Microgrid Frameworks: History
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Microgrids technologies are seen as a cost effective and reliable solution to handle numerous challenges, mainly related to climate change and power demand increase. This is mainly due to their potential for integrating available on-site renewable energy sources and their flexibility and scalability. The particularity of microgrids is related to their capacity to operate in synchronization with the main grid or in islanded mode to secure the power supply of nearby end-users after a grid failure thanks to storage solutions and an intelligent control system. 

  • microgrids
  • distributed generation systems
  • voltage source inverters
  • islanding

1. Introduction

The operation of the electrical power system is experiencing considerable mutations due to increasing energy demand, electricity market liberalization, fossil fuel scarcity, and the environmental impact of conventional generation methods. Consequently, the conventional electrical power system is not able to meet the balance between energy demand and production and face the aforementioned challenges [1]. Moreover, the mastery and evolution of technologies related to the exploitation of different energy sources and renewable energy sources in particular are pushing ahead the development of new power system paradigms [2]. To face these challenges, a competitive solution consists of producing energy based on the integration of more Distributed Generation (DG). These power challenges are transmuting the conventional electrical power generation concept and introducing an incipient trend coming mainly from the interconnection of distributed generation resources and the electric distribution system [1,2,3,4].
Benefits of a coupled DG-electric distribution system include:
  • Reduced greenhouse gas emissions;
  • Better energy system efficiency;
  • Increased system reliability;
  • Reduced congestion in distribution and transmission on the traditional power system;
  • Services provision, such as voltage support and demand response;
  • Increased integration of micro-generation systems along with the existing power generation schemes;
  • Bidirectional power flow that offers more energy utilization flexibility.
As the characteristics of DG units are different, connecting them to a distribution system requires the use of power electronic interfaces. These interfaces allow the user to control the instantaneous injection of power. Thus, high-power quality can be achieved through an appropriate control system [5,6,7,8,9].
In order to intensify DG integration into the grid, the development of the microgrid (MG) concept is of interest, as it can integrate multiple interconnected DG types, storage systems, and loads. The microgrid concept is introduced as a promising solution to remedy many technical challenges and benefit from the potential of DGs, notably renewable energy resources [7,10,11,12,13,14,15]. Currently, the MG concept has numerous definitions; one universal definition considers MGs as a platform for incorporating various DG resources with minimum disturbances to the main utility grid or to local loads. It is also seen as a small-scale demonstrator for Smart Grid development [7,16]. It is expected that new functionalities related to grid technologies and modern electric applications will open other horizons for the development of MG systems, such as the development of MG clusters and Smart Grid applications [10,11].
Figure 1 illustrates the basic architecture of a typical grid interactive AC MG. The use of DG units and MGs offers several benefits linked mainly to environmental and techno-economic aspects [14]. In addition, it improves reliability and power supply security, especially for isolated areas [13,14,15,17,18]. Therefore, these new paradigms are rapidly developing along with the technological development of power electronics interfaces and power generation systems. Thanks to these developments, the exploitation of renewable energy sources and storage systems becomes easier within the framework of DGs and MGs.
Figure 1. Typical architecture of grid interactive microgrid.
Microgrids can operate in Grid-Connected “GC” or Stand-Alone “SA” mode. Thus, MGs should be able to operate in mode transition between GC and SA. In other words, MGs should be controlled to provide GC and SA functions and to ensure smooth mode transition [8,9,16,19,20,21,22,23,24,25].
In the GC mode, the amplitude and frequency of the output inverter voltage are fixed by the main grid. Under fault conditions, DG units and local loads are isolated from the rest of the grid and need to operate in SA mode. In islanded MG, i.e., SA mode, the inverter control system is responsible for regulating the voltage characteristics (frequency and amplitude) [26,27,28,29]. Nevertheless, between GC and SA, there is a short period of time where the bus voltage is neither regulated by the inverter controller nor fixed by the main grid, which may result in a decrease in power quality and load malfunction [8,9,22,23,26]. This situation may result in abrupt changes in system state variables, which can affect the whole system [22,30].
Hence, increasing electric supply resilience is a major challenge that arises especially for MG applications, which accelerates the development of advanced control and protection mechanisms. This issue presents a potential new research horizon for many research and development program directions [31,32,33,34,35,36].
Thus, the goal for MG systems is to be able to function in complement with the utility grid and thus function in GC and SA modes and achieve smooth mode transition. This enthusiasm is reflected in a significant number of studies to remedy the widespread concern about mode transition topics (i.e., relieve the undesirable transient effect). Indeed, the problems experienced during the transition phase mainly concern frequency and voltage disruptions [25]:
  • An abrupt variation at the frequency level easily drives disturbances into the output angle of the DG inverter, which is contributing to the destabilization of the total MG system;
  • An abrupt variation at the voltage or current level leads to system destruction, notably at the local critical load.
In the literature, various control strategies have been proposed in order to handle mode transition [3,4,16,24]. Developed control strategies aim to guarantee a seamless disconnection or connection between all connected DG units and the utility [8,9,22,23,24].

2. Microgrid Frameworks

This section conducts a thorough literature review on the conceptual and operational aspects related to MGs. Figure 3 presents the typical functions and components of MGs. In addition, in order to achieve high levels of robustness, resilience, and reliability throughout all operational states and transitions, different control and management strategies need to be implemented. The theoretical field of sustainable transitions has grown rapidly in recent years [95,98,99,100,101,102,103], and various disciplines such as economics, sociology, economic geography, and engineering are committed to the development of new grid paradigms. In this sense, there are many MG demonstrators deployed all over. An overview of the existing MG systems is proposed in Table 1. Different information in relation to the geographical situation, type of loads, types of generation sources used, the existence of energy storage systems, and the operation modes of the targeted MG are listed in this table. As mentioned in Table 1, some energy development organizations realized their own MG projects, such as CERTS in the US [104], NEDO in New Mexico in collaboration with Japan [105], and MG projects in Europe as simulation and demonstration platforms [77].
The microgrid market in South America is experiencing rapid growth as well. According to a research report released by Triton Market Research, the Latin American microgrid market is projected to achieve a compounded annual growth rate of 10.61% between 2022 and 2028 [106]. This explains the existence of various laboratories and test systems dedicated to microgrids in South America. In [107], a concise portrayal of each microgrids general information, characteristics, and components. Additionally, it incorporates a discourse on the progress made in distributed generation within the Latin American region (in Brazil, Ecuador, Argentina, Mexico, Chile, Peru, etc.).
Currently, multiple demonstration and pilot projects are actively integrating renewable energy sources into isolated microgrid systems. These initiatives, discussed in [108], specifically focus on incorporating renewable energy resources into microgrids that serve remote communities in Canada. For instance, ongoing initiatives in Northern Ontario, such as Deer Lake and Fort Severn, involve the deployment of photovoltaic (PV) and hybrid systems to decrease reliance on diesel fuel. Additionally, the remote mine located in Diavik, Northwest Territories, has implemented a wind farm to reduce fuel consumption, whereas the Northwest Territories Power Corporation has installed a solar and battery system in Colville Lake. Additionally, research was conducted on the utilization of ocean energy. However, in 2020, Canada launched the Ocean Energy Smart Grid Integration Project, with the aim of integrating ocean energy into isolated microgrids on remote islands. The Pacific Regional Institute for Marine Energy Discovery at the University of Victoria received a grant of USD 730,000 to develop a microgrid powered by wave energy for Nootka Island [109].
Taking into account a country’s own needs, strategies, priorities, and assets that can be different from one region to another, different MG concepts can be developed. As a result, advancements in these technologies depend on the specificities, priorities, and potential of each country. Therefore, the majority of the MGs developed have at least one of the following aims:
  • ✓ Provide an appropriate remedy for delivering electricity to remote areas where it is difficult to connect rural communities, which is the case in many developing countries and isolated areas. Some cases can be found in Africa (Lucingweni, Diakha Medina), Asia (India, Vietnam, Nepal, Koh Jik, Sri-Lanka), and Europe (Akkan, Macedonia) [77,105,110];
  • ✓ For Europeans, the major concern is to develop energy communities in terms of renewable energy integration rates, i.e., integrating as much renewable energy on a large scale [99]. To achieve this, several EU countries have ambitious goals to decarbonize their energy systems (reduce gas emissions by 80–95% compared with 1990 by 2050). To achieve these goals, new policies and directions have been made to increase renewable energy production (mostly solar and wind energy) [100]. For example, in Feldheim, Germany, the power supply is 100% renewable [95]. To achieve a transition to 100% renewable energy by 2050, in Denmark, appropriate energy policies have been made to improve the energy efficiency of the residential sector [101]. In addition, numerous research programs focused on MGs’ development have been launched within EU research frameworks, such as the DISPOWER project or the MORE MICROGRIDS project, in order to develop advanced control schemes (decentralized and centralized control) and communication protocols. In this direction, many universities and technological institutes have also developed their own MG to carry out experiments while they are self-producers (Manchester, Aalborg, Fraunhofer, Chalmers, Illinois) [102].
  • ✓ The security of supply remains a major concern in the US in case of war or disaster [65]. The US Department of Defense focuses on the deployment of MGs in hot spots. The Navigant research predicts that the annual MGs implementation spending by the US Department of Defense is expected to exceed USD 1 billion annually by 2026 [65,111]. In this sense, many communities in the US have ambitious targets for the transition toward more renewable energy production, mainly from solar and wind energy. In this context, some initiating projects have been launched within US research frameworks., e.g., Sandia National Laboratories and the University of Buffalo have applied the social burden of power outages method in two projects to date [112].

This entry is adapted from the peer-reviewed paper 10.3390/en16135062

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