Power Systems and Restructuring

By Nouredine Hadjsaïd and Jean-Claude Sabonnadière

The development of electric power systems has been made up of incremental innovations from the end of the 19th century and throughout the 20th century. The creation of deregulated electricity markets has brought about an emerging paradigm in which the relationships between producers, power system operators and consumers have changed enormously compared to the monopolistic case.

The scope of this book is to provide fundamental concepts of the physics and operation of transmission and distribution lines, which is the content of Part 1, followed by the models and tools for the description and simulation of large electrical grids for steady state and transient operation. These advanced tools allow the physics and technology of power systems to be described and the algorithms of Ybus and Zbus matrices to be built for various studies such as short-circuit studies and load flow or transient phenomena analysis.

Part 3 deals with the new organization concepts in the frame of deregulated markets. In this part the restructuring of the power industry is presented where various actors interact together through market places or bilateral contracts. In addition, the operation of the power grids under this deregulated context is detailed and the relationships between power system operators and market actors (energy producers and providers, traders, etc.) is explained with several examples. The ancillary services, congestion management and grid access concepts are also described.

A large number of exercises and problems disseminated throughout the book with solutions at the end enable the reader to check his understanding of the content at any time.

• Language: English
• Publisher: Wiley
• Release date: Feb 7, 2013
• ISBN: 9781118599921

Book preview

PART 1

TRANSMISSION LINES AND ELECTRIC POWER NETWORKS

Chapter 1

The Two Paradigms of the World Electrical Power System

The term ‘electrical system’ is used to define the entire chain of electricity supply from the most distant generation centers to the load centers, while including electrical transmission and distribution systems. The distinction between electric transmission and distribution will be approached in later chapters. This system is the basis of the electric energy supply on which modern economies are strongly dependent. The paradigms governing this system, at the present time, rest on the following:

– the historical paradigm based on a vertical organization within the framework of integrated monopolistic operation, centralized production and electrical networks;

– the liberalization of energy markets within a competing framework.

1.1. Introduction

Before describing in detail the operation of lines and elements that constitute the transmission or distribution network, it is important for us to indicate to our readers very important changes that have occurred in the electric systems by worldwide evolution of legislation relating to the management of energy networks at the end of the 20th century. This change, known as liberalization of the energy markets, started in certain countries as early as 1980, following the example of Chile and the United Kingdom. The form of liberalization adopted then related mainly to the electrical energy generation sector. Thereafter, liberalization of the energy markets was adopted almost simultaneously in the United States, Europe and in other countries like Australia at the beginning of 1990.

For the United States, it is the National Energy Act of 1992 which truly launched the opening of the energy markets. In Europe, this opening was officially launched in the European Union by Directive 96/92/CE published on December 19, 1996. This document deals with the common rules for the internal electricity market, with an obligation for each country to transpose this directive in its national legislative system, which France did in February 2000 in the form of a law. It is interesting to note that this law founded a minimum opening level of 25% of the market of each Member State. Each state is free to proceed in fully opening its market if it so wishes. This opening was often progressive and was characterized by the concept of eligible consumers. This eligibility is related to the level of consumption.

For countries that initiated by opening 25% of the market, eligible consumers are 25% of the large-scale consumers. A threshold is thus fixed for each market share. Today, this movement of liberalization extends to the entire industrialized world and will undoubtedly soon spread to all electricity companies worldwide. Of course, this opening has had considerable consequences on exploitation, and to a lesser extent system planning. Part 3 of this book will be entirely devoted to economic effects, the effects on technical operation and evolution of management styles of transportation and distribution grids in the context of a liberalized energy market.

We, however, will consider in this first chapter the technical and economic factors that led to the construction of the electric systems on which the supply of electrical energy to private individuals and to companies is based. It is this model that we call the historical paradigm of the development of the electrical energy networks, a paradigm which is to be replaced in a way by that emerging from the liberalization of energy markets. Nevertheless, this model was created more than one century ago on scientific and technical bases, which remain valid in spite of the upheavals induced by the new economic deal. Indeed, the physical laws governing this system remain unaffected by the changing paradigms of exploitation of the system.

1.2. The historical paradigm

1.2.1. Grouped generation: scale effect

The basis of historical development of the networks and, more generally, of electrical systems of generation, transmission, distribution and energy utilization, lies mainly on two very important facts: the impact of electric generator size on the value of output; and the impact of the increase in operating voltage on losses by Joule effect in the cables. Indeed, Joule losses in the transmission of electric power are inversely proportional to the square of the voltage to which energy is conveyed:

[1.1]

where Pu represents transmitted power, U is the effective voltage, and ρ is the resistivity of the cables.

It should be noted that electric machines represent a big part of the electric system. Indeed, power generation is primarily done by rotating electric machines. Furthermore, an important part of power consumption is associated with electric motors. In an electric machine losses are much more complex, but at first approximation we can say that the smaller the size of a machine’s air gap, the lower the losses inside that machine. It then becomes obvious that a machine of a few watts, such as the alternator of a bicycle, will have very low efficiency — about 30%. An alternator of about 1,000MW, however, such as those found in nuclear power plants, reaches an efficiency of nearly 99.5%.

1.2.1.1. Electricity transmission in alternating current

The first electric generators built were direct current (DC) machines, but very quickly the impossibility of transmitting this energy sufficient distances led to the development of alternating current (AC), thanks to the invention of the transformer. Indeed, this latter device made it possible to transmit power given under increasingly high voltage, thus making it possible to limit losses during power transmission. As indicated in equation [1.1], the losses are inversely proportional to the square of the voltage. We thus may find it beneficial to transmit electricity at high voltages, which explains why electric grids operate at high or extra-high voltages.

In addition, it is possible to transmit electrical energy using three phases in AC networks. The use of three-phase systems allows both higher transmission capacity and power rating of the machines under favorable economic conditions. We will presume that the reader is familiar with the basic concepts of electrical engineering, and in particular with three-phase circuits. We will thus not review the characteristics of these systems.

1.2.1.2. Power generation and system frequency

The choice of operating frequency, a value chosen at 50Hz (Hertz) (60Hz in some other countries), results from a compromise between the size of the machines (inversely proportional to frequency) and losses in the machines and conductors (which both increase with frequency).

The above observation led to massive development of electric power networks with increasingly powerful generators over the 20th century. Energy produced was conveyed by lines whose rated voltage did not cease increasing with the growth of energy demand in the residential, commercial and industrial sectors. The same needs led to the construction of large hydraulic and thermal power stations on sites in which primary energy is easily accessible (rivers, coal mines, refineries, etc.), and transmission lines starting with high, then very-high voltages to convey the energy produced towards the load centers for consumption.

This historical tendency developed particularly in France, with the start up of nuclear plants with generating units ranging from 900–1,500MW. Today, these plants are distributed throughout the territory according to specific needs and the availability of means to cool the reactors. Figure 1.1 shows the geographical distribution of these nuclear power stations on the French territory.

1.2.2. Scattered consumption

Unlike power generation, energy consumption is generally scattered or dispersed over a given territory. Indeed, with regard to France, consumption is divided among various types of users, as indicated in Table 1.1. Moreover, it should be noted that the load curve is characterized by a substantial difference between the base load and peak load in a day, within a week or between seasons, as shown in Figure 1.2. This difference is about 30% and depends significantly on the weather, including temperature. Within Europe, a one degree difference between the temperature in a day and the normal seasonal temperature corresponds to a power production between 2,000MW and 3,000MW.

Table 1.1. Distribution of consumption of electrical energy in France

Figure 1.1. Distribution and capacity of nuclear power plants in France

By analyzing Table 1.1, it is clear that the main power users include domestic and industrial loads. This implies that the geographical distribution of power consumption follows the territorial establishment of these users, who are generally located in urban centers. This phenomenon is amplified by the power demand of electric transportation systems, which are very mobile in nature (e.g. high-speed trains) or in urban areas (e.g. subway, trams, etc.). It thus appears difficult to reconcile massive power generation on particular sites with a more or less densely distributed consumption within the territory.

1.2.3. Very limited means of energy storage

One of the main features of electrical energy is the impossibility of storing it in sufficient quantities to be able to meet an instantaneous demand at a given point. The only devices that can store electrical energy in a directly usable form are batteries and capacitors. In spite of the important progress made in the technology of these devices, it is not possible to use them to store a sufficient amount of energy to deliver in a quasi-immediate way a power of several megawatts (MW).

The only means of avoiding the dilemma of insufficient energy storage is to build a physical system that is able to effectively connect generation and consumption sites. Further, the power produced needs to be adjusted at every instant to match the quantity required, and should be conveyed economically under acceptable security conditions. This is the role of the transmission and distribution systems.

1.2.4. Transmission and distribution of electrical energy

The network is the essential component which, at every moment, has the task of balancing electrical power generation and its consumption by all customers connected to the system. It is also an effective means by which to carry out the economies of scale related to the generation-consumption pair. In addition, this balance must be established by respecting the norms of voltage regulation at any point in the territory. This allows users to reliably predict the functioning of their electrical appliances.

Figure 1.2. Typical winter and summer load curves (Source: RTE)

The balance between power produced and consumed is necessary in order to rigorously maintain a constant frequency. The ability to maintain a virtually constant voltage across the entire service is directly linked to the flow of power according to load distribution. This problem is very complex because it brings into play very important powers on a very dense set of links. Therefore, if we consider the French network while taking into account only the internal power exchange, the average value of power generated is about 70,000MW (with a peak value reaching about 90,000MW), which travels through almost 1,200,000km of lines (three times the distance between the Earth and the Moon).

The management of such a system, so as to ensure its operation under normal conditions (all variables in the system remain within the allowable range) and restore operation following accidental events at the earliest possible time, is an extremely complex task that requires several levels of automation. These levels act with different time constants and involve taking decisions whose effects range from milliseconds (e.g. faults) to the order of a decade (e.g. equipment planning for load forecast).

In fact, the network is composed of three related entities, but corresponds to different functions that are as follows:

– the delivery of electric energy from power plants to major load centers — this is the role of the transmission system;

– the distribution of this energy to different parts of each load center with a high level of reliability to all consumers — this is the role of subtransmission networks;

– direct feeding to all consumers from the distribution network — this is the role of distribution networks.

Figure 1.3. The 400kV French transmission network (Source: RTE)

Figure 1.3 represents the French transmission network, which operates at 400kV. Note that the 220kV lines are not shown to avoid overloading the schematic. To alleviate the effects of the loss of one or more generation centers or transmission lines, the French network is interconnected with networks of neighboring countries, as shown in the figure. This enables operators of different networks to provide mutual assistance in when needed, or simply to export their generation surplus.

These interconnections take an important part in the current context of open markets, but we shall see later that they are woefully inadequate. The subtransmission network is the link, near major load centers, between the transmission network and the distribution network.

The subtransmission networks are often arranged in a loop around the load centers in order to compensate, by a network reconfiguration, defects that occur on a transmission system. Distribution networks, which are either overhead or underground cables, are designed to cover the entire load center territory in order to serve all customers who request connection to the network.

Figure 1.4 below shows how all electricity networks have been constructed and operated from throughout the 20th century. Their organization and operation, from generation to consumption, were integrated within a single private or public company of monopolistic type.

Figure 1.4. Historical paradigm

The liberalization of energy markets has introduced considerable changes by imposing separate functions of generation, transmission and distribution. This led to the establishment of a new organization based on a model that is more suitable for competition. This model is described below as the new paradigm.

1.3. New paradigm

The objective pursued by the promoters of deregulation of electrical systems has always been to reorganize the electrical system so as to create conditions for free competition between different players. This benefits consumers by providing better financial conditions for their energy supply. The introduction of these new conditions occurred in a context in which geographical constraints play a role, by the nature of power grid locations as a de facto monopoly and by the delivery of the most competitive energy sources.

The new system was introduced in the early 1980s in the United Kingdom, followed by the United States and mainland Europe in the 1990s. It is now working in most industrialized countries. There have been some difficulties of adaptation in the physical functioning of a system designed and built to operate in an integrated manner on a defined territory which is now operating across continents without the infrastructure transport and interconnections being changed beforehand.

1.3.1. Electric system operation in liberalized world

The main functions required to meet customer demand are the same as those described above but their mode of interaction is different. These are provided by cooperation of players on five key links, as shown in Figure 1.5.

In this new organization, there is complete independence in both hierarchical and financial partnership between producers and the rest of the industry. The role of producers is to generate electricity to sell to consumers with different types of short- or medium-term contracts. The role of merchants or traders is to maintain the link between producers and consumers on a commercial basis. The main purpose of the electrical grid is, as before restructuring, to transmit and distribute electricity to end users while guaranteeing non-discriminatory access for all users. Transmission system operators (TSOs) and distribution system operators (DSOs) are responsible for ensuring this mission is accomplished.

The marketing and generation businesses are becoming increasingly important and are in support of the end of the operation of commoditization. It is the role of those involved in marketing to purchase energy from producers on energy markets or through bilateral contracts with suppliers, and then sell the energy to different consumers based on the financial conditions related to urgency and importance of immediate need.

This commercial activity has been enhanced with respect to the historical paradigm in which the price of electricity, seen as an essential element, has been stable over relatively long periods. This stable price has been achieved through a close cooperation between the companies and representatives of users who were connected to the government of the country in which the company operates.

Today, changes in energy price are related to the balance between supply and demand, which is governed by an energy market, similar to any commodity or classic good. Naturally, these price changes are often linked to the influence of climatic conditions on both supply and demand, especially for hydropower plants, the price of primary fuels and socioeconomic factors.

There are periods when energy price is subject to extreme variations due to the need for heating or air conditioning, when the availability of low-cost energy such as hydropower is very low. For example, the price of a megawatt hour (MWh) climbed from a nominal value of 25 Euros to over 1,000 Euros in the summer of 2003, and to the order of US$10,000 in the United States when the market had just opened.

This constraint, which new users of electricity are facing, leads them to anticipate these changes and write them into contracts with their suppliers; the contracts guaranteeing relatively stable prices over long periods. The internal management of energy in an industrial setting is now something that requires serious attention, or risks prohibitively high electric bills. The suppliers themselves must take out insurance to enable them to absorb the large variations in purchasing costs of energy that they secured at a set price to their customers. The producers themselves must be prepared, by proper investment, for changes in the price of primary energy. This can become a burden on the cost of electricity, e.g. the price of a barrel of oil has risen from $6 to nearly $150 per MWh in a relatively short period of time. This event had a dramatic impact on the price of a generated by oil-based power plants.

Figure 1.5. Links of the new power system organization

All these new concepts (to be discussed in Part 3) have led to a change in the economic operating conditions of the electrical system. They have also led to significant technical advancements in promoting the development of new generation technologies, especially the establishment of decentralized generation based on renewable energy that will continue to provide, in addition to mass production, a significant part of the total energy generated for consumers

1.4. Distributed generation

The conditions caused by the new regulations are incentives for consumers to install local generation means that enable them to dampen tariff turbulence issues due to deregulation. Moreover, the regulatory incentives that encourage generation from renewable sources have led to the emergence of this type of generation. This obviously means that limited power generation sources connected to distribution networks are susceptible to modulations in the power imported by one consumer who might even, if pricing conditions become favorable, export this power by injecting it into the network.

Aside from the classical basic means of power generation, the development of an important number of small power sources (including wind, solar, hydro, heat in the form of co-generation, additional production of heat or cold, etc) superimposed on the normal mode of distribution network operation has created a phenomenon associated with bi-directional energy flow.

This phenomenon is leading to new problems, such as: the management of renewable energy, by keeping the same level of security on the network; or the search for new concepts such as central, virtual real-energy cooperatives whose goal is to promote these energy sources despite their intermittency. The purpose of these cooperatives is to aggregate, in an optimal way, this energy mix by combining the profitability of each source while minimizing their adverse effects due to their randomness.

Implementation of the new paradigm, which integrates economic decentralized generation, will lead to a new operational diagram of electrical systems that will progressively replace the scheme in Figure 1.4. In the following chapters, we will describe the main features of the network components and electrical quantities, characterizing their operation in terms of voltage, current, active and reactive power.

We will then discuss the equations of operation in cases related to different types of networks:

– the distribution network where the short length of lines allows us to neglect the propagation phenomena and effects of line capacitance;

– the subtransmission and transmission networks in which the above items are paramount.

Figure 1.6. The new electrical system (source: Alstom)

Chapter 2

Production of Electrical Energy

Electric power generation is a transformation of the energy contained within primary sources, by means of conversion of different types (see Figure 2.1 below), in the form of electricity that is made available by an electromotive force across the generators terminals. When the generator is connected to a load, this electromotive force produces an electric current at a given voltage to provide the power required.

We can classify the primary sources in two groups:

– one is associated with fossil fuels extracted from the ground, such as:

- oil;

- gas;

- coal; and

- uranium, which is the basic mineral for nuclear energy;

– the other is based on renewable sources such as those from:

- the sun (solar, wind, hydro, biomass); or

- underground magma activity, i.e. geothermal.

The diagram below illustrates all sources of electric energy production.

Figure 2.1. Energy transformation

Energy conversion of primary sources to electric energy can be either direct, such as the case of photovoltaic systems, or indirect as it goes through some transformations.

The most common mode of conversion is the sequence of a thermal process that transforms the primary energy into heat. The thermal energy is then transformed into kinetic energy, which is transferred to the turbine shaft in the form of mechanical energy. Finally, electric generators convert mechanical energy into electric energy. We can also have a direct conversion combustion/turbine (crankshaft) as in the case of gas engines.

In practice today, worldwide electrical energy is mainly produced from thermal power plants (oil, gas or coal), hydropower plants, and nuclear power plants (particularly in France). Renewable energy sources like wind energy, geothermal or solar energy are fully developed but still remain marginal, as shown in Table 2.1 below, which lists current world electric energy production by type of primary energy. In Germany, however, the penetration of wind energy is quite significant (18GW today, 30GW in the near future).

When we study the traditional means of producing electrical energy today, they are primarily alternators driven by various types of turbines whose power ratings depend on the mode of energy conversion and turbine drives. It is important to remind the reader at this point that the frequency of the network is fixed at 50Hz (60HZ in certain countries). Further, the speed N (in rotations per minute, i.e. rpm) of each machine connected to the network is related to its number of pole pairs p by the following relation (for systems operating at 50Hz):

Table 2.2 lists typical sizes, rpm and pole pairs of alternators in different types of power plants.

Table 2.1. World electric power production (TW) by type of primary energy

Table 2.2. Characteristics of types of power production

Chapter 3

General Information on Electrical Power Networks

3.1. Transmission and distribution systems

As pointed out in an earlier chapter, an electric power system is composed of three main parts: generation, transmission, and distribution (as illustrated in Figure 3.1 below). Here we discuss each of these subsystems.

The transmission network carries energy produced at the power plants to major load centers. This network consists of lines that operate at very high voltage in order to reduce power loss and voltage drop, and help carry large quantities of energy in favorable economic conditions. Interconnections are also carried out through these networks. In key areas of consumption, e.g. cities and areas of heavy industrial activity, electric energy is often brought in by several transmission lines and must be distributed in a manner that ensures a secure supply in the event of an incident on one of these transmission lines or associated generation units. To achieve this, lower voltage lines are placed around these areas in a loop. This subsystem is referred to as a subtransmission network.

Transmission networks have a mesh network topology for higher reliability since it is essential that, after the loss of one transmission line or failure of a generator, the system continues to supply all consumers in an area regardless of their location. The practice of the so-called N-1 rule, which ensures network operation after the loss of one element, is the basis of the operation of electrical systems under optimal security conditions.

Figure 3.1. General architecture of an electric power network

It is important to note that the length of transmission and subtransmission lines, despite their strategic importance, add up to about 40,000km in France (source: RTE). However, the complete distribution network is approximately 1,100,000km long. These orders of magnitude are because the normal distribution system aims to deliver energy to each client regardless of his or her location within the service territory.

Finally, service to customers is provided through a medium- and low-voltage network, which is very often structured in a radial form for economic reasons. In some densely populated areas, however, meshed (or looped) networks are often used to ensure a higher reliability of the power supply.

3.2. Voltages

The voltage in a network is, with the operating frequency, one of the fundamental parameters of electrical operation because it determines the most characteristics, such as power transmitted, current line, losses, etc.

Electrical engineers have introduced several concepts that characterize voltages at which networks operate. The different levels of voltages are tightly defined by the International Electrotechnical Commission (IEC). The most important of these definitions concerns the nominal voltage of a network is:

– nominal voltage (Un): is the RMS value of the voltage between phases (i.e. line-to-line or simply line voltage) by which a network is designated and identified.

The IEC recommends a number of nominal voltage values for networks operating at 50Hz or 60Hz. These voltages vary from one country to another, especially between the states of North America and Europe. In France, the following voltage levels are designated by the Technical Union of Electricity:

– very low voltage (VLV): Un < 50V;

– low voltage A (LVA): 50V < Un < 500V;

– low voltage B (LVB): 500V < Un < 1 kV;

– high voltage A (HVA): 1kV < Un < 50kV;

– high voltage B (HVB): 50kV < Un.

The voltage levels utilized in distribution networks, subtransmission networks, and transmission networks in France are listed below:

– distribution: LVA (220V, 380V) and HVA (10kV, 20kV);

– subtransmission: HTA (45kV) and HTB (63kV, 90kV);

– transmission: HTB (150kV, 220kV, 400kV).

Note: We may question why these are the specific voltage levels within a power network, but they happen to be the result of historical development of the electrical grid and the exceptional longevity of power equipment (some have a life cycle in excess of 40 years). As will be discussed later, the ever-increasing demand for electricity worldwide requires larger blocks of power transfer, which in turn call for even higher transmission voltage levels.

Other definitions that have been standardized on the designation of voltage in a power network include the following:

– The highest voltage in a network is the highest RMS voltage value that is measured at any one moment and at any point of the network under normal conditions. This value does not take into account transient variations, for example due to network switching, or temporary voltage variations (e.g. ferroresonance). It characterizes the differences of voltage levels under normal operation. Thus in a transmission network that is rated at 400kV, the voltage may vary from 420kV at the generating plant down to 380kV in the consumption point.

– The highest voltage for equipment (Um) is the RMS value of the highest voltage specified for a piece of power equipment in relation to its insulation. This voltage, which should be less than the highest voltage in a network above, is used to determine the maximum stress the equipment is subject to under normal operation.

– The factor of ground fault is the ratio of the value of the highest RMS voltage between a phase and ground during a ground fault (affecting any phase at this node of the network) over the RMS voltage between a phase and ground, which would be obtained at the location considered in the absence of a fault. This factor of ground fault at a node of a three-phase network enables us to define for this point of the network the value of the highest voltage for equipment as a function of the over-voltage factor defined below.

– Over-voltage factor: the over-voltage factor between phase and ground is the ratio of peak values of an over-voltage between one phase and earth and the voltage between phase and ground, which corresponds to the highest voltage for the equipment (i.e. Um √ 2 / √ 3). This concept is useful when we compare a temporary over-voltage in one phase with the maximum voltage in steady operation.

3.3. Power transfer

The transmitted power is usually defined for a dipole or between networks. It characterizes the power that can be transferred without exceeding the constraints and without the placing the network in danger. For a given transmission line, this power depends on the following:

– voltage of the line (conditions of insulation);

– conductor cross-section area (acceptable current);

– acceptable voltage drop;

– possibility of load shedding (in the case of an incident);

– stability (in the case of an incident);

– etc.

It is generally difficult to predict with accuracy the capacity of a transmission line with a given length if the operating conditions of this line are not exactly known. Table 3.1 below gives an idea of the capacity of transmission lines according to their rated voltage and their length.

Note that two transmitted power values are listed under 400kV for two typical distances in France. The first concerns high-voltage transmission lines between large power plants, such as those found in mountainous regions and conurbations such as Paris and Marseilles. The second is characteristic of lines carrying energy produced by nuclear power plants that are located around a 100km from distribution networks. In the latter case, power produced by a nuclear plant is in the order of 4,000MW, thus this requires the use of four lines operating at 400kV.

Typical capacities of transmission lines rated at 750kV are also shown in Table 3.1. The extra-high-voltage lines where studied some years ago, but were not chosen for the French network as a result of the deployment of nuclear power plants to lessen the distance between the centers of production and pockets of consumption. Moreover, it is easy to realize that two lines operating at 750V are (in principle) enough to carry all the power produced by a nuclear power plant. The side-effect though is that they are unable to serve the load when one of the lines is out of service. Such voltage is used for long-range transmission in many countries, however, such as Russia, the United States, Canada and Brazil.

Table 3.1. Typical power transfer of transmission lines

Chapter 4

Network Architecture

We will now address the practical aspects of electrical networks and their components, including the lines and cables, generators, transformers, etc. Before considering these components, however, it is useful to focus on network architecture and topology.

4.1. Network architecture: mesh or radial layout

The architecture of a power network depends on the function assigned to this system since the operating conditions will affect the operating characteristics, which in turn justify the degree of complexity and cost that the designer is ready to recommend for construction and strengthening of the network. It should be noted that often work is done on an existing network rather than a new network that is in the planning stages. Hence, most studies are associated with the strengthening of an existing network. Examples of such studies include modifying network topology to improve system security or increasing the power transfer capability of some power lines as load demand increases.

The link between different parts of a network is via power transformers that modify the voltage from very high, to high, to medium, and finally to low levels, as illustrated in Figure 4.1. In this figure, we show the general structure of a network that links generation to different types of consumer who use different voltage levels. The system is typically broken down into three sub-networks: transmission, subtransmission and distribution. Each of these sub-systems is briefly described in the next paragraphs.

Figure 4.1. General structure of a power network

4.1.1. Transmission networks

The transmission network is responsible for carrying energy from remote generation plants to urban areas or industrial sites while maintaining a voltage level within allowable limits. This critical network must have a very high reliability in order to achieve a strong performance guarantee, not only under normal conditions but also during incidents that may result in the loss of one or more lines, or unexpected shutdown of a generating unit.

To fulfill its mission and deal with unforeseen events, a transmission network has a strong mesh structure that provides alternative routes for power flow in case of line loss, as well as good voltage regulation during times when larger power transfers take place. Figure 4.2 illustrates the topology of a transmission network that is strongly meshed.

The disadvantage of the above meshed structure is, as we shall see