5. Switching Control of Doubly-Fed Induction Generator-Based Wind Turbines for Transient Stability Enhancement of Wind Power Penetrated Power Systems

This chapter firstly proposes a four-loop switching controller (SC) for the DFIG to improve the transient stability of wind power penetrated power systems (WPPSs). The short-term resilience index is employed to reflect the dynamics of both system frequency and load bus voltage. A four-loop SC is designed to control the four outputs of a DFIG, i.e., the rotor speed deviation, the reactive power output of stator winding, the reactive power transferred through grid-side converter and the DC-link voltage. The SC switches between a LBCFC and a VC based continuous controller (CC) in each control loop. The LBCFC is robust to system nonlinearities, uncertainties and external disturbances. Simulation studies were carried out in a modified IEEE 16-generator 68-bus power system, where four DFIG-based wind farms were connected to provide 9.94% power supply. In the second place, this chapter proposes a switching angle controller (SAC) and an automatic generation controller (AGC) for the DFIG to control the frequency of DFIG-based WPPSs. The virtual rotor angle of the DFIG is defined, which is controlled by the SAC in a bang-bang manner such that the DFIG is controlled to offer frequency support to the external power grid. The SAC is also used to regulate the pitch angle to provide a short-term support of the mechanical power input. The long-term control of the mechanical power is realized by the AGC. Small-signal analysis was carried out to evaluate the modal performance of the SAC and the AGC. Simulation studies were undertaken in a two-machine power system and a modified Kundur four-machine two-area power system, respectively.