Nomenclature



97.95 s, in order to better see the stator voltage waveform during the fault. As demonstrated in Section 5, it is not possible to maintain both torque and reactive power constant during unbalanced conditions; therefore, the stator reactive power is affected (see Figure 7 VI) since the controller forces the electromagnetic torque to be constant. As a result, the controller injects a negative sequence current to the rotor in order to cancel torque oscillations out (see Figure 7 VII), it is worth mention that other control strategies as vector control requires a double control loop for controlling positive and negative sequence separated [13], which not only complicates the current regulation algorithm but also requires sequence decoupling of rotor current, a very

Figure 7. Simulation results: I stator voltage, II wind speed profile, III extracted aerodynamic wind power, IV rotor speed,

V electromagnetic torque, VI stator active and reactive power, VII rotor current, VIII rotor filtered voltage.

Finally, the filtered rotor voltage is displayed in Figure 7 VIII. The controller does not require modulation and automatically injects negative sequence voltage to regulate torque and reac-

A stator-flux–oriented sliding mode control, which regulates torque and reactive power in DFIG, is presented. The controller is not dependent on electric machine parameters and do not require modulation, injecting the desired voltage vector directly to a two-level power converter. Despite the proposed SMC controller has a variable switching frequency, which is not desired in practical applications, the switching frequency is limited by a hysteresis loop in the torque and reactive power controllers. The hysteresis value is calculated by means of the frequency domain characterization of the DFIG system with SMC, via Tsypkin's method.

complex issue that is avoided by using a robust control strategy.

tive power under unbalanced conditions.

7. Conclusions

70 Adaptive Robust Control Systems

Lr Rotor inductance [H]


VDC DC-link voltage [V]


ω<sup>m</sup> Rotor speed [rad/s]

