Handbook of Electrical Installation Practice - BeKnowledge

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.