State-feedback and observer-based control design of a hybrid system consisting of a steam power plant in cooperation with a doubly fed wind turbine

Abstract

This article presents a control strategy for a variable speed pitch-controlled wind turbine which supplies an autonomous system in cooperation with a small steam power unit. The wind turbine consists of a doubly fed induction generator and operates in variable speed mode in order to maximize the power absorbed from the wind and minimize the amount of steam consumed by the steam power unit. Many reliable control methods concerning doubly fed induction generator–based wind generation systems connected to the grid can be found in the literature. The proposed control schemes concern both of the generating units of the hybrid system and so there are two main controllers. The objectives of the control design are to deliver the total power from the wind turbine to the isolated load under constant voltage and frequency and suitably adjust the power from the steam unit at the same time. The pitch controller of the wind turbine drives the wind turbine at such value of mechanical velocity that the proper amount of power from the wind is absorbed according to the existing load so that the requirements of the voltage and the frequency are realized. The second controller is responsible for the regulation of the corresponding amount of flow of steam mass into the steam turbine. First, the whole system has been linearized according to the two-machine theory. In this model, there is also the electrical power of the load which is regarded as a disturbance in the linear model. In the first case, the controllers have been designed according to the state-feedback theory without taking the disturbance into consideration for the control design, and simple proportional–integral controllers have been developed. In the second case, an observer that estimates both the states of the disturbance and the system has been designed, in order for a better response of the power frequency to be achieved. The system has been tested with the real nonlinear model using SIMULINK software under two severe inputs: the first is a step (and almost unreal) change of the wind speed which brings about a huge alteration of the wind turbine output under constant load. The second is a step decrease of 40% of the load under constant wind speed. Under both tests, the controllers seem to cooperate very well and the variations of the voltage and the frequency are within acceptable limits.