Distribution networks represent a huge capital investment. To make sensible decisions about their investments, electricity utilities need to form clear-cut design policies and adopt the most accurate systemdesign procedures. Customers' expectations of the reliability of supply continue to rise, market pressures on the design engineer are growing stronger, and the increasing use of computers has changed the entire approach to distribution system design. Technical innovations have presented the design engineer with the means to improve system efficiency. Electricity Distribution Network Design was the first book to be entirely devoted to the planning and design of modern distribution systems, as apposed to the more general aspects of transmission and generation. This second edition has updated its treatment of computer-based planning and reliability. It also covers the implications of international standards, network information systems and distribution automation. With comprehensive and up-to-date bibliographies at the end of each chapter, the book will be useful both for students and for practising engineers involved in distribution network design.
Inspec keywords: power distribution economics; substation protection; power distribution protection; power distribution planning; power distribution reliability
Other keywords: distribution network planning; reliability; computer-based planning; HV network; load data; power system protection; electricity distribution network design; network voltage performance; LV network; MV network; distribution substation; economic principle
Subjects: Power system management, operation and economics; Power system planning and layout; Reliability; Distribution networks; Substations; Power system protection
The primary aim of the electricity supply system is to meet the customer demands for energy. Power generation is carried out wherever it gives the most overall economic selling cost. The transmission system is used to transfer large amounts of energy from the main generation areas to major load centres. Distribution systems carry the energy to the furthest customer, utilising the most appropriate voltage level. Thus an electricity supply system contains three different functions. Often individual supply organisations cover only one of these functions within a particular area or region.
The planning and design of electricity distribution networks can be divided into three areas. Strategic or long-term planning deals with future major investments and the main network configurations. Network planning or design covers individual investments in the near future while construction design includes the structural design of each network component taking account of the various materials available. The object of this chapter is to introduce those factors which should be taken into account when designing electricity distribution systems. The emphasis will be on general planning guidelines. Later chapters are devoted to more detailed considerations of technical and economic aspects, and specific engineering topics.
This chapter covers various technical aspects which should be considered for both normal and abnormal operating conditions. The planning engineer has to consider the effect of the loss of any item of equipment on the supplies to customers and on the quality of supply, e.g. voltage fluctuations, and the amount of time a customer may be off supply, as well as the safety of the public and the utility staff. It is also necessary to take account of the effect of transient and permanent system faults on both utilityand customer-owned equipment.
Reliability is an essential factor with regard to the quality of supply. The main factors used to judge reliability of supply to customers are the frequency of interruptions, the duration of each interruption and the value a customer places on the supply of electricity at the time that the service is not provided. These factors depend on variables such as the reliability of individual items of equipment, circuit length and loading, network configuration, distribution automation, load profile and available transfer capacity.
Asset management is a key issue in network business. It is essential in network design to consider not only the technical aspects but also the associated economics, since decisions based on both technical and economic assessments can have a significant impact on the financial stability of the utility. The necessary economic studies are normally an integral part of the overall appraisal carried out by the planning engineer.
In all other Chapters the objective has been to present the various theoretical, technical, economic and operational factors to be considered when planning and designing electrical distribution systems. This chapter is intended to provide some background information on the construction and operating characteristics of the main components installed on distribution networks, in order to provide the design engineer with basic data on the equipment which will be used to build up a functional system. With rapidly changing technologies affecting in some way the design of virtually all equipment, specific examples have not been illustrated; only general aspects of the main features of transformers, lines, cables and equipment are covered in this chapter. Protection aspects are covered in Chapter 7 and switchgear arrangements in Chapter 8.
A properly co-ordinated protection system is vital to ensure that an electricity distribution network can operate within preset requirements for safety for individual items of equipment, staff and public, and the network overall. Automatic operation is necessary to isolate faults on the networks in a minimum time in order to minimise damage. In addition, minimising the costs of non distributed energy is receiving increasing attention. The various items of switchgear and automatic disconnectors referred to in the previous chapter require sensing devices to activate them. These must determine whether some abnormal situation has arisen on the network which requires disconnection of any circuit, and are generally referred to as protective devices.
High-voltage systems provide a link between major transmission and medium voltage distribution systems. In addition, medium-sized power stations are connected into these HV networks. Many HV systems now operating primarily as distribution networks have earlier been used for transmission purposes until superseded by higher-voltage systems. High-voltage switching stations provide suitable node points for adjusting network configurations for system-operation purposes, and often are focal points for HV/MV transformation supplying the MV systems. Standardised layouts have tended to be adopted. These include singleand multi-busbar arrangements, often involving large open-air layouts or low-volume metal-clad switchgear in purpose-designed buildings. The trend is for enclosed construction requiring less space, often utilising simpler busbar layouts than the more conventional open-air arrangements.
In the early days of the commercial use of electricity, power was produced by small generators close to the customers of municipal or industrial electricity producers. As systems expanded it was necessary to distribute electrical energy over distances of some tens of kilometres with circuits operating in the 5-20 kV range. When HV systems were introduced to deal with the longer distances and increased power requirements, the earlier systems changed in nature to become intermediate MV systems mainly to be used for distribution. The use of a single higher-voltage system (40-250 kV) to supply local LV networks directly would lead to unacceptably high costs and amenity problems. Thus another voltage level, or levels, is used to interlink the HV and LV systems. Material and construction costs of 10-20 kV overhead lines are only slightly higher than those for a 400 V line of the same length, but are approximately one-tenth the costs of a 100 kV line. It is this large cost differential which economically justifies the inclusion of an MV network between the EHV/HV and LV systems, even when account is taken of the costs of HV/MV substations.
Comparison of low-voltage networks and distribution systems operating at higher voltage levels reveals a considerable number of similarities. Both are usually operated radially, and thus each network has only one infeed point. In the MV system the construction of a new 110/20 kV infeed substation can involve investment of the order of several million pounds and require extensive design work. However, such schemes are not so numerous as the provision of new 20/04 kV distribution substations, each costing only about £3000-£30 000, and the associated changes in operating arrangements of the low-voltage system. These latter schemes are carried out almost daily, depending on the size of the utility and the distribution network conditions. Although individual LV construction schemes are small, the large number of such jobs carried out each year tends to absorb a major part of a utility's capital and design resources. This leads to the need for good design practice involving an efficient organisation to oversee the large number of LV schemes, with standardised approaches to LV network design.
Of all the parameters affecting the network design and timing of major reinforcements, the forecast load is the most sensitive. It is therefore essential that special emphasis should be placed on developing effective and reliable routines to cover this aspect of the planning function. In determining the losses for a particular section of the network, the most critical time periods are during the peak demand for the utility as a whole. At these times the losses incurred will have to be purchased at peak-energy high price levels, and will themselves increase the peak demand charges. Usually the nearer the location of the section under consideration is to individual customers, the greater will be the diversity between the peak demand on that section and the total system peak demand. When studying voltage drops and voltage variations, the simultaneous loading of all items of plant from the voltage controlled busbar to the furthest load point need to be considered. The loads on distribution circuits are the instantaneous summation of the individual demands of many customers, and of the losses on each section behind the section under consideration. Since the pattern of electrical demand of each customer cannot be determined precisely, it is usually necessary to calculate system loadings on a statistical basis, whether considering existing loads or forecast values.
Customers expect an electricity supply of good quality. Consequently it is necessary to give special consideration to loads which may produce various irregularities on the supply voltage, resulting in interference with the correct operation of customer appliances or utility equipment. Typical of such loads are steel-making arc furnaces, welding equipment, induction furnaces, rolling mills and colliery winders, and railway traction, where rapid variations in load currents may result in fluctuations in the voltage at customers' intake points. While the larger industrial loads will often require individual attention, there are also items of equipment, mainly in use in domestic and commercial premises, which, while individually not causing problems, can collectively affect the quality of supply owing to the large number of items involved. In addition some installations, such as computers and process-control equipment, are themselves susceptible to the quality of the supply voltages. The overall effect of these 'disturbance loads' on individual supply voltages will depend on such factors as the magnitude, phase angle and rate of change of the currents taken by the load, and whether the load changes occur at regular or random intervals of time. The frequency of such load changes and whether they occur at time of peak demand, or at off-peak periods such as during the night, have a bearing on their interference with the operation of other equipment.
The quality of electricity supply is considerably influenced by the quality of the voltage provided to customers, which can be affected in various ways. There may be long periods of variation from the normal voltage, sudden changes in voltage, rapid fluctuations, or unbalance of 3-phase voltages. In addition, other irregularities such as variations in frequency and the presence of non-linear system or load impedances will distort the voltage waveform, and transient spikes and surges may be propagated along circuits in a supply system.
In distribution-network design a large amount of data is required, e.g. information on the present networks, design objectives, cost parameters and possible ways of reinforcement. Complicated calculations are necessary in some cases to optimise network configurations. The use of computers makes it possible to carry out sophisticated network-design calculations. The main aim of using computers here is to improve the quality of routine design. Common tasks for computer-aided network design are obtaining quantitative information on the status of networks or determining the most suitable future network configuration and the optimum circuit ratings. Computer programs can also act as an efficient tool for long-term planning and the study of more complex aspects such as network reliability.
A reliable electricity supply is one of the pre-requisites for the modern way of life. The distribution network plays an essential role in providing electricity supplies, requiring large capital investment. In building, or reinforcing, the distribution network a wide variety of problems is invariably encountered, often involving other organisations. For example, an electricity-distribution system has to be planned so that it can be extended into new housing areas at their construction stages. The extension of housing and industry into rural areas may lead to the need for underground networks. The diversion of a minor road can require the resiting of some poles and pole-mounted equipment, while major road works such as motorways can involve considerable diversions or under-grounding of circuits, especially at medium and low voltage but also occasionally at high voltage.