International Journal of Rotating Machinery

Volume 2017 (2017), Article ID 5651736, 9 pages

https://doi.org/10.1155/2017/5651736

## Variable Ratio Hydrostatic Transmission Simulator for Optimal Wind Power Drivetrains

^{1}Purdue University, West Lafayette, IN, USA^{2}Universidad Tecnológica de Querétaro, Santiago de Querétaro, QRO, Mexico^{3}Universidad de los Andes, Bogotá, Colombia

Correspondence should be addressed to Jose M. Garcia-Bravo; ude.eudrup@aicragmj

Received 20 May 2017; Accepted 20 July 2017; Published 28 August 2017

Academic Editor: Lei Chen

Copyright © 2017 Jose M. Garcia-Bravo et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

This work presents a hydromechanical transmission coupled to an electric AC motor and DC generator to simulate a wind power turbine drive train. The goal of this project was to demonstrate and simulate the ability of a hydrostatic variable ratio system to produce constant electric power at varying wind speeds. The experimental results show that the system can maintain a constant voltage when a 40% variation in input speed is produced. An accompanying computer simulation of the system was built and experimentally validated showing a discrete error no larger than 12%. Both the simulation and the experimental results show that the electrical power output can be regulated further if an energy storage device is used to absorb voltage spikes produced by abrupt changes in wind speed or wind direction.

#### 1. Introduction

Wind power generators are a technology that contributes to sustainable energy and low impact to the environment. However, some of the challenges that the current wind power industry face are high installation and maintenance costs and reliability issues [1–4]. To be able to reduce the cost of wind power energy in general, the wind power industry must invest resources in developing wind power technologies that decrease the total cost of the produced energy. This can be done if wind power turbines operate efficiently at broader wind speed ranges, while maintaining optimum electrical generator shaft speeds, so they can generate more electric power regardless of the wind speed. There is a great impact on the turbine reliability when major components of the wind power turbine like the gearbox or the generator fail because this creates an extended mean time to repair (MTTR) [4–6]. Therefore, reducing failure and maintenance of these major components reduces the cost of operation of the wind power turbine, which in turn reduces the cost of energy itself.

The power coefficient in (1) is a parameter used for measuring the efficiency of the wind energy captured by the turbine; this coefficient represents the ratio of the mechanical power output of the turbine to the power input from the wind.

is the aerodynamic efficiency of the turbine and and are the electric generator efficiency and the gearbox efficiency, respectively [7]. The maximum theoretical achievable is 59%; this condition, which is derived from fluid mechanics principles is known as the Betz limit [7]. The direct consequence of this limit is that no more than 59% of the available power in the wind can be captured.

The tip speed ratio (see (2)) is parameter used to define the ratio between the tangential velocity of the blade and the wind speed. The family of plots shown in Figure 1 illustrates the effect of the tip speed ratio on the power coefficient for various pitch angles of the same blade. where is the rotational speed of the blades,* R* is the radius of the wind turbine, and* V* is the wind speed. As the wind speed varies, the pitch angle of the blade needs to be adjusted to operate at the optimal . Inevitably, when the wind speed varies, the shaft speed of the generator also changes because the rotor’s shaft is directly (direct drive) or indirectly (gearbox) coupled to the generator’s shaft. That means that the electric generator will produce electric power at variable frequencies that need to be conditioned using power electronics to match the constant frequency standard of the electric grid. The challenge of using this approach is the increased complexity, cost, weight, and size of the wind turbine components and it reduces its efficiency and reliability. Additionally, most common current electric generators operate more efficiently at their constant rated shaft speed, which is normally a single value between 1800 and 2200 RPM. This is particularly true, for field induced generators, but it also affects permanent magnet motors because the amount of power produced is proportional to the input shaft speed.