The story of Electricity Generation from pipelines to pylons
Part 2: The problem of electrical power distribution: transformer basics explained.
From Pipelines to Pylons
Killingholme "A" is a gas-fired power station. Why gas? When the UK electricity marketplace was forcibly opened up to competition in 1990, the switch from coal to gas became all the rage in what became known as "the dash for gas". Whilst coal-fired power stations battled with the logistics of being constantly fed by trainloads of cheap coal, not to mention the enormous cost of upgrading plant to meet pollution targets, one thing which is still in plentiful supply is natural gas, provided from rigs in the nearby North Sea.
Several new power stations were therefore constructed in this locality, some being independently owned and others being built by both National Power and PowerGen. A gas fired power station is far cheaper and much more compact to build than a comparable coal-fired station, producing less carbon dioxide and virtually non-existent levels of sulphur dioxide, the compound which gives rise to acid rain.
Since the region's petro-chemical industries are handsomely served by major underground gas pipelines, then if there is an immediate need to construct power plants quickly and efficiently, gas is an obvious choice for fuel. Furthermore, by purchasing "off the shelf" power plant rather than attempting to design everything in-house, National Power enjoyed a greater choice of supplier and shorter lead times during the dash for gas.
Before we delve under the bonnet of Killingholme "A", it is worth relating a few fairly fundamental principles of electricity, which actually have a most profound impact on the way in which electricity must be distributed. When scaled up to the level of national electricity distribution, it soon becomes apparent why milliohms suddenly matter and kilovolts really count. A set of rules different from those which the microelectronics enthusiast usually concentrates on, exists in the field of generating and transmitting power and even the hardened electronics enthusiast cannot help being filled with awe when confronted with a 400,000V transformer or a 10,000 Amp circuit breaker!
Long distance transport
When electric current needs to be conducted over large distances (e.g. dozens of miles), several issues arise. The primary problem is that of unwanted electrical resistance, which results in heating effects (I2R) that are proportional to the square of the current: if a length of wire has a known resistance, then doubling the current will quadruple the power dissipated in the form of heat. Wasting power in this way is inefficient and equates directly to increased costs, so it is highly desirable to reduce these heating effects.
|Fig. 4 The resistance of a conductor is related to the cross-sectional area (CSA): the smaller the diameter, the higher the resistance.|
Since a conductor's resistance is directly proportional to its cross-sectional area, then in order to overcome the resistance inherent in long-distance power lines, the cross-sectional area of a conductor could obviously be increased (Fig. 4, left). This will reduce its resistance to current but will obviously increase costs because of the greater volume of conductor needed.
The solution is to step up the voltages being transmitted to much greater levels - tens or hundreds of thousands of volts. The higher the operating voltage, the lower the current, then the smaller the cross sectional area of power lines can be, to deliver the same level of power. This saves material costs, but then introduces yet another factor: the cost of insulating the environment from these extremely high voltages.
Transmitting electrical power economically around the country, then, is a finely-calculated compromise between several factors if power is to be transmitted efficiently and also at the most economical price: too thin a wire and the I2R heating losses become unacceptable; however too thick a wire results in a formidably high material cost; lastly, too high a voltage implies a greater cost in insulation and other technologies. It is also cheaper to suspend lengthy cables overhead than bury them underground (where heat dissipation and insulation become a problem).
Fig. 5 below shows the economics of this simple relationship in a graph. Incidentally, in case you've always wondered, those power transmission lines found hanging from pylons are usually made of aluminium alloy.
|Fig. 5 Illustrating the simple relationship between the cost of providing a supply versus the voltage and insulator costs.|
In order to transmit electrical power over considerable distances, great reliance is made on the transformer. Every reader will be familiar with a transformer, and exactly the same principle of "stepping up" or "stepping down" an alternating voltage is used throughout the power distribution network. It would of course not be at all feasible to route high d.c. voltages on overhead or underground cables due to the magnitudes of current involved. Imagine trying to transport 80 amperes per house at 230V a.c. [UK] and you can imagine that the conductors would have to be impossibly thick - several metres in diameter - to transmit such power levels to an entire town. (The Cross Channel Link does however run at d.c., as a way of separating the English and French power transmission systems: converter stations at both ends then produce alternating currents for onwards transmission.)
|Fig. 6(a) Step-down transformer symbol. The "spot" indicates the direction (start/ finish) of a winding (b) Step-up transformer (c) Auto-transformer|
The main function of a transformer is of course to step an alternating voltage up or down. Fig. 6 (above) shows the familiar circuit symbol of a typical mains transformer that would be found in a constructional project or consumer equipment. It consists of two or more coils wound on a laminated steel core. The primary winding can be considered as the input and the output is taken from the transformer's secondary winding. It is also often important to know the direction or phase of the windings: in electronics a spot-mark may sometimes be used to identify one end of each winding, or they may be labelled as, say, 230V [UK] and 0V on the primary, and 12V and 0V on the secondary winding.
Whether the transformer will increase (step up) the alternating voltage applied to the primary, or reduce it (step down) depends on the ratio of the number of turns of both windings. Regardless of which type the transformer actually is, at a simple level it can be assumed that the power (V x I) across the primary is roughly the same as that across the secondary. A step-down transformer (used in ordinary mains adapters for example) might have a 230V a.c. primary and, say, a 12V a.c. secondary. The turns ratio is therefore approximately 20:1. If the voltage across the primary is Vp and that across the secondary is Vs, then Vp/ Vs = Np/ Ns, where Np and Ns are the numbers of turns in the primary and secondary windings. As shown in Fig. 6a, the primary power (230V x 0.5A watts) is the same as the secondary (12V x 10A).
Therefore, the primary of a typical step-down mains transformer is at a higher voltage but carries a lower current than the secondary. The power (voltage x current) is the same in both windings. Importantly, this means that thin wire can be used for the high voltage side. However the secondary circuit operates at a lower voltage but a much higher current. A thicker gauge wire is needed on the secondary, to cope with these higher currents.
The auto-transformer can be considered as a single winding with a tapping made somewhere along its length. One terminal is therefore common to both the primary and the secondary. Scaled-down versions are used in workshops or laboratories, and have a moving contact which can be rotated to produce a variable a.c. voltage.
A key advantage of the auto transformer is that the secondary winding does not "see" all of the secondary current, which means that less copper wire is needed when compared with the classic "double wound" transformers of Fig. 6a and 6b. The use of auto transformers is quite widespread in the power industry, and these are classed as voltage transformers (VTs). One disadvantage to be remembered at consumer level is that they do not provide complete safety isolation from the mains.
A third type of transformer is also utilised in the power generation industry, in order that current flow can be measured. Since it would be impractical to directly measure the many kilo-amperes which can flow in certain parts of the electricity generation system, a current transformer (CT) is used to enable readings or measurements to be taken. A "doughnut" or toroidal-shaped secondary coil can be placed over a conductor which passes through the centre; the current-carrying conductor can then be deemed to be the "primary" of a current transformer whose secondary current can then be directly measured, or used in conjunction with protection equipment.
A series of CTs and VTs are used to constantly monitor the circuits of the power station; an enormous voltage transformer with a 15.75kV primary is positioned directly to measure the output of each generators. Transformers are also instrumental (literally) in alerting the power generation and distribution companies to any losses which may occur further downstream in the electrical grid.
In the power generation industry, thin wires at high voltages are used to transport power economically over great distances. Transformers will then be utilised at sub-stations in order to step down the voltages to something more appropriate, using thicker, more expensive wires to carry these higher "secondary" currents. We will look at the aspects of three phase power transmission and distribution later on.
Next, the basics of gas turbines and power generation are described along with unique photos taken during a shut-down.