Decentralized multivariable modeling and control of wind turbine with hydrostatic drive-train

Majid Deldar, Purdue University

Abstract

In this research, two types of hydrostatic transmission systems (HTS) were analyzed comprehensively, namely valve-controlled and displacement-controlled systems. The valve-controlled HTS utilized a pump driven by the wind turbine rotor, a proportional flow control valve, and two hydraulic motors. A 5th order nonlinear model of the system was developed, simulated using MATLAB/SIMULINK platform, and then validated on a test-bench. The validation results showed that the model accurately predicted the system performance. Then, the validated model was used to evaluate the performance of such a system with desired operation of a wind power plant. It was found that the valve-controlled system was not suitable for this application due to consistent power loss and operational conflicts between the HTS and desired outcome of a wind power plant. Next, displacement-control configurations were analyzed. One configuration comprised of a variable displacement pump with fixed displacement motor (variable-pump/ fixed-motor) and another configuration comprised of a fixed displacement pump with a variable displacement motor (xed-pump/variable-motor). Considering wind turbine speed/power characteristics and the hydraulic circuitry, it was assessed that the variable-pump/fixed-motor configuration yielded lower eciency at low wind speeds and required a larger pump unnecessarily. The fixed-pump/variable motor HTS showed potentials for wind power harvesting; therefore it was selected for further analysis in this research. The power and speed characteristics of the rotor as the prime mover of the pump and the need for efficient operation of the pump and the xiv motor required a unique system design that is very different than that of conventional hydrostatic systems. A component sizing methodology was proposed to improve annual energy production of a HTS. It was shown that with optimal operation planning, a wind turbine with hydrostatic transmission system (HTSWT) can generate equal or more energy than that of a geared WT. To further improve the modeling accuracy, a nonlinear multivariable model of the fixed-pump/variable motor HTS was developed. Two main control objectives were defined for the system, namely: maximum power point tracking (MPPT) of the instantaneous wind and stabilizing the generator speed tightly as mandated by the stringent grid connectivity codes. To design a robust control configuration in order to achieve these objectives, a relative gain array (RGA) analysis was performed. The RGA suggested a novel input-output pairing for the system. Considering results of the RGA analysis, a decentralized multivariable control system was designed. For wind MPPT, an innovative control law was derived independent of direct wind speed measurement. For generator speed control, a feedback linearization technique was implemented to maintain the generator speed precisely at the grid frequency. Operation of a community scale hypothetical HTSWT (600 kW) with the designed control system was simulated with MATLAB/SIMULINK software. It was shown the at any wind speeds and under very large wind perturbations, the maximum power is harvested from wind using the proposed decentralized control system. Additionally, the generator was driven with zero steady state error and less than 0.15 Hz frequency deviation during wind transients. Robustness of the control system was examined under wide range of plant uncertainties. Even for a turbine with unknown rotor specifications, the designed control law was able to track near-maximum wind power. To ensure that the maximum power is delivered by a HTSWT, a constrained optimal control formulation was investigated. A real-time optimal feedback control adapted from Pontryagin minimum principle was designed for the HTSWT system in order to maximize energy output for any given operating condition. The optimal control law minimized the overall loss at any operating points. Finally, a conceptual xv HTS design featuring variable-pump/variable-motor was studied. It was shown that the additional control input, pump displacement, enables the HTSWT to eliminate the trade-off between aerodynamic efficiency and transmission eciency at low wind speeds. Consequently, the output power from a variable-pump/variable-motor HTS is shown to exceed that from a fixed-pump/variable motor HTS. This output power improvement was significant at low wind speeds.

Degree

Ph.D.

Advisors

Chiu, Purdue University.

Subject Area

Mechanical engineering

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