Introduction

This chapter discusses key concepts and evaluation metrics used to describe and quantify the value proposition of a local area power and energy system (LAPES). Often the engineer is asked to evaluate multiple options and create a justification for a recommendation. Concepts of techno-economic analysis explored in this chapter are useful to compare the LAPES to a traditional system as well as to explore the cost-benefit tradeoff of various LAPES designs, configurations, and operating modes. Higher power quality, energy availability, and reliability of small-scale and distributed power systems are often touted as significant advantages over a conventional utility electrical system. While LAPES may indeed offer technological advantages, in order for it to be a practical engineering solution a comprehensive economic valuation of these benefits must occur in order to build the business case for a LAPES system. That is, higher availability and reliability may come at a cost that is unaffordable for a given application. Thus, this chapter explores tools and techniques to perform a techno-economic analysis that considers both the technological merits and the economic costs.

A LAPES is expected to provide decades of service with comparable or better availability than a commercial, large-scale, wide-area electrical utility. The downside of a decentralized, distributed system is that propagating intelligence and control to the edge of the grid tends to increase the number of components required and creates new failure modes. Consider a solid-state transformer (SST) compared to a traditional line-frequency transformer (LFT). Certainly the SST can perform more functions than the LFT, including real and reactive power control and volt-var control, harmonic current compensation, among others. Juxtapose this enhanced feature set against the fact that the LFT routinely provides decades of service life with minimal if any operation and maintenance costs (O&M) and that the purchase price of the SST far exceeds the price of the LFT. Techno-economic analysis provides a data-driven, fact-based way to evaluate the cost-benefit of these competing engineering solutions. Further, consider that the SST is comprised of a large plurality of individual electrical components, each with associated failure and wear-out modes.

This chapter discusses the structure of the interconnected system building blocks that make up a microgrid. These building blocks include the energy sources and storage technologies presented in Chapters 5 and 6, as well as the power electronic interfaces presented in Chapter 4, that typically link these components together into various classes of architectures. The discussion will initially cover various structures for LAPES, including the use of power electronic interfaces to provide more control flexibility. From a planning perspective, LAPES power distribution systems may rely on ac or dc buses. Hence, this chapter also includes a comparison of ac and dc microgrid performance and characteristics. Two key aspects related to LAPES architecture choices are also explored in this chapter: fault detection and behavior of microgrids with constant-power loads.

Microgrid realizations

Radial cabling is, perhaps, the most common approach for power distribution grids. In a radial distribution system, such as the one in Figure 7.1, there is one power path from local generators to each load. Since radial architectures are one of the most common approaches to design power distribution networks in large power grids, microgrids with such an architectural approach can be analyzed somewhat as an extension of conventional power grid distribution systems. One of the advantages of radial distribution systems is that circuit protection schemes are relatively simple to coordinate and design. Two other advantages of radial power distribution systems are that system components’ rating requirements are relatively simple to determine and that voltage drop compensation techniques are easily implementable. However, from a planning perspective, radial systems tend to have limited growing flexibility because load addition or new generation integration may require the installation of new cables or other components unless the originally installed cables and other components were oversized (an additional cost) when they were first installed. One other weakness in radial distribution systems is that the existence of a single power path from each load to the sources makes each power path to a load a single point of failure. Thus, power availability, as seen by each load, may be lower than that observed in equivalent LAPES with another power distribution configuration.

Basic concepts

Electrification is widely regarded as the single greatest engineering achievement of the twentieth century. The electrical power grid, the system that interconnects and distributes electrical energy, is the foundation of the built environment that supports modern societies and their economies. Yet, electric power grids are showing increased signs of deterioration due to aging equipment and usage that stresses these systems beyond their original design and safety margins. Simply stated, the current technology is inadequate to meet the evolving and growing needs of society. The last decade saw a number of poignant cases where problems in the energy system wreaked havoc and devastation. Singular instances, including the 2000 and 2001 California electricity energy crisis, the 2003 Northeast blackout, and widespread cases of expansive outages caused by natural disasters should all serve as wake-up calls for infrastructure improvement. Looking toward the future, simply patching the existing system will not alleviate the power-delivery congestion, which creates local markets with extremely high marginal pricing and prevents widespread penetration of renewable energy resources such as wind and residential-scale photovoltaic systems.

Many of the energy sources that powered the last century are dwindling. Even if new deposits of these finite energy resources are discovered, their conversion to electrical energy will remain one of the largest contributors to global greenhouse gas emissions and other environmental hazards. Nuclear power, once seen as an alternative to fossil fuels, has lost widespread support since the nuclear event at the Japanese Fukushima #1 Nuclear Power Plant. The move to more environmentally friendly technologies, so-called renewable energy sources, adds stresses to the electrical grid that were never previously considered and pushes existing electric systems to the edge of their design and operational envelope.

The design of current electric power systems is essentially unchanged from that of the first power grids developed over a century ago. This design is based on the centralized architecture illustrated in Figure 1.1, in which power is generated in relatively few large power plants from which it is delivered to many loads that can be hundreds and even thousands of miles away. Despite having mesh topologies and other sources of redundancy, this predominantly centralized architecture is relatively unreliable and inflexible because sensing, actuation, and control coordination does not generally permeate down to the distribution level, much less to individual loads.

It can be said that this book loosely originated at the University of Illinois at Urbana-Champaign where the three authors met during graduate studies under the advising of Professor Philip T. Krein. It was likely the combination of this enlightening environment and our combined perspectives from past industry experience that caused the three of us to gravitate toward studying microgrids. Although the concept of microgrids implied revisiting a ideas initially proposed by Thomas Edison, much of the technology had obviously changed since the late 1800s. Perhaps the most significant is that, power electronics now provides the ability to step-up or step-down dc voltage, a flexibility that Edison's concept lacked. Today, in modern large interconnected power grids, the need for integrating new technologies (such as renewable energy sources and energy storage) has also led to a search for new technological solutions and, in particular, modern microgrids have emerged. Still, during the first decade of the twenty-first century, when we were together at the University of Illinois, the concept of microgrids was not mainstream and the focus of most University power programs were staunchly divided between power systems and power electronics. Power systems studies tended to center their attention on large conventional interconnected power grids. Power electronics programs were mostly oriented to circuit topologies, devices, or control analysis. Microgrids represented, at the time, a bridge between these two large areas of study of power systems and power electronics in to a new research area that could be called power electronics systems. Hence, the study of microgrids and what we call in this book local area power and energy systems represented pioneering work, not only because it bridged power systems and power electronics, but because of the enormous potential applications for this technology and impact on society. For example, in the United States microgrids became a mainstream technology for improved power resilience to natural disasters after Superstorm Sandy affected New York City and the New Jersey Shore in late 2012, but this technological relevance of microgrids had already been identified by us in 2005 after studying the effects of Hurricane Katrina on critical power infrastructure.

This chapter discusses operation and control aspects related to the system architectures presented in Chapter 7. The basic focus in this chapter is on discussing control strategies that can mitigate stability issues found in systems in which most of the loads have power electronic interfaces that make them operate as CPLs. Another major focus of this chapter is on control systems architecture and methodologies, especially oriented toward autonomous controls. This chapter presents advanced control concepts, including nonlinear and game-theoretic techniques, and concepts related to linear and geometric controllers for converters operation with CPLs.

Autonomous controls of LAPES

Some of the goals in relying on a LAPES for powering loads are to achieve higher power supply availability than from a bulk power grid and to realize more flexible and optimal ways to utilize electric power at a local level. However, the mere use of local distributed generation units does not necessarily imply a local improvement in system availability or efficiency over conventional grids, as it is commonly implied [1]. As explained in Chapter 2, to achieve high availability a system needs to maintain full operation even when one or more failures occur, i.e., it needs to be fault tolerant. One of the main premises in the design of fault-tolerant systems is to avoid single points of failure.

Some of the microgrid architectures currently proposed have a centralized controller that controls the power flow from each source and monitors the overall system condition [2]. This control architecture has several disadvantages, including limited flexibility and high risk of failure due to the presence of a single point of failure in the controller [3]. In some cases, critical components of systems with centralized controllers are designed to maintain the configuration and operational state so the entire system does not fail if the controller fails. However, keeping the system functioning is often achieved at the expense of some operational features, such as energy efficiency. Implementation of other desired operational functions, such as current sharing among source interfaces, may also affect system availability. Current sharing is implemented to balance operational stresses in power sources’ electronic interfaces. In a system with current sharing, source interfaces are controlled so they all have output currents proportional to their power ratings.

Introduction

Power electronics is the enabling technology for efficient and controllable conversion of electrical energy. Prior to the invention of the transistor in 1947 at Bell Labs – and more importantly the first power semiconductor, the silicon-controlled rectifier invented by GE in 1957 [1] – electrical energy at high power levels was not controllable except through crude and inefficient methods. This chapter will present the fundamental topo-logies, analysis, and control-of-power electronic technologies used as the basic building blocks of local area power and energy systems. The basic concepts of ac-dc, dc-dc, and dc-ac converters will be discussed with application to multiple converter systems. Primary topics will include topologies, control, and practical implementation.

Treatment of ac to dc rectifiers will cover point of load aspects such as power quality and controllability in ac architectures, including single and multiphase systems. Conversion of dc to dc power will play a central role in small-scale power systems because most renewable energy (photovoltaics) and energy storage (batteries) are dc voltage. Then dc to dc conversion will be presented as a key enabling technology, including single and multi-input topologies, and bidirectional power flow. The final section of this chapter will be devoted to dc to ac converters (also known as inverters). This section will cover the three main circuit topologies for inverters (voltage, current, and impedance source). Then, discussion of modulating and control techniques will show how these topologies link to single and multiphase ac small-scale power systems. It must be noted that this chapter represents a basic overview of power electronics as a subject of study and not as a comprehensive guide. The reader is encouraged to consult a power electronics text such as [2]–[4] for more in-depth treatment of these subjects and a more comprehensive listing and treatment of converter topologies.

Power conversion concepts

In general, a power electronic interface enables a controllable bidirectional energy flow between electrical sources and loads, as illustrated in Figure 4.1. In LAPES, the distinction between “sources” and “loads” can sometimes be undefined and may depend on time and system conditions. This conversion can involve changes in voltage levels and galvanic isolation depending on the needs of the application.

This chapter discusses interconnection and interoperation requirements of LAPES with utility power grids. Local area power systems may be designed and operated as either grid-connected or grid-independent systems. This chapter will discuss the requirements for grid interconnectivity from the hardware and control perspective. The issues concerning island detection are also explored, including a review of common island detection techniques. This chapter also presents the main aspects relating to regulatory statues and standards pertaining to the operation of grid-connected and grid-independent systems.

Introduction

From the microgrid definition in Chapter 1, we know that LAPES may or may not be connected to a main grid. Local area power systems may be designed and operated as either grid-connected or grid-independent systems. Examples of both cases were also introduced in Chapter 1, including cases of stand-alone LAPES in remote villages where it is not possible to interact with a main grid, and cases such as the microgrid in Sendai, which is designed to interact with a main grid. Still, grid-connected microgrids must be able to operate disconnected from the grid either by an intentional or unintentional event. When microgrids operate disconnected from a main grid, they are said to be in islanded mode. Islanded operation may be the result of:

1. – A necessity, as happens with remote villages.

2. – An involuntary outcome caused by an external action, as happens during natural disasters when grid power outages are common and extensive.

3. – A desired design, planning, or operational choice, as happens in some LAPES in order to be able to achieve costs and energy savings. In this case, microgrids may be designed with the capability to freely disconnect from the grid – for example, if there is sufficient local power generation – or be connected to it – for example, in order to inject excess local power generation into the grid – or they may be designed as intentional stand-alone microgrids that cannot be connected to a main grid.

Both operational modes – islanded or grid-connected – have implications for the control and stability of the local area power system. Obviously, the operation of a microgrid may also have effects on the main power grid, ranging from planning to operational aspects, which also need to be considered in order to ensure a common adequate operation.

This chapter discusses electrical energy storage technologies, management, and conversion requirements. Thus, the focus is not just oriented toward electrochemistry or physical analysis but rather how specific characteristics of each particular technology affect microgrid design, planning, and operation. There are fundamental differences between microgrids and traditional backup systems with respect to the requirements and choices for energy storage technology. In general, energy storage devices can be characterized by operation in two distinct modes: used often and in short intervals (i.e., a power delivery profile) or used seldom for long intervals (i.e., an energy delivery profile). Energy storage with a power-delivery profile is commonly needed in microgrids to compensate for slow dynamic response of some local generation sources, such as fuel cells. One example of using an energy storage device with an energy delivery profile is powering a load at night in a stand-alone photovoltaic system. In this chapter, batteries are considered as devices with energy delivery profiles, whereas ultracapacitors and flywheels are two storage technologies with power delivery profiles. Other storage technologies, such as pumped hydro, are not considered here because they tend to be dependent on location and thus are not universal options for arbitrarily located LAPES. The main battery technologies are compared by considering some important characteristics, such as scalability, cost, lifetime, and cycle-life. Charging circuits and energy management are also discussed. Particular energy conversion requirements, such as the need for a constant current, are also explored. In addition to explaining their basic physical characteristics and features, this chapter discusses suitable energy conversion interfaces.

Introduction

A basic operational requirement in a conventional power grid – thus, without energy storage devices – is that electricity generation and consumption must be continuously balanced. This requirement imposes difficult operational constraints on the system when variability on both the load side and source side exists, such as with renewable energy sources like wind and solar that are variable. Energy storage is an important concept to ensure that electricity is generated when the source is available, and that the load continues to be supplied when the source is not present or insufficient. Batteries can improve the electric power system asset utilization, energy availability, and performance.