An Introduction to Wind Emulators

In recent years, wind power generation is one of the most promising technologies that enable electricity generation from renewable energy sources. The study of grid-connected wind systems is extremely imperative as wind energy promises free fuel for the lifetime of the project and beyond. To study the steady-state and dynamic behavior of the wind energy conversion system (WECS) without relying on natural wind resources and actual wind turbines, a system that represents the wind and wind turbine is essential [1].

Wind energy is one of the fast-growing technologies in the United States and the rest of the world. Based on the US DOE study concerning Wind Vision, wind energy is estimated to contribute towards 35% of the total electricity generation. Over the past years, significant research has been conducted on WTE by operating it using different motors and under varied wind speed characteristics. Several of these implementations were accomplished using pure modeling and simulation or real-time simulation using software tools like MATLAB/Simulink, PSCAD, or PSIM. DC motors are the most commonly employed hardware; however, permanent magnet synchronous motors (PMSMs) and induction motors are also used. The widely employed conversion technologies with DC motor drives are AC/DC thyristor-based control and DC/DC chopper.

A Wind Turbine Emulator (WTE) is generally designed and developed in the laboratory. It is an artificial, controllable, and flexible test bench that mimics the behavior of a real WECS. The main elements of a WECS are as shown in Figure 1. This serves as a platform for researchers, designers, and engineers to carry out research, observe and analyze the working of WECS. The emulation includes replication of system operation and control. This enables analyzing the system in a controlled environment similar to the real-world setup without relying on physical equipment and the actual renewable energy source [2]. The turbine characteristics are replicated so that the setup can be run at different wind speeds and pitch angles.

 

Figure 1: Main elements of a Wind Energy Conversion System [3]

 

Design and Working Sample of a Wind Emulator

The wind turbine can be controlled using speed or power control concepts. Variable speed control with a fast-pitch mechanism is commonly employed due to power limitation considerations. However, the drawback of using fixed-speed power control techniques is that the wind fluctuations that are converted into mechanical fluctuations, in turn, lead to electrical fluctuations [4].

The WTE can be built such that it comprises a DC motor coupled with a PMSM generator [5]. Control can be achieved using a traditional PI controller using PSIM software. Prebuilt blocks for motors and generators can be used for this purpose. The Particle Swarm Optimization (PSO) technique can be used to calculate the characteristics of the output power. It is noticed that several iterations are required to reach the optimized value, which leads to a larger settling time. Further, when each PSO code is run, several solutions are produced, which are then evaluated in the Simulink model to find the best estimate. This is a reliable design but might not be a feasible option for large wind turbines as it is computationally intensive.

Another setup option is the use of a separately excited DC motor model to emulate the characteristics of the wind turbine [6]. The variation of rotational speed with the wind speed can be observed with oscillations due to the inertia of the rotating disc and the use of a PI controller. This setup is decent for small wind turbines but not recommended for large ones as oscillations are not desirable in the WTE.

Wind energy is typically intermittent as it has random variation and is uncontrollable. When wind energy is served as an input to the WECS, it yields mechanical power delivered by the wind turbine. This enables the coupled generator to deliver variable power. The performance of a WECS needs to be analyzed in detail before installing it on-site. Thus, the design and implementation of a Wind Turbine Emulator (WTE) is a crucial step in installing a wind energy system.

 

Key references:

1. Anushree et. al., An Extremely Low-Cost Wind Emulator, 2018.
2. Bhayo et. al., Design, and Development of a Wind Turbine Emulator for Analyzing the Performance of Stand-alone Wind Energy Conversion System, 2017.
3. Shirazul, Application of Matrix Converter to Enhance the Performance of Wind Turbine Driven Asynchronous Generator: A Review, 2012.
4. Thomas Ackermann et. al., Wind Power in Power Systems, Wiley Publications, 2012.
5. Deab et. al., Optimizing control of Wind Energy Conversion System using Particle Swarm Optimization, 2013.
6. Arifujjaman et. al., Emulation of a small wind turbine system with a separately-excited dc machine, 2008.

 

Featured image used courtesy of Wind Systems