Simply, for its simplicity.
No synchronizing required, no pesky AVR to complicate the controls under gusty conditions, no complexity to invoke Murphy's law during unattended operation.
Minor aside — Some more recent wind-induction machines have used the equivalent of a 4-quadrant VF drive, claimed for PF improvement, and the characteristic of horsepower varying with the cube of wind speed.
Suggestion: The WTG are often powering loads in remote areas. They are not connected to the grid. Then, the advantage of induction generator is more visible. No synchronizatoin needed, no AVR, etc. This is however different for Wind Farms that deliver power to the grid. Induction generators must be synchronized with the grid if the Wind Farm is connected to the grid.
Whether it is in a remote area or part of a wind farm, the WTG must be connected to the grid in order to have a source of excitation. By saying that it didn't need to be synchronized, I meant that this is an asynchronous generator that is connected to the grid in the same way that an induction motor is - there is no need to match vectors prior to closing the switch.
Offhand, there are two practices for starting wind-induction machines. One is to connect each unit when wind velocity is near a point of exporting real power. The other is to motor the device from zero or very low speed, importing a portion of real power until [over-synchronous] negative slip is realized.
Suggestion to peterb (Electrical) May 11, 2003 marled ///\\Whether it is in a remote area or part of a wind farm, the WTG must be connected to the grid in order to have a source of excitation.
///The statement can easily be challenged since there are WTGs generating electricity in remote areas, where there is no grid. Apparently, an auxiliary power supply must be applied to have the WTG fully functioning and delivering power.\\\
Jbartos, if you want to split hairs, we can agree that the induction generator must be connected to an external source of reactive power in order to operate. However, I maintain that the induction generator is never "synchronized" to anything, as it is is by definition an asynchronous generator.
Jbartos, the paper you recommended looks like a fine academic paper. Do you know if there are any real systems where WTG's are connected to an islanded load? Here in the Pacific Northwest we have several wind farms, all connected to the main grid.
SidropoulosM -
Sounds like you are in a good position to further address the original question yourself. My direct experience in the field is limited to a single WTG unit, connected to a distribution feeder in the boondocks - this is the main source of my original comments, where operational simplicity and rugged construction were perceived to be the pre-eminent requirements.
Peterb: I do not have specialized knowledge of wind turbines. As a transmission planning engineer my primary concern is the impact of wind farms on the grid. My understanding is that the main reason for using induction generators is their simplicity, robustness and cost effectiveness. These are sufficiently important to outweigh their disadvantages, especially their voracious appetite for VARS.
I cannot see, however, wind generators supplying islanded loads without a back up supply. Wind generators can supply energy when the wind blows, but you can't rely on them for capacity.
Simplicity & the slip characteristic of induction generators provides some mechanical damping. Synchronous generators are "locked" to the grid and this could cause large mechanical loads in wind turbines in turbulent wind conditions.
Some wind turbines use "variable slip" induction generators to further smooth mechanical loads, while the same can be acheived with variable speed technology.
Suggestion to SidiropoulosM (Electrical) May 12, 2003 marked ///\\Jbartos, the paper you recommended looks like a fine academic paper. Do you know if there are any real systems where WTG's are connected to an islanded load?
///Visit
for small wind project examples.\\
Here in the Pacific Northwest we have several wind farms, all connected to the main grid.
///These are called "big" projects.\\\
jb, Please note that the Bergey products are DC generators operating through an inverter which provides the frequency control necessary for isolated operation. Induction generators require a fairly stiff bus to operate into or both the frequency and voltage will be subject to significant swings.
This is probably not relevent but the small turbines used for AE applications commonly are perminant magnet type. These produce variable frequency AC that is 3-phase rectified to dc to charge batteries.
Suggestion to RAMConsult (Electrical) Sep 23, 2003
jb, Please note that the Bergey products are DC generators operating through an inverter which provides the frequency control necessary for isolated operation.
///I noticed on
that:
The blades attach directly to a specially designed very-low-speed permanent magnet alternator which uses state-of-the-art neodymium super- magnets.
Normally, the permanent magnet alternator is not called the DC generator.\\
jb, thanks but my point was that the output of the Bergey WTG is not connected directly to the grid but passes through a two step energy conversion system that regulates the output frequency and voltage. WTG for large projects use induction generators which can, under the proper circumstances, be connected through normal switchgear directly to the existing utility grid. Generally speaking there is lower cost and higher reliabilty when fewer components are used. Each method has its place.
for:
"Design and Simulation of a Stand-alone Wind-Diesel Generator with a Flywheel Energy Storage System to Supply the Required Active and Reactive Power." (about 50kW)
