State Space Formulation and Stability Analysis of a Doubly-fed Induction Machine with a Flywheel Energy Storage System

The multi-functional flexible power conditioner (FPC) is a novel FACTS device, which consists of a large-mass varying-speed energy storage flywheel and a double-fed induction machine (DFIM). The FPC can be used to compensate the dynamic unbalanced power of the power system following disturbances using the energy stored in the flywheel by controlling the rotor speed. With an appropriate control strategy, the FPC is able to realize an independent active and reactive power exchanging with the connected power system. Similar to the superconductive magnetic energy storage (SMES), the FPC can be used to improve the stability of power system and the quality of the power supply. As it is difficult for the conventional stator flux oriented control (SFOC) to give satisfactory robustness of the FPC against the grid voltage fluctuation in this particular application, an improved SFOC is developed. A state space displacement model in terms of d-q components is deduced from the linearized equations, which is used to describe the small signal dynamic performance of the doubly fed induction machine. The system eigenvalue analysis result shows that the dynamic performance and the stability operation regions of the FPC are influenced mainly by the rotor speed, the grid voltage fluctuation and the rotor excitation control. The slip stability rather than the angular stability seems to dominant the stability characteristics of the FPC. A small slip absolute value will give a more stable FPC. Investigation results also show that the dynamics of the stator currents on the performance of the FPC is neglectable and therefore a third order reduced model is enough to represent FPC for approximation. This simplifies the design of the excitation control. Further more, it has been verified by simulation that the improved SFOC provides better system performance than the conventional SFOC during the grid voltage fluctuations.