Browsing by Subject "Hydrostatic Transmission"

Now showing 1 - 3 of 3
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    Analysis, Simulation, and Experiments of Dynamics and Control of a Hydrostatic Wind Turbine
    (2023) Leinberger, Mark
    Kω^2 control, also called torque control, is a popular tool for maximizing wind turbine power in region 2. For hydrostatic wind turbines, the Kω^2 law relates pressure and rotor speed because pressure is proportional to torque. The Kω2 control law becomes pressure control with pc=K'ω2. A new control law, Inverse Kω2 control, is proposed for rotor speed control with ωc=(p/K')1/2. Both pressure- and rotor speed-regulation methods are investigated using P-, PI- and PID-control. This work analyzes the nonlinear dynamic interaction between HST wind turbines and the two Kω2 control methods.Dimensionless, linearized models of these two approaches are used to investigate dynamics and control. Analysis shows that the mechanical rotor dynamics are much slower than the hydraulic transmission dynamics and that frictional and leakage losses have a negligible effect on system dynamics. Root locus analysis shows how systems responses change with variation of PID controller gains. Both control approaches require derivative controller action to sufficiently dampen their responses; both are also fundamentally limited in their speed of response by a slow stable pole regardless of their controller loop gains. Nonlinear system simulation shows that both control approaches track the maximum power point with nearly identical transient behavior and have nearly identical power losses when using suboptimal values of the control law gain K. Experiments using the power regenerative hydrostatic test stand at the University of Minnesota – Twin Cities show that the control approaches have different transient responses but capture comparable power within 2% under steady, turbulent and nonideal conditions.
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    Design and Control of Hydrostatic Continuous Variable Transmission for a Community Wind Turbine
    (2019-10) Mohanty, Biswaranjan
    This research aims to develop an efficient and reliable hydrostatic drive train (HST) for community wind turbines by implementing an advanced controller to maximize energy capture and stabilize the power grid. An HST is a continuously variable transmission (CVT) consisting of a hydraulic pump driving a variable displacement motor. HSTs are simple, light, cost-effective, and offer high power density. An HST drive train was designed for application in community wind turbines. To validate the performance of the HST, a novel power regenerative test platform was successfully designed, constructed and commissioned. It has two hydrostatic closed loops, coupled to each other. The research platform is capable of generating 100 kW output with only 55 kW of electrical input by taking advantage of power regeneration. The performance of each hydraulic component was measured on the test platform. The test platform is a multi-domain system, consisting of electrical, mechanical and hydraulic components. Using a bond graph-based method, a dynamic model of the system was developed. All possible pairings between inputs and outputs were studied in this multi input and multi output system to select the pair with the strongest coupling. A decentralized controller was later designed to control the rotor speed and pressure of the system. The start-up and shut down algorithms developed, enabled smooth operation of the testbed without a cavitation and pressure spikes. The performance of the HST was validated under various wind profile inputs. A pressure control strategy was developed to maximize power capture. To implement the above controller, one only needs measurement of the rotor speed for the reference command and pressure for tracking. The control law automatically drives the turbine to the optimum point, since the optimal parameters of the turbine are included in the control gain. A hardware-in-the-loop simulation was implemented to mimic the wind turbine environment. The turbine rotor dynamics are emulated on the test platform by implementing a decoupling controller. We examined the results of the HST drive train in transient and steady conditions. The efficiency of the HST wind turbine estimated from these experiments was comparable with that of a conventional turbine. Finally, the feasibility of connecting an HST wind turbine to the grid via a synchronous generator is studied. Furthermore, we investigated the electromechanical dynamics of the synchronous generator and performance of the system under large disturbances in incident wind, through detailed time-domain simulations. We found that the generator terminal voltage and frequency comply with the grid regulation band under all operating conditions. This strategy circumvents the need of expensive power electronics. This research have offered a novel insides of the HST for wind turbine applications. The outcomes of the project will stimulate industry to develop more efficient hydraulic components, system and control for wind applications and contribute to our green economy.
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    Methods to Improve the Efficiency of Hydrostatic Transmissions in Wind Turbines
    (2022-12) Escobar Naranjo, Daniel
    Our research group has previously proposed using Hydrostatic Transmissions (HST) for wind turbines. The results have been encouraging, but the system's efficiency has always been a concern compared to a conventional gearbox. This work aims to approach the problem through three different formulations, including blade pitch oscillations, HST wind turbine control using Extremum Seeking Control (ESC), and dynamic temperature control to optimize the efficiency of the HST. The first approach involves oscillating the blades of the turbine to increase the lift coefficient and, in turn, improve power capture. A series of CFD simulations and optimizations were performed on a simplified blade model to evaluate if this is beneficial for power capture in horizontal-axis wind turbines. The results show that the optimal conditions are the same as the static blade conditions. These results happen because the drag coefficient rises exponentially as the lift coefficient rises. Also, there is a power loss due to the power required to oscillate the three blades. The second approach involves using extremum seeking control (ESC) to continuously adapt the torque gain in a modified kω^2 control law. The k gain is a constant value that highly depends on wind turbine parameters, the C_p vs. λ curve, and uncertain wind conditions. The turbine will not operate under optimal conditions if these parameters change over time. Adapting k by using ESC allows for optimal operation under any conditions. For the conditions considered, simulations and experiments showed that ESC improves power capture by 2.8% to 12.3%. The third approach involves controlling the temperature of the hydraulic oil to optimize the viscosity, which improves efficiency. A simplified model based on the friction and leakage losses of a hydraulic pump and a hydraulic motor is used to find the optimum operating point. Two control strategies are evaluated through simulations, classic proportional plus integral control (PI) and Sliding Mode Control (SMC). SMC is chosen due to its quick and robust response and low computational needs. Experimental validations showed that this approach leads to a 0.8% to 0.9% efficiency improvement compared to constant temperature control, although the improvement depends on operating conditions. Overall, these three approaches show potential for improving the efficiency of HSTs in wind turbines, with the second and third approaches showing the most promise. However, further research and experimentation will be needed to fully understand and optimize the use of HSTs in wind turbines.