Management of Islanded Operation of Microgirds

Distributed generations with continuously growing penetration levels offer potential solutions to energy security and reliability with minimum environmental impacts. Distributed Generations when connected to the area electric power systems provide numerous advantages. However, grid integration of distributed generations presents several technical challenges which has forced the systems planners and operators to account for the repercussions on the distribution feeders which are no longer passive in the presence of distributed generations. Grid integration of distributed generations requires accurate and reliable islanding detection methodology for secure system operation. Two distributed generation islanding detection methodologies are proposed in this dissertation. First, a passive islanding detection technique for grid-connected distributed generations based on parallel decision trees is proposed. The proposed approach relies on capturing the underlying signature of a wide variety of system events on a set of critical system parameters and utilizes multiple optimal decision tress in a parallel network for classification of system events. Second, a hybrid islanding detection method for grid-connected inverter based distributed generations combining decision trees and Sandia frequency shift method is also proposed. The proposed method combines passive and active islanding detection techniques to aggregate their individual advantages and reduce or eliminate their drawbacks. In smart grid paradigm, microgrids are the enabling engine for systematic integration of distributed generations with the utility grid. A systematic approach for controlled islanding of grid-connected microgrids is also proposed in this dissertation. The objective of the proposed approach is to develop an adaptive controlled islanding methodology to be implemented as a preventive control component in emergency control strategy for microgrid operations. An emergency power management strategy for microgrid autonomous operation subsequent to inadvertent islanding events is also proposed in this dissertation. The proposed approach integrates microgrid resources such as energy storage systems, demand response resources, and controllable micro-sources to layout a comprehensive power management strategy for ensuring secure and stable microgrid operation following an unplanned islanding event. In this dissertation, various case studies are presented to validate the proposed methods. The simulation results demonstrate the effectiveness of the proposed methodologies

Controle coordenado em microrredes de baixa tensão baseado no algoritmo power-based control e conversor utility interface

Orientadores: José Antenor Pomilio, Fernando Pinhabel MarafãoTese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Elétrica e de ComputaçãoResumo: Esta tese apresenta uma possível arquitetura e sua respectiva estratégia de controle para microrredes de baixa tensão, considerando-se a existência de geradores distribuídos pela rede. A técnica explora totalmente a capacidade dos geradores distribuídos em ambos os modos de operação: conectado à rede e ilhado. Quando conectado à rede, sob o modo de otimização global, o controle busca a operação quase ótima da microrrede, reduzindo as perdas de distribuição e os desvios de tensão. Quando em modo ilhado, a técnica regula de forma eficaz os geradores distribuídos disponíveis, garantindo a operação autônoma, segura e suave da microrrede. A estratégia de controle é aplicada a uma estrutura de microrrede completamente despachável, baseada em uma arquitetura de controle mestre-escravo, em que as unidades distribuídas são coordenadas por meio do recém-desenvolvido algoritmo Power-Based Control. As principais vantagens da arquitetura proposta são a expansividade e a capacidade de operar sem sincronização ou sem conhecimento das impedâncias de linha. Além disso, a microrrede regula as interações com a rede por meio do conversor chamado de Utility Interface, o qual é um inversor trifásico com armazenador de energia. Esta estrutura de microrrede permite algumas vantagens como: compensação de desbalanço e reativo, rápida resposta aos transitórios de carga e de rede, e suave transição entre os modos de operação. Em contrapartida, para compartilhar a potência ativa e reativa proporcionalmente entre as unidades distribuídas, controlar a circulação de reativos, e maximizar a operação, a comunicação da microrrede requer em um canal de comunicação confiável, ainda que sem grandes exigências em termos de resolução ou velocidade de transmissão. Neste sentido, foi demonstrado que uma falha na comunicação não colapsa o sistema, apenas prejudica o modo de otimização global. Entretanto, o sistema continua a operar corretamente sob o modo de otimização local, que é baseado em um algoritmo de programação linear que visa otimizar a compensação de reativos, harmônicos e desbalanço de cargas por meio dos gerador distribuído, particularmente, quando sua capacidade de potência é limitada. Esta formulação consiste em atingir melhores índices de qualidade de energia, definidos pelo lado da rede e dentro de uma região factível em termos de capacidade do conversor. Baseado nas medições de tensão e corrente de carga e uma determinada função objetiva, o algoritmo rastreia as correntes da rede ótima, as quais são utilizadas para calcular os coeficientes escalares e finalmente estes são aplicados para encontrar as referências da corrente de compensação. Finalmente, ainda é proposta uma técnica eficiente para controlar os conversores monofásicos conectados arbitrariamente ao sistema de distribuição trifásico, sejam conectados entre fase e neutro ou entre fase e fase, com o objetivo de compensar o desbalanço de carga e controlar o fluxo de potência entre as diferentes fases da microrrede. Isto melhora a qualidade da energia elétrica no ponto de acoplamento comum, melhora o perfil de tensão nas linhas, e reduz as perdas de distribuição. A arquitetura da microrrede e a estratégia de controle foi analisada e validada através de simulações computacionais e resultados experimentais, sob condições de tensão senoidal/simétrica e não-senoidal/assimétrica, avaliando-se o comportamento em regime permanente e dinâmico do sistema. O algoritmo de programação linear que visa otimizar a compensação foi analisado por meio de resultados de simulaçãoAbstract: This thesis presents a flexible and robust architecture and corresponding control strategy for modern low voltage microgrids with distributed energy resources. The strategy fully exploits the potential of distributed energy resources, under grid-connected and islanded operating modes. In grid-connected mode, under global optimization mode, the control strategy pursues quasi-optimum operation of the microgrid, so as to reduce distribution loss and voltage deviations. In islanded mode, it effectively manages any available energy source to ensure a safe and smooth autonomous operation of the microgrid. Such strategy is applied to a fully-dispatchable microgrid structure, based on a master-slave control architecture, in which the distributed units are coordinated by means of the recently developed power-based control. The main advantages of the proposed architecture are the scalability (plug-and-play) and capability to run the distributed units without synchronization or knowledge of line impedances. Moreover, the proposed microgrid topology manages promptly the interaction with the mains by means of a utility interface, which is a grid-interactive inverter equipped with energy storage. This allows a number of advantages, including compensation of load unbalance, reduction of harmonic injection, fast reaction to load and line transients, and smooth transition between operating mode. On the other hand, in order to provide demand response, proportional power sharing, reactive power control, and full utilization of distributed energy resources, the microgrid employs a reliable communication link with limited bit rate that does not involve time-critical communications among distributed units. It has been shown that a communication failure does not jeopardize the system, and just impairs the global optimization mode. However, the system keeps properly operating under the local optimization mode, which is managed by a linear algorithm in order to optimize the compensation of reactive power, harmonic distortion and load unbalance by means of distributed electronic power processors, for example, active power filters and other grid-connected inverters, especially when their capability is limited. It consists in attain several power quality performance indexes, defined at the grid side and within a feasible power region in terms of the power converter capability. Based on measured load quantities and a certain objective function, the algorithm tracks the expected optimal source currents, which are thereupon used to calculate some scaling coefficients and, therefore, the optimal compensation current references. Finally, the thesis also proposes an efficient technique to control single-phase converters, arbitrarily connected to a three-phase distribution system (line-to-neutral or line-to-line), aiming for reduce unbalance load and control the power flow among different phases. It enhances the power quality at the point-of-common-coupling of the microgrid, improve voltage profile through the lines, and reduce the overall distribution loss. The master-slave microgrid architecture has been analyzed and validated by means of computer simulations and experimental results under sinusoidal/symmetrical and nonsinusoidal/asymmetrical voltage conditions, considering both the steady-state and dynamic performances. The local optimization mode, i.e., linear algorithm for optimized compensation, has been analyzed by simulation resultsDoutoradoEnergia EletricaDoutor em Engenharia Elétrica2012/24309-8, 2013/21922-3FAPES

Evolution of microgrids with converter-interfaced generations: Challenges and opportunities

© 2019 Elsevier Ltd Although microgrids facilitate the increased penetration of distributed generations (DGs) and improve the security of power supplies, they have some issues that need to be better understood and addressed before realising the full potential of microgrids. This paper presents a comprehensive list of challenges and opportunities supported by a literature review on the evolution of converter-based microgrids. The discussion in this paper presented with a view to establishing microgrids as distinct from the existing distribution systems. This is accomplished by, firstly, describing the challenges and benefits of using DG units in a distribution network and then those of microgrid ones. Also, the definitions, classifications and characteristics of microgrids are summarised to provide a sound basis for novice researchers to undertake ongoing research on microgrids

Modeling, Analyses and Assessment of Microgrids Considering High Renewable Energy Penetration

Microgrids are receiving attention due to the increasing need to integrate distributed generations and to ensure power quality and provide energy surety to the customers. Since renewables need to be in the mix for energy surety, a high renewable-energy penetrated microgrid is analyzed in this paper. The standard IEEE 34 bus distribution feeder is adapted and managed as a microgrid by adding distributed generation and load profiles. The 25kV system parameters are scaled down to 12kV and renewable sources including solar PV and wind turbines, an energy storage system, and a natural gas generator have been added to the 34-bus system. The distribution generations (DG) and renewables are modeled in detail using PSCAD software and practical constraints of the components are considered. The droop control and autonomous control for microgrid normal operation in islanded mode and grid-tied mode have been proposed and studied. A novel comprehensive supervisory control scheme has been defined to manage the microgrid transition from or to the bulk grid, and to minimize the transients on voltage and frequency. Detailed analyses for islanding, reconnection, and black start are presented for various conditions. The proposed control techniques accept inputs from local measurements and supervisory controls in order to manage the system voltage and frequency. The monitoring of the microgrid for measuring power quality and control requirements for DGs and storage are modeled. The power quality issues are discussed and indexes are calculated. A novel probabilistic assessment of microgrid reliability has been proposed. At last, several extended researches are presented. An experimental system has been built which includes three 250kW inverters emulating natural gas generator, energy storage, and renewable source. The simulation and experimental results are provided which verifies the analytical presentation of the hardware and control algorithms

Design, Implementation and Evaluation of a Microgrid in Island and Grid Connected Modes with a Fuel Cell Power Source

The ability to connect a microgrid to the grid is an important step in the development and evolution of the modern power system. The principle objectives of this research are (1) to simulate a simple microgrid consisting of a PEM hydrogen fuel cell, load and connection to the grid and (2) to evaluate the resulting microgrid control system on a corresponding experimental microgrid. The microgrid simulation demonstrated that the control algorithms can operate the microgrid in both islanded (VSC with voltage and frequency regulation) and grid connected (VSC with current control for power transfer). The experimental laboratory microgrid was constructed and operated in real-time performing its black start and managed transitions between island and grid connected modes of operation. The synchronization method adjusted the island microgrid to become in phase with the grid and tracked well under steady state and load changing conditions. The synchronization process brought the island in phase with the grid within 400 ms. Passive island detection was demonstrated with the restoration to grid operation. The grid connected voltage and current THD were under 1%

Virtual Droop Control Framework and Stability Analyses for Microgrids with High Penetration of Renewables

Microgrids can provide the most promising means of integrating large amounts of distributed sources into the power grid and can supply reliable power to critical loads. However, managing distributed sources and loads within a microgrid during island and grid-tie modes and during transitions is a challenge. Stable operation of a microgrid is a concern specifically during the starting of motor loads, switching of large loads, and in presence of high penetration of renewable resources. Hence, a generalized control framework is required to regulate microgrid voltage and frequency, maintain power quality, manage Distributed Generations (DG) and ensure microgrid stability. Several control methods have been developed for microgrid control. Majority of these techniques are based on natural droop control or modified natural droop control, which rely on voltage and frequency variations as inputs to control algorithms. At present, there are no methods available for sizing the capacities needed to ensure reliable operation and stability. A new microgrid control framework, Virtual Droop Control (VDC), for power management as well as for voltage and frequency regulation is proposed in this thesis. The proposed control method analyzes the effect of intermittent resources and dispatches the power commands to individual generation assets ensuring stable operation of the microgrid. The proposed method is described, formulated and compared with existing natural droop control technique in this dissertation. The unit commitment algorithm has also been implemented to manage non-renewable sources to improve system efficiency. The proposed technique operates the microgrid at a constant voltage and frequency and uses communications for power sharing. It also provides the means to operate the microgrid in case of lost communication or sabotage on the communication network. The modeling results of the Virtual VDC technique have been compared with exiting microgrid control methods including natural droop control technique. A laboratory setup, that consists of a 100kW natural gas generator, a 56 kWh Li-ion battery with a 250kW inverter, and a 100kW load bank, has been built and tested. The results of the setup have been provided, confirming the viability of the proposed technique. Detailed analysis for intentional islanding, unintentional islanding, and reconnection are presented. The state space model has been developed for the Fort Sill microgrid and the stability analysis has been performed to verify stability of a microgrid in various scenarios. The proposed method has been applied to the Fort Sill microgrid and examined for effectiveness and viability. A modified control technique is also proposed to regulate the voltage and frequency for high penetration of renewable energy, which can be used with VDC framework. This technique allows the improvement of efficiency and power quality indexes for critical loads while reducing greenhouse gas emissions. The standard IEEE 34 bus system is modified and adapted to function as a microgrid test bed. Three different cases were studied and analyzed. The CO2 emission, efficiency, and power quality indexes have been calculated and compared for all three cases in order to verify the performance of the proposed control technique