The installation at the Grosvenor Gallery, London, may be taken to
illustrate Class I. or the practical working of distribution by means
of transformers.
Fig. 11 represents the arrangement of primary and secondary circuits.
An alternating current is sent through the main L L¹, which is a closed
circuit, and a small portion is drawn off wherever there is a secondary
generator or transformer T; these instruments are placed in parallel
between the conductors in the same manner as a glow lamp; neither main
can be called positive or negative, as the current flows backwards and
forwards many times in a second. The house wires M M are joined to the
secondary circuits, and are quite distinct from the main, which they
do not even touch, although sufficiently near to receive an induced
current alternating the same as the primary, but of a much lower
electro-motive force.
[Illustration: FIG. 11.
D, alternating current dynamo;
E, continuous current dynamo for exciting;
L L¹, main primary conductors;
M M, secondary conductors;
T T, transformers;
S¹ S² S³ S⁴ S⁵ S⁶, lamps in parallel.]
The Goulard transformers were used at first, but have been superseded
by others designed by Mr. Ferranti; they are of the No. 2 kind, or
shell type, and have a core of hoop-iron, on which the two coils are
wound; the hoop-iron is then bent over, and the ends joined so as to
enclose the coils. The machinery is fixed in a basement excavated under
the Grosvenor Gallery; the foundations are of massive concrete, in which
stone supports for the engines and dynamos are embedded; the concrete
does not touch the walls of the building, but a space of about 1 foot
is left, which is filled in with clay; and by this simple plan all
vibration of the machinery is isolated from the building. The power
is obtained from two horizontal high-pressure engines, each of 600
indicated horse-power, in addition to the original two horizontal
high-pressure non-condensing engines, each of 35 nominal horse-power,
running at a speed of 55 revolutions per minute, which is maintained
constant by means of a governor directly attached to the expansion
slide-valve. The four engines drive on to a countershaft, which is
cut up into lengths; each section is coupled to a dynamo and exciter
by means of a conical friction-disk clutch; this permits of either
length being started or stopped without interfering with the other. The
speed of each engine is checked by means of a liquid speed-indicator,
designed by the author. Two Ferranti alternating current dynamos,
each capable of maintaining ten thousand lamps, are driven direct,
one dynamo by each length of shafting: they are excited by two
continuous current machines, the circuits from which are joined
to a regulating apparatus, which by altering resistance keeps the
electro-motive force of the large machines proportional to the number
of lamps which are to be maintained. At present hand regulation is
employed, but it is proposed to use automatic regulation, which
will increase the life of the lamps, as they are severely tried
by the variation of the current, which is more noticeable than in
continuous current installations. The current from the machines is at a
potential of 2400 volts, and that from the transformers is 100 volts.
The primary wire which carries this high electro-motive force does not
enter the houses, as the transformers are, as a rule, fixed in the
cellars, and from them the house branch is led in the form of a cable
of fine wires, having a total diameter of 7/16 inch; the lamps, which
are placed in parallel across this cable, are attached to single No. 18
or No. 20 B. W. G. wires in the usual manner. When first established,
the transformers presented an element of danger, in that they, in
common with all induction coils, were also condensers, and therefore
a dangerous shock might be given to any one touching some unguarded
portion of the lighting system. This has been prevented by the simple
plan of connecting one of the terminals of every secondary circuit to
earth, a method which, however, is not to be recommended, as it throws
an additional strain on the insulation of the primary circuit; in fact,
by earthing the secondary the insulation is practically reduced to
one-half. A safety device should be inserted, which would come into
operation on any leakage from primary to secondary, and immediately cut
out the transformer.
The primary-current conductor is led overhead, and still remains an
objectionable feature of the system, although the original trouble
with the neighbouring telephones and telegraphs has been overcome.
The primary circuit is a small carefully insulated cable of high
conductivity copper wire, nineteen strands of No. 15 B. W. G. It weighs
about 1¾ ton per mile, and is suspended, where it crosses the streets,
on a steel bearer whose tensile strength is 1⅕ ton. It is so arranged
according to the droop of the cable that the strain of the bearer never
exceeds 225 lbs., which means that the factor of safety is nearly 12 to
1. Double cut-outs or safety fuses, in many instances of the author’s
design, are placed on each pole of the primary, at the point where
it enters the house, so that, in the case of an excess current, the
mica-foils would fuse, and all connection between that house and the
supply main would cease.
Much credit is due to M. Goulard, who, in spite of great opposition
to the use of his transformer system, initiated the Grosvenor Gallery
installation three years ago. It has developed into not only the
largest and most important central-station in Europe, but, as regards
the transformer system, it supplies more lights than any in the United
States. The original company has been taken over by the London Electric
Supply Corporation, who are putting down plant capable of maintaining
30,000 lights, and are erecting another station at Deptford for 200,000
lights, which will be distributed by means of district transformers
from mains, which it is proposed to run from Deptford through the
Thames Tunnel and the Underground Railways. The electric current is
supplied by meter at the price of 7¼_d._ per Board of Trade Unit, a
price for light equal to gas at about 4_s._ 2_d._ per 1000 cubic feet.
The Eastbourne station is also on the transformer system. An
alternating current dynamo, by Ellwell Parker, maintains a pressure
in the primary circuit of 2000 volts, which is reduced by means of a
Lowrie Hall transformer to a working pressure of 100 volts. There is a
special arrangement for maintaining a constant electro-motive force in
the mains, independent of the number of lights in use. The mains are
carried underground, and have so far given no trouble as regards the
insulation of the high-tension current which passes through them.
The Eastbourne company commenced by lighting the parade only with
arc lamps, but now supply the incandescent light to all parts of the
town, and enjoy the unique position of having obtained power from the
corporation to run the mains in the streets prior to the passing of
the Act of 1882. Another small station has been successfully worked
for the last six years at Brighton; the group system was originally
adopted, the lamps, both arc and incandescent, being placed in
series or multiple series; the high-tension current is led through
overhead wires in a very similar manner to the installation at
Temesvar, Hungary, which is described at page 58, as an example of
multiple series lighting. The extensions at Brighton are to be carried
out on the transformer plan, which will necessitate the running of
separate circuits, the intention of the company, however, being to
gradually convert its whole system of supply to the transformer system.
The Brighton Company has regularly paid dividends to its shareholders
since its formation.
* * * * *
On the Continent the Goulard transformer is largely employed.
An important installation at Tours of 3500 lamps has been for some time
successfully working. Another at Tivoli has some additional points
of interest, in that the natural power of a waterfall is applied to
generate electricity. Two turbines constructed by Escher Wyss, of
Zurich, having an available head of 29·75 feet, give 80 horse-power
each, which is employed to drive two Siemens alternating current
dynamos, separately excited by two small continuous current machines.
Two distinct circuits of chromo-bronze naked wire, 3·7 millimetres in
diameter, are run overhead, in the same manner as telegraph wires,
through the town for a total length of about nineteen miles. The street
lamps are fixed alternately on each circuit, so that one-half can be
extinguished at a late hour without interfering with the others, or
having to turn out individual lamps. The number of lamps used in the
streets is two hundred glow lamps of 50 candle-power; also one hundred
and twenty glow lamps of 16 candle-power for the illumination of the
narrower streets. Arc lamps are also employed, as well as a large
reflector lamp, the rays from which are turned on the Temples of Vesta
and Sibilla. A house-to-house system is also being established, and
the company which has put up the work proposes to utilise the falls of
Tivoli in order to transmit 2000 horse-power for lighting purposes in
Rome.
* * * * *
The firm of Ganz, of Budapest, who are the manufacturers of the
Zippernowsky-Deri-Blathy system of transformers, have a similar
installation completed in order to light a portion of Lucerne. The
water power of Thorenburg 3·1 miles off, works the turbines, which
drive two self-exciting alternating current dynamos of the Ganz type,
similar to those shown at the Vienna Exhibition in 1884. The primary
current of 38 ampères, at an electro-motive force of 1800 volts, is led
by four uncovered wires, each six millimetres in diameter, to the
first station, which is 2·4 kilometres distant; here 1500 watts are
taken off, and at 2·3 kilometres further 7000 watts are utilised in
two of the hotels at Lucerne. A large installation on the same system
has been put down in Rome, and several Continental cities are adopting
this method of supplying electric light by small overhead wires. An
advantage claimed by the Zippernowsky system is the method of keeping
the strength of the magnetic field of the dynamos in accordance with
the external demand for current. The regulating apparatus employed
consists of a small transformer, the primary coil of which is traversed
by the whole, or by a proportionate part, of the main circuit, while
the secondary coil is inserted into the exciting circuit. Thus, if
the main current increases, the exciting current induced in the two
armature coils of the dynamo is reinforced by the inductive action of
the regulating transformer; and the field of the dynamo is strengthened
when more current is required. The opposite takes place when, through
the extinction of lamps on the external circuit, the demand for current
becomes less. In an experiment made with the transformers, which
supply some five hundred electric lamps for the Teatro dal Verone
and adjoining houses at Milan from the central electrical station
three-quarters of a mile away, the main current was often found to vary
from one ampère to thirty-five ampères; it was stated that no variation
in the service pressure could be detected, and the lamps burnt with
equal brightness whatever the number in use. In the experiments at the
Teatro dal Verone each transformer worked its own independent circuit
of lamps; but, if the conditions of the different circuits were alike,
they could be coupled up together in any manner desired, and thus a
group of transformers could become a centre of distribution.