This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. A Hydraulic Wind Power Transfer System: Operation and Modeling Afshin Izadian, Senior Member, IEEE, Sina Hamzehlouia, Majid Deldar, and Sohel Anwar Abstract—Conventional wind power plants employ a variable speed gearbox to run a generator housed on top of a tower. A new topology can remove some of the weight from the tower and centralize the wind power generation. This new topology uses a hydraulic wind power transfer system to connect several wind turbines to the generation unit. This paper demonstrates a mathe- matical modeling of this wind power transfer technology and its dynamic behavior. The flow response, angular velocity, and pres- sure of the system obtained from the mathematical model are compared with test results to demonstrate the accuracy of the mathematical model. Several speed-step responses of the system obtained from the mathematical model demonstrate a close agree- ment with the results from the prototype of the hydraulic wind power transfer unit. Index Terms—Hydraulic wind power transfer, mathematical modeling, wind turbine. I. INTRODUCTION U TILIZATION of renewable energies as an alternative for fossil fuels is growing considerably due to the ex- haustion of natural hydrocarbons and the related environmental concerns [1], [2]. Potential sources of renewable energy available around the world, if harvested, can meet all power demands and eliminate the negative effects of fossil fuels [3]. Recent advancements in wind turbine manufacturing have reduced production costs of the wind energy harvesting units and have resulted in the expansion of the application of wind power plants by 30% [5], [6]. Consequently, wind turbines can become one of the major power sources contributing to the world’s energy demands [4]. How- ever, the harvesting technology has remained in its traditional topology. Typical horizontal axis wind turbines include a rotor to convert the wind energy into the shaft momentum [7]. This rotor is connected to a drivetrain, a gearbox, and an electric generator, which are integrated in a nacelle located at the top of the tower. These components, specifically the variable speed gearbox, are expensive, bulky, and require regular maintenance, which keeps wind energy production expensive. In addition, since the gear- box and generator are located on the top of the tower, its installation and maintenance are time consuming and expensive. Moreover, although the typical expected lifetime of a utility wind turbine is 20 years, the gearboxes require an overhaul within 5–7 years of operation, and their replacements could cost approximately 10% of the turbine cost [8]. Accumulation of the wind energy from several wind turbines in one central unit at the ground level is an innovative solution to address the above deficiencies. In this novel system, each wind tower harvests wind energy and converts it to a high-pressure fluid. The flows from several wind turbine towers are combined and fed to the central unit. At this unit, the combined fluids are split between a main generator and an auxiliary generator. This technology will eliminate the weight from the tower which reduces the maintenance time and cost. Moreover, instead of having one generator and one variable gearbox for each wind tower, multiple wind turbines are integrated to ultimately reduce the capital costs. A hydraulic transmission system (HTS) is identified as an exceptional means of power transmission in applications with variable input or output velocities such as manufacturing, auto- mation, and heavy-duty vehicles [9]. It offers fast response time, maintains precise velocity under variable input and load condi- tions [10], and is capable of producing high forces at high speeds [11]. Moreover, HTS offers decoupled dynamics, allowing for multiple-input, single-output drivetrain energy transfer config- urations [12]. Earlier research has shown the possibility of using this type of power transfer technology in a wind power plant, even though it is not feasible in its electrical counterpart [20]–[22], [30]. Simulation tools have been developed for hydraulic circuits [14] and used for modeling and control of turbines [15] and hydraulic transmissions [16]. Closed-loop hydraulic transmis- sion lines have similarly been modeled by the use of governing equations [17], [18] and by modeling fluid compressibility [19]. Mathematical models of HTS wind turbine power plants are required to understand the dynamic behavior of the system, to investigate the performance of the plant, and to improve their design and controls. However, no validated mathematical model is available for the hydraulic transmission of wind power. This paper introduces a mathematical model of a hydraulic wind power transmission system and demonstrates the perfor- mance of its operation at different speed ratios. This model was developed based on the models and governing equations of hydraulic circuit components that include wind-driven pumps, generator-coupled hydraulic motors, hydraulic safety compo- nents, and proportional flow control elements. The dynamic operation and step response of the system were modeled and verified with the experimental results gained from a prototype of the wind power plant. Manuscript received February 09, 2013; revised June 24, 2013 and August 22, 2013; accepted November 05, 2013. This work was supported by Grants from IUPUI RSFG Funds, IUPUI Solution Center, and IUPUI FORCES Funds. This research was conducted at the Energy Systems and Power Electronics Laboratory at the Purdue School of Engineering and Technology, IUPUI. The authors are with the Purdue School of Engineering and Technology, Indiana University–Purdue University Indianapolis (IUPUI), Indianapolis, IN 46202 USA (e-mail: aizadian@iupui.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TSTE.2013.2291835 IEEE TRANSACTIONS ON SUSTAINABLE ENERGY 1 1949-3029 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.