Producing alternating current AC
We are all familiar with DC current from a battery in a torch. In electrical power generation it is alternating current that is the preferred and natural form of electricity. One advantage is that it can be transformed to a high voltage which enables transmission from power stations over the country to towns where it is transformed down for use in homes, offices and factories.
An electrical alternator/generator produces 'alternating current' (AC) at 50 cycles per second (hertz) (60 in the USA and Canada.)
The power driving the rotor, from a steam or gas turbine, is transferred to electrical power in the stator winding. In the case of a large generator in a coal or nuclear power station, to achieve 50 hertz the 'rotor' has two poles of a magnet N & S which rotate past the top red winding of the 'stator' 50 times a second (or 3000 revs per minute - rpm.) Each time the N passes the R (for red stator winding see diagram above) electrical current is induced into the red winding. The current builds up to a maximum when N is under R , it then decreases and when the S passes the R it is a maximum in the reverse direction. This is single phase AC.
The resulting waveform of current against time is shown below, a sine wave.
Power station Generation is in the form of three 'phases' and the N & S poles therefore pass each winding in turn each cycle (R-red Y-ellow and B-Blue at 120 deg apart as in the diagram below). (Only one cycle is shown here for each phase.)
The rotor magnet is electrically produced by a DC generator or 'exciter' and the whole assembly is known as a 'Synchronous Generator.'
Single phase used in a home requires two' wires' (line and neutral- return) as well as the earth.
Transmission of electrical power
Initially in the past power stations were located near to cities or industry except for hydro stations. As from the 1950's power stations were located near to coal fields, oil stations near to ports and in the future solar arrays, wind farms, tidal, wave and geothermal generation will be remote from places of demand. Thus there will be an increasing demand for electrical transmission over long distances and probably between countries, time zones and from desert areas to countries in colder climates.
Generally speaking, underground cable costs 20 times overhead transmission so the latter is favoured across country and rural areas.
Transmission lines usually carry two 'lines' each of 3 phases.
Three phases only require when transmitted one wire for each phase as the three neutral returns cancel out at the receiving end (and therefore do not have to be transmitted back). The three phase system has several advantages, it saves on transmission lines, is more efficient, enables 3 phase motors in industry which are low cost.
Note that the three wires, one for each phase, for the one 'line' circuit are usually carried on one side of the 400kv pylon spaced wide apart in air (see diag below). A second line of 3 phases is carried on the other side making two 'lines' in all. Each phase wire will be in the form of 2 or 4 cables strapped close together at these high voltages (to minimize weight due to 'skin effect.')
Back to the future - HVDC transmission over very long distances to distribute wind and solar.
In the future to enable interconnection of the diversified green options over very long distances, High Voltage DC transmission will be used which is more efficient and becomes economic over 100km. To obtain DC at High Voltage AC/DC/AC converters are necessary at the ends of these long lines. These include laser controlled semiconductors, Thyristors. Losses could be 3% over 1000 km. However superconductors might affect transmission lines in the future to reduce losses (ref 179). Existing examples are the Tasmanian - Melbourne link 180 miles across the Bass straight and the Yunnan hydroelectric plants 875 miles to Hong Kong and surrounding areas, capacity 5000 MW at 800 kv.
A downside of transmission items , EHV switchgear, transformers.
Use of wind power and DC transmission over long distances will result in a significant increase in EHV switchgear, transformers etc. Sulphur hexafluoride SF6 is used as an inert non toxic solution for cooling, insulation and arc quenching in high temp applications such as high voltage switchgear etc. but it is 22,000 times as potent as CO2 as a greenhouse gas and has a life in the upper atmosphere of 400 years. Close control over escape is necessary (ref 168).
Transmission line connections from power station to Grid UK.
Transmission line substation switchgear and connections UK
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