Pages

Wednesday, December 5, 2012

CHAPTER 7 TRANSMISSION THEORY




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.

No comments:

Post a Comment