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nonconsistant power conversion 4
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davidbeach
Electrical
Random voltage/frequency is rectified to DC and inverted back to the proper AC characteristics.
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davidbeach
Electrical
Of course there is. The laws of thermodynamics wouldn't have it any other way.
OperaHouse
Electrical
There are a lot of high efficiency claims for inverters, but when they operate over a wide range of conditions you should figure on 15% loss.
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Operahouse reckons about 15% losses in an inverter-based system.
One of the popular technologies for wind turbines is the 'double-fed induction generator' or DFIG which allows the variable speed turbine to connect to the fixed speed grid by injecting power at the difference between the grid frequency and the machine frequency. The overall efficiency of these machines is typically just over 90%, i.e. the losses are about half of that 15% figure mentioned earlier. It's quite a complex machine to understand but there are plenty references on Google which include diagrams to illustrate some of the concepts. One of the key features is that the inverter does not have to be large enough to handle the full generator output power, something which is significant when you're considering a machine producing a couple of megawatts.
One of the popular technologies for wind turbines is the 'double-fed induction generator' or DFIG which allows the variable speed turbine to connect to the fixed speed grid by injecting power at the difference between the grid frequency and the machine frequency. The overall efficiency of these machines is typically just over 90%, i.e. the losses are about half of that 15% figure mentioned earlier. It's quite a complex machine to understand but there are plenty references on Google which include diagrams to illustrate some of the concepts. One of the key features is that the inverter does not have to be large enough to handle the full generator output power, something which is significant when you're considering a machine producing a couple of megawatts.
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Not to offend, but if you are going to have a wind turbine installed in a nacelle that can yaw to follow the direction of the wind, how else would you do it but to have a generator connected to every turbine?
Are you thinking that numerous towers/turbines could be interconnected with shafting to one central generator? Not practical; wind turbines located in matrices have to be able to respond independently as wind gusts traverse the matrix, and with shafting interconnecting multiple turbines the differential torque would tear the shafting apart.
Depends what the output type is of each machine...AC output collected power is commonly stepped up to transmission or distribution system voltage...why would bulk/central "treatment" necessarily be any more efficient than local/distributed treatment?
CR
"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]
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In general, there are two strategies for generating synchronized AC electrical power from wind turbine generators.
The first, and older, strategy, is to make mechanical and electrical adjustments to the generating mechanism so that it is always generating the proper magnitude, frequency and phase. The mechanical adjustments are things like changing blade pitch. The electrical adjustments usually would vary the slip frequency of an induction generator so the output frequency is always proper.
This strategy avoids the capital expense of the rectifier and inverter, and the operating losses as well. However, the maintaining of line synchronization often means that significantly less power is generated than would otherwise be possible -- for example, much of the extra power from a gust would have to be spilled by the blades to keep from losing synchronicity. Also, the effective generating range, both at the high and low ends of wind speeds, can be reduced, because it is not practical to generate synchronous power at these extremes.
The other, more recent, strategy is to optimize the generating mechanism for maximum power generation under given conditions, allowing varying magnitude, frequency, and phase. These waveforms are rectified into DC electronically, and then inverted back into AC, with the inverter managing the full synchronization to the lines.
The key question is whether the additional energy that this strategy permits out of the generator itself is enough to overcome the losses in the rectifier and inverter, and to justify the capital expense of the power electronics.
One of the other factors pushing designers toward the second strategy is the fact that a permanent-magnet AC generator is significantly smaller and lighter than an induction generator of the same power capacity. This provides significant structural savings when it is so high up. But with these PM generators, it is not feasible at all to try to generate directly to the lines, because you do not have the fast "slip" adjustments of an induction generator.
So, like most things, it comes down to engineering trade-offs.
Curt Wilson
Omron Delta Tau
The first, and older, strategy, is to make mechanical and electrical adjustments to the generating mechanism so that it is always generating the proper magnitude, frequency and phase. The mechanical adjustments are things like changing blade pitch. The electrical adjustments usually would vary the slip frequency of an induction generator so the output frequency is always proper.
This strategy avoids the capital expense of the rectifier and inverter, and the operating losses as well. However, the maintaining of line synchronization often means that significantly less power is generated than would otherwise be possible -- for example, much of the extra power from a gust would have to be spilled by the blades to keep from losing synchronicity. Also, the effective generating range, both at the high and low ends of wind speeds, can be reduced, because it is not practical to generate synchronous power at these extremes.
The other, more recent, strategy is to optimize the generating mechanism for maximum power generation under given conditions, allowing varying magnitude, frequency, and phase. These waveforms are rectified into DC electronically, and then inverted back into AC, with the inverter managing the full synchronization to the lines.
The key question is whether the additional energy that this strategy permits out of the generator itself is enough to overcome the losses in the rectifier and inverter, and to justify the capital expense of the power electronics.
One of the other factors pushing designers toward the second strategy is the fact that a permanent-magnet AC generator is significantly smaller and lighter than an induction generator of the same power capacity. This provides significant structural savings when it is so high up. But with these PM generators, it is not feasible at all to try to generate directly to the lines, because you do not have the fast "slip" adjustments of an induction generator.
So, like most things, it comes down to engineering trade-offs.
Curt Wilson
Omron Delta Tau
electricpete
Electrical
Efficiency is not really relevant to the economics, since we don't have to pay for the input (wind).
What is relevant is the costs mentioned above, divided by the output power (example cents per kw-hr)
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(2B)+(2B)' ?
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Practically all modern megawatt –class wind turbines utilize some kind of frequency converter between the grid and the generator. Directly grid-connected wind generators using so-called stall-control (that cswilson was referring to) are very rare in new designs mainly due to being less efficient when considering the whole operating range.
Two of the most common concepts nowadays are the one where 100% of generator power goes through the frequency converter (generator is typically a squirrel-cage induction machine or a permanent magnet synchronous machine) and the one where only 30…40% goes through the converter and the rest goes directly to the grid (generator is doubly-fed induction generator –type). While the first one needs bigger and more expensive converter, its main benefits are better overall efficiency and brushless operation (less maintenance). Both of these generator types operate at 96…98% efficiency at the rated point, and the efficiency of the converter in MW range is around 96…97%.
Some turbine designs use so-called direct-drive concept, where turbine is directly rotating the generator. In this case, no gear-box is needed, and the only suitable generator type is synchronous machine either with electrical or permanent magnet excitation (induction machines cannot be designed to operate at such low speeds). Direct-drive systems don’t have gear-box losses, but such generators lack in efficiency when compared to high-speed ones (with gear-box), so there is not that big difference in overall efficiency. Additionally, direct-drive machines are huge in physical size. Main benefit of course is, that they don’t have gear-box related issues.
More common system is the one with the gear-box, where the generator speed is increased to 400…1500RPM (typically). As the efficiency of the gear-box is around 97…98%, total drive-train efficiency is typically around 90…92% when considering the gear-box, generator and frequency converter. Of course some power is lost on transformer(turbines often produce low voltage/medium voltage that needs to be stepped up) and turbines’s own consumption too.
I wouldn’t say that efficiency is not important since the wind is free. The higher the efficiency, the more money turbine generates. Furthermore, poor system efficiency means that blades, gear-box, generator etc must be designed to handle bigger input power (torque) which makes them heavier more expensive. One old rule of thumb says that one extra kilogram up in the nacelle means 3kg extra in total due to heavier foundations and tower structure.
Two of the most common concepts nowadays are the one where 100% of generator power goes through the frequency converter (generator is typically a squirrel-cage induction machine or a permanent magnet synchronous machine) and the one where only 30…40% goes through the converter and the rest goes directly to the grid (generator is doubly-fed induction generator –type). While the first one needs bigger and more expensive converter, its main benefits are better overall efficiency and brushless operation (less maintenance). Both of these generator types operate at 96…98% efficiency at the rated point, and the efficiency of the converter in MW range is around 96…97%.
Some turbine designs use so-called direct-drive concept, where turbine is directly rotating the generator. In this case, no gear-box is needed, and the only suitable generator type is synchronous machine either with electrical or permanent magnet excitation (induction machines cannot be designed to operate at such low speeds). Direct-drive systems don’t have gear-box losses, but such generators lack in efficiency when compared to high-speed ones (with gear-box), so there is not that big difference in overall efficiency. Additionally, direct-drive machines are huge in physical size. Main benefit of course is, that they don’t have gear-box related issues.
More common system is the one with the gear-box, where the generator speed is increased to 400…1500RPM (typically). As the efficiency of the gear-box is around 97…98%, total drive-train efficiency is typically around 90…92% when considering the gear-box, generator and frequency converter. Of course some power is lost on transformer(turbines often produce low voltage/medium voltage that needs to be stepped up) and turbines’s own consumption too.
I wouldn’t say that efficiency is not important since the wind is free. The higher the efficiency, the more money turbine generates. Furthermore, poor system efficiency means that blades, gear-box, generator etc must be designed to handle bigger input power (torque) which makes them heavier more expensive. One old rule of thumb says that one extra kilogram up in the nacelle means 3kg extra in total due to heavier foundations and tower structure.
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