J & P Transformer Book (eBook)

Martin Heathcote (Autor)

eBook Download: PDF | EPUB

2011 | 13. Auflage
992 Seiten
Elsevier Science (Verlag)
978-0-08-055178-4 (ISBN)

Infos

Maintaining appropriate power systems and equipment expertise is necessary for a utility to support the reliability, availability, and quality of service goals demanded by energy consumers now and into the future. However, transformer talent is at a premium today, and all aspects of the power industry are suffering a diminishing of the supply of knowledgeable and experienced engineers.

Now in print for over 80 years since initial publication in 1925 by Johnson & Phillips Ltd, the J & P Transformer Book continues to withstand the test of time as a key body of reference material for students, teachers, and all whose careers are involved in the engineering processes associated with power delivery, and particularly with transformer design, manufacture, testing, procurement, application, operation, maintenance, condition assessment and life extension.

Current experience and knowledge have been brought into this thirteenth edition with discussions on moisture equilibrium in the insulation system, vegetable based natural ester insulating fluids, industry concerns with corrosive sulphur in oil, geomagnetic induced current (GIC) impacts, transportation issues, new emphasis on measurement of load related noise, and enhanced treatment of dielectric testing (including Frequency Response Analysis), Dissolved Gas analysis (DGA) techniques and tools, vacuum LTCs, shunt and series reactors, and HVDC converter transformers. These changes in the thirteenth edition together with updates of IEC reference Standards documentation and inclusion for the first time of IEEE reference Standards, provide recognition that the transformer industry and market is truly global in scale.

-- From the foreword by Donald J. Fallon

Martin Heathcote is a consultant specializing in power transformers, primarily working for utilities. In this context he has established working relationships with transformer manufacturers on several continents. His background with Ferranti and the UK's Central Electricity Generating Board (CEGB) included transformer design and the management and maintenance of transformer-based systems.

*The definitive reference for all involved in designing, installing, monitoring and maintaining high-voltage systems using power transformers (electricity generation and distribution sector, large-scale industrial applications)
*The classic reference work on power transformers and their applications: first published in 1925, now brought fully up to date in this thirteenth edition
*A truly practical engineering approach to design, monitoring and maintenance of power transformers - in electricity generation, substations, and industrial applications.

Inhalt

Front Cover 1
The J & P Transformer Book
Copyright page 5
Contents 6
Foreword 10
Preface 12
Acknowledgements 14
Chapter 1 Transformer theory 16
1.1 Introduction 16
1.2 The ideal transformer: voltage ratio 17
1.3 Leakage reactance: transformer impedance 19
1.4 Losses in core and windings 20
1.5 Rated quantities 24
1.6 Regulation 26
Chapter 2 Design fundamentals 29
2.1 Types of transformers 29
2.2 Phase relationships: phasor groups 32
2.3 Volts per turn and flux density 37
2.4 Tappings 39
2.5 Impedance 40
2.6 Multi-winding transformers including tertiary windings 42
2.7 Zero-sequence impedance 48
2.8 Double secondary transformers 49
2.9 General case of three-winding transformer 51
Chapter 3 Basic materials 56
3.1 Dielectrics 56
3.2 Core steel 57
3.3 Winding conductors 69
3.4 Insulation 75
3.5 Transformer oil 90
Chapter 4 Transformer construction 120
4.1 Core construction 121
4.2 Transformer windings 134
4.3 Dispositions of windings 158
4.4 Impulse strength 164
4.5 Thermal considerations 173
4.6 Tappings and tapchangers 184
4.7 Winding forces and performance under short-circuit 245
4.8 Tanks and ancillary equipment 265
4.9 Processing and dry out 300
Chapter 5 Testing of transformers 334
5.1 Testing and quality assurance during manufacture 334
5.2 Final testing 336
5.3 Possible additional testing for important transformers 399
5.4 Transport, installation and commissioning 407
Chapter 6 Operation and maintenance 421
6.1 Design and layout of transformer installations 421
6.2 Neutral earthing 430
6.3 Transformer noise 445
6.4 Parallel operation 469
6.5 Transient phenomena occurring in transformers 510
6.6 Transformer protection 545
6.7 Maintenance in service 603
6.8 Operation under abnormal conditions 637
6.9 The influence of transformer connections upon third-harmonic voltages and currents 661
Chapter 7 Special features of transformers for particular purposes 685
7.1 Generator transformers 685
7.2 Other power station transformers 697
7.3 Transmission transformers and autotransformers 703
7.4 Transformers for HVDC converters 704
7.5 Phase shifting transformers and quadrature boosters 716
7.6 System transformers 725
7.7 Interconnected star earthing transformers 729
7.8 Distribution transformers 733
7.9 Scott- and Le Blanc-connected transformers 758
7.10 Rectifier transformers 765
7.11 AC arc Furnace transformers 767
7.12 Traction transformers 772
7.13 Generator neutral earthing transformers 779
7.14 Transformers for electrostatic precipitators 784
7.15 Reactors 786
Chapter 8 Transformer enquiries and tenders 797
8.1 Transformer enquiries 797
8.2 Assessment of tenders 822
8.3 Economics of ownership and operation 827
Appendices 837
1 Transformer equivalent circuit 837
2 Geometry of the transformer phasor diagram 848
3 The transformer circle diagram 854
4 Transformer regulation 859
5 Symmetrical components in unbalanced three-phase systems 863
6 A symmetrical component study of earth faults in transformers in parallel 886
7 The use of finite-element analysis in the calculation of leakage flux and dielectric stress distributions 936
8 List of national and international standards relating to power transformers 965
9 List of principal CIGRE reports and papers relating to transformers 976
10 List of reports available from ERA Technology Ltd 979
Index 984
A 984
B 984
C 984
D 985
E 985
F 985
G 985
H 985
I 985
K 986
L 986
M 986
N 986
O 986
P 986
Q 987
R 987
S 987
T 988
U 988
V 988
W 989

Textauszug

1 Transformer theory

1.1 INTRODUCTION

The invention of the power transformer towards the end of the nineteenth century made possible the development of the modern constant voltage AC supply system, with power stations often located many miles from centres of electrical load. Before that, in the early days of public electricity supplies, these were DC systems with the source of generation, of necessity, close to the point of loading.

Pioneers of the electricity supply industry were quick to recognise the benefits of a device which could take the high current relatively low voltage output of an electrical generator and transform this to a voltage level which would enable it to be transmitted in a cable of practical dimensions to consumers who, at that time, might be a mile or more away and could do this with an efficiency which, by the standards of the time, was nothing less than phenomenal.

Todays transmission and distribution systems are, of course, vastly more extensive and greatly dependent on transformers which themselves are very much more efficient than those of a century ago; from the enormous generator transformers such as the one illustrated in Fig. 7.5, stepping up the output of up to 19 000 A at 23.5 kV, of a large generating unit in the UK, to 400 kV, thereby reducing the current to a more manageable 1200 A or so, to the thousands of small distribution units which operate almost continuously day in day out, with little or no attention, to provide supplies to industrial and domestic consumers.

The main purpose of this book is to examine the current state of transformer technology, inevitably from a UK viewpoint, but in the rapidly shrinking and ever more competitive world of technology it is not possible to retain one’s place in it without a knowledge of all that is going on the other side of the globe, so the viewpoint will, hopefully, not be an entirely parochial one.

For a reasonable understanding of the subject it is necessary to make a brief review of transformer theory together with the basic formulae and simple phasor diagrams.

1.2 THE IDEAL TRANSFORMER: VOLTAGE RATIO

A power transformer normally consists of a pair of windings, primary and secondary, linked by a magnetic circuit or core. When an alternating voltage is applied to one of these windings, generally by definition the primary, a current will flow which sets up an alternating m.m.f. and hence an alternating flux in the core. This alternating flux in linking both windings induces an e.m.f. in each of them. In the primary winding this is the ‘back-e.m.f’ and, if the transformer were perfect, it would oppose the primary applied voltage to the extent that no current would flow. In reality, the current which flows is the transformer magnetising current. In the secondary winding the induced e.m.f. is the secondary open-circuit voltage. If a load is connected to the secondary winding which permits the flow of secondary current, then this current creates a demagnetising m.m.f. thus destroying the balance between primary applied voltage and back-e.m.f. To restore the balance an increased primary current must be drawn from the supply to provide an exactly equivalent m.m.f. so that equilibrium is once more established when this additional primary current creates ampere-turns balance with those of the secondary. Since there is no difference between the voltage induced in a single turn whether it is part of either the primary or the secondary winding, then the total voltage induced in each of the windings by the common flux must be proportional to the number of turns. Thus the well-known relationship is established that:

and, in view of the need for ampere-turns balance:

where E, I and N are the induced voltages, the currents and number of turns respectively in the windings identified by the appropriate subscripts. Hence, the voltage is transformed in proportion to the number of turns in the respective windings and the currents are in inverse proportion (and the relationship holds true for both instantaneous and r.m.s. quantities).

The relationship between the induced voltage and the flux is given by reference to Faraday’s law which states that its magnitude is proportional to the rate of change of flux linkage and Lenz’s law which states that its polarity such as to oppose that flux linkage change if current were allowed to flow. This is normally expressed in the form

but, for the practical transformer, it can be shown that the voltage induced per turn is

where K is a constant, Φm is the maximum value of total flux in Webers linking that turn and f is the supply frequency in Hertz.

The above expression holds good for the voltage induced in either primary or secondary windings, and it is only a matter of inserting the correct value of N for the winding under consideration. Figure 1.1 shows the simple phasor diagram corresponding to a transformer on no-load (neglecting for the moment the fact that the transformer has reactance) and the symbols have the significance shown on the diagram. Usually in the practical design of transformer, the small drop in voltage due to the flow of the no-load current in the primary winding is neglected.

Figure 1.1 Phasor diagram for a single-phase transformer on open circuit. Assumed turns ratio 1:1

If the voltage is sinusoidal, which, of course, is always assumed, K is 4.44 and Eq. (1.3) becomes

For design calculations the designer is more interested in volts per turn and flux density in the core rather than total flux, so the expression can be rewritten in terms of these quantities thus:

where E/N = volts per turn, which is the same in both windings

Bm = maximum value of flux density in the core, Tesla

A = net cross-sectional area of the core, mm2

f = frequency of supply, Hz.

For practical designs Bm will be set by the core material which the designer selects and the operating conditions for the transformer, A will be selected from a range of cross-sections relating to the standard range of core sizes produced by the manufacturer, whilst f is dictated by the customer’s system, so that the volts per turn are simply derived. It is then an easy matter to determine the number of turns in each winding from the specified voltage of the winding.

1.3 LEAKAGE REACTANCE: TRANSFORMER IMPEDANCE

Mention has already been made in the introduction of the fact that the transformation between primary and secondary is not perfect. Firstly, not all of the flux produced by the primary winding links the secondary so the transformer can be said to possess leakage reactance. Early transformer designers saw leakage reactance as a shortcoming of their transformers to be minimised to as great an extent as possible subject to the normal economic constraints. With the growth in size and complexity of power stations and transmission and distribution systems, leakage reactance – or in practical terms since transformer windings also have resistance – impedance, gradually came to be recognised as a valuable aid in the limitation of fault currents. The normal method of expressing transformer impedance is as a percentage voltage drop in the transformer at full-load current and this reflects the way in which it is seen by system designers. For example, an impedance of 10 per cent means that the voltage drop at full-load current is 10 per cent of the open-circuit voltage, or, alternatively, neglecting any other impedance in the system, at 10 times full-load current, the voltage drop in the transformer is equal to the total system voltage. Expressed in symbols this is:

where Z is R and X being the transformer resistance and leakage reactance respectively and IFL and E are the full-load current and open-circuit voltage of either primary or secondary windings. Of course, R and X may themselves be expressed as percentage voltage drops, as explained below. The ‘natural’ value for percentage impedance tends to increase as the rating of the transformer increases with a typical value for a medium sized power transformer being about 9 or 10 per cent. Occasionally some transformers are deliberately designed to have impedances as high as 22.5 per cent. More will be said about transformer impedance in the following chapter.

1.4 LOSSES IN CORE AND WINDINGS

The transformer also experiences losses. The magnetising current is required to take the core through the alternating cycles of flux at a rate determined by system frequency. In doing so energy is dissipated. This is known variously as the core loss, no-load loss or iron loss. The core loss is present whenever the transformer is energised. On open circuit the transformer acts as a single winding of high self-inductance, and the open-circuit power factor averages about 0.15 lagging. The flow of load current in the secondary of the transformer and the m.m.f. which this produces is balanced by an equivalent primary load current and its m.m.f., which explains why the iron loss is independent of the load.

The flow of a current in any electrical system, however, also generates loss dependent upon the magnitude of that current and the resistance of the system. Transformer windings are no exception and these give rise to the load loss or copper loss of the transformer. Load loss is present only...

Produkt-Details

• Erscheint lt. Verlag: 1.4.2011
• Sprache: englisch
• Themenwelt: Technik ► Elektrotechnik / Energietechnik
• ISBN-10: 0-08-055178-5 / 0080551785
• ISBN-13: 978-0-08-055178-4 / 9780080551784