Below, a graphical representation of the different transformations of the current that is produced from the generator is shown.
Generating Alternating Current (AC) at variable frequency
Most wind turbines run at almost constant speed with direct grid connection. With indirect grid connection, however, the wind turbine generator runs in its own, separate mini AC-grid, as illustrated in the graphic. This grid is controlled electronically using an inverter, so that the frequency of the alternating current in the stator of the generator may be varied. In this way it is possible to run the turbine at variable rotational speed. Thus the turbine will generate alternating current at exactly the variable frequency applied to the stator.
The generator may be either a synchronous generator or an asynchronous generator, and the turbine may have a gearbox, as in the image above, or run without a gearbox if the generator has many poles, as explained on previous pages.
Conversion to Direct Current (DC)
AC current with a variable frequency cannot be handled by the public electrical grid. We therefore start by rectifying it, i.e. we convert it into direct current, DC. The conversion from variable frequency AC to DC can be done using thyristors or large power transistors.
Conversion to Fixed Frequency AC
We then convert the (fluctuating) direct current to an alternating current (using an inverter) with exactly the same frequency as the public electrical grid. This conversion to AC in the inverter can also be done using either thyristors or transistors.
Thyristors or power transistors are large semiconductor switches that operate without mechanical parts. The kind of alternating current one gets out of an inverter looks quite ugly at first sight - nothing like the smooth sinusoidal curve we learned about when studying alternating current. Instead, we get a series of sudden jumps in the voltage and current, as you saw in the animation above.
Filtering the AC
The rectangular shaped waves can be smoothed out, however, using appropriate inductances and capacitors, in a so-called AC filter mechanism. The somewhat jagged appearance of the voltage does not disappear completely, however, as explained below.
Advantages of indirect grid connection: variable speed
The advantage of indirect grid connection is that it is possible to run the wind turbine at variable speed. The primary advantage of this is that gusts of wind can be allowed to make the rotor turn faster, thus storing part of the excess energy as rotational energy until the gust is over. Obviously, this requires an intelligent control strategy, since we have to be able to differentiate between gusts and higher wind speed in general. Thus it is possible to reduce the peak torque, reducing wear on the gearbox and generator, and we may also reduce the fatigue loads on the tower and rotor blades.
The secondary advantage is that with power electronics one may control reactive power (i.e. the phase shifting of current relative to voltage in the AC grid), so as to improve the power quality in the electrical grid. This may be useful, particularly if a turbine is running on a weak electrical grid.
Theoretically, variable speed may also give a slight advantage in terms of annual production, since it is possible to run the machine at an optimal rotational speed, depending on the wind speed. From an economic point of view that advantage is so small, however, that it is hardly worth mentioning.
Disadvantages of indirect grid connection
The basic disadvantage of indirect grid connection is cost. As said before, the turbine will need a rectifier and two inverters, one to control the stator current, and another to generate the output current. Presently, it seems that the cost of power electronics exceeds the gains to be made in building lighter turbines, but that may change as the cost of power electronics decreases. Looking at operating statistics from wind turbines using power electronics (published by the the German ISET Institute), it also seems that availability rates for these machines tend to be somewhat lower than conventional machines, due to failures in the power electronics.
Other disadvantages are the energy lost in the AC-DC-AC conversion process, and the fact that power electronics may introduce harmonic distortion of the alternating current in the electrical grid, thus reducing power quality. The problem of harmonic distortion arises because the filtering process mentioned above is not perfect, and it may leave some "overtones" (multiples of the grid frequency) in the output current.