The story of Electricity Generation from pipelines to pylons
How a CCGT power station works. Turbines introduced.
Having introduced these fundamental electrical aspects, let's return to our power station at Killingholme "A" and explore in more detail where electrical power actually comes from. Our adopted power station is fuelled by gas brought in from the North Sea and transported in an underground pipeline. The actual compound area where the natural gas arrives contains just a little surface pipework and is remarkably ordinary-looking, all things considered!
National Power's Killingholme "A" plant is known as a Combined Cycle Gas Turbine (CCGT) plant, which utilises gas turbines to drive electrical generators. In a CCGT plant, surplus heat created by the gas turbines is further utilised to produce steam. This drives a steam turbine to generate yet more electricity. The steam turbine is driven by "waste" heat from the gas turbine which results in a vast improvement in overall power plant efficiency. A diagram explaining the overall process is shown in Fig. 7. | Click here to enlarge |
Fig. 7 Schematic representation of a Combined Cycle Gas Turbine (CCGT) power plant. A gas turbine drives a generator and the exhaust is used to heat purified water and generate superheated steam. A steam turbine is then used to generate more electricity. A cooling tower enables the steam to be condensed back into purified water. See text for more details. (Adapted from original ABB material.) © Copyright.
A 3D cutaway illustration of this type of CCGT power station is available, with key to identify major areas. (Courtesy National Power.)
Large grilles on the front of the building are actually air inlets for the gas turbines. Each turbine requires about half a tonne of air per second, so atmospheric air is sucked in and compressed by many stages of spinning blades located at the front of each turbine shaft.
The resultant high pressure air is "swirled" along with natural gas within a combustion unit fitted on top of the turbine. Within this "silo combustor" are 54 separate burners which act as gas jets. The burning mixture reaches temperatures of over 1,000°C.
|Photographed during shut-down, a silo combustor bank of 54 gas turbine burners undergoing maintenance.||The other two silos removed from the gas turbines, during a shutdown.||A gas turbine during inspection, with the stripped-out turbine compressor blades nearby.|
|A gas turbine being inspected and serviced. The combustor bank (above) fits where the technician is working.||The gas turbine opened up, with the turbine and compressor blades removed.||The turbine blade assembly completely removed. Note the size of the technicians for comparison.|
|The steam turbine under maintenance.||The rotor of the steam turbine.||Steam turbine blades|
Photographs © Alan Winstanley 1999-2011
Larger Royalty-Free uncropped versions of these images can be purchased for non-commercial use - please enquire.
In the same way that in an internal combustion engine the petrol/ air mixture ignites and expands to force down a piston, the resulting continuous expanding force from the burning gas mixture passes over and spins the gas turbine blades. These drive a generator through a shaft, which also drives the air compressor blades.
In A Spin
Looking at the generator in more detail, it is much easier to use stationary coils rather than attempt to rotate them, so the electrical generator consists of a comparatively small rotating electromagnet (the rotor) surrounded by a series of large fixed coils (stators) in which electrical energy is induced. They output up to 145 megawatts at 15.75kV. The Killingholme power plant has three gas-turbine driven generators plus a steam turbine as well. We look in greater detail at what happens to the generator's output later on.
|Fig. 8 A static starting device (SSD) is used to convert a generator into a "starter motor" (consuming 4MW). This causes the rotor to spin, which in turn, turns over the turbine.|
To start the system, a "static starting device" (SSD) is utilised (Fig. 8) in which the generator is actually used in reverse, as a starter motor (consuming some four megawatts of power in the process!). Acting as an induction motor, the stator is energised by a variable voltage, variable frequency a.c. supply; the generator's inner rotating windings are powered with a d.c. current (called "exciting" the rotor) through brushes and moving contacts called sliprings. Variations on this theme include the use of rectified a.c. exciters, or brushless excitement systems which use a.c. generators and eliminate the need for sliprings altogether.
At a certain point, the rotor's magnetic field "locks" together with the field created in the stator, and the generator (still behaving as a motor) achieves synchronous operation: the two magnetic fields are synchronised with each other. Then the supply to the stator is increased in frequency, which causes the rotor to be dragged along at a higher rotational speed. Thus the shaft is forced to rotate.
At 2,500 r.p.m. the gas turbine manages to sustain itself and the SSD is disabled, the turbine's compressor blades now spinning fast enough to maintain the combustion process. The rotor's speed will then be automatically governed up to the critical speed of 3,000 r.p.m. and electricity can then be generated. To give you an idea of scale, the rotor shaft typically weighs 100 tonnes and is machined from one solid casting. It will become apparent later why a speed of 3,000 r.p.m. is significant to electricity users!
The power generation process does not stop at the gas turbine. Having passed over the spinning gas turbine blades, the exhaust gases still have a temperature of some 500°C. Rather than letting this go to waste, in a CCGT system this is put to further use in a heat exchange boiler or "heat recovery steam generator" (HRSG).
Each heat exchanger contains over 100 kilometres of finned tubing, which functions like a heatsink in reverse: the hot exhaust gas is used to heat water which is pumped through the core of the heat exchanger. The water turns to steam. The chimney-like structures or stacks vent exhaust from the gas turbines after it's passed through the heat exchanger.
The "bonus" superheated steam produced by the heat recovery steam generators is completely free of water vapour and is invisible, and is piped to a steam turbine to drive a fourth 227 MW generator. The steam exhausted from this turbine is condensed by passing it over a bank of titanium tubing through which cooling water is pumped (originally extracted from the nearby River Humber). The resultant condensed water is extremely pure and is recycled for use back in the heat recovery boilers, to be heated back into steam again.
Lastly, the cooling water that has now been heated by the steam turbine's condenser, has to be cooled down and this is achieved in a cooling tower, where it is spraying it over a large surface area in the face of a rising column of air. The cooled water is then pumped back to the steam turbine's condenser for re-use. Sometimes water vapour is produced during this cooling-down process which will be seen billowing from power station cooling towers. As readers will know, coal-fired power stations rely on steam turbines and require much larger cooling towers for reducing the temperature of their condenser cooling water.
In a CCGT plant, it can be seen that much use is made of recycling and utilising the by-products of the combined cycle process. Exhaust heat from the gas turbine is used to create steam which generates "bonus" power with a steam turbine; the steam is then condensed back into water for further use in the heat exchanger, where it is re-heated by the gas turbine's exhaust to make more steam. The heat recovery cycle has a phenomenal effect on throughput: it increases the overall efficiency of the plant from approximately 33% to 50% or so.
In Part Four, the fundamentals of three-phase electricity are explored and we delve more deeply into our adopted CCGT power station, along with more photos.