<p>In this research, a Split-Shaft Wind Energy Conversion System (SS-WECS) is investigated</p>
<p>to improve the performance and cost of the system and reduce the wind power</p>
<p>uncertainty influences on the power grid. This system utilizes a lightweight Hydraulic Transmission</p>
<p>System (HTS) instead of the traditional gearbox and uses a Doubly-Fed Induction</p>
<p>Generator (DFIG) instead of a synchronous generator. This type of wind turbine provides</p>
<p>several benefits, including decoupling the shaft speed controls at the turbine and the generator.</p>
<p>Hence, maintaining the generator’s frequency and seeking maximum power point</p>
<p>can be accomplished independently. The frequency control relies on the mechanical torque</p>
<p>adjustment on the hydraulic motor that is coupled with the generator. This research provides</p>
<p>modeling of an SS-WECS to show its dependence on mechanical torque and a control</p>
<p>technique to realize the mechanical torque adjustments utilizing a Doubly-Fed Induction</p>
<p>Generator (DFIG). To this end, a vector control technique is employed, and the generator</p>
<p>electrical torque is controlled to adjust the frequency while the wind turbine dynamics</p>
<p>influence the system operation. The results demonstrate that the generator’s frequency is</p>
<p>maintained under any wind speed experienced at the turbine.</p>
<p>Next, to reduce the size of power converters required for controlling DFIG, this research</p>
<p>introduces a control technique that allows achieving MPPT in a narrow window of generator</p>
<p>speed in an SS-WECS. Consequently, the size of the power converters is reduced</p>
<p>significantly. The proposed configuration is investigated by analytical calculations and simulations</p>
<p>to demonstrate the reduced size of the converter and dynamic performance of the</p>
<p>power generation. Furthermore, a new configuration is proposed to eliminate the Grid-</p>
<p>Side Converter (GSC). This configuration employs only a reduced-size Rotor-Side Converter</p>
<p>(RSC) in tandem with a supercapacitor. This is accomplished by employing the hydraulic</p>
<p>transmission system (HTS) as a continuously variable and shaft decoupling transmission</p>
<p>unit. In this configuration, the speed of the DFIG is controlled by the RSC to regulate the</p>
<p>supercapacitor voltage without GSC. The proposed system is investigated and simulated in</p>
<p>MATLAB Simulink at various wind speeds to validate the results.</p>
<p>Next, to reduce the wind power uncertainty, this research introduces an SS-WECS where the system’s inertia is adjusted to store the energy. Accordingly, a flywheel is mechanically</p>
<p>coupled with the rotor of the DFIG. Employing the HTS in such a configuration allows the</p>
<p>turbine controller to track the point of maximum power (MPPT) while the generator controller</p>
<p>can adjust the generator speed. As a result, the flywheel, which is directly connected</p>
<p>to the shaft of the generator, can be charged and discharged by controlling the generator</p>
<p>speed. In this process, the flywheel energy can be used to modify the electric power generation</p>
<p>of the generator on-demand. This improves the quality of injected power to the</p>
<p>grid. Furthermore, the structure of the flywheel energy storage is simplified by removing</p>
<p>its dedicated motor/generator and the power electronics driver. Two separate supervisory</p>
<p>controllers are developed using fuzzy logic regulators to generate a real-time output power</p>
<p>reference. Furthermore, small-signal models are developed to analyze and improve the MPPT</p>
<p>controller. Extensive simulation results demonstrate the feasibility of such a system and its</p>
<p>improved quality of power generation.</p>
<p>Next, an integrated Hybrid Energy Storage System (HESS) is developed to support the</p>
<p>new DFIG excitation system in the SS-WECS. The goal is to improve the power quality</p>
<p>while significantly reducing the generator excitation power rating and component counts.</p>
<p>Therefore, the rotor excitation circuit is modified to add the storage to its DC link directly.</p>
<p>In this configuration, the output power fluctuation is attenuated solely by utilizing the RSC,</p>
<p>making it self-sufficient from the grid connection. The storage characteristics are identified</p>
<p>based on several system design parameters, including the system inertia, inverter capacity,</p>
<p>and energy storage capacity. The obtained power generation characteristics suggest an energy</p>
<p>storage system as a mix of fast-acting types and a high energy capacity with moderate</p>
<p>acting time. Then, a feedback controller is designed to maintain the charge in the storage</p>
<p>within the required limits. Additionally, an adaptive model-predictive controller is developed</p>
<p>to reduce power generation fluctuations. The proposed system is investigated and simulated</p>
<p>in MATLAB Simulink at various wind speeds to validate the results and demonstrate the</p>
<p>system’s dynamic performance. It is shown that the system’s inertia is critical to damping</p>
<p>the high-frequency oscillations of the wind power fluctuations. Then, an optimization approach</p>
<p>using the Response Surface Method (RSM) is conducted to minimize the annualized</p>
<p>cost of the Hybrid Energy Storage System (HESS); consisting of a flywheel, supercapacitor, and battery. The goal is to smooth out the output power fluctuations by the optimal</p>
<p>size of the HESS. Thus, a 1.5 MW hydraulic wind turbine is simulated, and the HESS is</p>
<p>configured and optimized. The direct connection of the flywheel allows reaching a suitable</p>
<p>level of smoothness at a reasonable cost. The proposed configuration is compared with the</p>
<p>conventional storage, and the results demonstrate that the proposed integrated HESS can</p>
<p>decrease the annualized storage cost by 71 %.</p>
<p>Finally, this research investigates the effects of the reduced-size RSC on the Low Voltage</p>
<p>Ride Through (LVRT) capabilities required from all wind turbines. One of the significant</p>
<p>achievements of an SS-WECS is the reduced size excitation circuit. The grid side converter is</p>
<p>eliminated, and the size of the rotor side converter (RSC) can be safely reduced to a fraction</p>
<p>of a full-size excitation. Therefore, this low-power-rated converter operates at low voltage</p>
<p>and handles the regular operation well. However, the fault conditions may expose conditions</p>
<p>on the converter and push it to its limits. Therefore, four different protection circuits are</p>
<p>employed, and their effects are investigated and compared to evaluate their performance.</p>
<p>These four protection circuits include the active crowbar, active crowbar along a resistorinductor</p>
<p>circuit (C-RL), series dynamic resistor (SDR), and new-bridge fault current limiter</p>
<p>(NBFCL). The wind turbine controllers are also adapted to reduce the impact of the fault</p>
<p>on the power electronic converters. One of the effective methods is to store the excess energy</p>
<p>in the generator’s rotor. Finally, the proposed LVRT strategies are simulated in MATLAB</p>
<p>Simulink to validate the results and demonstrate their effectiveness and functionality.</p>
| Identifer | oai:union.ndltd.org:purdue.edu/oai:figshare.com:article/20380038 |
| Date | 27 July 2022 |
| Creators | Rasoul Akbari (13157394) |
| Source Sets | Purdue University |
| Detected Language | English |
| Type | Text, Thesis |
| Rights | CC BY 4.0 |
| Relation | https://figshare.com/articles/thesis/DFIG-BASED_SPLIT-SHAFT_WIND_ENERGY_CONVERSION_SYSTEMS/20380038 |
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