Wind Energy Reference Manual Part 4: Electricity
Voltage
In order to make a current flow through a cable you need to have a voltage difference between the two ends of the cable - just like if you want to make air move through a pipe, you need to have different pressure at the two ends of the pipe.
If you have a large voltage difference, you may move larger amounts of energy through the wire every second, i.e. you may move larger amounts of power. (Remember that power = energy per unit of time, cf. the page on Energy and Power Definitions ).
Alternating Current
The electricity that comes out of a battery is direct current (DC), i.e. the electrons flow in one direction only. Most electrical grids in the world are alternating current (AC) grids, however.
One reason for using alternating current is that it is fairly cheap to transform the current up and down to different voltages, and when you want to transport the current over longer distances you have much lower energy losses when you use a high voltage. Another reason is that it is difficult and expensive to build circuit breakers (switches) for high DC voltages which do not produce huge sparks.
Grid Frequency
Alternating current sinusoidal curve With an alternating current in the electrical grid, the current changes direction very rapidly, as illustrated on the graph above: Ordinary household current in most of the world is 230 Volts alternating current with 50 cycles per second = 50 Hz ("Hertz" named after the German Physicist H.R. Hertz (1857-1894)). The number of cycles per second is also called the frequency of the grid. In America household current is 130 volts with 60 cycles per second (60 Hz).
In a 50 Hz system a full cycle lasts 20 milliseconds (ms), i.e. 0.020 seconds. During that time the voltage actually takes a full cycle between +325 Volts and -325 Volts. The reason why we call this a 230 volt system is that the electrical energy per second (the power) on average is equivalent to what you would get out of a 230 volt DC system.As you can see in the graph, the voltage has a nice, smooth variation. This type of wave shape is called a sinusoidal curve, because you can derive it from the mathematical formulavoltage = vmax * sin(360 * t * f), where vmax is the maximum voltage (amplitude), t is the time measured in seconds, and f is the frequency in Hertz, in our case f = 50. 360 is the number of degrees around a circle. (If you prefer measuring angles in radians, then replace 360 by 2*pi).
Phase
Since the voltage in an alternating current system keeps oscillating up and down you cannot connect a generator safely to the grid, unless the current from the generator oscillates with exactly the same frequency, and is exactly "in step" with the grid, i.e. that the timing of the voltage cycles from the generator coincides exactly with those of the grid. Being "in step" with the grid is normally called being in phase with the grid.
If the currents are not in phase, there will be a huge power surge which will result in huge sparks, and ultimately damage to the circuit breaker (the switch), and/or the generator.
In other words, connecting two live AC lines is a bit like jumping onto a moving seesaw. If you do not have exactly the same speed and direction as the seesaw, both you and the people on the seesaw are likely to get hurt.
The page on Power Quality Issues explains how wind turbines manage to connect safely to the grid.
Alternating Current and Electromagnetism
To learn about electromagnetism, turn to the next pages.
© Copyright 1997-2003 Danish Wind Industry Association
Updated 12 May 2003
http://www.windpower.org/en/stat/unitsac.htm
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