【正文】 stem. Despite the wind turbines have different initial setpoints, the windmills show a coherent response at the failure event in the external work so that the fluctuations are inphase and at the same frequency. The fluctuation frequency is the torsional mode of the windmill shafts. When the voltage is reestablished, fluctuations in any electrical or mechanical properties are no longer seen. There is no selfexcitation of the wind farm with a large number of wind turbines equipped with induction generators because the induction generators are passive systems in that no synchronizing torque and fast control have been applied. 7 6. Dynamic stability improvements within conventional technology The movement equation of a windmill in terms of the lumpedmass system is )(2)( GM EML HH TTdtd ????, (1a) Where MT and ET are the mechanical torque of the rotating mill and the electric torque, respectively, and L? is the lumpedmass system speed ,GM GGMML HH HH ??? ??? (1b) Where M? and G? are the mill mechanical speed and the electric speed of the generator, respectively, and MMM wPT ?)(? at the given wind, w. The dynamic stability limit of the windmill is found from the movement equations (1a) and (1b) as the speed L? above the kipspeed where EM TT ? . This solution is the critical speed of the windmill, C? , so that exceeding the critical speed, CL ?? ? , leads to protective disconnection of windmills caused by overspeeding (prevention of voltage instability). Theoretical explanation for this definition can be found in Ref. and its graphical illustration is shown in . From the definition of the dynamic stability limit, a number of stability improvement methods can be introduced in terms of conventional windmill technology that are given in the following. . Generator parameters The shape of the electric torque versus speed curve, )( GET ? , is influenced by the windmill induction generator parameters in accordance with )()( )()()( 222GTGTGTGGSGE XR RVT ?? ?? ?? ?? (2)Where SV is the windmill generator terminal voltage as a function of the generator speed, and the machine impedance )()( GTGT jXR ?? ? with 8 is given by the induction generator electrical parameters such as the stator resistance, SR , the stator reactance, SX , the magizing reactance, MX , the rotor resistance, R ,and the rotor reactance, RX , as given in Ref. The shortterm voltage stability will be always improved when the critical speed of the windmill is expanded. This can be reached when: 1. the values of MSS XXR , and RX are reduced, 2. the value of the rotor resistance, R ,is increased. Graphically this is illustrated in case of increasing the rotor resistance value, R ,is increasing the rotor resistance value, R ,see . 9 Increasing the rotor resistance by the factor of 2, as in the example, leads to significant expanding of the critical windmill speed, C? , and the dynamic reactive pensation demands are reduced significantly. When the rotor resistance is .. 0 upRR ? , there will only be necessary to use 25MVAr dynamic reactive pensation The voltage in the wind farm connection point is shown in . The 25MVAr dynamic reactive pensation shall be pared with the reactive pensation demands in case of the rotor resistance value of 0R that are in Section 5 found to be 100MVAr. The dynamic reactive pensation demands are reduced significantly. On the other hand, this solution leads to increasing the power losses in the rotor circuit when the power system is in normal operation as well. Enforcing mechanical construction It is a mon opinion that when the inertia of the rotating system is higher, the more 10 stable operation is expected in the power system in postfault situations. In terms of the dynamic stability limit definition, the inertia value does not influence on the windmill critical speed.