System configuration, fault detection, location, isolation and restoration: a review on LVDC Microgrid protections

Low voltage direct current (LVDC) distribution has gained the significant interest of research due to the advancements in power conversion technologies. However, the use of converters has given rise to several technical issues regarding their protections and controls of such devices under faulty conditions. Post-fault behaviour of converter-fed LVDC system involves both active converter control and passive circuit transient of similar time scale, which makes the protection for LVDC distribution significantly different and more challenging than low voltage AC. These protection and operational issues have handicapped the practical applications of DC distribution. This paper presents state-of-the-art protection schemes developed for DC Microgrids. With a close look at practical limitations such as the dependency on modelling accuracy, requirement on communications and so forth, a comprehensive evaluation is carried out on those system approaches in terms of system configurations， fault detection, location, isolation and restoration

Dynamic modeling, stability analysis, and controller design for DC distribution systems

The dc distribution systems or dc microgrids are known to be best suited for integration of renewable energy sources into the current power grid and are considered to be the key enabling technology for the development of future smart grid. Dc microgrids also benefit from better current capabilities of dc power lines, better short circuit protection, and transformer-less conversion of voltage levels, which result in higher efficiency, flexibility, and lower cost. While the idea of using a dc microgrid to interface distributed energy sources and modern loads to the power grid seems appealing at first, several issues must be addressed before this idea can be implemented fully. The configuration, stability, protection, economic operation, active management, and control of future dc microgrids are among the topics of interest for many researchers. The purpose of this dissertation is to investigate the dynamic behavior and stability of a future dc microgrid and to introduce new controller design techniques for the Line Regulating Converters (LRC) in a dc distribution system. Paper I is devoted to dynamic modeling of power converters in a dc distribution system. The terminal characteristics of tightly regulated power converters which are an important factor for stability analysis and controller design are modeled in this paper. Paper II derives the simplified model of a dc distribution system and employs the model for analyzing stability of the system. Paper III introduces two controller design methods for stabilizing the operation of the LRC in presence of downstream constant power loads in a dc distribution system. Paper IV builds upon paper III and introduces another controller design method which uses an external feedback loop between converters to improve performance and stability of the dc grid. --Abstract, page iv

Islanding operation of hybrid microgrids with high integration of wind driven cage induction generators

This paper proposes two control strategies for the islanding operation of hybrid microgrid with a high penetration of wind driven cage induction generators. The control strategies combine approaches traditionally applied to self-excited cage induction generators with recent approaches for microgrid's islanding operation. The proposed control strategies aim to facilitate the higher integration of cage induction generators in microgrids. The first strategy is based on direct frequency and reactive power control while the second one uses an artificial grid to regulate the voltage amplitude and frequency. The proposed schemes are tested in PSCAD/EMTDC using a real wind speed pattern measured at Hokkaido Island of Japan. Simulation results show the successful operation of both schemes. The implementation simplicity and cost-effectiveness of both schemes are explained as well

Solid state transformer technologies and applications: a bibliographical survey

This paper presents a bibliographical survey of the work carried out to date on the solid state transformer (SST). The paper provides a list of references that cover most work related to this device and a short discussion about several aspects. The sections of the paper are respectively dedicated to summarize configurations and control strategies for each SST stage, the work carried out for optimizing the design of high-frequency transformers that could adequately work in the isolation stage of a SST, the efficiency of this device, the various modelling approaches and simulation tools used to analyze the performance of a SST (working a component of a microgrid, a distribution system or just in a standalone scenario), and the potential applications that this device is offering as a component of a power grid, a smart house, or a traction system.Peer ReviewedPostprint (published version

Improving the storage capability of a microgrid with a vehicle-to-grid interface

In the emergent deployment of microgrids, storage systems play an important role providing ancillary services, such as backup power and reactive power support. This concept becomes crucial in the context of microgrids with a high penetration of renewable energy resources, where storage systems may be used to smooth the intermittency and variability of most of them. Plug-in electric vehicles provide an enormous distributed storage capability, which favours the technical and economical exploitation of such systems. This paper presents a comprehensive implementation and control of a bidirectional power converter for Vehicle-to-Grid integration, based on a bidirectional DC/DC converter followed by a full bridge DC/AC converter. The evaluation of the adopted topology and its control is performed through MATLAB/Simulink simulation.info:eu-repo/semantics/publishedVersio

A Novel Power Sharing Control Method for Distributed Generators in DC Networks

The power sharing control method is a desirable solution to integrate multiple renewable energy generators into the grid and to keep them working synchronously. Power sharing control between different distributed generators is an important consideration for the stabilized operation of the power grid network. In this thesis work, a novel method is used with the concept of droop control technique and is designed to control power from each individual generator in DC network particularly. The proposed power sharing control method can be widely applied to grid connected network and to islanded power grid network for obtaining high efficiency of power distribution and also provides higher stability. An efficient power control method to share the load demand power is designed based on the concept of droop control. This method does not follow sequential or predefined topology of power sharing but uses the availability of power from each generator as a factor of control. The proposed controller can be applied to an individual distributed generator to regulate its output power quickly and accurately. The power sharing control method was formulated, modeled and verified by simulation studies of steady state and transient stability tests. The optimal coupling resistance for power sharing was also identified. The interaction of the controller and the communication delay was also studied. The interference of communication delay is negligible for the power sharing controller. The system is simulated in MATLAB/SIMULINK environment

Centralized and Decentralized control of Microgrids

ABSTRACT Microgrid can be seen as an important controllable sub-system in future power systems. As a part of distribution network, the microgrid can operate in grid-connected or islanded mode to supply its local loads, and it consists of different renewable and non-renewable distribution generations that are connected to the system through power electronics (PE) interfaces. However, the control of microgrids is one of the important issues to focus on in order to overcome the challenges raised by high penetration of of renewable energy sources (RES). Depending on the responsibilities assumed by the different control levels, the microgrid can be controlled in centralized or decentralized modes. In centralized approach, the microgrid central controller (MGCC) is mainly responsible for the maximization of the microgrid value and optinization of its operation, and the MGCC determines the amount of power that the microgrid should import or export from the upstream distribution system by optimizing the local production or consumption capabilities. However, the MGCC should always consider the market prices of electricity, grid security concerns and ancillary services requested by the DSO when taking decisions. In this case an optimized operating scenario is realized by controlling the microsources and controllable loads within the microgrid, where non-critical, flexible loads can be shed, when profitable. Furthermore, the actual active and reactive power of the components are monitored. When a full decentralized control is implemented, the Management Center (MC) takes responsibilities and it competes or collaborates to optimize the production, satisfy the demand and provide the maximum possible export to the grid but all is done by considering the real time market prices. This thesis discusses the concepts of centralized and decentralized control of MG, where the main chapters introduce different control methods and PE interfaces that are involved in the microgrid control, while the final work presents simulation models that demonstrate how microgrids are controlled through inverters and the results. Using MATLAB/Simulink environment, PQ and V/f control modes of inverter are simulated and the results are discussed to point out their significant effect on balancing the voltage magnitude, maintaining the frequency and power sharing

Ancillary Services in Hybrid AC/DC Low Voltage Distribution Networks

In the last decade, distribution systems are experiencing a drastic transformation with the advent of new technologies. In fact, distribution networks are no longer passive systems, considering the current integration rates of new agents such as distributed generation, electrical vehicles and energy storage, which are greatly influencing the way these systems are operated. In addition, the intrinsic DC nature of these components, interfaced to the AC system through power electronics converters, is unlocking the possibility for new distribution topologies based on AC/DC networks. This paper analyzes the evolution of AC distribution systems, the advantages of AC/DC hybrid arrangements and the active role that the new distributed agents may play in the upcoming decarbonized paradigm by providing different ancillary services.Ministerio de Economía y Competitividad ENE2017-84813-RUnión Europea (Programa Horizonte 2020) 76409

A novel power management and control design framework for resilient operation of microgrids

This thesis concerns the investigation of the integration of the microgrid, a form of future electric grids, with renewable energy sources, and electric vehicles. It presents an innovative modular tri-level hierarchical management and control design framework for the future grid as a radical departure from the ‘centralised’ paradigm in conventional systems, by capturing and exploiting the unique characteristics of a host of new actors in the energy arena - renewable energy sources, storage systems and electric vehicles. The formulation of the tri-level hierarchical management and control design framework involves a new perspective on the problem description of the power management of EVs within a microgrid, with the consideration of, among others, the bi-directional energy flow between storage and renewable sources. The chronological structure of the tri-level hierarchical management operation facilitates a modular power management and control framework from three levels: Microgrid Operator (MGO), Charging Station Operator (CSO), and Electric Vehicle Operator (EVO). At the top level is the MGO that handles long-term decisions of balancing the power flow between the Distributed Generators (DGs) and the electrical demand for a restructure realistic microgrid model. Optimal scheduling operation of the DGs and EVs is used within the MGO to minimise the total combined operating and emission costs of a hybrid microgrid including the unit commitment strategy. The results have convincingly revealed that discharging EVs could reduce the total cost of the microgrid operation. At the middle level is the CSO that manages medium-term decisions of centralising the operation of aggregated EVs connected to the bus-bar of the microgrid. An energy management concept of charging or discharging the power of EVs in different situations includes the impacts of frequency and voltage deviation on the system, which is developed upon the MGO model above. Comprehensive case studies show that the EVs can act as a regulator of the microgrid, and can control their participating role by discharging active or reactive power in mitigating frequency and/or voltage deviations. Finally, at the low level is the EVO that handles the short-term decisions of decentralising the functioning of an EV and essential power interfacing circuitry, as well as the generation of low-level switching functions. EVO level is a novel Power and Energy Management System (PEMS), which is further structured into three modular, hierarchical processes: Energy Management Shell (EMS), Power Management Shell (PMS), and Power Electronic Shell (PES). The shells operate chronologically with a different object and a different period term. Controlling the power electronics interfacing circuitry is an essential part of the integration of EVs into the microgrid within the EMS. A modified, multi-level, H-bridge cascade inverter without the use of a main (bulky) inductor is proposed to achieve good performance, high power density, and high efficiency. The proposed inverter can operate with multiple energy resources connected in series to create a synergized energy system. In addition, the integration of EVs into a simulated microgrid environment via a modified multi-level architecture with a novel method of Space Vector Modulation (SVM) by the PES is implemented and validated experimentally. The results from the SVM implementation demonstrate a viable alternative switching scheme for high-performance inverters in EV applications. The comprehensive simulation results from the MGO and CSO models, together with the experimental results at the EVO level, not only validate the distinctive functionality of each layer within a novel synergy to harness multiple energy resources, but also serve to provide compelling evidence for the potential of the proposed energy management and control framework in the design of future electric grids. The design framework provides an essential design to for grid modernisation