DFIG-BASED SPLIT-SHAFT WIND ENERGY CONVERSION SYSTEMS

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.