[Reason077]

> "solar generates DC but wind generates AC."

Yes, but a wind turbine is allowed to spin at variable speeds - its rotation is not synchronized to the grid frequency in the same way that hydroelectric and thermal turbines are.

In order to get a wind turbine's power output to match the grid frequency, it goes through an AC -> DC -> AC conversion in a component known as a double-fed induction generator (DFIG).

[sandworm101]

Wind turbines, the big ones, regulate themselves to turn in sync with the grid. The adjust thier blades as needed to keep pace/power. That's why huge fields of them all turn in perfect lockstep.

[upofadown]

From some random reading on the net, it is the smaller ones that can be directly connected. Above 2MW things are not as easy and it is better and more efficient to convert to DC.

[Gravityloss]

Wind turbines have frequency converters. They are semiconductor devices. In one 3 MW turbine I visited, it was at the base of the tower.

The blade angle is adjusted to get optimum power extraction. Rotor RPM is completely independent of produced power frequency.

References: https://www.vestas.com/en/products/offshore/V236-15MW and https://library.e.abb.com/public/bf09cdf11d234241845c79ac343...

[mlyle]

You're talking past each other.

The electronics perform two key functions:

* Choosing pitch angles for efficiency and turbine safety. You can, for smaller turbines, just synchronize the turbine to the grid, but this is becoming uncommon practice.

* Converting the produced AC power to DC, and then choosing the proper frequency output and voltage to feed power to the grid, and inverting the DC to make this power. This should usually be trying to "speed up" the frequency of the grid a little if it's not already way too fast and regulate the voltage appropriately.

The second link you have, on page 3, shows (active) rectification (d) of the wind turbine AC power to make positive and negative DC buses, and then inversion of that DC (f) to make 3 phase AC output power.

[Gravityloss]

Yeah, I don't know if for example in big wind offshore wind parks, since it's connected with a DC cable to mainland anyway, you would convert all the turbine generator outputs just to DC.

Then at the ground station where it's connected to the grid, convert to AC, whatever kilovolts are needed.

[mlyle]

> you would convert all the turbine generator outputs just to DC.

You could. But you probably still need a DC/DC conversion step or boosting in order to let the power flow from each of the turbines fairly. (The synchronous conversion to DC from AC provides opportunity to slightly change the voltages you get out, but not terrifically so).

[julosflb]

Blade pitching first use is to reduce the aerodynamics power above the rater power point of the turbine and avoid overload.

[mlyle]

Yup, that's why the word "safety" is in there. I probably could have put that better. (It also includes e.g. feathering rather than just trying to stay close to rated power, speed, and torque limits)

[Reason077]

> "That's why huge fields of them all turn in perfect lockstep."

Not true for any modern wind farm. While they might appear to be turning together, that's just because the wind conditions are relatively consistent across the whole area. Not because they are actually synchronised! Most modern turbines will target a certain rated/optimal/maximum rpm once a certain wind speed has been reached, but are free to rotate more slowly (while still generating power) in light wind conditions.

[Upitor]

They are still not syncing directly to the AC grid. They are not going 50 rounds per second

[YZF]

They wouldn't necessarily need to do 50 rounds per second to generate 50 Hz. There could be any number of poles on the generator. At least in theory... Not saying you're wrong about not being synced but just that they wouldn't need to do 50 rps...

[sandworm101]

Gearboxes. Windmills of the same design will turn at the same rate in lockstep with each other. A gearbox converts that rate into what is necessary for the generator. The generator turns in sync with the grid.

[colanderman]

Likewise for solar -- the DC voltage and current a panel puts out is dependent on both insolation and load -- only in the most simple applications can you just hook in directly to a panel, without involving a DC-DC converter. Even grid-tie installations often use DC-DC converters, called "power optimizers" [1], to optimally tie multiple panels to a single bus.

[1] https://en.wikipedia.org/wiki/Power_optimizer

[KennyBlanken]

I have personally worked on a tens-of-kW-scale wind turbine which was directly connected to the grid and was fixed speed and blade angle. It's quite common at lower power ratings because the cost/complexity of pitch control isn't justifiable.

It wasn't much more complicated than a three phase electric motor bolted to high-angle-of-attack blades, connected to the grid via contactors controlled by a microcontroller and a grid-intertie monitor.