Handbook of Electrical Installation Practice - BeKnowledge

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Distribution Transformers 341 Fig. 13.11 Designation of connections of some separate winding three-phase transformers by connection symbols.
342 Handbook of Electrical Installation Practice Local power supplies are often obtained from diesel engine or gas turbine generating sets. Voltages generated are often too low for economic power distribution, and transformers are used to raise the generated voltage to a satisfactory value for distribution, then reduce it back to domestic levels. Contractors often order two identical delta-star transformers for this application, which results in an unearthed and floating distribution system and a phase displacement between generated voltage and user voltage of 60°. It is necessary in this case to arrange for the generator-transformer to have a delta-connected winding connected to the generator and a star connection for the transmission voltage. With the connection Yd 1 there would be no phase displacement between the generated voltage and the user voltage, and the higher voltage distribution system would have a star point for earthing. There are still occasionally cases where unusual phase relationships exist from old privately owned systems. Linking these into modern networks can be made easier with the delta-interconnected star transformer or the star/interconnected-star transformer. The interconnected star winding, by varying the values of the winding sections, can be made to produce any phase relationship between primary and secondary windings. Large supplies of d.c. are essential for certain industrial application. Furthermore 12- and 24-phase systems of rectifier supply are necessary if harmonics in the supply are to be kept to a minimum. The interconnected star transformer winding provides the solution. Terminations Electrical connections to transformers are made with cable which is either carried overhead on poles or pylons or buried in trenches in the ground. The former uses fairly simple steel reinforced copper or aluminium conductors and relies on air spacing between conductors for insulation. Electrical cable for underground power distribution has solid insulating material between conductors and usually has a metal protection on the outer surface which is again protected against corrosion by an outer covering. At system voltage above 3300V the underground cable becomes very much more complex and is beyond the scope of this chapter. Generally speaking, distribution systems above 33kV are by the overhead system and require bushing terminations on transformers.Distribution voltages of below 600 V in most of the cities and towns of Western Europe are by the underground method. Underground cables have to be terminated by a method which does not expose the inner conductor insulation of the cable to the elements and this inevitably means some form of cable terminating box. In the early days of power distribution the insulation most common to l.v. underground cables usually consisted of paper, cotton, jute, wax and hemp, etc. Although thoroughly impregnated with wax and oil these materials were quite prone to moisture absorption. Cable terminating boxes were compound filled. Cable jointing and cable box filling was a fairly complicated business which required considerable skill. In spite of every effort failures in cable boxes were fairly common.
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HANDBOOK OF ELECTRICAL INSTALLATION
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© 2003 by Blackwell Science Ltd, a
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vi Contents System operation 79 Ide
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viii Contents Training and systems
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x Contents 18 Lighting 475 h.r. kin
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Preface My first task as editor is
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INTRODUCTION CHAPTER 4 Cable Manage
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CHAPTER 6 Standby Power Supplies D.
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INTRODUCTION CHAPTER 7 Ground Earth
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INTRODUCTION CHAPTER 8 Cathodic Pro
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CHAPTER 10 Special Installations or
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CHAPTER 15 Rotating Machines D.B. M
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CHAPTER 19 Mains Cables G.A. Bowie,
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APPENDIX I Main Authoritative Docum
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670 Index oil, 364, 366 oil filling
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672 Index earthing, 234 earth termi
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674 Index swimming pools, 252 switc
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