Nobleza del Andaluzía

Several key factors heavily influence the development and operation of the elec-tricity transmission and distribution systems as described below.

First, since electricity cannot be stored in reasonable quantities, the elec-tricity generated must continually be adjusted to meet the continually varying demand, plus the system losses. Failure to maintain this balance results in a change in system frequency, which could in turn result in widespread demand loss across the system. It is the role of the control engineer to continuously meet the demand by scheduling sufficient generation to maintain the system voltage and frequency. An illustration of the dimensions of this task is given in Figures 1.8 and 1.9, which show impact of major events on demand and typical seasonal demand curves.

Second, the demand for electricity is growing. Although in some parts of the world, electrical energy consumption may have slowed down or declined during certain periods of economic uncertainty, the overall trend still shows annual increase in the 1–10% range. This means that the transmission and distribution system planners have to accurately anticipate this growth several years in advance to ensure the networks are capable of meeting the increased requirements. Furthermore, the long economic life of the transmission and distribution equipment requires particular attention to selection of investments and technologies best suited to meet the system need over such periods.

Finally, since electricity is generated from primary fuel sources, the nature and location of these resources are important factors in influencing the structure of a

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transmission and distribution system. This is due to the fact that transportation of energy in the form of electricity over transmission lines is more economic than the transportation of other primary energy sources. Large power stations also have a requirement for large amounts of cooling water for use within the steam cycle and thus tend to be sited on the coast or by large rivers. Furthermore, other environ-mental factors such as emissions of harmful gases and acceptability of certain types of tower structures in the countryside are increasingly playing an important part in the siting, physical characteristics and installation principles of generation, trans-mission and distribution systems.

1.3.1 Technical factors influencing the structure of transmission and distribution systems

Modern AC transmission and distribution systems transport the electricity gener-ated at a frequency of 50 Hz or 60 Hz from the generators connected via generator transformers. The choice of either frequency was governed by the necessity to ensure that there is no annoying visual flicker in incandescent lighting. This choice was also compatible with the manufacture of transmission and distribution equip-ment based on technologies at near optimal overall cost to meet public needs and present least cost penalties in the establishment of such systems. The DC trans-mission systems within the AC transtrans-mission utilise converters at their points of connection with the AC system. These operate in the rectifier mode for conversion

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Figure 1.9 Typical seasonal power demand variations

Source: National Grid – National Electricity Transmission System Seven Year Statement May 2010

to DC and inverter mode for re-conversion back to AC. Due to the specific cap-abilities of the converters the DC connections are capable of connecting two AC systems operating at different nominal frequencies or providing asynchronous connection between the systems.

One of the most important characteristics of a transmission or distribution system is the security of supply which, by implication, means a supply of electricity that is continuous and of the required quantity and quality especially in terms of frequency and voltage. This in turn means that the generation, transmission and distribution systems must have sufficient in-built flexibility by design to maintain supplies under conditions of plant breakdown or weather induced failures for a wide range of demand conditions.

The frequency of the system is governed by the speed at which the generating plant operates. The quality of the frequency generated is conditional on the trans-mission system operator scheduling sufficient generation to meet the demand plus the losses of the transmission and distribution systems. This quality is further assured by holding generation in reserve for immediate availability in the event of unplanned plant losses. In this way, the level of the frequency can be maintained to predetermined levels. In the UK, the statutory frequency is 50 Hz with1% per-mitted variation. The DC interconnection with France provides an asynchronous connection in that the systems at either side of the connections are isolated from the frequency variations in the other system.

The other important characteristic of transmission and distribution systems is the voltage quality, which is also governed by the generating plant. While the frequency of a system will be substantially constant at all points, the voltage levels will differ at different locations on the system. This difference is governed by the capacitive and inductive characteristics of the ‘wires’ i.e. the overhead lines and underground cables, forming the systems and the power factor of customer demand. At times of low power flow through the wires, the capacitive effect dominates and causes the voltage at the end of transmission and distribution lines to rise. At times of high power flow, the inductive effect dominates and the voltage tends to fall along the length of a line. Through the installation of capacitive or inductive compensation plant termed ‘reactive compensation’ it is possible to limit variations in voltage beyond pre-defined limits. This ensures the customer equip-ment does not suffer any undue interruptions and the customer does not have to invest undue amount in purchasing equipment designed to operate over very wide voltage variations. Statutory voltage variations permitted on transmission and dis-tribution systems are usually around5% and 10% respectively. The manual and automatic control facilities put in place to achieve compliance with these perfor-mance criteria are shown in Figure 1.10.

The above technical principles and characteristics are applicable to virtually any transmission and distribution system around the world. The only difference between transmission and distribution utilities is related to the organisational structures of the companies that control the various types of systems.

Electric power transmission and distribution systems 33

1.3.2 Organisational structures

In essence, the electricity supply industries or utilities around the world based on their structure and ownership can be classified under four main headings:

1. vertically integrated 2. horizontally separated 3. privately owned 4. publicly owned.

In a vertically integrated industry, the whole industry, i.e. generation, trans-mission and distribution, is controlled by a single authority. This authority may be

400 and 275 kV transmission system No tap-chargers on 400/275 kV transformers

Generator (G) 400 or 275 kV/132 kV 240 MVA

LV on-load tap-changer

Figure 1.10 Transmission and distribution system voltage control facilities

wholly or partially owned publicly or privately. Essentially it is able to carry out integrated generation, transmission and distribution planning and operation within a defined geographical area. After the UK privatisation in 1990, the only vertically integrated electricity utilities were two in Scotland (Scottish Power and Scottish Hydro Electric) and one in Northern Ireland (Northern Ireland Electricity), and all these entities were in private ownership.

For the privately owned utility companies that are vertically integrated within a certain geographical area the regulatory pressures on the ‘natural monopolies’

of transmission and distribution result in a clear definition of the costs of each part of the company activity. The traditional single tariff structures in the publicly owned vertically integrated utilities are not designed to clearly differentiate between the costs of generation transmission and distribution activities. Cross-subsidies between these activities are also more common in such utilities.

The prevailing electricity industry structure in England and Wales from 1990 to 2001, with both energy trading and transmission/distribution infrastructure components shown in Figure 1.11, is horizontally separated by virtue of privately owned generation, transmission and distribution companies being separated from each other. This structure allows for the private ownership of all or parts of each function in ‘wires’ and ‘supply’. However, it is the independence of the transmis-sion utility from all others that tends to be the essential mechanism by which the horizontally separated system structure operates in a secure and economic manner.

The natural pivotal role of a transmission company is to provide a transmission and operational infrastructure by which power can be securely transported across the system and hence facilitate the trading between generators, distributors and suppliers. This also allows access for other interconnected electricity utilities outside

First

Figure 1.11 Electricity supply industry structure in England and Wales from 1990 to 2001

Electric power transmission and distribution systems 35

the immediate geographical area. Two further models of horizontally separated industry structures implemented around the world are shown in Figures 1.12 and 1.13 for Argentina and State of Victoria, Australia, respectively.

Transmission system interconnections extend customer access to more sources of electricity supply. Availability of interconnection technologies at economic cost means that systems with different technical characteristics in their choice of voltage and frequency can be interconnected without problems over long distances. The interconnection may be in the form of a high-voltage AC overhead transmission line, as in the case of many interconnections across European countries and

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Figure 1.12 The new electricity industry structure in Argentina

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Figure 1.13 The new electricity industry structure in State of Victoria, Australia

utilities, or it may be a DC link as in the case of the cross-channel interconnection of England with France.

Although three-phase AC overhead lines are the usual method of interconnec-tion between utilities, in cases where long transmission distances and/or sea crossings are involved, ‘high-voltage direct current (HVDC)’ transmission is economic despite the relatively higher costs of converter and terminal equipment. HVDC transmission is also utilised for interconnecting utilities with different supply frequencies, e.g. 50 and 60 Hz and in cases where an asynchronous link between systems is required. The

‘back-to-back’ HVDC links in Japan, with both their converters situated at the same site, are examples of this type of interconnection.

The horizontally separated industry structures are ideal for introduction of competition in generation and supply. This is more difficult, if not impossible to achieve with vertically integrated structures especially if they are publicly owned.

The attempt by the 1983 Electricity Act to oblige CEGB and area boards to set use of system tariffs giving access to other competitive generation sources to its system has failed to attract new connections although such tariffs were produced. A similar attempt in France has also met the same fate due to lack of regulatory/commercial incentives and pressures on the vertically integrated company.

Ownership of the assets forming a transmission and/or distribution is no longer important as the legally binding technical and commercial codes ensure safe, secure and stable operation of a multiplicity of utilities within country and over interna-tional boundaries.

Since 1990 the UK electricity industry structure has gone through two further stages of development. First, in 2001, ‘New Electricity Trading Arrangements (NETA)’ was introduced to eliminate the perceived weaknesses of the ‘Pool’

trading structure, which was in place from 1990 onwards. In the NETA trading structure shown in Figure 1.14, while the infrastructure arrangements remained unchanged, both generation and demand could make firm bids to a ‘balancing market’ and would be paid based upon their ‘bid price’ instead of the ‘marginal price’ paid to all plants in the previous ‘pool’ trading structure.

Forwards

Figure 1.14 New Electricity Trading Arrangements (NETA) in UK in 2001 Electric power transmission and distribution systems 37

Second significant development occurred through the ‘Energy Act 2004’ in July 2004 and created a ‘single electricity market for Great Britain (GB – England, Wales and Scotland)’ as shown in Figure 1.15. Under this arrangement National Grid as the single ‘GB System Operator (GBSO)’ would be responsible for:

● contracting for connection/use of system

● generation despatch and balancing

● planning and co-ordination of transmission and generation outages

● transmission charging

● co-ordination of GB transmission investment planning.

Three ‘transmission operators (TOs)’ of the ‘wires’ namely:

● National Grid Electricity Transmission (NGET)

● Scottish Power Transmission (SPT)

● Scottish Hydro Electric Transmission Ltd (SHETL).

would be responsible for maintenance, investment planning and carrying out system switching as directed by the system operator GBSO. These infrastructure and trading arrangements are unlikely to be subject to further major development in the foreseeable future.