I am currently studying wind power and the power electronics used for it. In wind power a generator is driven by wind, thus the resulting power is of widely varying frequency and amplitude. The power grid, in turn, has strict requirements for the input power in terms of frequency, phaseshift and sinusoidal form. For this reason, power converters are today used routinely in wind power.
The predominant way to get the power into the grid is to use an AC-DC converter followed by a DC-DC converter and a DC-AC converter. This seems rather complicated instead of using a single direct AC-AC converter. Why is the indirect conversion via the DC "in-between" route preferable?
(This is actually a repost from Engineering, since I only found out later that there is a more active, thematically fitting, non-beta Electrical Engineering.)
Best Answer
There is a type of converter which can do this: the matrix converter.
In theory it can take many phases in and produce many phases out at quite a wide range of frequencies. It also has the additional benefit of not needing any power passives (in theory), or no large capacitor, no large inductors.
However, there are two golden rules with matrix converters
It is point #2 that makes the topology impractical as a simple loss of power will cause the inverter to blow up.
There is a variant of the matrix converter called the cycloconverter which uses thyristors and does not suffer the same issues as a full matrix converter. It, however, has a limitation of only being able to synthesise an output frequency around 1/10th of the input frequency. This limitation is fine for marine which typically uses 400Hz electrical supplies so generating 40Hz isn't too limiting for propulsion
So why AC-DC-AC instead of direct AC-AC ... The complications and limitations. A six switch inverter is extremely versatile.