Background and Objectives: Cascaded doubly fed induction generators (CDFIGs) can directly connected to isolated load or power grid without any brushes which are needed in conventional DFIGs. Output control targets before grid connection of CDFIGs are voltage and frequency control and after that are active and reactive power control. In control aspect, output control of CDFIG is a multi-input multi-output (MIMO) subject. In this paper, Relative Gain Array (RGA) methodology, as a MIMO interaction index, is used to show the degree of relevance between the control inputs and output targets, in both voltage control mode (before grid connection) and active-reactive power control mode (after grid connection). Based on RGA results, conventional PI controllers cannot be used to decouple control of generator outputs in grid connected mode. So, a powerful method based on sliding mode approach is proposed to generate the proper control voltages for output control of CDFIG in both islanded and grid connected mode. Simulation and experimental results using Matlab and TMS320F28335 based prototype of CDFIG are provided to demonstrate the effectiveness and robustness of the proposed method.
Methods: A mathematical method based on RGA matrix is used to evaluate the amount of interactions between output targets and input control variables in CDFIGs in islanded and grid connected mode.
Results: Conventional PI controller is a proper method to control the output voltage of Power Machine (PM) in CDFIG but is not a suitable technique for active and reactive power control in grid-tied mode.
Conclusion: Sliding mode control can be used to decouple control of CDFIGs in both before and after grid connection. As well as, robustness against the wind speed variation and parameters uncertainties is proved via both simulation and experimental tests.