HVDC Grids

For Offshore and Supergrid of the Future

Dirk Van Hertem, Oriol Gomis-Bellmunt, Jun Liang

• Publisher: Wiley-IEEE Press
• Year: 2016
• ISBN:
  9781119115236
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Power Resources
• Index: Technology & Engineering

This book discusses HVDC grids based on multi-terminal voltage-source converters (VSC), which is suitable for the connection of offshore wind farms and a possible solution for a continent wide overlay grid. HVDC Grids: For Offshore and Supergrid of the Future begins by introducing and analyzing the motivations and energy policy drives for developing offshore grids and the European Supergrid. HVDC transmission technology and offshore equipment are described in the second part of the book. The third part of the book discusses how HVDC grids can be developed and integrated in the existing power system. The fourth part of the book focuses on HVDC grid integration, in studies, for different time domains of electric power systems. The book concludes by discussing developments of advanced control methods and control devices for enabling DC grids.

• Presents the technology of the future offshore and HVDC grid
• Explains how offshore and HVDC grids can be integrated in the existing power system
• Provides the required models to analyse the different time domains of power system studies: from steady-state to electromagnetic transients

This book is intended for power system engineers and academics with an interest in HVDC or power systems, and policy makers. The book also provides a solid background for researchers working with VSC-HVDC technologies, power electronic devices, offshore wind farm integration, and DC grid protection.

PART I
HVDC GRIDS IN THE ENERGY VISION OF THE FUTURE

THE RADICAL CHANGES experienced by the transmission system in recent decades, along with the fast increase of renewable (including offshore) power generation in the energy mix, have triggered the interest for offshore wind connections and for overlay HVDC grids.

Part I analyzes the main drivers that have contributed to the development of the HVDC grid concept and discusses the main economic and technical challenges that the development of such technologies will face. The worldwide energy scenarios driving investments in transmission infrastructure are presented, with special attention to the Europe, where the first initiatives are ongoing.

Part I sets the scene, where the technology, planning and operation, modeling, and some applications of HVDC grids for offshore grids and the supergrid will be developed in further parts.

CHAPTER 1
DRIVERS FOR THE DEVELOPMENT OF HVDC GRIDS

Dirk Van Hertem

Research Group Electa, Department of Electrical Engineering, University of Leuven, Leuven, Belgium

1.1 INTRODUCTION

For a long time, the electric power industry was seen as a stable and fully matured sector, which was operating well without much innovation. Over recent decades, the situation has changed drastically, with a rapid development and innovation process. There is a strong interest from the general public for a cleaner and cheaper supply of electric energy. At the same time, the security of energy supply must be kept at the existing level, or should even be increased. The consequences are seen throughout the industry and throughout the world, with as a consequence fundamental changes in the generation of electrical energy. From a transmission point of view, this results in the need to fundamentally invest in the transmission system and to move towards a smarter and more flexible use of the grid. Different geographic, social, and historic influences have led to a different evolution over the continents. In this chapter, the different drivers of the energy revolution are discussed and the effect that these drivers have on the development of the electrical energy system is sketched. The focus is on the developments on the bulk power transmission level.

This chapter describes the evolution and the drivers in energy policy, resulting in a strong move towards the development of the offshore grid and supergrid. As the perceived drive is strongest in Europe, the main focus is on the evolutions there.

1.2 FROM THE VERTICALLY INTEGRATED INDUSTRY TO FAST MOVING LIBERALIZED MARKET

1.2.1 Brief History of the Transmission System Before Liberalization

After the introduction of electricity, the power industry has been in continuous evolution throughout the twentieth century. The earliest systems consisted of a single generator connected to one or more nearby loads. A generator company owned the generation unit and the grid connecting the loads. This generator company sold electricity directly to its customers. In order to increase reliability, facilitate a growing demand, and provide a sufficiently flexible grid operation with a minimum of assets, these local grids were connected to form interconnected grids. The outage of a single generator no longer led to supply interruption. As loads increased, more systems were interconnected, the total generated power grew and higher voltage levels had to be used. Longer distances were covered. Step by step, entire countries were electrified and smaller independently operated power systems were connected.

In continental Europe, the establishment of international interconnections really started after World War II. This led in 1951 to the establishment of the “Union for the Co-ordination of Production and Transmission of Electricity” (UCPTE) [1]. Since then, an international transmission system was developed with strong interconnections at the 380-kV level. UCPTE originally oversaw the development of economic activity through the improved exploitation of primary energy resources associated with the interconnection of electricity systems. Gradually, UCPTE also organized the international cooperation between the electricity system operators and set common operational rules, amongst which is a strict frequency control. NERC¹ fulfilled a somehow similar role in the United States, where different synchronous zones cooperated from the early 1960s.

On a local level, a consolidation of power companies led to the situation in the 1990s in which each country in Europe had one or more vertically integrated companies, each dealing with their own zone. They were responsible for generation and transmission of electricity, and in some countries also for distribution. This included the planning, operation, maintenance, and exploitation of the transmission system as well as the power plants. Often these companies were state-owned or state-controlled. The power system is still operated as such in a significant part of the world.

In the vertically integrated system, operation is done from a best engineering practice point of view. To ensure a high reliability, investments are done in a coordinated manner: Generation and grid development are planned by the same group of engineers or at least within one firm. The focus is to provide an adequate energy supply with a grid which is sufficiently secure. Also, the management of the entire power system is within a single entity. As the entire supply chain is within a single company, there is a tendency to favor generators which are “grid friendly.” This generally means large, controllable generators with a high availability and predictable energy supply. In such a utility, “economies of scale” resulted in large centrally planned power plants, mostly using fossil fuels or nuclear energy, or hydro power where available.

As stated before, connections between zones also existed in the vertically integrated system. On the one hand, they allowed for support during events that threatened the secure operation of the power system, enabling each zone to operate more economically, with fewer reserves needed. On the other hand, long-term contracts between countries existed, allowing international trade. This made the cheap hydro power from the Alps and the cheap and abundant nuclear power from France available to other European countries. Mainly AC links were used, connecting the different zones into synchronous zones, but also asynchronous links existed in the form of HVDC connections between different synchronous zones.

As a whole, the vertically integrated power sector in industrialized countries was a slowly changing and well-established business, with limited, well-defined problems. This was especially so after the economic development in the industrial world became mature and the increase in energy consumption dropped to a few percent per year. However, as there was no competition, the power system was not necessarily operated at the highest techno-economic optimum.

1.3 DRIVERS FOR CHANGE

Although that the implementation and the priorities concerning energy policy differs between countries and regions, they are generally built around the same main pillars. Energy policy has the objective to ensure an energy provision that is reliable, cost-effective (cheap), and sustainable (Figure 1.1).

Figure 1.1 Policy drivers for the energy supply of the future: competitiveness, sustainable development, and security of supply [2].

Given the importance of energy to the overal economy, a secure energy supply is crucial to the modern industry and society as a whole. The security of supply (SoS) requires the system to have sufficient resources available (adequacy), the infrastructure available to transmit the energy from the source to the consumer with sufficient redundancy, and the ability to deliver the energy at any given moment. Failure to do so leads to short-term economic and national security concerns in the form of (local or wide-scale) blackouts. The relative economic position of a country or region can be compromised if the security of supply is threatened over a longer period of time.

The second aspect is a cost-effective energy supply, which also has a direct impact on the economy of a country or region. A...