Distributed Energy Resources in Microgrids

Integration, Challenges and Optimization

Rajeev Kumar Chauhan, Kalpana Chauhan, Rajeev Kumar Chauhan, Kalpana Chauhan

• Publisher: Academic Press
• Year: 2019
• ISBN:
  9780128177754
• Topic: Tecnologia e ingegneria
• Subtopic: Risorse energetiche rinnovabili

Distributed Energy Resources in Microgrids: Integration, Challenges and Optimization unifies classically unconnected aspects of microgrids by considering them alongside economic analysis and stability testing. In addition, the book presents well-founded mathematical analyses on how to technically and economically optimize microgrids via distributed energy resource integration. Researchers and engineers in the power and energy sector will find this information useful for combined scientific and economical approaches to microgrid integration.

Specific sections cover microgrid performance, including key technical elements, such as control design, stability analysis, power quality, reliability and resiliency in microgrid operation.

• Addresses the challenges related to the integration of renewable energy resources
• Includes examples of control algorithms adopted during integration
• Presents detailed methods of optimization to enhance successful integration

Chapter 1

Microgrids architectures

Vijay K. Sood and Haytham Abdelgawad, University of Ontario Institute of Technology (UOIT), Oshawa, ON, Canada

Abstract

In recent years, there has been a growing interest in the concept of microgrids being used to integrate distributed generation systems like solar photovoltaic and wind to reduce greenhouse gas emissions, provide higher reliability for critical loads and supply electricity in areas not served by existing grid infrastructure. A microgrid (MG) is a portion of the electrical system which views generation and associated loads as a subsystem, with the ability to operate both grids connected or islanded from grid, thus maintaining a high level of service and reliability. The existing grid infrastructure, the distributed energy resources to be integrated, as well as specific customer-oriented requirements will determine the best-fitting architecture to constitute a MG. Several MG demonstration projects have been implemented further investigate this emerging concept. In this chapter, the most common MGs architectures based on alternating current, direct current and hybrid AC/DC buses are analyzed. Furthermore, comparisons are made between different MG architectures. Positive and negative features of different architectures are given as a guide for further MG system studies.

Keywords

Microgrid; distributed generation; architecture; AC microgrid; DC microgrid; hybrid AC/DC microgrid

1.1 Introduction

Recently, it has been noted that the world’s electricity systems are starting to decentralize, decarbonize, and democratize in many cases [1]. These features, known as the “three Ds,” are driven by the need to reduce greenhouse gas (GHG) emissions to alleviate climate change, provide higher reliability and resilience for critical loads, reduce electricity costs, substitute aging grid infrastructure, and supply electricity in areas not served by existing grid infrastructure. While the compromise between the technical driving factors and the details of specific solution may differ from one place to another, microgrids came to the picture as a flexible solution for managing distributed energy resources (DERs) that can meet the varying requirements of different communities.

In recent decades, the research, development, and implementation of renewable energy sources (RESs) have been strongly propelled, mainly as distributed generation (DG). Due to variability and intermittence of RESs, their large penetration over traditional energy systems (especially wind and solar) involves operational difficulties that limit their implementation, such as variations of supplied voltage magnitude or imbalances between active and reactive power among generators. Moreover, new flow patterns may require changes to the distribution grid infrastructure with the application of enhanced distribution automation, adapted protection and control strategies, and improved voltage management techniques [2]. A possible way to conduct the emerging potential of DG is to take a systematic approach which views generation and associated loads as a subsystem, or a “microgrid” [3]. The MG can operate either connected to or separated from the distribution system, thereby maintaining a high level of service and reliability. In this sense, distributed energy storage systems (ESSs) become necessary to improve the reliability of the overall system, supporting the distributed generators’ power capability in cases when they cannot supply the full power required by the consumers. MGs can also operate isolated in those areas with no access to the utility.

The modularity of emerging generation technologies permits generators to be placed and sized optimally to maximize the reliability, security, and economic benefits of DG deployment. For example, installing microgeneration on the customer’s side provides an opportunity for the utilization, locally, of the waste heat from the conversion of primary fuel to electricity (combined production of heat and power—CHP), thus accomplishing a better overall efficiency. In recent years, there has been significant progress in developing small, kW-scale CHP applications, which are expected to play a very significant role in upcoming microgrid implementations [4].

A key feature that distinguishes microgrids from active distribution networks with DG is the implementation of the control system. MGs are considered to be the building blocks of the “smart grids,” thus integrating the actions of all the DERs including distributed generators and storage devices, plus local loads and the main distribution grid. The target is to deliver sustainable, economic, and secure electricity supply through intelligent monitoring, control, communication, and self-healing technologies, with cost-competitive information and communication technologies (ICT) playing a fundamental role.

Microgrids can be considered as vital components in the smart grid environment, which is being developed to improve reliability and power quality, and to facilitate the integration of DERs. These are defined as medium or small power systems comprising DERs and controllable and uncontrollable loads; either isolated and meeting their own demand needs or connected to the external grid to supplement their supply requirements. Microgrids are connected to a distribution grid at a single point known as the point of common coupling (PCC). Fig. 1.1 shows a conceptual topology of a future smart grid connecting to a small microgrid [5].

Figure 1.1 A conceptual topology of future smart grid [5].

Microgrids are considered to be efficient and resilient since they not only allow for a high penetration level of RESs, but also because of their ability to operate in isolated mode when faults occur in the main grid [6]. Consequently, microgrids will offer greater reliability, especially when integrated with smart grids, because, during outages, they continue to operate in islanded mode or even put power back into the wider grid. Finally, microgrids can offer flexibility, because a variety of resources, including CHP and diesel back-ups can be integrated into the grid, providing reliability, using waste heat used for other purposes, and smoothing out supply/demand spikes. However, the implementation of microgrids encounters a number of challenges. For instance, maintaining demand-supply balance in the presence of RESs is a complex task because of generation intermittencies, load mismatches, and voltage instabilities [7,8].

The microgrid concept is gaining rapid acceptance because of its environmentally friendly energy provision, cost effectiveness, improvement in power quality and reliability, reduction in line congestion and losses, and reduction in infrastructure investment needs. From the customer’s point of view, the microgrid is designed to meet their electrical and heat energy demands and avoid loadshedding [9].

In order to integrate DERs into a MG effectively, proper architectures should be performed, based on alternating current (AC), direct current (DC) and hybrid AC/DC systems, seeking the highest reliability and efficiency [10]. The existing grid infrastructure, the DERs to be integrated, as well as specific customer-oriented requirements will determine the most suitable electrical architecture of the MG. Since the late 19th century, AC has been the standard choice for commercial energy systems, based mainly on the ease of transforming AC voltage into different levels, the capability of transmitting power over long distances. Therefore AC distribution is the most popular and commonly used structure for MG studies and implementations. By utilizing the existing AC network infrastructure (distribution, transformers, protections, etc.), AC microgrids are easier to design and implement, and are built on proven and thus reliable technology. However, DC distribution has shown a resurgence in recent years due to the development and deployment of RES based on DC power sources, and the rapid growth of DC loads which today constitute the vast majority of loads in most power systems. DC distribution presents several advantages, such as reduction of the power losses and voltage drops, and an increase of capacity of power lines, mainly due to the lack of reactive power flows, the absence of voltage drops in lines reactance, and the nonexistence of skin and proximity effects which reduce the ohmic resistance of lines. As such, the associated planning, implementation, and operation is simpler and less expensive [11].

This chapter depicts different architectures of microgrids, such as AC, DC, and hybrid AC/DC microgrids, including a general definition of the electrical microgrid, and comparisons are made between different microgrid architectures. The pros and cons of different architectures are given to guide further microgrid system studies.

1.2 Literature review of microgrid studies

This paper references [12] a three-phase power-flow algorithm in the sequence-component frame for the microgrid and active distribution system (ADS) applications. In addition, it presents steady-state sequence-component frame models of DER units for the developed power-flow approach under balanced/unbalanced conditions and develops sequence-component models of directly coupled synchronous machine-based and electronically coupled DER units. The validity and accuracy of the power-flow algorith...