Emulation of Wind Turbine System Using Vector Controlled Induction Motor Drive

Depleting fossil fuels and concerns about global warming have forced a paradigm shift toward renewable energy resources in meeting our energy requirements. Wind energy utilization has been growing at a rapid rate fuelling research and development in high power wind turbines (WTs). To advance research and education in wind energy conversion systems, a controlled test bed is necessary that does not depend on wind availability. Majority of the existing WT emulation schemes employ a torque compensation loop to account for the inertia difference between the WT and motor used for emulation. However, the loop suffers from noise and instability issues. Also, the torque compensation term is sensitive to parameter estimates. This article proposes one degree of freedom (1-DOF) and 2-DOF control structures for WT emulation. It first illustrates the design and performance of 1-DOF control structure. It further proposes two controllers along with their prefilters in 2-DOF structure, which has superior performance than 1-DOF structure. The design of the controllers and prefilters is explained with the help of root locus. The performance of the WT emulator with the proposed control structures is demonstrated through simulations and experiments on a 7.5 kW doubly fed induction generator driven by a 5.5 kW squirrel cage induction motor, where the dynamics and steady-state operation of a WT–generator system are emulated.

I. Introduction

The installed wind power capacity has been increasing [1]–[3]. In 2015, the installed capacity was predicted to reach 765 GW by 2020 [1]; this has already reached 597 GW by 2019 [4]. Major factors for this impressive growth rate are advancements in offshore wind technology, reduction in cost of system components, and government incentive programs [1]. Compared to a set of small wind turbines (WTs), a single large WT of the same cumulative capacity reduces installation and maintenance costs [1], [5]. Hence, ratings of individual WTs have been increasing over the years; 12 MW WT with a diameter of 105 m is among the largest currently [6]; 20 MW turbines with diameters exceeding 150 m are being predicted in near future [1]. Generation of electric power from wind in an efficient and reliable manner and in compliance with modern grid codes demands research and development of advanced control strategies. Wind energy industry is also providing massive job opportunities, leading to significant economic development [1]. It is, therefore, imperative that the operation and control of wind energy conversion systems be taught to engineers to develop skilled man power for the field. Hence, to facilitate research and education, it is essential to develop controlled test beds that can emulate large-inertia WT using small-inertia electric machines, without relying on the availabilty of wind.