31.4 : Wind Turbine Machine Models

In the growing field of wind energy, incorporating wind turbine models into transient stability analysis is essential. Induction and synchronous machines are the primary models used, with induction machines being prevalent due to their simplicity and reliability.

Induction machines interact through the rotating magnetic field generated by the stator and the rotor. The key parameter is slip, which is the difference between synchronous speed and rotor speed relative to synchronous speed. Slip is zero at synchronous speed, positive when motoring, and negative when generating. The mechanical dynamics involve the inertia constant (H) and the torque difference (T_m-T_e).

A simplified electrical model for a single-cage induction machine represents the equivalent voltage behind stator resistance and transient reactance. The key parameters include the open-circuit time constant for the rotor and the synchronous reactance derived from leakage reactance and magnetizing reactance.

The electrical torque and terminal real power injection are determined by the machine's internal voltages and currents. Induction machines typically consume reactive power, indicated by a negative value.

Wind Turbine Models are of four different types:

Type 1 and Type 2: These models use induction generators. Type 1 has a fixed rotor resistance, while Type 2 uses variable rotor resistance for better control, affecting the machine's time constant and power output.

Type 3 and Type 4: These advanced models (Doubly-Fed Asynchronous Generators and Full Converter Systems) allow for control of both real and reactive power. Type 3 uses converters for rotor current control, providing a wide speed range. Type 4 decouples the generator from the grid, offering flexible control and eliminating mechanical coupling with turbine dynamics.

Understanding wind turbine machine models involves analyzing the interaction of electrical and mechanical components for accurate stability analysis.