-Most wind turbines are equipped with line-connected induction generators. Induction generators are very attractive as wind turbine generators due to their low cost, ruggedness, and the need for little or no maintenance. At constant frequency, the induction generator operates in a small range of speeds and, therefore, it operates with a small range of slips with respect to synchronous speed. Compared to a synchronous generator, an induction generator provides lower stiffness, thus alleviating the mechanical stress.In a weak power system network, an unbalanced load at the distribution lines can cause unbalanced voltage conditions. If an induction generator is connected to an unbalanced voltage, the resulting stator current will be unbalanced. The unbalanced current creates unequal heating (hot spots) on the stator winding. The heat may increase the winding temperature, which degrades the insulation of the winding, i.e., the life expectancy of the winding. Unbalanced currents also create torque pulsation on the shaft resulting in audible noise and extra mechanical stress. This paper explores the unbalanced voltage problem in induction generator. The levels of unbalance and the loads are varied. Experimental and predicted results are presented in this paper.
INDEX TERMSUnbalanced-voltages, induction generators, wind energy.
1331per unit length and D the diameter of the conductor and finally f the frequency of vibration.Using this formula (with a friction coefficient of = 0:2), a gross-slip amplitude of usip = 2:7 mm has been calculated for f = 34 Hz, which seems to coincide well with a pronounced change in the slope of the curve shown in Fig. 7 of the paper. 1 Can the authors report, whether they have detected a similar effect for the other frequencies and/or conductors where they have carried out self-damping measurements? G. Diana et al.We thank Chuck B. Rawlins and Konstantin O. Papailiou for their kind discussions.Considering Dr. Rawlins interesting remarks, we consider separately the three points in which the discussion can divided. The first point is about the need for measuring two points close to the nodes or elsewhere: in our experience if the cable self damping alone is considered, we can say that the different '(x)=u functions (Figs. 2 and 3 of the paper 1 ) depending on the non dimensional damping h, are well separated only close to the nodes (in the considered case a few centimeters). Far from the nodes the curves tend to collapse independent from the cable self damping and they cannot give any reliable information to identify the cable self damping. We agree that when a more important power flows happen, as when dampers are on the cable, the different curves keep rather well separated also not so close to the nodes.Concerning the second point, we did not check the mentioned variation of the testing procedure based on the vector product of the conductor slope and the transverse vibration velocity, coming out from the closely spaced vibration sensors: our aim was just to evaluate the node amplitude of vibration, which, in our opinion, gives raise to the most robust identification procedure.The third point is about a different representation of the self damping, as the fraction " of the maximum bending energy stored in the cable that is lost to self damping each cycle. We made use of h because it is a universal well known parameter, giving the energy losses in mechanical systems, such as cables.Manuscript received 1999.
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