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Operating Principle
Figure 1 shows the schematic structure of a doubly fed converter. The rotor is excited by two back-to-back connected VSC converters with PWM. This arrangement is referred to as generator side converter and grid side converter respectively. The doubly PWM converters supply excitation current to rotor winding in order to realize the maximum capture of wind energy and the adjustment of static reactive power output. When the turbine operates at a sub-synchronous speed, power is fed into the rotor and the grid side converter runs as a rectifier while the rotor side converter runs as an inverter, providing the excitation current to the turbine. When the turbine operates at super-synchronous speed, stator and rotor can both feed energy to the grid. If the turbine operates in synchronous status, the generator runs as a synchronous motor and the converter system provides DC excitation to the rotor.
The grid side converter and the generator side converter are controlled by two control units. The grid side control unit is used to maintain DC busbar voltage stability and ensure high quality input current waveform and unit power factor. The generator side control unit is used to controls the rotor current torque and the excitation components of a doubly fed motor to adjust its active power and reactive power output, and tracks active power order and reactive power order., So that the doubly fed motor can run on the optimum power curve of wind turbine to realize the maximum capture of wind power.
