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This book presents a panoramic look at the transformation of the transmission network in the context of the energy transition. It provides readers with basic definitions as well as details on current challenges and emerging technologies. In-depth chapters cover the integration of renewables, the particularities of planning large-scale systems, efficient reduction and solution methods, the possibilities of HVDC and super grids, distributed generation, smart grids, demand response, and new regulatory schemes. The content is complemented with case studies that highlight the importance of the power transmission network as the backbone of modern energy systems. This book will be a comprehensive reference that will be useful to both academics and practitioners.



Chapter 1. Introduction: The Key Role of the Transmission Network

The transmission network, bridging the generation and distribution of energy, plays a pivotal role in the power system. This system must be capable of long-term and continuous energy transfer [1], which, given the long distances covered, leads to networks that are built and operated in AC or DC at high voltage levels [2]. The increasing growth in energy consumption has led to significant changes in the types of consumption as well as power generation sources, most importantly increasing the penetration of renewables in recent decades [2–4]. This trend is set to continue: Europe aims to achieve a 30% improvement in energy efficiency and a 27% increase in the use of renewable energies by making a 40% reduction in greenhouse gas (GHG) emissions by 2030, with the ultimate goal of completely eliminating GHG from power generation by 2050 [4–6]. Renewables can be located anywhere on the grid, causing dramatic and direct changes in the need for transmission. Besides, the need for tighter market integration calls for the expansion of cross-border capacity. These two main drivers have led to an increased need for transmission expansion that should be undertaken in the next couple of decades.
Sara Lumbreras, Hamdi Abdi, Andrés Ramos, Mansour Moradi

Chapter 2. Metaheuristics for Transmission Network Expansion Planning

This chapter presents the characteristics of the metaheuristic algorithms used to solve the transmission network expansion planning (TNEP) problem. The algorithms used to handle single or multiple objectives are discussed on the basis of selected literature contributions. Besides the main objective given by the costs of the transmission system infrastructure, various other objectives are taken into account, representing generation, demand, reliability and environmental aspects. In the single-objective case, many metaheuristics have been proposed, in general without making strong comparisons with other solution methods and without providing superior results with respect to classical mathematical programming. In the multi-objective case, there is a better convenience of using metaheuristics able to handle conflicting objectives, in particular with a Pareto front-based approach. In all cases, improvements are still expected in the definition of benchmark functions, benchmark networks and robust comparison criteria.
Gianfranco Chicco, Andrea Mazza

Chapter 3. Transmission Network Expansion Planning of a Large Power System

The unbundling of the electricity companies, which means the separation of generation, transport, and distribution activities, imposed over the past two decades in many countries has led to a review of planning methods, be it generation or networks.
Chap. 3 presents the evolution of transport network planning. In particular, the use of scenarios has gradually become necessary to manage the multiple uncertainties over the evolution over time of different parameters to represent the future need for networks.
After the general presentation of the methods used by most TSOs, the chapter introduces an enlarged vision of network development studies by describing the methods used in Europe for interconnections between countries.
Chap. 3 develops in particular prospective methods developed within the framework of a European research project, e-Highway250 project, which aims to identify the European transport network necessary to integrate large volumes of renewable energy by 2050.
Gerald Sanchís

Chapter 4. Reduction Techniques for TEP Problems

One of the most common representations of the TEP problem is a mixed-integer linear programming (MILP) problem, in which operation costs are modeled by linear variables, and investment decisions are represented by integer variables. The large size of the TEP problem, together with the lumpiness of investment decisions, makes the problem very hard to solve. To cope with this issue, techniques to represent the TEP problem in a compact way, i.e., reduction techniques, can lessen the size of the problem and make it easier to solve. The TEP problem can be reduced in three dimensions: the power grid representation (spatial dimension), the operating point representation (temporal representation), and the number of candidate grid elements to consider. An interesting methodology to reduce the TEP problem in each of its three dimensions is to guide the reduction using the information from the solution of a more tractable version of the problem. The first step consists in solving a linear relaxation of the TEP problem. Then, we make use of the information associated with this approximate solution to formulate the TEP problem in a compact way for each dimension. This chapter introduces three efficient reduction techniques, one reduction technique for each problem dimension, that are based on this linear relaxation.
This chapter is organized as follows. Section 4.1 presents a methodology to reduce the size of the temporal representation of the TEP problem, also called snapshot selection method. Section 4.2 presents a methodology to reduce the size of the spatial representation of the problem, also called network reduction method. Section 4.3 introduces a methodology to identify promising candidate grid elements, also called search-space reduction method. Finally, Sect. 4.4 concludes.
Quentin Ploussard

Chapter 5. Offshore Grid Development as a Particular Case of TEP

Offshore grids transmission lines of different technologies interconnect onshore power systems and connect offshore wind farms to shore. In an integrated offshore grid, some transmission lines realize both functions simultaneously. Integrated offshore grids may provide additional socio-economic benefits, including non-monetary ones such as innovation, industrial development, reduced environmental impacts and improved cooperation between countries. However, an integrated grid may entail additional costs to a non-integrated one, and several risks may lead to suboptimal offshore expansion planning. Nonetheless, research and cooperation between actors to develop such grids in Europe, Asia and North America is advancing. This chapter surveys the challenges to the expansion planning of offshore grid, structured in the building blocks of governance, planning, ownership, pricing and finance, and operation. Some solutions are drawn from current research and practical developments, especially in Europe where developments are most advanced, providing options which can be tailored to the characteristics of each offshore region.
João Gorenstein Dedecca

Chapter 6. HVDC in the Future Power Systems

High-voltage direct current (HVDC) systems are called to play an important role in future power systems, and therefore, transmission system operators (TSOs) are keen to explore design and analysis tools for those systems embedded in traditional high-voltage alternating current (HVAC) systems. This chapter presents the fundamentals of HVDC systems and illustrates how optimal power flow (OPF) calculations can be carried out in hybrid AC/DC systems when the two sides are interfaced through voltage source converters (VSCs), which is an arrangement with a promising future for its many advantages. Tools for OPF studies could help TSOs assess the profitability of modern high-voltage AC/DC systems at the transmission expansion planning (TEP) stage and to operate them optimally once they are installed. The OPF algorithm presented in this chapter can deal with large-scale hybrid power networks with a fairly detailed representation of the generation and transmission systems (both AC and DC) including detailed loss models. Two alternative formulations of the algorithm are described here in detail: (a) a nonlinear approach to OPF and (b) a linear one. The two alternatives are compared thoroughly in a case study. Results obtained with the nonlinear OPF model are more accurate than those obtained with the linear OPF model, but the latter presents a much lower computational burden. The linear OPF model could be useful when analyzing large-scale systems in which only active power flows are of interest.
Quanyu Zhao, Javier García-González, Aurelio García-Cerrada, Javier Renedo, Luis Rouco

Chapter 7. Transmission Expansion Planning Outside the Box: A Bilevel Approach

Ever since the beginning of the liberalization process of the energy sector and the arrival of electricity markets, decision-making has gone away from centralized planners and has become the responsibility of many different entities such as market operators, private generation companies, transmission system operators, and many more. The interaction and sequence in which these entities make decisions in liberalized market frameworks have led to a renaissance of Stackelberg-type games referred to as bilevel problems, which capture the natural hierarchy in many decision-making processes in power systems. This chapter aims at demonstrating the important insights that such models can provide with respect to transmission expansion planning. Finally, a numerical example is included for illustrative purposes.
Sonja Wogrin, Salvador Pineda, Diego A. Tejada-Arango, Isaac C. Gonzalez-Romero

Chapter 8. The Impact of Distributed Energy Resources on the Networks

Traditionally, generation was mainly produced on large generation plants, and the main driver for investments in transmission and distribution networks was demand growth in the form of new consumers being connected to the networks, or existing consumers increasing their demand. This scenario is subject to a paradigm shift nowadays, where new distributed energy resources (DER) are appearing, such as distributed generation, electric vehicles, demand response, and storage. On one hand, distributed generation and electric vehicles bring new opportunities, to comply with environmental targets and reduce carbon emissions. However, they also pose new challenges in terms of the operation and planning of the networks. On the other hand, demand response and storage can help to increase the flexibility of the power system. The impact of DER on the operation and planning of the networks, as well as in crosscutting topics like resiliency and cybersecurity are analyzed in this chapter. Finally a case study is used to compare the traditional scenario driven mainly by demand growth, versus scenarios with distributed generation and electric vehicles. The results show the different tendency of energy losses in the presence of distributed generation and the importance of avoiding peak charging in electric vehicles.
Carlos Mateo, Fernando Postigo, Álvaro Sánchez-Miralles

Chapter 9. Stability Considerations for Transmission System Planning

Transmission system operators are required to elaborate the planning schedule for the network in various time horizons, from short-term to long-term. Traditionally stability studies have been carried out as short-term and/or mid-term planning tasks, but they have been not considered for long-term planning, where the main goal is generation-demand balance. However, the paradigm is shifting due to diverse factors, such as the sustained growth of the demand, the expansion of transmission networks, the increasing number of power electronics associated with renewable energy resources and advance control devices, or higher inter-area energy interchanges, among others. Due to these facts, power system stability and dynamic studies need to receive more attention in long-term planning to prevent future incidents and guarantee the robustness of the system. The objective of this chapter is to provide an overview of the power system stability phenomena and their increasing relevance for transmission planning issues.
Francisco M. Echavarren, Lukas Sigrist

Chapter 10. Energy Storage Systems in Transmission Expansion Planning

This chapter presents a framework to demonstrate the impacts of energy storage systems (ESSs) on transmission expansion planning (TEP). In order to integrate the ESSs into TEP, a typical test network, i.e., IEEE 24-Bus RTS, is adopted as case study, and TEP is carried out on this network. The TEP is integrated with ESSs and the impacts of ESSs are investigated. The optimal sizing, siting, and charging-discharging regime of the ESSs are determined to achieve the best and optimal operation. The TEP including ESSs is modeled as mixed integer linear programming and solved by means of GAMS/CPLEX. The objective function is regarded as the investment cost on new lines and new ESSs. The constraints are included as security constraints of the network as well as the operational constraints of the ESSs. The planning horizon is considered as 6 years, and dynamic TEP is carried out to find the best location, capacity, time, and number of new lines. The simulation results demonstrate that the TEP without ESSs installs more lines to handle load growth, but the TEP with ESSs installs less lines and uses ESSs to supply peak demand. The ESSs properly shift energy over the day hours and shave the peak load. As a result, the congestion in the lines is relieved and the network requirement for new lines is deferred. The ESSs can efficiently defer the investment on new lines and improve the congestion in the network lines.
Reza Hemmati

Chapter 11. Regulation of the Expansion of Electricity Transmission

Widely different schemes have been designed to rule the expansion of the electricity transmission grid, both at the local, or national, level and at the regional one. We review the most relevant ones here. Some of them rely on the planning of the expansion of the grid by central authorities looking after the interest of users in the whole system or region; others rely in different forms of participation of the private initiative. The features of the electricity transmission activity, making the planning of the expansion and operation of this grid a natural monopoly, as well as the experience gathered, have shown that the development of the grid should primarily rely on regulated, centrally planned investments, possibly complemented with those promoted by associations of network users and merchant, for-profit investors. The planning and regulatory authorities should have appropriate tools and processes to define the network investment proposals and assess them according to their expected benefits and costs.
Defining an appropriate mechanism to allocate the cost of the network reinforcements is central to achieving an efficient development of the grid to be coordinated with that of generation and demand. Besides, in a regional context, this may probably be critical to getting the regulatory approval for the construction of reinforcements. We argue in our chapter that grid cost allocation should be based on the distribution of the benefits to be produced by each transmission expansion project among network users and the countries, or areas, in a region.
At regional level, different regions have opted for significantly different schemes for the management of the expansion of their grids and the allocation of their cost. Normally, a tension exists between the central and local decision-making levels in this regard. This must be adequately dealt with. We review the experience gathered with the development of their grids in three of the most representative regions, Europe, the USA, and Central America. Normally, history is conditioning the regional schemes adopted.
Luis Olmos, Michel Rivier


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