Mathematical Modeling and Control of Standalone DFIG-DC System in Rotor Flux Reference Frame

The mathematical modeling of doubly fed induction machine (DFIG)-dc system, including dominant harmonic (fifth and seventh), along with its control in rotor flux reference frame is presented in this paper. Modeling of the system provides the feedforward terms that consist of the fundamental and oscillatory components of the current. The use of nonderivative feedforward terms with proportional integral can reduce the steady-state current ripple, but for controlling the transient current ripple use of resonant controller is suggested such that the use of derivative feedforward terms can be avoided. The resonant controller is capable of controlling the current ripple by itself, but its use with suitable feedforward terms can enhance the performance of the system. DC voltage regulation of standalone DFIG-dc system is obtained by four combinations of above mentioned controllers and feedforward terms. A comparison is made on performance of these control structures. Feedforward terms used in the control structure require decoupled currents. Therefore, decoupling of various currents in different d–q axes are obtained with mathematical understanding. The proposed control schemes are verified on 5.5-KW slip ring induction machine for standalone application.