Electrical Power Technology

D Tyler

• Publisher: Routledge
• Year: 2016
• ISBN:
  9781136075896
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

This book is a comprehensive introductory text on electrical power, encompassing generation, electrical machines, motors, electrical materials, etc. David Tyler's approach is designed for independent or classroom study, with plenty of learning checks and activities throughout. The content is designed to cover Advanced GNVQ and BTEC NII syllabuses, but it is also ideal as an introduction for first year degree students or for professionals seeking to reinforce their grasp of the fundamentals.

1	Electricity supply

Summary

This chapter looks at energy sources for generation of electrical energy and the basic cycle employed. It explains why transmission is carried out at very high voltages and shows how the various pieces of equipment are interconnected using switchgear and transformers to give maximum efficiency together with practical user voltages. It compares the use of underground cables with overhead lines. The purpose of switchgear is described together with the equipment found in distribution substations. The reasons for the adoption of the three-phase system is explained and the losses in feeder systems examined.

Synchronous generators

Virtually all the generation of electrical energy throughout the world is done using three-phase synchronous generators. Almost invariably the synchronous generator has its magnetic field produced electrically by passing direct current through a winding on an iron core which rotates between the three windings or phases of the machine. These windings are embedded in slots in an iron stator and one end of each winding is connected to a common point and earthed. The output from the generator is taken from the other three ends of the windings. The output from a three-phase generator is therefore carried on three wires. In many three-phase diagrams single line representation is used when each line on the diagram represents three identical conductors. Figure 1.2 is drawn using this method.

All such generators connected to a single system must rotate at exactly the same speed, hence the description synchronous generator.

They are driven by prime movers using steam generated by burning coal or oil, by nuclear reactors, water falling from a higher to a lower level, or aircraft-type gas turbines burning oil or gas. A very small amount of generation is carried out using diesel engines.

Generators range in size from 70 MVA (60 MW at 0.85 power factor) at a line voltage of 11 kV which were mostly installed in the 1950s, through an intermediate size of 235 MVA (200 MW at 0.85 power factor) to more recent machines of 660 and 1000 MW which generate at 25.6 kV. The very large machines are quite rare; many modern power stations employ a combined cycle where gas turbines running on gas produce electricity, and the extremely hot exhaust gases raise steam in a boiler which is used in a further turbine to generate more power. Such combined cycle stations have an efficiency about 50 per cent greater than a straight steam power station but individually are of lower rating.

Energy sources

• Coal. Coal is plentiful worldwide and there are sufficient known reserves to last for centuries. It may be deep mined or extracted from open cast mines. Open cast coal is generally of lower quality than deep mined coal but is more easily extracted since once the top overburden is removed it can be scraped out by very large diggers. Open cast mining is opposed on environmental grounds because of the huge areas of land involved and the dust produced. When coal is burnt it produces sulphur oxides which combine with water to form sulphurous and sulphuric acids which are very corrosive. The term acid rain is used in this context. Large amounts of ash are produced which need to be disposed of, filling in quarries and marshes etc. To plan and build a coal-fired power station takes several years and it must be accessible to bulk transport to bring in the coal (5 million tonnes annually typically for a major station) and to get rid of the ash which may be up to one tenth of the coal input.

• Oil. This may be residual oil from refineries or refined products.

(a)

Residual oil. When oil from the well has been refined and the petrol, paraffin and diesel oil taken off, the refinery is left with a tar-like substance which is only liquid when kept hot. This may be burnt in power stations to produce steam in the same manner as coal. It should be cheap since the refineries need to get rid of it but is sometimes costed as coal equivalent for heat production. It contains all the impurities of the original raw oil and produces large amounts of sulphur oxides but little ash. It needs special transport to keep it hot and must be maintained hot at the power station since once cold it cannot be pumped.

(b)

Refined oil. This may be kerosene for use in gas turbines or diesel oil for use in reciprocating engines. This is a very clean fuel but relatively expensive. It burns mainly to water vapour and carbon dioxide although in common with all high-temperature processes there will be some nitrogen oxides produced which are also associated with acid rain. Stations burning refined fuel are generally much simpler to construct than those involving solid fuels.

• Natural gas. This is comparable to refined oil but is richer in hydrogen so that when it burns it produces more water vapour and less carbon dioxide and is therefore favoured by those concerned with the effects of carbon dioxide on the global atmosphere (global warming). Stations using refined fuel are quick to start and shut down and so are often used for supplying short peak demands.

• Nuclear. Heat produced by nuclear fission of uranium derivatives is used to produce steam and the cycle then continues as with burning any other fuel in a boiler. Nuclear power stations are immensely expensive to plan, construct, fuel up the initial charge and to bring on line. However, once running they produce electricity at virtually zero incremental cost, the main charge being paying off the investment. Therefore nuclear stations are ideally suited to run on base load, maintaining their output 24 hours a day for months at a time. The principal disadvantage will probably turn out to be the cost of decommissioning the stations at the end of their life. Once the turbines and external pipework have been removed the reactors will need to be boxed up and kept secure for centuries before anyone can finally dismant...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• 1 Electricity supply
• 2 Transformers
• 3 DC machines
• 4 Induction motors
• 5 Materials and their applications in the electrical industry
• 6 Illumination
• 7 Fuse protection
• 8 Tariffs and power factor correction
• 9 Regulations
• Appendix to Chapter 1 The three-phase delta connection
• Answers
• Index