Abstract
The Doubly Fed Induction Generator based Wind Turbine (WT-DFIG) suffers from instability while steady-state operation if improperly controlled. This paper proposes and evaluates three Scalar strategies that ensure Maximum Power Point Tracking (MPPT) and stabilize the WT-DFIG.
Both of the generator electromagnetic torque – speed and the wind turbine mechanical torque-speed characteristics are taken into account. The DFIG steady-state stabilized MPPT characteristic is first examined and evaluated in conjunction with the ratio of the stator voltage to rotor voltage magnitude. Then a scalar control strategy based on Rotor Voltage and Frequency Magnitude Control (RVFMC) function as related to the selected stator voltage is proposed. Stabilized MPPT for any specified speed range at different operating conditions are presented. For DFIG steady state instability declaration, its developed torque is defined in forms of two main components, specified as induction torque components and synchronous torque components. The induction torque impacts on stability while MPPT at steady state is presented, and further approved by the WT-DFIG MPPT dynamic model. The three Scalar MPPT control algorithms are constructed based on a constrained Rotor Voltage Angle (RVA). The first one deal with maximizing the synchronous developed power (MSP), the second strategy optimizes the synchronous power to achieve higher stability at higher power capability (OSP) and the third strategy deals with gaining higher power at maximum efficiency (ME). The RVA is the shift angle with the stator voltage which may be considered as a load angle has been defined for each algorithm. Detailed performance characteristics are presented, compared and evaluated to recognize the gained power ratio, stability, and efficiency enhancement. The proposed techniques implementation is explained; the RVC controller design and a simplified block diagram of the prescribed independent voltage and frequency reference generator block diagram are provided.