A study on the impact of control on PV curve associated with doubly fed induction generators

The Power/Voltage characteristics (PV curve) of a Doubly Fed Induction Generator (DFIG) is not as well understood as those of synchronous generators. This is becoming a serious concern because of the fast growth of large-scale wind generation, which uses the DFIG technology. A PV curve describes the relation between injected active (real) power and voltage at a receiving bus, taking into consideration the dynamics of generator(s). A PV curve is often used for the characterization of the stable operating region for both generator and transmission systems. Although possessing some characteristics of a synchronous generator, by nature a DFIG is an induction machine, which means it has more complicated active and reactive power dynamics due to the coupling between its electrical and mechanical systems, and between the double feedings of its active and reactive powers from both its stator and rotor circuits. This paper presents the results of a study on the impact of rotor voltage control on the PV curve of a DFIG. Voltage dynamics, power angle dynamics, and the dynamics of the electro-mechanical system are modeled as a third-order nonlinear differential equation. A PV curve controller is also proposed, which uses an exact nonlinear cancellation strategy, as opposed to the commonly used “linear approximation” that is less accurate. This controller is used to study the impact of rotor voltage control on a PV curve. It is found that the unstable operating region of the PV curve, which exhibits bifurcation, can be circumvented through appropriate and timely regulation of the rotor voltage of the DFIG. This finding suggests that an advanced control technology not only provides an improved stable operating region for wind generation, it also enables effective real-time control during emergency.