DC Distribution Systems and Microgrids
2: Department of Electrical and Electronics Engineering, University of Nottingham, Nottingham, UK
DC electric power distribution systems have higher efficiency, better current carrying capacity and faster response when compared to conventional AC systems. They also provide a more natural interface with many types of renewable energy sources. Furthermore, there are fewer issues with reactive power flow, power quality and frequency regulation, resulting in a notably less complex control system. All these facts lead to increased applications of DC systems in modern power systems. Still, design and operation of these systems imposes a number of specific challenges, mostly related to lack of mature protection technology and operational experience, as well as very early development stage of standards regarding DC based power infrastructure. This book provides an up-to-date overview of recent research activities in the control, protection and architectural design of a number of different types of DC distribution systems and microgrids. Practical requirements and implementation details of several types of DC distribution systems used in the real world industrial applications are also presented. Several types of coordinated control design concepts are shown, with concepts of stabilization being explained in detail. The book reviews the shortcomings and future developments concerning the practical DC system integration issues.
Inspec keywords: photovoltaic power systems; computer centres; aircraft power systems; electric vehicles; marine power systems; power distribution control; distributed power generation
Other keywords: DC distribution systems; redundancy; aircraft; residential buildings; converters; power architectures; data centers; DC microgrid architectures; control circuits; microgrids; photovoltaic powered systems; shipboard systems; electrical vehicles
Subjects: General electrical engineering topics; Aerospace power systems; Distribution networks; Solar power stations and photovoltaic power systems; Power system control; Distributed power generation
- Book DOI: 10.1049/PBPO115E
- Chapter DOI: 10.1049/PBPO115E
- ISBN: 9781785613821
- e-ISBN: 9781785613838
- Page count: 400
- Format: PDF
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Front Matter
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1 DC microgrid control principles - hierarchical control diagram
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In this chapter, the hierarchical control of DC microgrids (MGs) is introduced. The definitions for each control level have been discussed. Primary control is responsible for distributed generator (DG) load sharing and is predominately implemented using the droop control. The droop control can be perceived as a virtual resistance, and its value can affect system stability and maximum DC bus voltage deviation. Two inherent issues with conventional droop control are discussed. Both terminal DC voltage control accuracy and cable resistance have impacts on the power sharing among DGs. Droop control can be mainly divided into two groups: current-based droop and voltage-based droop. The difference is the implication for system stability. In addition, another nonlinear droop curve is also mentioned with adaptive droop gain. It can reduce power sharing error in heavy load. Furthermore, dynamic power sharing during load transition is also discussed. Different load dynamic spectrums are assigned to different DGs according to their dynamic response. Finally, to receive commands from upper control levels, interfaces are pointed out without changing the basic primary control loop structure. The purpose of the secondary control is to restore the DC bus voltage deviation caused by conventional droop control from the primary control level. It can be implemented remotely in an MG central controller (Centralized approach) or locally inside each DG (distributed approach). Both dedicated analogue and digital communication links can be used in a decentralized secondary control to transmit current signals. To enhance the reliability of the control, a communication link based on power lines is also possible. The tertiary control level is responsible for the connecting process of MGs to the upper grid. A basic mode structure based on DBS is introduced to accommodate energy-management algorithms.
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2 Distributed and decentralized control of dc microgrids
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In this chapter, distributed and decentralized control approaches for dc microgrids are discussed. Distributed approaches employ a communication system among different converters in order to regulate the dc voltage and improve the load sharing accuracy. Some of the distributed methods utilize point-to-point communication links among converters; however, some of them use a sparse communication system based on consensus protocol. Sparse communication-based control approaches are more reliable and resilient than the fully communicated methods. On the contrary, the decentralized methods use no communication (physical link) between converters to reach the power sharing objectives. In these approaches, the control system of each converter uses local voltage and current information to control the output power (current) of the corresponding converter. Since these converters do not need to communicate with other converters, the overall stability and reliability can be enhanced. The centralized control approaches can be categorized as mode-adaptive (autonomous) droop control, nonlinear droop control and frequency droop control, and these methods are conceptually discussed in this chapter.
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3 Stability analysis and stabilization of DC microgrids
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This chapter discusses the effects of constant power loads ( CPLs) in DC-microgrids. Such loads are commonly observed in DC microgrids because of the extensive use of power electronic circuits to interface loads. As it is explained, due to their nonlinear nature, CPL creates a complex dynamic system that present two possible behaviors with an unstable equilibrium point unless power source convertor interfaces are properly controlled. In this chapter, both time domain and geometric control approaches that achieve stable equilibrium points in the presence of CPLs are explained. Passive approaches to compensate the destabilizing effect of CPLs are also discussed, but these approaches often lead to costly and bulky designs. Complex dynamics are also observed in rectifiers with CPLs, which often present excessively large output voltage variations.
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4 Coordinated protection of DC microgrids
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DC power systems have been investigated for transmission and distribution systems due to the advantages such as improved efficiency, reliability, and simpler configuration, as well as increasing DC-based distributed generation, energy storage, and loads. Along with comparatively short operating history and insufficient guidelines and standards, one of the current challenges is system protection techniques to deal with the highly dynamic and nonzero crossing nature of DC power. In this chapter, various coordinated protection schemes and applications for DC power systems have been reviewed in conjunction with the analysis of DC fault currents dynamics and topological bus configurations.
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5 Energy management systems for dc microgrids
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With the application of dc microgrids at different scales and topologies, it is possible to integrate different energy sources (such PV systems, fuel cells and batteries) into the energy mix of a larger system with lower conversion requirements. Moreover, it is much easier and cheaper to convert ac power into dc power which, in turn, helps economic operation of future energy systems. By using dc microgrids, it is also possible to achieve energy savings up to a great extent (~15%) and to improve system reliability by reducing the number of devices required and the total points of failure. The dc architecture also enables cost-effective and green solutions for operation and control of zero-net energy residential/office buildings as well as data centers. However, such optimal performance mainly depends on the proper design and application of EMSs which effectively manage the process of energy production and consumption based on predefined objectives and constraints. There are, definitely, a number of challenges in this regard which have to be suitably addressed. Currently, there is a lack of approved standards and technical codes for dc equipment and distribution networks at low voltage. There is also a lack of approved and recognized dc architectures at low-to-medium voltage levels which in turn necessitates different safety and protection practices in comparison with conventional ac systems. Last but not least, there is a strong need for upgrading the existing infrastructure to accommodate dc systems and interfaces.
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6 Control of solid-state transformer-enabled DC microgrids
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With the world-wide energy shortage and deterioration of existing power grid, microgrid becomes one of the hottest research directions in the power engineering area. Considering DC nature of many key components in the smart grid, such as photovoltaic (PV), battery, fuel cell, super capacitor, etc., as well as many DC type loads, such as light-emitting diode, DC microgrid has received more attention recently since it brings the opportunity for boosting the efficiency by eliminating the unnecessary power conversion stages. However, the existing DC microgrid can only interface with the distribution system by using a heavy and bulky passive line frequency transformer plus a rectifier, which has large space and heavy weight. Developing a more compact and active grid interface to enable an intelligent DC microgrid system is still a research focus. In this chapter, the solid-state transformer (SST)-enabled DC microgrid is presented. In addition, two system control strategies, namely, the centralized power management and hierarchical power management strategies, are proposed. In addition, an improved control strategy is proposed for increasing the penetration of distributed renewable energy resources (DRER) integration, which controls SST-enabled DC microgrid as a solid-state synchronous machine (SSSM). With the proposed control concept, frequency and voltage stability are improved in case of high-power intermittence at either DC load or DRER side. Design examples are given to illustrate the main characteristics of the presented system and control schemes.
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7 The load as a controllable energy asset in dc microgrids
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In a traditional power grid system, the operator had total control of generation and distribution assets while the load was viewed as a disturbance. Thus, planning and operation necessitated always being prepared for unforeseen changes in the load consumption. The result is that the US power grid is amazingly resilient, robust, and expensive. As we consider a new paradigm of dc microgrid systems, overcapacity may not be feasible for technological and/or economic reason. Yet, high power quality and availability is more important than ever particularly to support the digital economy and information age. This is compounded as renewable sources become increasingly utilized, and the system operator no longer has total and arbitrary control of the generation. In this chapter, we introduced a framework for load control in a LAPES. In this paradigm, the load is considered to be an energy asset which can be controlled not just by the end user but also by the system. As such, a new degree of freedom is introduced in the control problem of balancing electrical supply and demand. Key to implementing any of these concepts is to strike the balance between the opportunity and cost, which would provide the techno-economic trade-off needed to implement these concepts practically. Important application-specific design considerations are the desired energy availability and the price willing to be paid for that availability as well as the tolerable amount of control relinquished. Optimizing this is not straightforward and may lead to solutions that change with time and conditions. To anticipate this, the chapter discussed the concept of load prioritization and a method to change the allowable control of the load.
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8 Electric vehicle charging infrastructure and dc microgrids
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With substantial growth in sales of electric vehicles (EVs) globally, there is a push for expansion of the recharging infrastructure to service these vehicles. Over 2 million of electric cars (battery-electric and plug-in hybrid electric), 200 million electric motorcycles and 345 thousand buses (primarily in China) were deployed worldwide by the end of 2016, and over 1.2 million of electric cars were sold globally in 2017 alone. However, the global electric car stock made only a 0.2% of the total number of passenger cars globally in 2017. Assessments of country targets, original equipment manufacturer announcements and deployment scenarios for electric cars indicate that the number of EVs will range between 9 and 20 million by 2020 and between 40 and 70 million by 2025. Furthermore, a number of countries have decided to end the sales of fossil-fuel-powered cars in the near future (Norway by 2025, India and Netherlands by 2030, Scotland by 2032, France and rest of the United Kingdom by 2040), further accelerating the shift to electric transportation. The electric vehicle supply equipment (EVSE) is closely following the EV stock growth, with 2.3 million EVSE outlets (including 110,000 publicly available fast-charging outlets) available globally in 2016, and predicted six-fold increase in the available outlets by 2025. The fastest growing EVSE market is the Chinese market, with over 88,000 publicly available fast-charging outlets in 2016.
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9 Overview and design of solid-state transformers
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The SST has several advantages when applied in the modern electric distribution grid. Among them, the availability of the dc link for connection of dc microgrid is mentioned in this chapter. There are many different possibilities to implement the architecture of the SST, and an overview and classification have been presented. For electric distribution, the three power processing stages architecture is cited to be the best choice, because it provides the decoupling between the MV side and LV side and also providing dc link connectivity. Furthermore, the dc-dc stage plays an important role on the SST architecture design, because it is responsible for the major losses of the system, besides to be in charge for controlling the LVDC link. Therefore, the SRC and the DAB have been pointed out as the most promising choice. The SRC can provide a very high efficiency, but it is not able to control the power flow and the regulate the LVDC link properly. The DAB converter, on the other hand, can provide a high efficiency solution and provide full control. The design aspects of the power stage and control stage of the DAB converter focusing on the SST application has also been discussed in this chapter.
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10 Bipolar-type DC microgrids for high-quality power distribution
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During the last decades, AC systems dominated the power transmission and distribution applications almost exclusively. However, a recent convergence of needs originated in different sectors (renewable energy conversion, information technology and transportation) have accelerated the development of DC systems. Nowadays, DC systems are present at both transmission and distribution levels, offering high-performance solutions with enhanced efficiency and reliability, besides reducing the number of power conversion stages involved and uninterrupted power delivery. For LVDC active networks, two kinds of architectures are possible: unipolar and bipolar. Despite being a more sophisticated and technically complex solution, bipolar structure provide several advantages over conventional unipolar ones. Higher availability, efficiency and flexibility are just a few advantages featured by bipolar systems. This chapter presented a brief overview covering the different aspects of bipolar LVDC networks. Distribution converter topologies, balancing stages and also their control schemes are discussed in order to highlight the efforts being made in this growing architecture.
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11 Aircraft DC microgrids
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This chapter will provide an overview of aircraft DC microgrids. It introduces the aircraft EPS, covering the topics from power generation, distribution and utilization. It also reviews aircraft electrical system standards and highlights the power quality and power factor requirement for aircraft applications. The chapter presents the concept of aircraft electric starter/generator system. Control and stability analysis of aircraft DC microgrids are performed.
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12 Shipboard MVDC microgrids
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In this chapter, the shipboard MVDC microgrid system is introduced. First, the architecture and components of the system are described, and then the modeling and control are dealt with. Next, power management based on hierarchical control is described and then fault and protection schemes are discussed. Some simulation results for the case study are provided.
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13 DC-based EVs and hybrid EVs
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In this chapter, the power electronics converters and inverter in vehicle application are introduced. Several possible topologies for PFC, isolated DC/DC converter, APM, and APF are presented. The practical design of DC bus bars and the selection of power switch devices and DC-link capacitors are given. Finally, the state-of-theart topological reconfigurations of DC systems in electrified vehicles are presented. One of the biggest challenges for the next generation of power electronic systems in vehicle application will be the cost reduction to provide more affordable solutions. This has become one of the major barriers for electrified vehicle for mass commercialization. These dual-mode integration approaches are dedicated to vehicle application and can reduce the cost, size, and weight of the system. Therefore, they will also help promoting and accelerating the paradigm shift to the transportation 2.0.
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14 DC data centers
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As high-performance computing and data storage transition toward becoming Internet-based services, the world has witnessed an ever-increasing demand for both size and capacity of data centers. The growth of cloud-based services and applications shows no sign of slowing down, with additional custom-hardware for machine learning algorithms beginning to be deployed at scale in dedicated data centers. Today's data centers accommodate many pieces of information technology (IT) equipment such as data-processing units, data storage units, and communication devices. A recent report estimated the energy usage of data centers in the United States (US) alone at 70 billion kW h in 2014, corresponding to 1.8% of the total electric energy consumed in the country [1]. Since the IT equipment requires low DC voltage (typically ranging from a few volts to a few dozen volts) to operate, various power delivery architectures are established to provide low DC voltage from utility and renewable resources. In this case, the power delivery infrastructure in data centers can be considered as a microgrid due to the high installed power capacity and dynamic loads. However, data centers are also quite different than typical DC microgrids in many regards, both in the characteristics of the loads (extraordinarily rapid transients, but all controlled/managed from a central load scheduling interface) and the extreme up-time requirements. This chapter addresses major aspects of power-delivery architectures in data centers such as efficiency, reliability, integration with renewable resources, and protection. A critical evaluation of the technical and commercial barriers to widespread DC power distribution in data centers will be performed, along with a few examples of existing DC data centers.
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15 DC microgrid in residential buildings
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In this chapter, analysis and comparison between AC and DC microgrid in residential buildings have been done based on appliances, converters and their power losses in both systems. The layouts for LVDC distribution network have been discussed. Both unipolar and bipolar layouts of LVDC system have been discussed. Two microgrid system configurations have been discussed: AC residential building [i.e. AC distribution system (ACDS) with DC appliances] and DC residential building (i.e. distribution network with DC appliances).
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16 DC microgrids for photovoltaic powered systems
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This chapter first overviews the dc microgrid architecture, where the primary energy sources are PV panels. Then, the focus is on power electronic converter technologies for PV dc microgrid systems, where an industrial case is also given. Regarding the control dc microgrids for PV collection plants, it is discussed in this chapter as well. A design example is also provided.
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17 Demonstration sites of dc microgrids
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In recent years, several demonstration projects using dc microgrids have been implemented across the world due to some distinct advantages of dc over ac systems. The purpose of such initiatives is to validate the theoretically predicted benefits of dc distribution in practical scenarios. This chapter presents a noncomprehensive overview of existing demonstrations and pilot projects for a wide range of applications such as off-grid microgrids, transportation electrification, datacenters, residential and industrial purposes. For each application, key aspects such as architecture, components, control, protection and socioeconomic impacts are highlighted. A short discussion is offered on trends in voltage levels, capacity and topology, progressing toward possible standardization approaches based on recognized best practice.
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Back Matter
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