DFIG-Based Split-Shaft Wind Energy Conversion Systems

Abstract

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