Controlling Wind Farms

On a windy day, I noticed some private windmills on a farm spinning like crazy. Other times, I’ve seen such things making lazier circles. This got me to wonder how are wind farms controlled? If the wind is really howling, wouldn’t the voltage generated become excessive to a dangerous level? Furthermore, how do operators deal with all the low and high fluctuation in production? And, do solar cells also exhibit fluctuations as the sun’s angle changes? How is that regulated?

They regulate the voltage with…wait for it…a voltage regulator.

OK, that’s understandable within a predetermined voltage range, but there must be an upper limit. Is there an equivalent of a circuit breaker to electrically disengage the windmill spinning blades from the generator, so that the windmill is freewheeling?

When you have a rotating generator like a windmill (or like the alternator in a car), regulating the voltage is pretty easy. These generators work by spinning a coil of wire through an electromagnet, which is just another coil of wire. Technically it’s multiple coils of wire, but you get the idea. You can control the output by changing the current that is going through the electromagnet.

Solar cells put out DC, which then goes through an inverter to convert it to AC. The inverter handles the voltage regulation.

People like wind and solar because it’s green. Power companies don’t like wind and solar because they aren’t constant, and often aren’t anywhere near as green as people think they are as a result. This is because power companies often need to have fossil fuel generators online to back up the green generators, and since fossil fuel plants can’t ramp their output power up and down quickly, those fossil fuel plants tend to just waste a lot of fuel so that they will have enough capacity when the sun and wind die down.

There is another type of solar plant that, instead of directly converting the sun’s energy into electricity, instead focuses the sun’s energy on something like salt, heating the salt until it melts. By pouring solar energy into the salt during the day, the molten salt retains enough heat to power the plant at night when there is no sun. Water is run through the salt, which gets converted into steam from the heat, and is used to power a conventional steam turbine generator. The obvious advantage is that these plants constantly produce power, instead of being hit or miss depending on the available sunlight or wind at any given moment.

Molten salt solar plants are generally only practical an areas that get a lot of sun, like the southwestern desert areas of the U.S.

Another energy storage technique is to take the output from whatever your green generator is (solar, wind, or whatever) and use that to pump water up into a reservoir. Water from that reservoir then flows down through generators to produce a constant output. Your reservoir needs to be big enough that it won’t drain dry when your green generator isn’t outputting power. You also lose energy to evaporation from your reservoir, so some water gets pumped up and just lost. But again, it’s another way to take something that does not have a constant output and make it’s output more constant.

This wasn’t there when I started typing my first response.

If you drop the electromagnet coil’s output down to zero, the generator will produce zero energy, so these generators can be controlled from all the way to zero all the way up to their maximum output.

Wind turbines also can sometimes adjust the pitch of their blades, which they often have to do so that they don’t overspeed and come apart from the rotational forces. If they can’t adjust the blade pitch, they will instead have some sort of braking system to prevent overspeed.

This is what happens when a wind turbine spins out of control and its braking system fails:


The generators will have protection systems as well, and can trip offline for a number of reasons, like overvoltage or undervoltage, regulator failure, overspeed, excessive heat, etc.

I believe that large windmills adjust the blade pitch depending on the wind speed, so that gives them some control over how fast they turn. If you look where the blade connects to the hub, it’s usually a circle. Imagine rotating that circle so the entire blade turns and encounters the moving air at a different angle. That’s the blade pitch.

Airplanes propellers do the same thing. By changing the pitch they operate at the most efficient speed and angle. If the engine stops, the pilot turns the blades directly into the wind so the stopped propeller creates as little drag as possible.

By turning the blades to the optimum pitch, the windmill can generate the maximum amount of power for the available wind. Rather than let the blades freewheel in a high wind, I suspect the control mechanism turns the blades into the wind so they won’t be damaged by turning too fast.

Another way to deal with inconstant supply is with smart metering. You let the price for your electricity change as the supply does, on a minute-by-minute or even second-by-second basis. You communicate that to your customers, and they in turn set certain loads (like an air conditioner or a car charger) to only run when the power is cheap. So when the wind is blowing, people use up the excess power, and when it’s not, they don’t.

The industrial sized windmill farm I see out my front door never lets the blades spin over 15 or16 rpm. Speed of the blade tips at those revs are near mach 1. The controllers will brake them in strong winds so that they are not turning 15-20 minutes before the storms or winds hit. They can also angle them away from the wind.

That’s done in part to prevent the turbine blades from shearing off, which has happened a few times and there are people who thought it was because they were hit by UFOs. :smack: Here’s a well-publicized example.

I believe it’s also true that the braking/blade pitch systems are used when winds are too light to make it worth trying to run - at very low speeds, a wind turbine may still be racking up wear and maintenance, without generating the power to pay its way, so it can be more cost-effective to shut it off.

If the wind is really howling, wind generators are turned off.

If they generate more or less power, the hydro-electric generators are turned down or up. If more range is needed, the gas turbines are turned down or up. If more range is needed, the coal-fired steam-turbines are turned down or up.

To give you more time to adjust the other sources, batteries and flywheels are used.to even out demand. Steam turbines are quite heavy, so they help keep the frequency correct, and system voltage just drops a bit if there is more demand than supply.

If there is too much demand, and not enough supply, everything stops working, and everything turns off. Last year in Aus, it took weeks to get everything turned on again.

Power from solar cells varies with time of day, and clouds, and temperature. When solar cells were very expensive, people put them on rotating mounts, to keep them facing the sun. Now, it’s cheaper just to buy more cells. Cloudiness is predictable, and movement of the sun is very predictable, so you just turn up the other generators to compensate.

Variability of supply is only part of the equation. Demand varies enormously during each day and month. Managing supply variations is much the same is managing demand variations.

Another option to regulate supply is an industrial scale battery bank. The Australian state of South Australia has a focus on renewable energy, gaining ~43% of its electricity from renewable sources. After some recent issues with consistent supply, in conjunction with Tesla they had a 100Mwh capacity battery facility completed. It smooths out the peaks and troughs from renewable sources.

A point that needs making about wind turbines. They generate AC, and the AC they generate needs to be sync’ed to the grid. You could use an inverter, but at those powers the price and difficulty making one gets serious. What they do is to use a variable frequency inverter to power the field coils. What this does is to provide a rotating magnetic field that can be made to rotate at a fixed rate relative to the rotating generator element - thus yielding an AC output that is of a constant frequency and phase locked to the grid. How much current you run though the field coils controls the power generated, and can be balanced against the grid’s needs and maintaining a safe/efficient speed for the turbine.
What are coming are control systems that provide frequency stability to the grid, and thus are slaved not to the grid, but to an external frequency standard, and can act to stabilise the frequency on the grid. Thus they provide the “inertia” the grid has traditionally had from the traditional huge spinning generators powered by steam or water.

Typically turbines have DC generators in the nacelles, and that power passes through inverters on the ground to produce AC output. Line frequency is not dependent on rotor speed. The facility can control fluctuations in frequency with batteries, or sometimes flywheels, to inject or absorb grid power. Solar farms have those too, to dampen out fluctuations from cloud cover etc.

There’s a minimum speed to get the turbine spinning, called “cut-in speed”, based on blade aerodynamics. Above “cut-out speed”, blades are feathered like an airplane propeller’s to limit drag and possible damage. Yes, it’s ironic that a turbine can’t use all that energy when there’s the most of it. Of course, wind farms are planned based on long-term wind surveys that define what the most economical approach is.

A power utility uses a mix of wind, solar, and gas-turbine production to provide the required power throughout the day. The Duck Curve illustrates load vs. output.

nitpick- alternators typically spin the electromagnet (the field/rotor) inside the coils of wire in which current is to be induced (the stator.) reason being is that the spinning element needs to have current passed through slip rings/brushes of some sort, and since the field only needs a relatively small amount of current its easier and longer-lived to have it be the moving element. trying to pass the alternator’s peak output through slip rings and brushes would burn them out in short order. this is one of several reasons cars switched from DC generators to alternators.

Huh? From Google "The B75 turbine blade itself is 75 meters long, while the entire rotor assembly measures 154 meters in diameter. As it spins, the blades cover an area of 18,600 square meters—that’s roughly two and a half soccer fields—at a brisk 80 meters per second, or 180 MPH at the tips.

That’s a long way from Mach 1. Some propeller planes have tips that occassionly break the sound barrier (they are very loud when they do) and spin at much much higher rates.

Right. At 16 RPM (1.68 rad/s), a turbine blade would have to be about 200 meters long to be close to the speed of sound at sea level (344 m/s).

The Vestas V164, which Wikipedia claims is the largest wind turbine currently running, has a blade length of 82 meters. That means that at 16 RPM, the tips are going 138 m/s, or 310 miles per hour. That’s Mach 0.4.

I didn’t see a rotational operating speed for the V164, so it may not turn as fast as 16 RPM. But the tips can’t get very close to Mach 1 in any case, as parts of the flow over the blade can locally exceed Mach 1 even if the tip is technically below that. This is the transonic regime; it caused a lot of problems in aircraft design between WWII and the 1950s/1960s.

As MikeF points out, things get really loud when anything exceeds Mach 1. They also get pretty inefficient unless the airfoil is designed for supersonic flows. Wind turbines are generally designed for low noise and high efficiency, so they’re not likely to ever approach Mach 1 no matter how big they get.

Dang I doubled the diameter instead of the radius. Might have even been told the wrong dimensions any way.

Sorry; I don’t quite follow. What were you trying to calculate? Why would you double either the diameter or the radius? The only things that matter are the radius and omega (rotational velocity in rad/s).

2piR=Circumference of a circle should get me feet/second which I could do the math to get mph. I slept through the rad/s part of math and never got a good feel for it.