Control of High-Power Slip Ring Induction Generator Wind Turbines at Variable Wind Speeds in Optimal and Reliable Modes  
[PERSON]  
1 Department of Automation and Applied Informatics, Politehnica University of Timisoara, 300223 Timisoara, Romania 11 Department of Automation and Applied Informatics, Politehnica UniversityWTs capture and convert the wind energy to electric energy that is meant to be delivered to some electrical grid to which the WTs are connected. To this end, the main research topics revolve around modeling [1; 2; 3; 4; 5], control [6; 7; 8; 9] and operation analysis [10; 11; 12; 13]. On the power transformation pipeline, a series of energy transformations occur which must be optimized from an efficiency point of view by the designers/manufacters [10; 11; 12; 13; 14; 15]. In this respect, there are WTs with or without a reducer. In reducer-based ones with ratios beyond 1:100 with the generator having nominal MAS of about 3000 [rpm], power losses occur; however, the generator mass is small [16; 17; 18; 19; 20]. In those without a reducer mechanism, there are no additional power losses, but the generator mass must be carefully designed in order to work at such small MAS, while the materials needed for producing such a device have significantly higher costs (supermagnet alloys from neodymium, iron, boron--Nd2 Fe14B) [21; 22; 23]. Modern WTs are equipped with blade pitch angle control systems; therefore, when the wind speeds are too high, the angle of attack of the blade is adapted to keep the MAS within safe bounds. The blade angle typically varies from 0 to 90 angular degrees, with the higher-end value being applied to stop the WT's rotor during violent windstorms. A blade pitch angle control system bases its feedback on anemometer measurements for wind speed and on the MAS (or the generated electrical power). **Remark 1**. : _Automatic blade pitch angle control also keeps the energy conversion in reliable condition by avoiding mechanical stress in the WT's tower structure, within bearing and within the transmission elements. Hence, it increases the WT lifespan and decreases the maintenance costs. It is due to this that the blades pitch angle \(\beta\) is based on three independent actuation systems powered by alternative energy sources._  
In the specialized literature, only induction generators with wound rotors having a single power converter in the rotor are generally analyzed (see works [10; 11; 12; 13; 17; 18; 19; 20]). In this work, we analyze the induction generator with two power converters in both the stator and in the rotor. In the Oravita (Romania) region, there are six Fuhrhander FL-MD-70 WTs of 1.5 [MW] capacity, each equipped with an induction generator with a wound rotor [24]. In this case, by imposing the rotor voltage \(U_{R}\), the optimal MAS can be tracked. The value \(U_{R}\) is intended to control the additional rotor resistance \(R_{ad}\). The group of six WTs from Oravita work at wind speeds ranging from 3 up to 25 [m/s]\({}_{L}\) then the MAS, depending linearly on the wind speed, should adapt within its own range at a ratio of about \(3/25=1/8.3333\) set by the wind speed variation. At this high wind speed variation, two power converters are needed: one in the stator circuit controllable by the stator frequency \(f\), which delivers the stator power \(P_{stator}\), and a second converter for the rotor, which is responsible for the rotor delivered power \(P_{rotor}\)[25; 26; 27]. **Remark 2**. : _The power converter from the rotor circuit changes the rotor resistance through the rotor voltage in a way that, apparently, an additional \(R_{ad}\) resistance is introduced._  
The wind speed variation is divided into two regions. The first one ranges within [3; 11] [m/s] and corresponds to small to medium wind speeds. The second one is [m/s] and corresponds to high wind speeds. In the first interval, the WT operates in MPP, with optimal MAS at the value \(n_{OPTIM}\), with optimal stator frequency value \(f_{OPTIM}\) and with optimal rotor additional resistance \(R_{ad-OPTIM}\).