There are wind turbines scattered all over Cornwall. Some of them in big 'farms', but many are isolated singles or small groups. While the generators on the big farms all seem to rotate together at the same speed (but differing speeds depending on the wind strength), the speeds of the small groups can vary one from another within a group. How is the electricity from these varying-speed turbines synchronised with the 50-cycle mains frequency? Do they just generate DC, which is then converted to AC in much the same way as for solar panels?
yes. I think earlier and larger ones are geared to have a generator that can actually do mains frequencies, but the smaller ones are all rectified and inverted.
OK, I can understand the smaller ones working with a DC to AC inverter, but that still doesn't explain how the geared or direct drive ones keep synchronised. On days with little wind, they turn slowly; when it's breezy, they spin round at a merry rate. And those are quite large 'farms' with a dozen or so big turbines. Whether the increase in speed is continuous or incremental (the latter might mean some sort of synchronisation), I don't know.
Was there not some clever hydraulic system that could drive a generator remotely and the speed was governed, in such a way that instead of the speed altering on the generator, the power it could generate before it slowed varied with the windspeed. I do not profess to understand how this might work, I'm just remembering the news item from a few years ago.
I'm sure there are any number of ways to do such things, and all must have been tried by now. Brian
All rotating "generators" work on AC. In the case of wind turbines, for maximum efficiency, they need to work at different speeds depending on wind speed So they generate AC which is rectified to DC and then they are linked to the mains with a "grid tie" inverter" that automatically converts the DC to mains frequency AC. Very similat to the ones used for PVpanels.
Wind speed is higher at higher altitudes. Small wind turbines spin faster than large ones. There are different designs of blade that rotate at different speeds. Some have variable pitch, some don't.
The latest ones have no gearbox making them cheaper, more reliable and quieter. (Made possible by high strength rare earth permanent magnets and improved inverters)
After quite a lot of fruitless searching I eventually found this:
formatting link
Scroll down to Doubly Fed Induction Generator - DFIG, about 2/3rds the way down. See also
formatting link
Although I won't claim to understand it in detail, at least it tells me that there's a clever system for coping with variable wind speeds and producing an AC output synchronised with the mains frequency.
Thanks for that. I couldn't see how a direct or geared drive to a generator could produce AC that's synchronous with the mains and still be of variable speed. More than I need in that book, and more than I can understand! But thanks anyway.
Then there's the question of electricity sales between countries with different national grids. They convert to DC then back again. That reduces transmission losses as well.
DC transmission is used extensively for undersea cables, where capacitative losses are much lower than for AC transmission especially over long distances.
formatting link
Map of major European interconnectors, existing and planned, here
formatting link
Click on any heading in the side table for enlarged map and details.
But AIUI overland interconnectors in Europe are AC.
The simplest and most efficient method for extracting wind power with small scale generators used in domestic/off grid setups (circa 1 to 10 KW) is to use a permanent magnet rotor[1] (preferably of the rare earth neodymium magnet type to maximise efficiency - or for those in danger of being conned into believing the false concept of 'free power from magnets', minimise the inevitable losses in the generator from winding resistance and windage and bearing friction - stronger magnets simply means that fewer turns of thicker wire can be used to achieve the same output voltage for any given rotational speed).
The variable frequency and voltage output from such basic generator machinery is simply rectified (and usually smoothed) to provide a variable DC voltage source feeding a switching converter which provides a voltage stabilised feed to an efficient sinewave inverter with a frequency to match local ac power standards (either simply to provide a local supply that meets the requirements of the electrical appliances and gadgets designed to work at that locality's mains supply voltage and frequency or else to synchronise via a grid tie controller with the local public supply which can fractionally adjust the sinewave inverter's frequency to allow it to synch up to the grid frequency before locking said inverter to the supply frequency once a synced up connection has been established).
The electronic power control functions are usually combined into a single monolithic control unit with seperate connectors for the generator and a local power feed which, except in the most basic of setups, will provide a grid tie link to the local incoming mains supply to allow automatic exporting of any surplus not immediately required by the local loads which remain plugged into the normal mains distribution wiring of the house.
Using very simple high efficiency PM generators to provide a source of 'wild' power (variable voltage and frequency) means that you can locate the required electronics to somewhere in the home that's accessible and sheltered from the elements, leaving just the wind turbine and its generator head exposed to the worst the local weather has to offer.
Designing a weatherproof generator is somewhat simplified when all it consists of is a bunch of magnets on a rotating shaft and several turns of enamelled copper wire in the stator slots with no delicate electronic components whatsoever in sight.
Since switching converters that only buck or boost the generator voltage have better efficiencies than buck/boost converters, it's normal practice to arrange for the generator's working output voltage to range entirely above or below the target voltage for the sine inverter's input rating.
I rather suspect that, in the case of the smaller 1.5 to 3KW wind turbine range intended for urban locations, the generator will be designed to run on the low voltage range ( 240v rms) simply due to the safety issue involved with higher voltages.
The high voltage option reduces I^2 R losses in the cabling between the generator head and the controller but since the cable loss at the low voltage level in the 1 to 3KW range is limited by the mercifully short cable run of a typical urban house installation (20 to 30 metres at most) and the improved 'fill factor' that is conferred upon the generator stator by low voltage high current windings, the safety aspect remains the dominant factor in the choice of generator maximum output voltage rating.
For those in more rural areas blessed by having access to a very windy high point, wishing to gain a lot more than the delusional 'feel good factor' out of their purchase, there's always the option to site the wind turbine several hundreds of meters away from their home by stringing up a high voltage transmission line with transformers at each end which will limit the losses to about the same as what might be expected with a directly connected 30 metre cabling run in an urban setting.
The transformers only need to be designed for the maximum voltage and current capabilities of the generator and a turns per volt ratio that just avoids core saturation. Since the generator frequency is in direct proportion to its voltage output, the transformer will always be operating at the same magnetic flux level throughout the full generator's voltage and frequency range.
There'll be an increase in core losses with rising frequency but since these losses can be readily kept to less than 1 or 2 percent in the worst case, the transformer will be able to maintain maximum utility of its magnetic core materials throughout the generator's entire operational speed range. In effect, the transformers can be designed to match the capabilities of a permanent magnet rotor alternator without the need for any 'over engineering' requirement in its design.
[1] The permanent magnetic rotor neatly avoids the parasitic losses of the more conventionally energised field winding types exemplified by the classic car alternator design. The unregulated nature of the output voltage of a PM generator now gets to be very efficiently resolved by a seperate switching regulator in the associated control box rather than within the generator itself.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.