An intelligent capacity management system for interface converter in AC-DC hybrid microgrids

An interface converter (IC) is used in an AC-DC hybrid microgrid (HMG) and its main tasks are frequency regulation in the AC side, adjusting the DC voltage, and controlling the power flow between AC/DC sides based on the droop control method. The IC should be capable of providing ancillary services such as reactive power supply and compensation of unbalanced and harmonic components in the AC side. However, the use of the IC to provide ancillary services occupies its capacity, which may interfere with the main tasks of the IC. In addition, it is shown in this paper that in unbalanced conditions, the effective power capacity of the IC is reduced by considering the current limit of the converter. In this case, the converter may not be able to perform the main task and provide all the necessary ancillary services at the same time, otherwise, it may be exposed to an overcurrent condition. Therefore, an efficient strategy is needed to manage the IC converter capacity to facilitate optimal use of the entire IC capacity even in unbalanced conditions. Given this challenge, this paper proposes an intelligent strategy for managing the IC capacity, which prioritizes the realization of the main task and the provision of ancillary services. The proposed strategy is evaluated, and its effectiveness is proven by simulation results in Matlab/Simulink

Hierarchical-power-flow-based energy management for alternative/direct current hybrid microgrids

Modern microgrids are systems comprising both Alternative Current (AC) and Direct Current (DC) subgrids, integrated with Distributed Generations (DGs), storage systems, and Electric Vehicles (EVs) parking facilities. Achieving stable and reliable load flow control amidst varying load, generation, and charging/discharging strategies requires a hierarchical control scheme. This paper proposes an hourly power flow (PF) analysis within an Energy Management System (EMS) for AC/DC Hybrid Microgrids interconnected via an Interlinking Converter (IC) in both grid-connected and islanded modes. The framework operates within a two-level hierarchically controlled platform. Tertiary control at the top level optimizes DGs' reference power for generation and consumption, minimizing power purchase costs and load shedding in grid-connected and islanded modes, respectively. DG converters employ current control mode to share their power references as the primary controller. While no secondary controller is adopted in this scheme, the Battery Energy Storage System (BESS) in islanded mode utilizes P/Q droop control to maintain voltage and frequency in the AC subsystem. Power sharing between AC and DC subgrids through IC is determined by the difference between AC grid frequency and DC link voltage. Integration of controlled converters’ buses into PF equations enables solving the unified system using the traditional Newton-Raphson (NR) method. A segment of a real distribution grid planned for installation in Italy under the HYPERRIDE project serves as a case study. Comparison with MATLAB/Simulink results confirms the effectiveness, precision, and convergence speed of the proposed model and control schemes, demonstrating efficient load distribution and voltage/frequency restoration in islanded mode

Analysis of Load-Flow Solution Methods for AC/DC Distribution Systems

RÉSUMÉ Les systèmes de distribution CA/CC ont récemment reçu plus d'attention car ils sont capables de fonctionner à la fois en mode connecté au réseau et en mode isolé. Comparés aux systèmes de distribution CA conventionnels, ils offrent une intégration plus pratique des composants à base de CC tels que les sources d'énergie renouvelables, les systèmes de stockage et les véhicules électriques. Plusieurs travaux de recherche ont porté sur l'analyse d’écoulement de puissance des systèmes d'alimentation CA/CC à différents niveaux de tension. Cette analyse comprend l'analyse d’écoulement de puissance CA/CC pour les systèmes à courant continu haute tension (CCHT), moyenne et basse tension des réseaux de distribution CA/CC et microréseaux dans les deux modes de fonctionnement connectés au reseau et en îloté. L'objectif de ce projet de recherche est d'examiner en détail les méthodes d’écoulement de puissance CA/CC en fonction de leurs approches de modélisation de système et de leurs procédures de solution. De plus, cette thèse valide deux méthodes d’écoulement de puissance récemment proposées pour les systèmes de distribution CA/CC avec différentes approches de modélisation et manières de solution en utilisant un logiciel de simulation de domaine temporel (EMTP). Suivi de la validation, les méthodes d’écoulement de puissance CA/CC sont comparées entre elles en termes de performances. Enfin, deux techniques d'amélioration sont proposées et appliquées à ces méthodes d’écoulement de puissance pour améliorer leurs performances. ----------ABSTRACT AC/DC distribution systems have recently received more attention as they are able to operate in both grid-connected and isolated modes. Compared to conventional AC distribution systems, they offer more convenient integration of DC-based components such as renewable energy sources, storage systems, and electric vehicles. Several research works have been dealt with load-flow analysis of AC/DC power systems at different voltage levels. This includes AC/DC load-flow analysis for high voltage direct current (HVDC) systems, medium and low voltage AC/DC distribution systems and microgrids in both grid-connected and islanded modes of operation. The objective of this research project is to comprehensively review AC/DC load-flow methods based on their system modeling approaches and solution procedures. Moreover, this thesis validates two recently proposed load-flow methods for AC/DC distribution systems with different modeling approaches and solution manners utilizing a time-domain simulation software (i.e. EMTP). Followed by the validation, the AC/DC load-flow methods are compared to each other in terms of their performances. Finally, two improvement techniques are proposed and applied into those AC/DC load-flow methods to enhance their performances

Fluxo de Potência para Redes de Distribuição Radiais, Ativas e Ilhadas

O presente trabalho tem por objetivo principal apresentar uma ferramenta de análise de redes de distribuição radiais ativas conectadas a rede básica e funcionando de maneira ilhada. Os objetivos específicos são avaliar o impacto de fontes renováveis em sistemas radiais de distribuição, considerando impactos da pequena variação a da frequência nos elementos do sistema. A metodologia utilizada é baseada no modelo da varredura considerando droop, sendo o modelo da varredura responsável pelo cálculo do fluxo de potência e o método droop responsável pela inserção de geradores despacháveis de energia. Inicialmente é apresentado objetivo, a motivação, a estrutura da dissertação, seguida da contextualização da geração distribuída no Brasil. Apresenta-se o estado da arte do fluxo de potência para redes de distribuição bem como uma bibliografia auxiliar a respeito de controle para redes de distribuição. Em seguida o método estático da varredura direta e inversa é explicado. Então, a inserção da variação dos parâmetros de rede em função de pequenas variações da frequência é explicada. E por fim, as simulações e suas respectivas análises são feitas em diferentes cenários. Chegou-se à conclusão que a metodologia proposta é eficiente e apresenta resultados coerentes, considerando a dependência com a variação da frequência, a topologia e impactos da geração distribuída

Upgrading Plan for Conventional Distribution Networks Considering Virtual Microgrid Systems

It is widely agreed that the integration of distributed generators (DGs) to power systems is an inevitable trend, which can help to solve many issues in conventional power systems, such as environmental pollution and load demand increasing. According to the study of European Liaison on Electricity grid Committed Towards long-term Research Activities (ELECTRA), in the future, the control center of power systems might transfer from transmission networks to distribution networks since most of DGs will be integrated to distribution networks. However, the infrastructure of conventional distribution networks (CDNs) has not enough capabilities to face challenges from DG integration. Therefore, it is necessary to make a long-term planning to construct smart distribution networks (SDNs). Although many planning strategies are already proposed for constructing SDNs, most of them are passive methods which are based on traditional control and operating mechanisms. In this thesis, an active planning framework for upgrading CDNs to SDNs is introduced by considering both current infrastructure of CDNs and future requirements of SDNs. Since conventional centralised control methods have limited capabilities to deal with huge amount of information and manage flexible structure of SDNs, virtual microgrids (VMs) are designed as basic units to realise decentralised control in this framework. Based on the idea of cyber-physical-socioeconomic system (CPSS), the structure and interaction of cyber system layer, physical system layer as well as socioeconomic system layer are considered in this framework to improve the performance of electrical networks. Since physical system layer is the most fundamental and important part in the active planning framework, and it affects the function of the other two layers, a two-phase strategy to construct the physical system layer is proposed. In the two-phase strategy, phase 1 is to partition CDNs and determine VM boundaries, and phase 2 is to determine DG allocation based on the partitioning results obtained in phase 1. In phase 1, a partitioning method considering structural characteristics of electrical networks rather than operating states is proposed. Considering specific characteristics of electrical networks, electrical coupling strength (ECS) is defined to describe electrical connection among buses. Based on the modularity in complex network theories, electrical modularity is defined to judge the performance of partitioning results. The effectiveness of this method is tested in three popular distribution networks. The partitioning method can detect VM boundaries and partitioning results are in accord with structural characteristics of distribution networks. Based on the partitioning results obtained in phase 1, phase 2 is to optimise DG allocation in electrical networks. A bi-level optimisation method is proposed, including an outer optimisation and an inner optimisation. The outer optimisation focus on long-term planning goals to realise autonomy of VMs while the inner optimisation focus on improving the ability of active energy management. Both genetic algorithm and probabilistic optimal power flow are applied to determine the type, size, location and number of DGs. The feasibility of this method is verified by applying it to PG&E 69-bus distribution network. The operation of SDNs with VMs is a very important topic since the integration of DGs will lead to bidirectional power flow and fault current variation in networks. Considering the similarity between microgrids and VMs, a hybrid control and protection scheme for microgrids is introduced, and its effectiveness is tested through Power Systems Computer Aided Design (PSCAD) simulation. Although more research is needed because SDNs are more complicated than microgrids, the hybrid scheme has great potential to be applied to VMs