Direct controls of converters for renewable energy sources

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Dr. Veszprémi Károly
Department of Electric Power Engineering

In my thesis I examined the applicability of direct control methods for converters in renewable power plants. I studied the theoretical background for the topics including the available direct control methods and the coordinate systems and conventions to describe electrical machines and drives. I have chosen the classical Direct Torque Control ( DTC ) to be used for the machine-side converter of the wind power generators, and its variant, Direct Power Control ( DPC ) for the grid-side converters of wind power generators and for photovoltaic panels.

I examined the characteristics of the renewable energy sources and build up the models of wind turbines and PV panels. I investigated the specific control techniques for wind turbine and chose Optimal Torque Controller to be used as upper layer controller for the plant due to its superior features and fitting to DTC. I created the models of the three widely used generator types of wind power plants: permanent magnet synchronous machine ( PMSM ), squirrel-cage induction machine and doubly-fed induction machine. Knowing the properties of these different types of machines I adapted DTC for each of them. Putting together the machines, the controllers and the load, I carried out simulations in Matlab Simulink to evaluate their operation and verify the applicability of the controllers. Both simple angular speed control ( for evaluation of dynamics ) and Optimal Torque Control ( used for wind turbines ) was covered.

I also modeled the grid, reduced to the busbar to which the plant is connected. I created the model of DPC based on DTC and combined it with the model of the grid and the converter. A DC voltage controller and ( optionally ) a busbar voltage controller were used as upper level controllers to command DPC. The system was tested with simulation in different modes of operation. The applicability of this system for PV panels was also covered.

Based on the results I concluded that direct control techniques can be successfully applied for renewable power plants as they are a robust, yet simple alternative to Field Oriented Control.


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