Energy Systems Security (eBook)

John G. Voeller (Herausgeber)

eBook Download: EPUB

2014 | 1. Auflage
85 Seiten
Wiley (Verlag)
978-1-118-65174-2 (ISBN)

Infos

Energy Systems Security features articles from the Wiley Handbook of Science and Technology for Homeland Security covering topics related to electricity transmission grids and their protection, risk assessment of energy systems, analysis of interdependent energy networks. Methods to manage electricity transmission disturbances so as to avoid blackouts are discussed, and self-healing energy system and a nano-enabled power source are presented.

Textauszug

2 Large-Scale Electricity Transmission Grids: Lessons Learned from the European Electricity Blackouts

Hans Glavitsch

Swiss Federal Institute of Technology, Zurich, Switzerland

2.1 Introduction

Electricity as the most versatile form of energy is the commodity of civilization, which has become something without which modern life is unthinkable. It is not a primary form of energy, but rather a secondary one, which has to be converted from various primary forms. The locations where these are available may be at distances to those where they are consumed; for example, hydraulic sources or technical constraints may require distant placements of generating stations, although primary sources would allow their site anywhere. Further, electricity requires a transport by conductors or better by transmission lines. Thus, transmission is a basic means for providing electricity to consumers. Since the transportation loss is a function of the current, the transportation over long distances is done at high voltages as high voltages allow low currents, which produce low losses. Single transmission lines are not enough as they do not guarantee enough reserves. Hence, the practice has led to the formation of interconnected transmission networks, which provide reserves, contribute to the economy of the operation and equalize between deficiencies and surplus.

The interconnection, however, implies the propagation of disturbances over wide areas. Hence, deficiencies or surplus of power are felt in the overall system. In extreme condition, a disturbance with all possible internal corrections may evolve to a blackout or near blackout as experienced in recent years in the European interconnected system. There are various causes for blackouts, such as technical, conceptual, due to misunderstanding of phenomena, or simply due to human error.

2.2 Basic Mechanism of Electric Power Transmission in a Large Grid

Electric power transmission is predominantly realized by the system of alternating currents (AC system), in particular by three-phase currents. The alternating mode allows transformation of voltages by the relatively simple transformer. A single-phase system generates a stream of pulsating power. However, if three single-phase systems—as a three-phase system consists of three single phase systems—are combined in such a way that the three single phase systems are shifted one third time period each the shifted pulsating powers result in one constant power stream. Alternating voltages and currents create synchronizing forces between generators such that all machines rotate at the same speed yielding one unique frequency in the system.

The sum of the input powers, thus the generated output, is balanced by the total of the consumed load. The level is adjusted such that the speed that is directly proportional to the frequency of the voltage stays at the nominal level, in terms of frequency 60 or 50 Hz. Any disturbance in the power balance causes a change in frequency. Thus, a drop in frequency is a signal that there is not enough primary power or an excess of load. The Union for the Co-ordination of Transmission of Electricity (UCTE) has established rules [1] for the contribution of generators in subsystems (areas) to the correction or maintenance of frequency. Should there be a major drop in frequency, each subsystem has to be adjusted such that it contributes in terms of the so-called primary control an amount of power proportional to its annual consumption. The amount is derived from an assumed maximum loss of generation of 3000 MW, which does not cause more than 180 mHz of frequency deviation. Besides global changes in frequency, there are local changes on transmission circuits, which may cause overloads.

Another important phenomenon is the change in system voltage. The transport of large amounts of power over long distances leads to a decrease in voltage, which may cause instabilities. Generally, the whole system is an oscillatory system that breaks up if the amplitudes of the swings exceed a limit, and then there is a loss of generation, which could cause a chain reaction, that is, further losses. In order to counteract undesired oscillations, damping measures are installed in generators, explicitly in voltage regulators. The magnetic field in the generators is mainly responsible for the voltage, and the excitation system controlled by the voltage regulator reacts to any change in the voltage.

2.3 Power Flows in Interconnected Grids

Power flows in the grid are determined by the injected nodal powers, that is generation and consumption, and by the laws of the network acting on meshes and nodes, which consist of balances of voltages generated by currents times impedances in a loop or by balances of nodal currents. Impedances are characteristics of transmission lines. Because of mechanical properties, transmission lines can carry flows up to a predetermined thermal limit (conductor temperature determining the sag of conductors). Since the flows from one location to a distant one is fixed by the injected powers, the loss of a transmission circuit causes a redistribution of local flows. In extreme condition, the loss of one of the several parallel circuits causes a shift of the flow to the remaining ones, which can lead to overloads. A numerical example of a flow situation is given in Figure 2.1.

Figure 2.1 Effect of outageof one circuit (A–C): black figures flow in nondisturbednetwork and grey figures flow when circuit A–C is tripped.

The total of flows is given by the injections at nodes A and B equaling the output at nodes C and D, that is 490 MW. In the normal operation, the flows in the circuits A–C, A–D, and B–D are also 490 MW. When the line A–C is lost, the loading on A–D reaches 279.4 MW and on B–D 210.6 MW without any change in the total of input/output at the nodes.

2.4 The European Interconnected System—the UCTE System

The European interconnected system consists of three major parts that are connected by high voltage direct current (HVDC), namely, the central UCTE system, the Scandinavian network, and the network on the British Islands. Here, the interconnected synchronous AC system, the UCTE system, is of interest. It extends from Denmark to Spain and from France to Greece and the East European countries. It consists of 380- and 220-kV-transmission lines operated to a maximum of 420 and 245 kV, respectively. The structure of the grid is characterized by substations where a large number of transmission lines terminate and a larger number of substations where just three or four lines are connected. Typical line lengths for 380 kV are in the range of 100–150 km, sometimes 200 km. For 220 kV, they are considerably shorter (50 km). In France, the line lengths are above 200 km for 380 kV. In Germany, Switzerland, and Northern Italy, the grid is highly interconnected. Tie-lines (circuits crossing borders) are not numerous, except in Germany and Switzerland. The number of tie-lines to East European countries is relatively less.

Originally, the UCTE system had the function to provide the security of supply in continental Europe. For this purpose, the system has been developed over the last 50 years with a view of assuring mutual assistance between national subsystems. However, there has been a fundamental change of paradigms over the past one or two decades. The transmission infrastructure is no longer just a tool for mutual assistance, but has become a platform for shifting ever growing power volumes all across the continent. On the other hand, the development of the system is more and more affected by stricter constraints and limitations in terms of licensing procedures and construction times.

In the UCTE system, the annual production in the year 2006 amounted to 2584.6 TWh, the maximum load 390.6 GW (third Wednesday in December) and the annual load reached 2530.1 TWh. Electricity is produced in nuclear stations (37%), conventional thermal stations (47%), and in hydro stations (16%, figures of 1999).

Within a country, one or more control areas operated by independent transmission system operators (TSOs) and a large number of market participants (traders) are in existence. Today there are 29 TSOs in 24 countries. Energy is exchanged for various reasons, hydro to thermal, day to night and vice versa, as well as for economic benefits. The annual exchange 2006 among UCTE countries reached 296,822 GWh, that is 11.7% of the consumption. This is an increase over the time before the opening of the market as the exchanged energy is typically in the order of 9–10%.

2.5 Management of the System

2.5.1 Before Opening of the Market

The vertically organized utilities were focused on their system and consumers. Tariffs for the exchange between voltage levels were fixed and state controlled. On the transmission level, an exchange of energy and power took place for the benefit of reducing reserves, peaking power, system regulation, better control of frequency, area control, and coordinated scheduling. The TSOs were the traders and the actors. The operation was coordinated by the rules of UCTE, which comprised the reserve management, primary, secondary, and tertiary frequency control, as well as security management.

2.5.2 In the Open Market

The Directives of the European Community introduced the liberalization of the electricity market [2], which was implemented step by step and is now nearly complete. The aim is a fully liberalized electricity market for all consumers...

Produkt-Details

• Erscheint lt. Verlag: 16.1.2014
• Sprache: englisch
• Themenwelt: Technik ► Elektrotechnik / Energietechnik
• ISBN-10: 1-118-65174-X / 111865174X
• ISBN-13: 978-1-118-65174-2 / 9781118651742