Detailed Model of the Grid-Connected Cascaded Doubly Fed Induction Machine

The cascaded (brushless) doubly fed induction machine (CDFIM) is a promising substitute for the commonly used doubly fed induction machine (DFIM) for wind power application. The CDFIM offers reliable performance and low maintenance due to the absence of slip rings and graphite brushes. In this study, a detailed analytical model for the CDFIM is proposed in order to clearly show the controllable input (i.e., current from the inverter side) and output (i.e., total output torque) relationship. The proposed model is expressed in the frequency (Laplace) domain. Field-oriented (i.e., vector) control approach is adopted in order to achieve a precise dynamic model for the grid-connected CDFIM. The steady-state and dynamic behaviors of derived torque terms are presented and explained individually. Dynamic response of the total output torque is calculated both analytically and numerically in a simulation, and the results are shown to be consistent. Next, the torque-sharing and power-sharing concepts of the CDFIM are presented to further study the system behavior under different rotor velocities. Finally, to verify the proposed model, an experimental setup using a cascaded DFIM was used, and the results are reported.