7.1 TRANSMISSION AND DISTRIBUTION
Both ashore and on platforms electricity is generated in
bulk in a relatively few generator sets of high capacity. It is transmitted, still in bulk, to a number
of distribution centres, called ‘substations’, where it is further divided into
smaller parcels and sent on either to the individual consumers or, in large
shore networks, to a number of smaller substations local to consumer centres
for further subdivision.
In shore networks the transporting of bulk power from
the generating station to the principal substations is referred to as
‘transmission’, whereas the further spreading of power from those points is
referred to as ‘distribution’. As will
be explained below, transmission is usually at a much higher voltage than
distribution. In the shore networks in
England and Wales generation and transmission are the responsibility of the
Central Electricity Generating Board (CEGB), but distribution is in the hands
of the various Area Boards who pass the power on to the consumers. In Scotland however generation, transmission
and distribution are all covered by the North of Scotland Hydro Electricity
Board (NOSHEB) and the South of Scotland Electricity Board (SSEB) who deal
direct with the consumers.
On platforms the distance between generators and
substations is so short that the passing of bulk power between the two, even though
it is at high voltage, is not regarded separately as ‘transmission’, and the
whole network is considered as distribution.
It will be noticed that on most platforms the power is
generated at high voltage, 6 600V or 4 160V. In shore installations such as refineries and
fractionating plants power is also brought in in bulk from the Area Board, or
NOSHEB or SSEB, at high voltage, typically at 11 000V.
Why, it may be asked, is it done this way when the bulk
of power-consuming equipments require only 415V or 440V?
The answer lies in one word - ‘current’.
7.2 WHY USE HIGH VOLTAGE?
This question is best answered by considering a d.c.
situation, although the argument applies equally to a.c.
It is one of the basic facts of electrical life that
power (measured in watts) is the product of voltage and current (measured in
amperes). This is the exact equivalent
of what happens in the mechanical world: e.g., hydraulic power transmitted is
the product of pressure and oil volume flow.
Putting figures to this (and assuming d.c. for the
moment), a 200 hp motor is equivalent to 150kW or 150 000 watts. If it is supplied at 440V, then the current
that it draws is
150 000
440
= 340A
But suppose the motor were rated 2 000 hp, equivalent to 1 500kW or 1 500 000
watts, and that it is still supplied at 440V, the current would then be ten
times as great, namely
1 500 000
440
= 3 400A
The copper windings of a machine, and the cores of the
connecting cables, require a cross-section of something more than one square
inch for every 1 000 amperes that they carry. So in this case the windings would need to be
more than 3½ square inches in section, as would the cores of the connecting
cables. Such a size is simply not
practical. The machines would be
enormous and heavy, and the cables would be as stiff as pipes, to say nothing
of cost. Further, the very heavy
starting currents of these motors, up to five times normal (see manual
‘Electric Motors’), would present an additional problem.
If however the voltage were increased, say, ten times to
4 400V, for the same power the current needed would be
1 500 000
4 400
= 340A
This is no more than the current taken by a 200 hp motor
fed at 440V. It is quite practical,
requiring as it does only one-third of a square inch section of winding or
cable core. True, the higher voltage
would call for thicker insulation, but this adds little to the weight and size
of the machine.
So the principle is that, where powers are such that the
currents become unmanageable, the operating voltage is raised so as to reduce
the currents again to manageable levels.
This applies particularly to transmission lines, where the power is
handled in bulk and the currents are consequently heavy.
The current limit of a 440V motor is reached at about
400 hp (300kW), above which a higher voltage must be used. For a 440V generator the practical limit is
about 2 000kW.
In theory the higher voltage used need only be enough to
reduce the current to a manageable level, but in practice the voltage steps are
fewer and coarser. This is because
British and International Standards have laid down certain standard operating
voltages, and manufacturers design their equipment to these standards. At the lower end of the scale the standard
voltages of a.c. systems, and the US equivalents, are:
UK/European US
380/415/440V 440V
3 300V
4 160V
6 600V
11 000V 13 800V
On shore installations in the UK
11kV, 6.6kV and 3.3kV may all be found, with distribution at 415V. On most
platforms generation is normally at 6.6kV and distribution at 440V, though some
of them generate at the US standard of 416kV.
Motors rated above about 400 hp operate directly at high voltage. Exceptionally drilling equipment operates at
600V.
With the coming of SI units the horsepower (equal to
0.746 or approximately ¾kW) has been superseded by the kilowatt not only in
electrical plant but increasingly in mechanical plant also. Nevertheless many motor rating plates will
still be found stamped in hp.
Examples
A motor is rated at 50 hp. What is its output in
kilowatts?
50hp = 50 x ¾
=
37.5kW
A diesel engine has an output of
1 200 hp. What is the equivalent in
kilowatts?
1 200
hp = 1 200 x ¾
=
900kW.
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