecasts with a sufficient accuracy, whereas, when longer horizon time are considered, significantly higher errors are calculated when persistent prediction techniques are adopted. Independently of the uncertainties considered for weather forecasts, the improved control logics demonstrated a potential for energy savings and improvements in indoor temperature stability when compared with a reference case of variable speed compressor with PID controller.
EffSys Expand P18: Smart Cotnrol Strategies for Heat Pump Systems

The present work reports a study related to the potential improvement of the energy performances of a heat pump based heating system for a Swedish single-family house. The analysis is focused on the design of new rule-based control strategies which employ perfect predictions of weather forecast and human behaviour information. In particular, the considered signals are the outdoor temperature, the solar radiation, the internal gain due to inhabitants’ activities and the Domestic Hot Water (DHW) consumption. The study is performed by means of the TRNSYS® simulation software in which the model of the heating system is implemented. More specifically, it is composed by a Ground Source Heat Pump (GSHP) unit, a stratified storage tank of three hundred litres and the building element. The performances of the developed control logics are evaluated using a degree-minute on/off controller as reference case. The results show that the improved control logics yield to an increase of the energy efficiency of the system as well as an enhancement of the indoor and DHW temperatures stability.
EffSys Expand P18: Smart Cotnrol Strategies for Heat Pump Systems

För att uppnå Sveriges miljömål begränsad klimatpåverkan behöver åtgärder göras för att minska energianvändningen inom bostads- och servicesektorn, vilken stod för drygt en tredjedel av Sveriges totala energianvändning år 2014. Dålig styrning av värme-, ventilations- och kylsystem är ofta en bidragande orsak till onödigt hög energianvändning inom hushåll och lokalbyggnader. Med hjälp av fastighetsautomation samlas byggnadens tekniska system i ett styr- och övervakningssystem, vilket kan användas för att optimera styrningen. Dock är marknaden för fastighetsautomation dåligt kartlagd och information saknas angående styrsystemens funktioner och användningsområden. Rapporten inleds med grundläggande teori för att skapa ökad förståelse för senare delar. Därefter har en undersökning gjorts för ett antal företags styrsystem i syfte att klargöra dess funktioner, användningsområden samt vilken energibesparing styrsystemen ger upphov till. I rapporten beskrivs också tillvägagångssättet för en verklig installation i lokalbyggnaden Kv Slottet 4 och hur energianvändning samt inneklimat har påverkats efter att styrsystemet installerats. Resultatet för den svenska marknadsöversikten visar på att fastighetsautomation är etablerat inom byggnadstyperna flerbostadshus och lokalbyggnader medan småhus är en outforskad marknad. Vanliga funktioner som implementeras i styrsystem är drifttidsstyrning, prognosstyrning och effektbegränsning. Undersökningen har visat att fastighetsautomation ger upphov till energibesparingar på i snitt 10 – 40 %. Besparingen varierar dock och beror på flera faktorer som byggnadstyp, geografisk placering och tidigare styrning. Företagen använder också olika metoder för att beräkna energibesparingen varför det är problematiskt att rättvist jämföra besparingen för olika styrsystem. Energibesparingen för de olika byggnadstyperna och installationssystemen har i många fall varit svåra att få tillgång till. Generellt gäller att de företag som tillhandahåller obligatoriskt driftavtal har bättre koll på den besparing som styrsystemet ger upphov till. I examensarbetet har fyra företag kartlagts men för en mer djupgående analys över den svenska marknaden bör fler företag undersökas. Efter installationen av det automatiska styrsystemet Ecopilot® på Kv Slottet 4 erhölls en värmeenergibesparing för vald period på upp till 20,6 MWh, vilket motsvarar en procentuell besparing på 53 %. Energibesparingen varierar dock till viss del beroende på vilken beräkningsmetod som används. Byggnadens totala elbesparing uppgick till 6,5 MWh, vilket motsvarar 20 % minskning av elanvändningen för undersökt period. Den beräknade energibesparingen kan inte helt tillskrivas Ecopilot® eftersom ett annat driftfall rådde under jämförd period än det som var precis innan installationen. Via styrsystemets gränssnitt kunde emellertid en sänkt energianvändning härledas till Ecopilot®. Bland annat tack vare effektivare reglering, samkörning av installationssystem, effektivare värmeåtervinning, sänkta ventilationsflöden samt lägre inomhus- och flödestemperaturer. Intervjuer med verksamma personer i byggnaden påvisar att ingen skillnad har märkts på inomhusklimatet före och efter installationen av Ecopilot®, varken med avseende på det termiska klimatet eller på luftkvaliten. För större säkerhet gällande styrsystemets inverkan på inomhusklimatet och energianvändningen bör en längre tidsperiod utvärderas.
Sweden has a number of environmental objectives, of which one is reduced climate impact. To achieve this goal measures have to be taken in order to reduce building energy use in residential and services sectors, sectors that accounted for over one third of Sweden’s total energy use in 2014. Poor regulation of heating, ventilation, and air conditioning (HVAC) is a common reason for often unnecessary high energy use in residential buildings and non-residential premises. Building automation integrates the buildings technical systems, such as HVAC, into one centralized system for monitoring and controlling, which then can be used to optimize the regulation for these systems. The market is however poorly mapped and information is missing regarding functions and area of use for these control systems. This report begins with a description of basic theory that is relevant for understanding parts later on. An analysis has then been made for a number of different building automation systems to clarify their functions, area of use, and potentials in energy savings. The report also describes the procedure for a real installation on Kv Slottet 4 and how the energy use and indoor climate has been affected after the building automation system has been installed. Results regarding the Swedish market overview shows that building automation is established in apartment buildings and non-residential premises while building automation is still unused in smaller houses. Some of the most common functions that are implemented in control systems are time clock operation, forecast control and power limitation. The analysis has shown that building automation can lead to energy savings between 10 to 40 %. The size of the saved energy depends on a number of factors like type of building, geographical location and former regulation of HVAC. The building automation companies are using different methods to calculate the energy saving why it is a problem to fairly compare different control systems. The information on energy savings for different type of buildings and technical systems are in many cases hard to get hold of. Generally speaking, the companies that provide mandatory agreements for operating the control system have more detailed information regarding energy savings. Four different companies were covered in this report. More companies should be mapped in order to make a more profound analysis. After the installation of Ecopilot® on Kv Slottet 4, a heat reduction of up to 20.6 MWh for the chosen period of time was achieved. That corresponds to a relative energy saving of 53 %. The energy saving varies depending on what method is used for calculation. The buildings total energy savings for electricity was 6.5 MWh, which corresponds to a 20 % reduction. The calculated energy saving is not all due to the building automation system but there has been some reduction. Ecopilot® has lowered the energy use by more efficient regulation, co-operating heating and cooling, more efficient heat recovery, lowered ventilation flows, and lowered temperatures. No change in indoor climate has been noticed by the people that were interviewed. The control system should be evaluated for a longer period of time to ensure how much it affects the indoor climate and the energy use in the building.

ITC/USA 2011 Conference Proceedings / The Forty-Seventh Annual International Telemetering Conference and Technical Exhibition / October 24-27, 2011 / Bally's Las Vegas, Las Vegas, Nevada
Much of the energy consumed in developed countries is for residential heating and cooling. Substantial savings are possible if one can monitor the indoor environment at many locations, and then actively control the heating, ventilation and air conditioning (HVAC) system. This project uses a wireless sensor array and dedicated microcontroller system to control a residential HVAC system. A low data rate, ad-hoc network of sensors is deployed throughout a residence, with the data sent to a central controller. A graphical user interface allows the resident to monitor the system status, and to set parameters.

The South African economy, which is largely based on heavy industry such as minerals extraction and processing, is by nature very energy intensive. Based on the abundance of coal resources, electricity in South Africa remains amongst the cheapest in the world. Whilst the low electricity price has contributed towards a competitive position, it has also meant that our existing electricity supply is often taken for granted. The economic and environmental benefits of energy efficiency have been well documented. Worldwide, nations are beginning to face up to the challenge of sustainable energy - in other words to alter the way that energy is utilised so that social, environmental and economic aims of sustainable development are supported. South Africa as a developing nation recognises the need for energy efficiency, as it is the most cost effective way of meeting the demands of sustainable development. South Africa, with its unique economic, environmental and social challenges, stands to benefit the most from implementing energy efficiency practices. The Energy Efficiency Strategy for South Africa takes its mandate from the South African White Paper on Energy Policy. It is the first consolidated governmental effort geared towards energy efficiency practices throughout South Africa. The strategy allows for the immediate implementation of low-cost and no-cost interventions, as well as those higher-cost measures with short payback periods. An initial target has been set for an across sector energy efficiency improvement of 12% by 2014. Thermal and energy system simulation is globally recognised as one of the most effective and powerful tools to improve overall energy efficiency. However, because of the usual extreme mathematical nature of most simulation algorithms, coupled with the historically academic environment in which most simulation software is developed, valid perceptions exist that system simulation is too time consuming and cumbersome. It is also commonly known that system simulation is only effective in the hands of highly skilled operators, which are specialists in their prospective fields. Through previous work done in the field, and the design of a dynamic thermal and energy system simulation scheme for cross industry applications, it was shown that system simulation has evolved to such an extent that these perceptions are not valid any more. The South African mining and commercial building industries are two of the major consumers of electricity within South Africa. By improving energy efficiency practices within the building and mining industry, large savings can be realised. An extensive investigation of the literature showed that no general suitable computer simulation software for cross industry mining and building thermal and energy system simulation could be found. Because the heating, ventilation and air conditioning (HVAC) of buildings, closely relate to the ventilation and cooling systems of mines, valuable knowledge from this field was used to identify the requirements and specifications for the design of a new single cross industry dynamic integrated thermal and energy system simulation tool. VISUALQEC was designed and implemented to comply with the needs and requirements identified. A new explicit system component model and explicit system simulation engine, combined with a new improved simulation of mass flow through a system procedure, suggested a marked improvement on overall simulation stability, efficiency and speed. The commercial usability of the new simulation tool was verified for building applications by doing an extensive building energy savings audit. The new simulation tool was further verified by simulating the ventilation and cooling (VC) and underground pumping system of a typical South African gold mine. Initial results proved satisfactory but, more case studies to further verify the accuracy of the implemented cross industry thermal and energy system simulation tool are needed. Because of the stable nature of the new VISUALQEC simulation engine, the power of the simulation process can be further extended to the mathematical optimisation of various system variables. In conclusion, this study highlighted the need for new simulation procedures and system designs for the successful implementation and creation of a single dynamic thermal and energy system simulation tool for cross industry applications. South Africa should take full advantage of the power of thermal and energy system simulation towards creating a more energy efficient society.
Thesis (Ph.D. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2005.

The North Sea has a vast amount of wind energy with largest energy per area densities located about 100-300Km of distance from shore. Should this energy be tapped by offshore wind farms, HVDC transmission would be the more feasible solution at such long subsea distances. On the other hand Norwegian oil/gas platforms in the North Sea use electricity from gas fired turbines at offshore sites. These gas turbines have much less efficiency than onshore generation of electricity and also release large amounts of green house gases. Therefore supplying the platforms with power from onshore transmitted by HVDC will result in benefits both from economic and environmental protection perspectives. Given these two interests for HVDC in the Norwegian offshore, the use of Multiterminal HVDC (MTDC) is a potential solution for the integration of the wind farms and oil/gas platforms into the onshore grid system. Hence, this thesis focuses on the operation and control of MTDC systems. The MTDC system is desired to be capable of interfacing with all kinds of AC grids namely: stiff, weak and passive grid systems. Compared to the classical thyristor based converter, VSC has several features that make it the most suitable converter for making of MTDC, the most decisive being its ability of bidirectional power transfer for fixed voltage polarity. VSC-HVDC is also suitable for implementing control of active and reactive current in synchronously rotating d-q reference frame which in turn results in decoupled control of active and reactive power. In the first two chapters of the thesis literatures are reviewed to understand operation of VSC and its use in HVDC systems. Afterwards controllers are developed for different AC connections (stiff, weak and passive) and for different DC parameter (power, DC voltage) control modes. DC voltage and active power control are implemented by active current control and AC voltage and reactive power control are achieved by reactive power compensation. Tuning techniques for the PI controllers are discussed and used in the simulation models. Finally control techniques for reliable operation of MTDC are developed. In order to validate theoretical arguments, each of the control schemes was developed and simulated in PSCAD/EMTDC simulation software. Simulation results indicate that satisfactory performance of VSC-HVDC was obtained with the proposed active/reactive power controllers, AC/DC voltage controllers, frequency and DC overvoltage controllers. For coordinated multiterminal operation, voltage margin control method and DC voltage droop characteristic were used. These are control methods based upon realization of desired P-UDC characteristic curves of converter terminals. Four-terminal MTDC system with different AC grid connections was used to study the multiterminal operation. Simulations have shown that voltage margin control method results in reliable operation of MTDC during loss of a terminal connection without the need for communication between terminals. The use of DC voltage droop control along with voltage margin control enabled load sharing among VSC-HVDC terminals in DC voltage control mode according to predetermined participation factor.

Thèse (Ph.D.)-- École de technologie supérieure, Montréal, 2005.
"Thesis presented to the École de technologie supérieure in partial fulfiliment [i.e. fulfillment] of the thesis requirement for the degree of philosophiae doctor in engineering". Bibliogr.: f. [178]-184. Également disponible en version électronique.

Dynamic security limits the power transfer capacity between regions and therefore has an economic impact. The power modulation control of high-voltage direct current (HVDC) links can improve the dynamic security of the power system. Having several HVDC links in a system creates the opportunity to coordinate such control, and coordination also ensures that negative interactions do not occur among the controllable devices. This thesis aims to increase dynamic security by coordinating HVDC links, as an alternative to decreasing the transfer capacity. This thesis contributes four control approaches for increasing the dynamic stability, based on feedforward control, adaptive control, optimal control, and exact-feedback linearization control. Depending on the available measurements, dynamic system model, and system topology, one of the developed methods can be applied. The wide-area measurement system provides the central controller with real-time data and sends control signals to the HVDC links. The feedforward controller applies rapid power dispatch, and the strategy used here is to link the N-1 criterion between two systems. The adaptive controller uses the modal analysis approach; based on forecasted load paths, the controller gains are adaptively adjusted to maximize the damping in the system. The optimal controller is designed based on an estimated reduced-order model; system identification develops the model based on the system response. The exact-feedback linearization approach uses a pre-feedback loop to cancel the nonlinearities; a stabilizing controller is designed for the remaining linear system. The conclusion is that coordinating the HVDC links improves the dynamic stability, which makes it possible to increase the transfer capacity. This conclusion is also supported by simulations of each control approach.
QC 20110302

Commissioning of HVAC systems has potential for significant improvements in occupant satisfaction, comfort and energy consumption, but is very labour-intensive and expensive as practiced at this time. Previous investigators have capitalized on digital control systems' capability of logging and storing data and of interfacing with external computers for open loop control by developing methods of automated fault detection and diagnosis during normal operation. Some investigators have also considered the application of this technique in commissioning. This thesis investigates the possibility of utilizing first principles and empirical models of air-handling unit components to represent correct operation of the unit during commissioning. The models have parameters whose values can be determined from engineering design intent information contained in the construction drawings and other data available at commissioning time. Quasi-dynamic models are developed and tested. The models are tested against design intent information and also against data from a real system operating without known faults. The results show the models agree well with the measured data except for some false positive indications, particularly in the damper and fan models, during transients. A procedure for estimating uncertainty in the instrumentation and the models is developed. The models are also tested against artificial faults and are able to detect all of the faults. Methods of diagnosing the faults are discussed.

With recent advances in power electronic technology, high-voltage direct current (HVDC) transmission system has become an alternative for transmitting power, especially over long distances. Multi-terminal HVDC (MTDC) systems are proposed as HVDC systems with more than two terminals. In addition, the wind is becoming one of the most important sources of renewable energy in the world, with vast sources available in offshore areas. MTDC systems are attractive solutions for connecting offshore wind farms to AC grids.   This thesis discusses three scopes of MTDC systems: primary control, secondary control, and AC-DC transmission expansion planning.  In the primary control part, sliding mode control and multi-agent control are proposed. The sliding mode control can control the system fast and with very small overshoot and compared to proposed methods in the literature, it is less sensitive to changes in parameters. In the proposed multi-agent control strategy, we aim to find a solution for the problems caused by lack of global signal in the control of MTDC systems.   In the secondary control part, we propose a controller, based on multi-agent systems, which follows the variations of wind and minimize the DC transmission and conversion losses, while considering the price of energy in each AC system and the scheduled injected power to each AC grid. The controller operates in both centralized and distributed modes. In the expansion planning part, we aim to propose a methodology to determine the optimal configuration of the MTDC system. The goal is to maximize the transferred power from the wind farms to the onshore grids while minimizing the investment cost. We propose a two-stage mixed-integer second order cone program (MISOCP) for optimal expansion of both DC and AC networks. The two-stage MISOCP is solved using the parallelized Benders decomposition algorithm.

QC 20170908

This thesis investigates the robust and coordinated design of multiple damping controllers to ameliorate the damping characteristics of a bulky power system. A new methodology is proposed in this thesis for VSC-MTDC and FACTS damping controllers based on multiple control objectives and system multi-model. The key feature of the methodology is the robust and coordinated performance of the damping controllers. The formulated BMI-based optimization problem is solved systematically via a two- step approach. System multi-model is established in the design for the robustness of the controllers under system disturbances and changing operating conditions. The sequential design of a series of SISO controllers with properly selected feedback signals minimizes the negative interactions among the controllers. The approach is applied to a three-terminal VSC-MTDC and subsequently exerted with one terminal of VSC-MTDC and a TCSC to incorporate multiple devices and examine the generality and feasibility of the design. Given the flexible internal control configuration of VSC converter, the assessment of the impact of the d-q decoupled control modes on the effectiveness and flexibility of the damping controllers is carried out. Real-Time Digital Simulator is used to examine the effectiveness and robustness of the damping controllers under various system operating conditions and disturbances.

A novel hybrid HVDC system is proposed based on the traditional LCC HVDC system. The proposed system is able to achieve full elimination of commutation failures which cannot be achieved in traditional LCC HVDC systems. In addition, reactive power controller is designed for the hybrid HVDC system. The controller is able to achieve zero reactive power exchange with the connected AC system at inverter side. It can also facilitate a faster fault recovery. Finally, the black start capability of the hybrid system is investigated. The black start sequence and inverter AC voltage controller are designed to achieve smooth and reliable black start of inverter AC system. The performances of the proposed system and controller are validated through detailed simulations in Real Time Digital Simulator (RTDS).

Voltage source converter high voltage direct current (VSC-HVDC) technology has become increasingly cost-effective and technically feasible in recent years. It is likely to play a vital role in integrating remotely-located renewable generation and reinforcing existing power systems. Multi-terminal VSC-HVDC (MTDC) systems, with superior reliability, redundancy and flexibility over the conventional point-to-point HVDC, have attracted a great deal of attention globally. MTDC however remains an area where little standardisation has taken place, and a series of challenges need to be fully understood and tackled before moving towards more complex DC grids. This thesis investigates modelling, control and stability of MTDC systems. DC voltage, which indicates power balance and stability of DC systems, is of paramount importance in MTDC control. Further investigation is required to understand the dynamic and steady-state behaviours of various DC voltage and active power control schemes in previous literature. This work provides a detailed comparative study of modelling and control methodologies of MTDC systems, with a key focus on the control of grid side converters and DC voltage coordination. A generalised algorithm is proposed to enable MTDC power flow calculations when complex DC voltage control characteristics are employed. Analysis based upon linearised power flow equations and equivalent circuit of droop control is performed to provide further intuitive understanding of the steady-state behaviours of MTDC systems. Information of key constraints on the stability and robustness of MTDC control systems has been limited. A main focus of this thesis is to examine these potential stability limitations and to increase the understanding of MTDC dynamics. In order to perform comprehensive open-loop and closed-loop stability studies, a systematic procedure is developed for mathematical modelling of MTDC systems. The resulting analytical models and frequency domain tools are employed in this thesis to assess the stability, dynamic performance and robustness of active power and DC voltage control of VSC-HVDC. Limitations imposed by weak AC systems, DC system parameters, converter operating point, controller structure, and controller bandwidth on the closed-loop MTDC stability are identified and investigated in detail. Large DC reactors, which are required by DC breaker systems, are identified in this research to have detrimental effects on the controllability, stability and robustness of MTDC voltage control. This could impose a serious challenge for existing control designs. A DC voltage damping controller is proposed to cope with the transient performance issues caused by the DC reactors. Furthermore, two active stabilising controllers are developed to enhance the controllability and robust stability of DC voltage control in a DC grid.

Recent issues such as priority access of renewable resources recommended by European energy directives and increase the electricity trading among countries lead to new requirements on the operation and expansion of transmission grids. Since AC grid expansions are limited by legislative issues and long distance transmission capacity, there is a considerable attention drawn to application of HVDC transmission grids on top of, or in complement to, existing AC power systems. The secure operation of HVDC grids requires a hierarchical control system. In HVDC grids, the primary control action to deal with power or DC voltage deviations is communication-free and local. In addition to primary control, the higher supervisory control actions are needed to guarantee the optimal operation of HVDC grids. However, the implementation of supervisory control functions is linked to the arrangement of system operators; i.e. an individual HVDC operator (central structure) or sharing tasks among AC system operators (distributed structure). This thesis presents distributed control of an HVDC grid. To this end, three possible supervisory functions are investigated; coordination of power injection set-points, DC slack bus selection and network topology identification. In this thesis, all three functions are first studied for the central structure. For the distributed solution, two algorithms based on Alternating Direction Method of Multipliers (ADMM) and Auxiliary Problem Principle (APP) are adopted to solve the coordination of power injection. For distributed selection of DC slack bus, the choice of parameters for quantitative ranking of converters is important. These parameters should be calculated based on local measurements if distributed decision is desired. To this end, the short circuit capacity of connected AC grid and power margin of converters are considered. To estimate the short circuit capacity as one of the required selection parameters, the result shows that the recursive least square algorithm can be very efficiently used. Besides, it is possible to intelligently use a naturally occurring droop response in HVDC grids as a local measurement for this estimation algorithm. Regarding the network topology, a two-stage distributed algorithm is introduced to use the abstract information about the neighbouring substation topology to determine the grid connectivity.

QC 20170306

Meshed high-voltage direct current grids are becoming an increasingly important technology for integrating renewable energies into the power system. To control the grids in the best possible way, optimal converter and grid control strategies are needed. This project studies how a four-terminal high-voltage direct current grid is operated and controlled by implementing different grid and converter control strategies. The grid control strategies examined are centralized voltage control and distributed voltage control with and without deadband. Simulations are made in the software PSCAD. Different fault types on the grid are studied to investigate how the power flow and voltage level are affected. An optimal value for both the deadband width and droop constant has been identified. Moreover, the results indicate that centralized droop control is not a suitable grid control strategy, whereas distributed voltage control with and without deadband are. The fault study indicates no differences between distributed voltage control with and without deadband. The power flow and voltage levels are identical for all fault types.
Högspända likströmsnät spelar en allt större roll med att integrera förnyelsebar energi i våra elnät. För att styra dessa nät på bästa möjliga sätt krävs optimala omvandlar- och nätkontrollstrategier. I detta projekt studeras hur ett fyrterminalt högspänt likströmsnät kan styras och drivas genom att implementera olika omvandlar- och nätkontrollstrategier. De nätkontrollstrategier som studerats är centraliserad spänningskontroll och distribuerad spänningskontroll med och utan ett spänningsintervall. Alla simuleringar har utförts i programmet PSCAD. Olika fel i nätet har även studerats för att undersöka hur effektflödet och spänningsnivån påverkas. Ett optimalt värde på både spänningsintervallet och droop konstanten har identifierats. Dessutom har resultat som indikerar att centraliserad spänningskontroll inte är en lämplig nätkontrollstrategi erhållits, medan distribuerad spänningskontroll med och utan spänningsintervall är det. Felsimuleringarna påvisar ingen skillnad mellan distribuerad spänningskontroll med och utan spänningsintervall. Effektflödet och spänningsnivån är identiska för alla fel.

In this master thesis control systems for a voltage-source converter HVDC connected to weak ac networks are investigated. HVDC stands for high voltage direct current and is a way to transfer power in the electrical power system. A HVDC uses direct current (dc) instead of alternate current (ac) to transfer power, which requires transformation between ac and dc since most power grids are ac networks. The HVDC uses converters to transform ac to dc and dc to ac and the converter requires a control system. A complete control system of a voltage source converter HVDC contains many different parts. The part investigated in this thesis is the active power control. Different structures containing PID controllers have been tested and evaluated with respect to stability and performance using control theory. The impact of weak ac networks has been evaluated in regards to the different control structures. The investigations have been conducted using mainly steady-state simulations. Based on the simulation and analyzes of the simulation results a promising control structure has been obtained and suggested for further investigation.

L'intégration des liaisons à courant continu dans les systèmes électriques permet d’accroitre les possibilités de pilotage des réseaux, ce qui permet d’en améliorer la sûreté et de raccorder de nouveaux moyens de production. Pour cela la technologie VSC-HVDC est de plus en plus plébiscitée pour interconnecter des réseaux non synchrones, raccorder des parcs éoliens offshore, ou contrôler le flux d’énergie notamment sur des longues distances au travers de liaisons sous-marines (liaison NorNed). Les travaux de cette thèse portent sur la modélisation, la commande non-linéaire et la stabilisation des systèmes VSC–HVDC, avec deux axes de travail. Le premier se focalise sur la conception et la synthèse des lois de commandes non-linéaires avancées basées sur des systèmes de structures variables (VSS). Ainsi, les commandes par modes glissants (SMC) et le suivi asymptotique de trajectoire des sorties (AOT) ont été proposées afin d’assurer un degré désiré de stabilité en utilisant des fonctions de Lyapunov convenables. Ensuite, la robustesse de ces commandes face à des perturbations et/ou incertitudes paramétriques a été étudiée. Le compromis nécessaire entre la robustesse et le comportement dynamique requis dépend du choix approprié des gains. Ces approches robustes, qui sont facile à mettre en œuvre, ont été appliquées avec succès afin d’atteindre des performances dynamiques élevées et un niveau raisonnable de stabilité vis-à-vis des diverses conditions anormales de fonctionnement, pour des longueurs différentes de liaison DC. Le deuxième vise à étudier l’influence de la commande du convertisseur VSC-HVDC sur l'amélioration de la performance dynamique du réseau de courant alternatif en cas d’oscillations. Après une modélisation analytique d’un système de référence constitué d’un groupe connecté à un convertisseur VSC-HVDC via un transformateur et une ligne, un contrôleur conventionnel simple PI est appliqué au niveau du convertisseur du système pour agir sur les oscillations rotoriques de la machine synchrone. Cette commande classique garantie une amélioration acceptable des performances dynamiques du système; surtout pour l'amortissement des oscillations de l'angle de puissance de la machine synchrone lors de défauts
The integration of nonlinear VSC-HVDC transmission systems in power grids becomes very important for environmental, technical, and economic reasons. These systems have enabled the interconnection of asynchronous networks, the connection of offshore wind farms, and the control of power flow especially for long distances. This thesis aims the non-linear control and stabilization of VSC-HVDC systems, with two main themes. The first theme focuses on the design and synthesis of nonlinear control laws based on Variable Structure Systems (VSS) for VSC-HVDC systems. Thus, the Sliding Mode Control (SMC) and the Asymptotic Output Tracking (AOT) have been proposed to provide an adequate degree of stability via suitable Lyapunov functions. Then, the robustness of these commands has been studied in presence of parameter uncertainties and/or disturbances. The compromise between controller’s robustness and the system’s dynamic behavior depends on the gain settings. These control approaches, which are robust and can be easily implemented, have been applied to enhance the system dynamic performance and stability level in presence of different abnormal conditions for different DC link lengths. The second theme concerns the influence of VSC-HVDC control on improving the AC network dynamic performance during transients. After modeling the Single Machine via VSC-HVDC system in which the detailed synchronous generator model is considered, the conventional PI controller is applied to the converter side to act on damping the synchronous machine power angle oscillations. This simple control guarantees the reinforcement of the system dynamic performance and the power angle oscillations damping of the synchronous machine in presence of faults

This thesis investigates issues affecting grid synchronisation of VSC-HVDC systems with particular regard to, but not limited to, offshore wind power generation during the complex but potentially serious behaviours following solar storms. An averaged value model (AVM) for the contemporary modular multilevel converter (MMC) based VSC-HVDC system is developed and is used in combination with different phase-locked loop (PLL) models and the unified magnetic equivalent circuit (UMEC) transformer model to assess the impacts of geomagnetically induced current (GIC) on grid synchronisation of an offshore VSC-HVDC system. GIC is DC current flowing in the earth caused by strong geomagnetic disturbance events. GIC enters the electric utility grid via the grounded transformer neutral and can cause severe saturation to transformers. This in turn causes disruptions to grid synchronisation. The main contribution of this thesis is that effects of GIC are studied using the UMEC transformer model, which can model saturation. The assessment leads to the development of enhanced fundamental positive sequence control (EFPSC) which is capable of reducing the stress on the system during GIC events. The methods developed can also be applied to other non-symmetrical AC events occurring in VSC-HVDC such as single-phase faults. Additional contributions of the thesis are:A mathematical model of the MMC is derived and forms the foundation of the AVM. The AVM is verified against a detailed equivalent-circuit-based model and shows good accuracy. The PLL is the essential component for grid synchronisation of VSC-HVDC system. Different PLLs are studied in detail. Their performance is compared both qualitatively and quantitatively. This appears to have been done for the first time systematically in the public literature. The UMEC model is verified using hand calculation. Its saturation characteristic is matched to a predefined B-H curve and is also verified. The verifications show that this model is capable of modelling transformer saturation and thus is suitable for this study. The consolidation of the AVM, PLL, UMEC, GIC and EFPSC provides an insight into the how the MMC based VSC-HVDC system behaves under severe geomagnetic disturbances and the possible methods to mitigate the risks and impacts to the power grid.

Ces dernières années, les technologies à courant continu haute tension (en anglais, HVDC) basées sur les convertisseurs modulaires multiniveaux (MMC) sont adoptées comme solution pour l'intégration efficace des énergies renouvelables dans les réseaux électriques. Cependant, ces technologies présentent de nouveaux défis dans la façon dont les systèmes de transmission de puissance sont contrôlés et exploités, car des stratégies de contrôle plus rapides et plus complexes seront nécessaires dans un domaine qui repose aujourd'hui fortement sur la décision humaine. Dans ce contexte, la modélisation des systèmes à événements discrets (SED) et la théorie du contrôle par supervision (TCS) sont des outils puissants pour la synthèse de superviseurs qui assurent que le système à contrôler respecte un ensemble de spécifications comportementales, imposées par le concepteur, dans ses limites physiques. Ce travail propose une méthode pour le développement complet, de la conception à la mise en œuvre, du contrôle par supervision d'un système Multi-Terminal DC (MTDC). Une analyse du système considéré a été effectuée afin d'identifier les principaux composants et modes de fonctionnement du réseau. La solution proposée repose sur la modélisation par événements discrets du comportement en temps continu des composants du système. A partir de là, les concepts de la TCS sont appliqués de manière à obtenir une architecture de contrôle hiérarchique prenant en compte la priorité de certaines actions de contrôle à traiter au niveau local. De plus, les contrôleurs discrets obtenus présentent une structure de commutation de mode afin de réaliser une gestion de mode pendant le fonctionnement du réseau MTDC. Enfin, une méthode pour la mise en œuvre des contrôleurs obtenus dans un logiciel de simulation de système électrique répandu est proposée. L'ensemble dutravail a été validé par la simulation d'une étude de cas impliquant la gestion des modes d'un système MTDC bipolaire à trois terminaux
The growth of renewable energy production is changing the future of power transmission systems. In recent years, High-Voltage Direct Current (HVDC) technologies based on Modular Multilevel Converters (MMC) are embraced by industry and academia as a solution for the efficient integration of renewable energies into electrical grids. However, this type of technology introduces new challenges in the way power transmission systems are controlled and operated, as faster and more complex control strategies will be needed in a domain which nowadays relies heavily on human decision. In this context, Discrete Event Systems (DES) modeling and Supervisory Control Theory (SCT) are powerful tools for the synthesis of supervisors ensuring that the system to be controlled respects a set of behavioral specifications, imposed by the designer, within its physical limitations. This work proposes a method for the full development, from conception to implementation, of the supervisory control of a multi-terminal DC (MTDC) system. A functional analysis on the considered system has been done so as to identify the main components and operational modes of the grid. Then, the proposed solution is based on the discrete-event modeling of the continuous-time behavior of the components in the system. From there, SCT concepts are applied so as to obtain a hierarchical control architecture taking into account the priority of some control actions that should be treated at the local level. Furthermore, the obtained discrete controllers present a mode-switching structure in order to realize mode management during the operation of the MTDC grid. Finally, a method for the implementation of the obtained controllers in widespread power system simulation software is proposed. The whole work has been validated through the simulation of a case study, involving the mode management of a 3-terminal bipolar MTDC system

In recent years, there has been an increased development and deployment of renewable energy resources to meet the ever increasing electric power demand and to limit the use of fossil fuels. This has spurred offshore wind farm development, particularly in the North Sea, due to the vast offshore wind energy potential. Large scale wind farms in the North Sea pose grid integration challenges such as the need for long distance sub sea power transmission and managing the variability of wind power variation on in the power grid. These challenges can be properly met by the use of multi-terminal voltage source converter high voltage dc transmission (MTDC) grid. Even though the North Sea region is envisioned as the immediate target of application, MTDC can also be used as the highway of power in onshore systems, thereby connecting loads and generation sites involving very long distances. MTDC system consists of three or more HVDC converter stations connected to a common dc transmission network. Currently there are two types of converters used in HVDC, namely: line commutated converter (LCC) type and voltage source converter (VSC) type. VSC-HVDC is superior to LCC-HVDC for MTDC applications due to its flexibility and relative simplicity in power control, its bi-directional power transmission capability while keeping the dc voltage polarity unchanged and due to its reactive power support capabilities. Hence most recent research works in the area of MTDC have focused on VSC based systems only. Several R&D works have been done in the area of MTDC transmission, especially in its control aspects, in the past. In most cases however, only qualitative approaches have been used to describe the operational characteristics of the various proposed control strategies. In particular, studies exploring the quantitative analysis of the steady-state and dynamic operational characteristics of MTDC grids have been missing in the literature. The research work described in this thesis was started with the objective of filling some of these gaps, i.e. (1) to investigate the various control strategies of VSC for use in dc grids using both qualitative and quantitative approaches, (2) to propose improvements in the control and operation of MTDC systems and (3) to increase understanding of the dynamic behavior of MTDC systems. The main contributions of the research work can be put into three areas, namely MTDC control, MTDC operation and MTDC analysis. In the area of MTDC control grid frequency support strategy by MTDC has been proposed and analyzed. The control strategy enables exchange of primary reserves between asynchronous ac grids connected by MTDC without involving the use of communication systems between converter stations. In the area of MTDC operation a method for precise control of steady-state power flow in MTDC has been proposed and tested with simulation models. The method enables power injection control in MTDC based on the power dispatch and is applicable to any MTDC transmission topology. The contributions in the area of MTDC analysis comprise of three parts, namely: (1) steady-state MTDC interaction, (2) state-space modeling of a generalized MTDC topology and (3) large-disturbance stability analysis of MTDC. Methods have been proposed for each of the aforementioned MTDC analysis aspects and tested with various simulation models. In the integrated ac/dc system events, such as load insertion/ rejection in the ac grid will have impacts on grid frequencies as well as dc-bus voltages in the MTDC. The steady-state interaction analysis is used to estimate the steady-state changes (of frequencies, dc-bus voltages, transfered nodal powers) that come as a result of such events. The state-space modeling approach has been used to study the dynamic aspect of MTDC. The large-disturbance stability analysis has been principally proposed to study the impact of ac short circuit fault close to a converter terminal in rectifier mode of operation. Ac faults occurring close to inverters connected to MTDC transmission result in excess power in the MTDC during the fault duration. This however can be tackled by using fast acting dc voltage controllers and hence excess power in the dc grid may not as sever problem as shortage of power. In addition, secondary control of MTDC based upon steady-state sensitivity analysis has been proposed in the thesis. The method enables accurate compensation of power flow deviations in MTDC occurring due to the action of primary control. Various representative case models have been used to show the merits claimed for each of the proposed control methods, and to verify the validities of the proposed operational strategies and MTDC analysis approaches. Finally a hypothetical dc grid scenario has been used to demonstrate the various potential benefits of employing MTDC in the North Sea region.

With recent advances in power electronic technology, High-Voltage Direct Current (HVDC) transmission system has become an alternative for transmitting power especially over long distances. Multi-Terminal HVDC (MTDC) systems are proposed as HVDC systems with more than two terminals. These systems can be geographically wide. While in AC grids, frequency is a global variable, in MTDC systems, DC voltage can be considered as its dual. However, unlike frequency, DC voltage can not be equal across the MTDC system. Control of DC voltage in MTDC systems is one of the important challenges in MTDC systems. Since the dynamic of MTDC system is very fast, DC voltage control methods cannot rely only on remote information. Therefore, they can work based on either local information or a combination of local and remote information. In this thesis, first, the MTDC system is modeled. One of the models presented in this thesis considers only the DC grid, and effects of the AC grids are modeled with DC current sources, while in the other one, the connections of the DC grid to the AC grids are also considered. Next, the proposed methods in the literature for controlling the DC voltage are described and in addition to these methods, some control methods are proposed to control the DC voltage in MTDC system. These control methods include two groups. The first group (such as Multi-Agent Control methods) uses remote and local information, while the second group (such as Sliding Mode Control and H¥ control) uses local information.The proposed multi-agent control uses local information for immediate response, while uses remote information for a better fast response. Application of Multi-Agent Control systems leads to equal deviation of DC voltages from their reference values. Using remote information leads to better results comparing to the case only local information is used. Moreover, the proposed methods can also work in the absence of remote information. When AC grid is considered in the modeling, the MTDC system has anon-linear dynamic. Sliding Mode Control, a non-linear control method with high disturbance rejection capability, which is non-sensitive to the parameter variations, is applied to the MTDC system. It controls the DC voltage very fast and with small or without overshoot. Afterward, a static state feedback H¥ control is applied to the system which minimizes the voltage deviation after a disturbance and keeps the injected power of the terminals within the limits. Finally, some case studies are presented and the effectiveness of the proposed methods are shown. All simulations have been done in MATLAB and SIMULINK.

QC 20140911

This thesis focuses on the advanced control methods for multi-terminal High Voltage Direct Current (HVDC) systems integrating offshore wind farms. Several key issues are investigated in this thesis, including controller design to improve the system dynamic performance, power loss reduction with controller optimization, system stability and dynamics assessment. A DC voltage backstepping control method is designed considering the cable dynamics and controller delay effects. DC cable and converter current loop dynamics are included in the voltage controller design. This control method is applied to a point-to-point and a 4-terminal HVDC system with a conjunction point. Simulation results show that the controller performance can be improved in terms of the disturbance rejection., The relation between Voltage Source Converter (VSC) control action and power losses in the multi-terminal HVDC systems is investigated. For a 4-terminal system, it is shown that the transmission loss can be reduced by properly setting the droop gain ratio between different terminals. For each converter, it is demonstrated by simulation that through a proper controller design, the power loss can be significantly reduced while controller performance can be maintained. A new droop setting design method is proposed. It is shown that due to the existence of droop control, DC voltage deviation will affect the power flow accuracy when the steady state is changed. The impact of DC voltage deviation on the power flow accuracy is studied to tackle this problem, and the DC voltage deviation can be kept unchanged, without affecting the steady state power flow. A droop gain selection procedure is proposed to satisfy the system stability requirement. A state feedback enhanced droop controller is proposed to improve the dynamic performance and stability requirement. With the proposed method, it is shown that the system stability can be guaranteed under both small and large droop gains.

Les nouvelles politiques adoptées par les autorités nationales ont encouragé pendant les dernières années l'intégration à grande échelle des systèmes d'énergie renouvelable (RES). L'intégration à grande échelle des RES aura inévitablement des conséquences sur le réseau de transport d'électricité tel qu'il est conçu aujourd'hui, car le transport de l'électricité massif sur de longues distances pourrait amener les réseaux de transport à fonctionner près de leurs limites, réduisant ainsi leurs marges de sécurité. Des systèmes de transport d’électricité plus complexes seront donc nécessaires.Dans ce scénario, les systèmes de transmission à Courant Continu Haute Tension (HVDC) constituent la solution la plus intéressante pour le renforcement et l'amélioration des réseaux à Courant Alternatif (AC) existants, non seulement en utilisant des configurations point à point, mais aussi dans des configurations multi-terminales. L'introduction des systèmes HVDC aboutira à terme à un réseau électrique hybride haute tension AC/DC, qui doit être analysé comme un système unique afin de mieux comprendre les interactions entre le réseau AC et le réseau DC.Cette thèse porte sur l'analyse de la stabilité transitoire des systèmes de transmission électrique hybrides AC/DC. Plus particulièrement, deux questions ont été abordées: Quel est l'impact d'un défaut du réseau DC sur la stabilité transitoire du réseau AC? Comment est-il possible de se servir des systèmes de transmission DC en tant qu'actionneurs afin d'améliorer la stabilité transitoire AC ?Dans la première partie de ce travail, les modèles mathématiques du réseau hybride AC/DC sont décrits ainsi que les outils nécessaires à l'analyse du système en tenant compte de sa nature non linéaire. Ensuite, une analyse approfondie de la stabilité transitoire du réseau électrique dans le cas particulier d'un court-circuit dans le réseau DC et l'exécution des stratégies de protection correspondantes sont effectuées. En complément, des indicateurs de stabilité et des outils pour dimensionner les futurs réseaux de la MTDC afin de respecter les contraintes des stratégies de protection existantes sont proposés.La deuxième partie de la thèse porte sur les propositions de commande pour la modulation des références de puissance des systèmes de transmission HVDC dans le but d'améliorer la stabilité transitoire du système AC connecté à ce réseau DC. Tout d'abord, nous axons notre étude sur le contrôle non linéaire des liaisons HVDC point à point dans des liaisons hybrides AC/DC. La compensation rapide des perturbations de puissance, l'injection de puissance d'amortissement et l'injection de puissance de synchronisation sont identifiées comme des mécanismes par lesquels les systèmes HVDC peuvent améliorer les marges de stabilité des réseaux AC.Enfin, une stratégie de contrôle pour l'amélioration de la stabilité transitoire par injection de puissance active dans par un réseau MTDC est proposée. Grâce à la communication entre les stations, la commande décentralisée proposée injecte la puissance d'amortissement et de synchronisation entre chaque paire de convertisseurs en utilisant uniquement des mesures au niveau des convertisseurs. L'implémentation proposée permet d'utiliser au maximum la capacité disponible des convertisseurs en gérant les limites de puissance d'une manière décentralisée
The new policy frameworks adopted by national authorities has encouraged the large scale-integration of Renewable Energy Systems (RES) into bulk power systems. The large-scale integration of RES will have consequences on the electricity transmission system as it is conceived today, since the transmission of bulk power over long distances could lead the existing transmission systems to work close to their limits, thus decreasing their dynamic security margins. Therefore more complex transmissions systems are needed.Under this scenario, HVDC transmission systems raise as the most attractive solution for the reinforcement and improvement of existing AC networks, not only using point-to-point configurations, but also in a Multi-Terminal configuration. The introduction of HVDC transmission systems will eventually result in a hybrid high voltage AC/DC power system, which requires to be analyzed as a unique system in order to understand the interactions between the AC network and the DC grid.This thesis addresses the transient stability analysis of hybrid AC/DC electric transmission systems. More in particular, two questions sought to be investigated: What is the impact of a DC contingency on AC transient stability? How can we take advantage of the of DC transmission systems as control inputs in order to enhance AC transient stability?In the first part of this work, the mathematical models of the hybrid AC/DC grid are described as well as the necessary tools for the analysis of the system taking into account its nonlinear nature. Then, a thorough analysis of transient stability of the power system in the particular case of a DC fault and the execution of the corresponding protection strategies is done. As a complement, stability indicators and tools for sizing future MTDC grids in order to respect the constraints of existing protection strategies are proposed.The second part of the thesis addresses the control proposals for the modulation of power references of the HVDC transmission systems with the purpose of transient stability enhancement of the surrounding AC system. Firstly, we focus our study in the nonlinear control of point-to-point HVDC links in hybrid corridors. Fast power compensation, injection of damping power and injection of synchronizing power are identified as the mechanisms through which HVDC systems can improve stability margins.Finally, a control strategy for transient stability enhancement via active power injections of an MTDC grid is proposed. Using communication between the stations, the proposed decentralized control injects damping and synchronizing power between each pair of converters using only measurements at the converters level. The proposed implementation allows to fully use the available headroom of the converters by dealing with power limits in a decentralized way