The story of Electricity Generation from pipelines to pylons
Part 4: Generators, frequency control, pylons, three phase distribution
Let's return to the process of electrical power generation and examine it in greater detail. Previously it was mentioned that each of the three gas turbines in our adopted power station drives an a.c. generator; a steam turbine drives a fourth. A power generator consists of an electromagnet (rotor) which is rotated directly by the turbine shaft. Each revolution of the turbine turns over the generator once. The rotor is surrounded by stator coils in which the moving rotor induces a voltage that will ultimately be delivered to the consumer.
The generator's electromagnetic rotor is d.c. controlled and since the rotor's currents are lower than those in the stator coils, it is easier to engineer the moving contacts (the sliprings and brushgear) which are needed to power or "excite" the rotor.
|Fig. 9 A sinewave is produced in the stator coil by the spinning rotor. The voltage level depends on the rotor's angle of rotation, and it reverses polarity every 180°|
If a simple two-pole rotor is used, this could be likened to a simple electromagnet having a North and a South Pole. The spinning electromagnet induces a voltage in the stator coil each time it passes by, and the stator voltage will therefore reverse polarity with every half-revolution of the rotor. The voltage level generated in the stator coil depends on how far the rotor has travelled during one revolution (its rotational angle).
Figure 9 (above) shows why a sine wave is generated. If the voltage is plotted against the angle of rotation, then the stator voltage peaks at 90 degrees of rotation, falls back to zero after 180 degrees and then peaks in the reverse direction at 270 degrees before completing a full cycle at 360 degrees. Because it delivers alternating voltage, this generator is more correctly called an alternator. (A dynamo produces a d.c. voltage instead.)
|Fig. 10 A three-phase generator has three fixed stator windings spaced 120° apart around a spinning rotor, which itself is an electromagnet having North and South poles.|
To get the most out of each revolution of the rotor, several stator coils are deployed so that multiple sine wave voltages are generated per revolution. In fact three coils are spaced at 120° apart (see Fig. 10) and the coils are designated by a colour code familiar to every electrician: they are red, yellow and blue. The generator windings produce 15.75kV (15,750V) between the phases.
|Fig. 11 Three phase electricity generated by the alternator of Fig. 10. The three phases are 120° apart.|
Obviously each stator voltage will rise and fall as the rotor passes by, and the overall result can be plotted as a three-phase voltage in Fig. 11. It can be seen that the voltage in the blue phase is 120° behind the yellow phase, which lags 120° behind the red phase.
By increasing the rotor's speed, the voltages induced in the stator coils will occur more frequently, although the three phases will always be 120° apart. This simplified approach assumes that there is only one pair of magnetic poles on the spinning rotor as shown.
If the rotor spins once per second, then the a.c. voltage generated in each phase will have a frequency of 1 Hertz (1 Hz.), changing polarity every half second. The frequency of the generated voltage is calculated by:
Frequency in Hertz = (no. of pairs of poles x revs. per minute) / 60
From the above formula, a generator with one pair of poles as illustrated must rotate at 3,000 r.p.m. to produce electricity at 50 Hertz, which is the declared system frequency [UK].
The statutory limits defined in the Electricity Supply Regulations of 1937 are 50 Hz. +/- 1% (i.e. 49.5 - 50.5 Hz) although the National Grid (NGC) strives for a variation of no more than 0.1% as best practice. The turbines operate at this speed, 24 hours a day for months on end.
In the mid 1920's before electrical power generation was standardised in the UK, several frequencies could be used - anything from what was probably a migraine-inducing 25 Hz and must have been murder to read by, all the way up to 80 Hz. The Electricity Supply Act of 1926 resulted in a standardisation of supply frequency across Great Britain, at 50 Hz., although in the USA and elsewhere the supply has been set at 60Hz.
In fact most generators tend to have two poles although certain types e.g. hydro-electric generators may have four or more poles. This allows for a slower rotor speed of 1,500 r.p.m for a four pole machine which is more appropriate for the medium involved, whilst still generating a 50Hz sine wave.
The actual method of controlling the supply frequency ultimately boils down to speeding up or slowing down all the generators on the system, by increasing or decreasing their load. The frequency of the voltage is the power station's ultimate yardstick of quality, and frequency itself plays a much more fundamental role in the country's entire electricity system than may generally be realised. Great effort is made to maintain this value and to eliminate cumulative errors in the consumer's supply, which might otherwise affect electric clocks, time switches, audio equipment etc., and any minor change in frequency is compensated for later on, in order to enable frequency-sensitive equipment to catch up (or slow down).
All power plants interconnected by the National Grid can be considered as part of an enormous "pool" of electricity hooked together on an "infinite busbar", which runs at a set frequency. Every power station thus connected operates at this frequency.
If at this time a small isolated power station was not connected to the busbar, but was then hooked in later, the frequency of the existing "pool" would easily dominate the generator of the newly-connected power plant. The net result is that all parts of an interconnected system operate at the same frequency.
The operating frequency of the rest of the grid is thus physically applied to an individual generator, in what is effectively a contest of wills. Since a generator's stator is synchronised to its rotor (and turbine shaft), it is necessary to ensure that a gas turbine runs at a speed which enables the generator's frequency to be matched to the rest of the grid. Hence the challenge is to supply just enough fuel to the turbines to ensure that the generator runs at the prevailing system frequency adopted by the rest of the grid.
Any increase in the fuel supply will not necessarily cause the turbine to run any faster, because the generator is already synchronised or locked to the frequency of the grid: instead the turbine will simply be "loaded", which is undesirable.
If the overall amount of generated power is insufficient to meet the demands placed on the system by consumers, then every turbine/ generator on that system will tend to slow down because insufficient input energy is being supplied. Hence the frequency would start to fall.
The system frequency can therefore be best controlled by ensuring that the generator's MW output constantly matches the consumer MW demand. It is ultimately the demand placed on a generator by the rest of the system which determines whether a generator speeds up or slows down. This, in turn, contributes to the quality of the frequency of the system.
The best analogy of this is to consider a car which is being driven at a fixed speed. If the car encounters a hill it will slow down, making it necessary to open the throttle to maintain engine speed. If the hill levels out, the throttle can be closed again. If it goes downhill, the engine can be used as a brake to slow the car.
A power station may be requested by the National Grid Control Centre to run in a "frequency insensitive" mode, meaning that the power plant simply runs at a fixed load irrespective of frequency. Conversely, in "frequency-sensitive" mode, the plant will be expected to operate at an automatically-variable load at a particular target frequency which is being enforced at the time. This compensates for variations in the consumer demand and whether the system frequency is starting to rise or fall as a consequence.
The other key parameter is of course voltage. For consumers, the statutory limits on their 230V [UK] supply is +/- 6%. Unlike the system frequency, the voltage levels can vary in different parts of the transmission system. The voltage output of a generator is directly related to the rotor voltage - i.e. the excitation voltage, which is controlled by a complex automatic voltage regulation (AVR) system. Every aspect of the generator and the turbine's performance is constantly monitored by the power plant's fully computerised control room.
The next part of the electricity generation process relates to the way in which the power generated in the stator coils is transmitted to the user. Typically, the generator outputs 15.75kV and is rated for more than two hundred megawatts.
The three phases - red, yellow and blue - are carried outdoors from the generator using large ducts which resemble pipelines. These pipes are pressurised in order to prevent corrosion or water ingress. The ducts are also colour coded to identify the phases, and this same theme is used all the way through to the end-user's premises.
|The power station's main circuit breaker enclosure. Ducts are lightly pressurised to prevent ingress of water etc.||End-on view of the main circuit breaker housing - note the colour coded dots for each phase. They start here!||A circuit breaker 'bus duct' filled with a compound called SF6 (sulphur hexafluoride) to suppress arcing.|
|Not your average toggle switch rating...||High voltage circuit breaker contact bank||Circuit breaker contact (close-up)|
One major problem is, how to actually switch such high magnitudes of voltage? Since these extremely high potential voltages can arc across considerable distances (see the section entitled "Leaps and Volts"), one can image the nightmarish problems of trying to switch many thousands of volts. The switchgear concerned must be able to withstand not only their full loads but six-fold overloads which occur when motors are starting; they must be capable of carrying or interrupting fault currents and must also cope with 17,500 Volts peak across the contact terminals. Evidently we are not talking 1/4" toggle switches here!
The solution lies in the use of special gas-filled circuit breakers. These are spring-loaded and motor driven and are designed to quench the high tension arc which develops between opening contacts. The compound sulphur hexafluoride (SF6) is used which is 6 times less conductive than air. Earlier types used oil-filled contacts or compressed air to snuff out the arc.
^ The three 3-phase lightly pressurised ducts leave the building and are connected to three 400kV transformers outdoors.
The 15.75kV (phase-to-phase) generator voltages are stepped up to 400kV by an external transformer - one per generator - for onwards transmission to the National Grid.
There are yet more amazing facts waiting to be "unearthed". Many readers will be aware that large transformers are oil-cooled in order to aid heat dissipation. In the largest types, oil is circulated by pumps and heat will be exchanged with a water-filled coolant circuit. The author was intrigued by several small instruments on these transformers: one small device - a Hydran - measures the level of hydrogen which accumulates in the oil.
|The bank of three 400kV transformers outdoors||A single 400kV oil-cooled transformer||A "Hydran" detects the early presence of hydrogen gas in the transformer's cooling oil, warning of a developing fault.|
|A Buchholz relay mounted on each transformer detects any dangerous build-up of gases in the oil (e.g. due to a fault), and acts as a safety switch.||This digital counter displays the number of times (20) the transformer's been hit by lightning! Unfortunately the readout was obscured by the railings.||The 400kV transformer output is then switched by the open-air Banking Compound - the air here is full of buzzes and crackles.|
In the event of a transformer internal failure (e.g. winding shorts, or contacts starting to burn out), hydrogen is one of the first gases to be produced, so by using a Hydran to test for this gas any trends can be spotted at an early stage.
A device known as a Buchholz relay is used as an automatic switch that responds to increasing levels of gas build-up in the oil. More accurate tests of oil samples are also undertaken by National Power and other gases such as acetylene can be measured over, say, a month and a good estimate made of the nature of an internal fault. Ultimately the oil can be drained and then the fault can be repaired.
Also worth mentioning is a small digital counter on the transformer's perimeter steel fence. It displayed "20": when this item was queried, the author was cheerfully told that this meant the transformer had been hit by lightning twenty times..... Oh, er, OK...
|The Banking Compund is designed to switch the ultra high tension voltages generated by the power station: it's the stations multi-way on-off switch. An instrumentation building is nearby. Current transformers (CTs) monitor the output.||Note the open contacts mounted in mid-air - they rotate to connect the supplies on busbars.|
An isolated area called the "banking compound" adjacent to the main power transformers contains an array of insulators and busbar isolating switches, which in my photos are open-circuit: on one of the author's visits, the entire system had been shut down for maintenance purposes and this is done by the physical act of rotating large isolators to break the circuit. (See Leaps and Volts page for a video.) Usually though, when Killingholme "A" in full swing, the crackle of high tension voltages fills the air around this bus-bar area and you can almost feel the high tension above you. The same compound contains current transformers which monitors the station's output.
^ The power station's 400kV output is fed underground to a sub-station, from where it rises on pylons to be distributed around to the countryside. Note the "terminal tower", a special pylon that routes the overhead wires in the right direction. The stack on the left belongs to a neighbouring 1,000MW power station.
From there, the 400kV supply is fed underground to a nearby sub-station, before finding its way on to a transmission tower, the very first in a series of many hundreds which will be used to distribute the power around the countryside. Some of these feature in my photographs.
It's... Super Grid!
The enormous 400kV supply - known as the Super Grid (275kV in certain regions) will be found hanging off the largest of pylons (as a general rule, the larger the pylon, and the bigger the insulators , then the higher the voltage being carried). The same pylons also carry Super Grid voltages generated by other neighbouring power stations. If ever one wondered why there are three arms to each side of a pylon, the answer is now obvious: there is one wire per phase, with each tower usually carrying two circuits. Sometimes, wires may be electrically paralleled, which will be witnessed by two or more wires running next to each other to share the load. (See my separate feature, "Electricity Pylons").
Fig. 12 The transmission and distribution of the electricity supply from the power station generator to the 33kV and 11kV needed by industry, and finally to the 415V / 230V used in domestic residences (UK).
These extremely high voltages are transmitted over considerable distances to regional sub-stations, where they are progressively stepped down by transformers (auto transformers are usually used on the Super Grid). Figure 12 outlines the structure of the electricity distribution system. | Click here to enlarge |
In the UK, the Super Grid is first reduced to a 132kV grid system and then 33kV for use by industrial estates and heavy industries. Light industries may require an 11kV supply which is provided by a sub-station. The final reduction occurs in residential areas, where the 11kV is stepped down to three-phase 415V from which single phase 230V a.c. is produced, as we shall see later. (Officially, UK domestic supplies have been "harmonised" at 230V for reasons best understood by the European Union. In reality, UK supplies are 240V just as they always have been, evidenced by taking a quick measurement of 247V!).
In many cases, the customer (say, a farm in a remote location) will have his own 11kV-to-415V step-down transformer and these are a common site in the English countryside, perched on top of a wooden pole. Actually, the one in my photo had replaced one that had caught fire and set the pole and the field alight: firefighters refused to go anywhere near it until the power generation board isolated the entire circuit.
It is the job of the Regional Electricity Boards to distribute power to commercial and residential properties, and sub-stations with suitable step-down transformers will be used as appropriate: from Fig. 12 it can be seen how the Super Grid voltage is systematically stepped-down as the end users' locality is approached. | Click here to enlarge |
In Part 5 the use of star and delta conenctions are explained, the neutral and ground (earth) are explored and we show how electricity is distributed to the home.