The operating speed range is 1% above the synchronous speed. The power electronics is used to realize soft starter. The Type 1 WECS uses SCIG, three-stage gearbox, soft starter for smooth grid connection, capacitor banks for reactive power compensation, and active stall or pitch control for aerodynamic power regulation. The main features of all five types of WECS configurations are summarized in Table 1 with respect to type of wind generator, number of stages in gearbox, aerodynamic power regulation method employed, operating speed range, MPPT operation, contribution of power electronics, converter capacity, requirement for soft starter, reactive power compensation approach, and market penetration. Such constant speed wind operated IGs have been installed in large numbers in recent decades in several countries with good wind regimes, typically with a peak capacity of about 1 MW. A schematic of a typical wind system is shown in Figure 12.44. Therefore utilities insist on good PF (say 0.9) at the point of power evacuation needing the wind developers to restrict the VAR drain from the grid by installing terminal capacitors. As we learnt earlier IG feeds active power ( P) to the grid and draws reactive power ( Q) from the grid, which adversely affects the transmission system with low voltage profiles. Normally the rated IG line voltage (typically 230 V or 415 V) is lower than the grid voltage (typically 11 kV or 33 kV) and a step-up transformer between the IG and the grid is inevitable. Average power may be about 20% of the peak installed power depending on the wind regime, which affects the IG performance. Thus, there is wide variation in wind power during operation due to randomly varying wind speed. There is also a cut-out speed, typically about 25 m/s, beyond which the machine may be disconnected due to mechanical considerations. Above a certain wind speed (about 12 m/s), power may go beyond rated value and hence blade pitch is changed to regulate the power to be nearly constant as shown. Typical variation of power with wind speed is shown in Figure 12.43. For each wind turbine there is a cut-in wind speed, typically 4–5 m/s below which it is infeasible to operate due to low power. Here ρ is the air density and C p is a constant.
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