Why/when is AC-DC-AC conversion superior to direct AC-AC conversion?
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
- Thou shalt not short circuit the supply
- Thou shalt not open-circuit the load
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.
When two routes are possible, there is rarely a good answer for why one particular one was chosen. It's often accidents of history, or advantages to one or the other depending on local industries, or common components.
There is an all electronic route directly from 3 phase AC at one frequency to another, it's called a Matrix Converter. It contains 9 switches in a 3x3 matrix, to connect any phase to any other. With suitable timing of the switch instants, and suitable input and output filters, it can create a similar output voltage to the input. They are becoming increasingly used for motor drives.
However, I can think of many advantages to using an intermediate DC link.
AC-DC and DC-AC converters are being made in large numbers, in large sizes for DC links where long distance transmission is a factor. This will lead to economies of scale. They are more mature than matrix converters, so with the long planning involved in electrical infrastructure are more likely to have been chosen. Wind turbines tend to be connected in short hops to hubs before being connected to a single long distance transmission line (very long in the case of offshore). It's easier to pool power at a nominal DC intermediate voltage, simplifying control. It's easier to stay DC for the long transmission.
The reason for direct AC-AC conversion is the size and mass of the DC choke coil (or capacitor array). You don't want to have that e.g. in a rubber-wheeled subway car or aircraft. In iron-wheeled trains it depends, because more mass means better friction.
That doesn't apply to buildings.
You cannot save on valves (transistors or thyristors). In contrary, AC-AC converters tend to have more valves (though smaller ones) than AC-DC-AC converters. The control concept is also much more complicated.