A direct driven wind turbine has the following details: A 12.6 kV. 2.5 MVA, 60-p

Question

A direct driven wind turbine has the following details: A 12.6 kV. 2.5 MVA, 60-pole. 50 Hz, permanent magnet synchronous generator. The mechanical and electrical efficiencies are 87 % and 92 % respectively. Rotor swept area is 2000 m^2 with a rotor efficiency of 42%. Full size WT side AC/DC uncontrolled full bridge converter. Grid side 11 kV, 2.5 kVA, three phase DC/AC Inverter. The Inverter is controlled to achieve power factor of 0.95 lagging at the point of common coupling. The inverter is coupled to the grid via 3 mil per phase inductance. At 14 m/s wind speed and 106 rpm rotor speed, determine the following: (assume rho _air = 1.225 kg/m^3) a) The AC output voltage of the synchronous machine. b) The DC link voltage. c) The magnitude and the phase shift of the ac phase voltage at the inverter terminals. d) The inverter modulation index of the inverter.

Explanation / Answer

Only the quantities which significantly affect the generator performance are included in the specification. A complete specification for the final design of a generator will include many more detailed requirements. The rated power used here is the mechanical power from the turbine, not the electrical power to the grid. The reason for using mechanical power is that generators designed for the same turbine should be compared with each other, not with generators of the same output power at rated load. The difference in generator efficiency is included as a cost of the losses. The size of a generator depends to a very large extent on the required rated torque. Consequently, the rated torque is one of the most important parts of the specification. The rated torque differs for different wind turbines of the same rated power, because of different turbine speeds. Data from 25 wind energy converters from Bindner et al. (1995) and Anon. (1994b) were used to find the typical values of the rated torque for different sizes of wind energy converters. A curve fit to the data was made to find an analytical expression of the rated torque. The rated torque can be approximated as

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