Microgrid (MG) concept has been emerged to enable integration of renewable energy sources and storage devices using power electronic converters. An MG can be grid connected to exchange power with the main grid, isolated that is completely separated from the grid, or islanded that is temporarily separated from the grid. The P and Q-V drooping approach is commonly used to control and achieve power sharing among the generators.\\ This study presents an approach for systematically modeling a class of microgrid (MG) systems. The derived model 1) accommodates grid-connected and islanded operation of the MG simultaneously, and 2) allows modeling of converter-based as well as directly-interfaced resources. The originally nonlinear model is then converted to a linear model whose eigenvalues determine local stability of the MG. \\The model is used to analyze the impacts of adding cross-coupling droop terms (P-V and Q) on an MG's performance. Various performance aspects such as stability, stability robustness, transmission power loss, voltage profile, and power sharing are considered. The conclusions are as follows. (1) Addition of a small portion of cross-coupling will reduce the losses without compromising other aspects in a grid-connected MG. Larger cross-coupling terms will compromise the system stability. (2) Large cross-coupling terms can be added to reduce the power loss and to improve the system stability in an isolated MG. Simulation and experimental results are presented to verify the derivations.

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A growing world population has resulted in an increasing number of people being homeless or living in inadequate housing. In addition, the threatening climate crisis and the world’s limited resources calls for a more sustainable way of living. The organization Stockholm Tiny House Expo aims to contribute a solution to these issues: an artificial island with several tiny houses, able to adapt to rising sea levels, having net-zero-waste and completely self-sufficient regarding energy. This island will symbolize the sustainable development goals and will be an attraction for tourists, as well as create several job opportunities. In order to realize this vision, research needs to be conducted to find solutions to make this island become reality. Therefore, the aim of this project is to evaluate the economic and environmental feasibility of a high degree of self-sufficiency regarding energy, by locally producing heat and power, on an artificial island in Stockholm. Firstly, a literature review is conducted to find suitable technologies to supply the island with heat and power. Subsequently, the software tools IDA ICE and HOMER Pro are used to simulate the energy demand and supply of the island. Eight different scenarios, with different types of supply and demand, are created to investigate different possibilities of the island. The scenarios are evaluated using technical-, economic- and environmental key performance indicators. A scenario where the demand is reduced and heat and power are supplied only by resources on the island, is deemed most relevant based on Stockholm Tiny House Expo’s vision. A sensitivity analysis is therefore performed on this scenario. The results indicate technical and environmental feasibility; however, the economic evaluation showed that this scenario will be non-profitable. Although the scenario is non-profitable, if further measures are taken to create a pricing model to customers, it could be possible. In conclusion, the results of this research indicate that it is possible for Stockholm Tiny House Expo to be self-sufficient regarding heat and power solely utilizing renewable energy. The evaluation of the results, however, showed that it is not economically feasible. In addition, the national grid did not contribute to an impact on the surrounding environment, nor to a considerable amount of greenhouse gas emissions. Hence, grid connection is recommended for Stockholm Tiny House Expo. / En växande världspopulation har resulterat i att ett ökande antal människor är hemlösa eller bor i bristfälliga bostäder. Den hotande klimatkrisen och jordens begränsade tillgångar kräver dessutom en hållbarare livsstil. Organisationen Stockholm Tiny House Expo vill bidra med en lösning till dessa problem: en artificiell ö med flera småhus, som kan anpassa sig till stigande havsnivåer, har noll nettoavfall och är helt självförsörjande gällande energi. Denna ö kommer symbolisera FN:s hållbarhetsmål och vara en turistattraktion så väl som skapa ett flertal arbetsmöjligheter. För att förverkliga denna vision krävs forskning för att hitta lösningar och göra denna ö till verklighet. Därmed är syftet med detta projekt att undersöka den ekonomiska och miljömässiga genomförbarheten av en hög grad av självförsörjning av energi, genom att lokalt producera kraft och värme, på en artificiell ö i Stockholm. Först utförs en litteraturstudie för att hitta lämpliga teknologier för att försörja ön med kraft och värme. Därefter används programvarorna IDA ICE och HOMER Pro för att simulera energibehovet och energiförsörjningen för ön. Åtta olika scenarier, med olika typer av försörjning och behov, konstrueras för att undersöka olika möjligheter för ön. Scenarierna utvärderas med hjälp av tekniska-, ekonomiska- och miljömässiga nyckeltal (key performance indicators). Ett scenario där behovet är reducerat samt att kraft och värme endast försörjs av resurser på ön, bedöms vara mest relevant baserat på Stockholm Tiny House Expos vision. En känslighetsanalys utförs därför på detta scenario. Resultaten tyder på att scenariot är tekniskt och miljömässigt genomförbart; dock visade den ekonomiska utvärderingen att det inte är lönsamt. Trots detta så skulle det kunna vara möjligt om vidare åtgärder tas för att skapa en prissättningsmodell mot kunderna. Sammanfattningsvis så tyder resultaten på att det är möjligt för Stockholm Tiny House Expo att vara självförsörjande gällande kraft och värme som endast utnyttjar förnybar energi. Utvärderingen av resultatet visade dock att det inte är ekonomiskt genomförbart. Det nationella kraftnätet bidrog dessutom inte till påverkan på den omgivande miljön och inte heller någon betydande mängd växthusgasutsläpp. Följaktligen rekommenderas nätanslutning för Stockholm Tiny House Expo.

<p>The advent of small-scale, distributed generators of energy has resulted in the problem of integrating them in the conventional electric power system, which is characterized by large-scale, centralized energy generators. MicroGrids have emerged as a promising solution to the integration problem and have duly received increasing research attention. Microgrids are semi-autonomous collections of controllable microsources and loads, which present themselves to the utility grid as single, controlled entities. In order to achieve the semi-autonomous and controlled nature of microgrids, especially,overcoming the challenge of balancing demand and power generation, an intelligent energy management scheme is required.</p> <p>Developing an energy management scheme is an interesting and challenging task, which provides the potential to exploit ideas from a plethora of fields like Artificial Intelligence and Machine Learning, Constrained Optimization, etc. However, testing energy management strategies on a microgrid would pose a multitude of problems,the most important of them being the unreliability and inconvenience of testing an energy management strategy, which is not optimal, on a functional microgrid. Errors in a test strategy might cause power outages and damage installed devices. Hence it is necessary to test energy management strategies on simulated microgrids.</p> <p>This thesis presents a Software Testbed of MicroGrids, specifically designed to suit the purposes of development of energy management strategies. The testbed consists of two components: Simulation Framework and Analysis Tool. The modular simulation framework enables simulation of a microgrid with microsources and loads,whose configurations can be specified by the user. The analysis tool enables visual analysis of data generated using simulations, which would enable the improvement of not only the management strategy and prediction techniques, but also the computer models used in the simulation framework. A demonstration of the software testbed's simulation and analysis capabilities is presented and possible directions for future research are suggested.</p> / Master of Science (MSc)

<p>An increasing concern over environmental impacts of fossil fuels and sustainability of energy resources is leading to significant changes in the electric power systems. Decentralized power generation, in particular, is emerging as one of the most effective and promising tools in addressing these concerns.</p> <p>Microgrids are small-scale electricity grids with elements of load, generation and storage. Microgrids have emerged as an essential building block of a future smart grid, and an enabling technology for distributed power generation and control. This thesis presents an optimization-based approach for the design and control of energy management systems (EMS) for electric microgrids. A linear programming formulation of power/energy management is proposed to minimize energy cost for a microgrid with energy storage and renewable energy generation, by taking advantage of time-of-use (TOU) pricing. The thesis also addresses the issue of sizing of the battery storage and solar power generation capacity by formulating and solving a mixed integer linear programming (MILP) problem. The aim of the optimization is to minimize the combined capital and electricity usage cost subject to applicable physical constraints. Several case scenarios are analyzed for grid-connected microgrids in residential, commercial and industrial settings, as well as a case of an islanded microgrid intended for a remote community.</p> <p>Finally, the thesis investigates circuit level control of a microgrid with EMS. A finite state machine based control logic is proposed that enables outage ride through and smooth transition between islanded and grid connected operation. Simulation results are provided to demonstrate the effectiveness of the proposed controller under various possible scenarios.</p> / Master of Applied Science (MASc)

The changing nature of the energy grid in recent years has prompted key stakeholders to think of ways to address incoming challenges. Transactive energy is an approach that promises to dynamically align active grid elements coming up in the previously inactive consumers' side to achieve a reliable and smarter grid. This work models the distribution grid structure as a combination of microgrids. A blockchain-in-the loop simulation framework is modelled and simulated for a residential microgrid using power system simulators and transactive agents. Blockchain smart contracts are used to coordinate peer-to-peer energy transactions in the microgrid. The model is used to test three market coordination schemes: a simple auction-less scheme, an auction-less scheme with a normalized sorting metric and an hour ahead single auction scheme with penalties for unfulfilled bids. Case studies are presented of a microgrid with 30 homes, at different levels of solar and energy storage penetration within the microgrid, all equipped with responsive and unresponsive appliances and transactive agents for the HVAC systems. The auction-less scheme with a normalized sorting metric is observed to provide a fairer advantage to smaller solar installations in comparison to the simple auction-less method. It is then concluded that the auction-less schemes are most beneficial to users, as they would not need sophisticated forecasting technology to reduce penalties from bid quantity inaccuracies, as long as the energy mix within the microgrid is diverse enough. / Master of Science / The legacy energy industry involved the bulk transfer of energy from huge generation plants through long transmission lines to the end consumers. However, with the onset of improved renewable energy and information technologies, energy is now being generated closer to the consumer side with appliances capable of actively participating in the energy system now widely available. Transactive energy with blockchain has been proposed in order to dynamically coordinate these systems to work towards a more reliable and smarter grid using economic value in a transparent and secure way. This work models a transactive power grid as a combination of microgrids using a blockchain network to coordinate hourly peer-to-peer energy transactions. The blockchain-in-the-loop simulation model is used to compare three different market mechanisms in a residential microgrid of 30 homes with varying levels of solar panels, batteries and transactive thermostats installed. Two auction-less schemes - one with a normalized sorting metric - and an hour ahead single auction mechanism are analyzed. While the auction-less scheme with the normalized metric is seen to be fairer than the simple auction-less scheme, it is concluded that the auction-less schemes are most beneficial to residents. This is because sophisticated forecasting technology would not be needed like in the hour ahead auction scheme, provided that the microgrid has participants with diverse energy consumption and production profiles throughout the day.

DC power distribution has gained popularity in sustainable buildings, renewable energy utilization, transportation electrification and high-efficiency data centers. This dissertation focuses on two aspects of facilitating the application of dc systems: (a) system-level control to improve load sharing, voltage regulation and efficiency; (b) design of a high-efficiency interface converter to connect dc microgrids with the existing low-voltage ac distributions, with a special focus on common-mode (CM) voltage attenuation. Droop control has been used in dc microgrids to share loads among multiple sources. However, line resistance and sensor discrepancy deteriorate the performance. The quantitative relation between the droop voltage range and the load sharing accuracy is derived to help create droop design guidelines. DC system designers can use the guidelines to choose the minimum droop voltage range and guarantee that the sharing error is within a defined range even under the worst cases. A nonlinear droop method is proposed to improve the performance of droop control. The droop resistance is a function of the output current and increases when the output current increases. Experiments demonstrate that the nonlinear droop achieves better load sharing under heavy load and tighter bus voltage regulation. The control needs only local information, so the advantages of droop control are preserved. The output impedances of the droop-controlled power converters are also modeled and measured for the system stability analysis. Communication-based control is developed to further improve the performance of dc microgrids. A generic dc microgrid is modeled and the static power flow is solved. A secondary control system is presented to achieve the benefits of restored bus voltage, enhanced load sharing and high system efficiency. The considered method only needs the information from its adjacent node; hence system expendability is guaranteed. A high-efficiency two-stage single-phase ac-dc converter is designed to connect a 380 V bipolar dc microgrid with a 240 V split-phase single-phase ac system. The converter efficiencies using different two-level and three-level topologies with state-of-the-art semiconductor devices are compared, based on which a two-level interleaved topology using silicon carbide (SiC) MOSFETs is chosen. The volt-second applied on each inductive component is analyzed and the interleaving angles are optimized. A 10 kW converter prototype is built and achieves an efficiency higher than 97% for the first time. An active CM duty cycle injection method is proposed to control the dc and low-frequency CM voltage for grounded systems interconnected with power converters. Experiments with resistive and constant power loads in rectification and regeneration modes validate the performance and stability of the control method. The dc bus voltages are rendered symmetric with respect to ground, and the leakage current is reduced. The control method is generalized to three-phase ac-dc converters for larger power systems. / Ph. D.