Optimal Control with Stochastic Principles for Voltage Decoupling Control of a Wind Turbine

Abstract

This paper presents new control strategies for a variable speed wind turbine (WT) which consists of a doubly fed asynchronous generator. In this kind of generators, the rotor windings are supplied with an appropriate voltage vr through a dc link. There are 2 converters: the rotor-side and the grid-side converter in the dc link. The right pulsation of the rotor-side converter defines the rotor voltage vr in such a way that both real power maximization from the wind speed and nominal voltage are secured at the same time. With all the flux dynamics inside the machine having been included in the generator model, there is a 2-input-2-output system. Based on the dq electric machines analysis theory, the proposed control scheme manages to create 2 single-input-single-output systems with the rotor voltage component in each axis responsible only for real power or voltage nominal value respectively. This is achieved through optimal control theory with the right definition of the cost function. A new control scheme that contains stochastic control principles is proposed for the grid-side controller in which the decoupling of the loops is achieved by means of the grid frequency being regarded as a ‘random’ variable to the voltage control loop and so it has no impact on it. The nonlinear system has been tested in the SIMULINK software under 3 tests: In the first test the wind speed changes and brings about an analogous alteration of the WT output. In the second test there is a short circuit in the grid. According to the new rules, the WT is not disconnected from the grid during the short circuit and continues to produce energy with a very smooth behavior. In the third test, there is a large load disturbance in the grid. Again the WT manages to supply the grid with electrical energy in a nice way. Under all tests the controllers seem to cooperate very well and the variations of the voltage and the frequency are within acceptable limits.