The placement of wind turbines in wind farms is economically beneficial to the industry. Consequently, the wake generated by the upstream turbines introduces interactions with downstream turbines that significantly affect power generation. However, it is common practice to control the individual turbines to operate at their optimal settings and neglect the wake effect. Turbines in a wake experience lower wind speeds and an increase in turbulence. This results in lower power generation and increased fatigue loads on the downstream turbine. Wind farm control aims to increase overall power generation and reduce fatigue loads by considering all turbines in the wind farm. Recently, numerous studies have been conducted to control the wake itself. The helix approach is one such strategy and seeks to initiate better mixing of the wake. For this, it uses IPC to create sinusoidal blade bending moments acting on the rotor disc. This in turn creates a helical structure in the velocity of the wind, allowing it to better mix with the surrounding energy-rich wind. As a result, the waked downstream turbine experiences an increase in power due to the reduced velocity deficit in the wake. An extensive load analysis of the helix method has shown that the rotational frequency of this helical wake can be measured in the bending moments of the waked downstream turbine. To further develop the field of dynamic wake mixing, this work investigated the initialization of the helix method in a downstream turbine by amplifying these signals through synchronization with the incoming helical wake. This method was tested in simulation environments with increasing levels of fidelity and number of turbines simulated. The results with low fidelity showed that synchronization could be achieved without loss of performance. In addition, less pitch actuation was required to generate large bending moments due to amplification. The final experiment was conducted for a three-turbine setup and was performed in a high-fidelity model using CFD methods. Here it was found that -90 degrees out-of-phase synchronization with the incoming helical wake resulted in a 9.8% increase in power generation in a turbine further downstream. These results demonstrate the clear potential of wake synchronization in a multi-turbine setting. This study, therefore, contributes to the field of dynamic wake mixing by introducing a novel field of wake synchronization.

To accelerate the growth of offshore wind power, its Levelised Cost Of Energy (LCOE) must be reduced. A means of reducing the LCOE of offshore wind power is by mitigating the adverse wake effect by employing Wind Farm Control (WFC) for power maximization. A recent WFC technology is the helix approach, which uses Individual Pitch Control (IPC) to initiate early wake mixing. Research regarding the helix approach is scarce, yet early results suggest that significant power gains are achievable. Simultaneously, increases in wind turbine loads are expected. To aid future researchers in making a trade-off between increased loading and performance, an extensive load analysis is conducted, aiming to analyze and quantify the load impact of the helix approach on the SG 11.0-200 DD offshore wind turbine. A sensitivity analysis is conducted to investigate the influence of the helix parameters on the load impact on the wind turbine tower, blades, main bearing and pitch bearing. It is found that the load impact on all components is mostly sensitive to the pitch amplitude. Most components show some sensitivity to the Strouhal number as well, except for the main bearing. For the blade pitch bearings, an upward and downward trend can be observed for Counter-Clockwise (CCW) and Clockwise (CW) helix directions, respectively, which are related to the pitch frequency. The helix approach is part of the plans for the Hollandse Kust Noord (HKN) wind farm. For this reason, a lifetime fatigue analysis is conducted to quantify the load impact on a wind turbine at the HKN wind farm over its full lifetime for several helix implementations. The fact that the helix approach is only employed under certain conditions is taken into account. It was found that the load impact on the pitch bearings is significant. The load impacts on the tower and main bearing become considerable for helix implementations with a large pitch amplitude. All other components are mildly affected. Finally, the load impact of the novel concept of Closed-Loop Helix Control (CLHC) is compared to conventional helix. This approach considers asymmetric loading on the rotor (caused by e.g. wind shear and tower shadow) when employing helix. Results found that CLHC reduces blade loads with respect to the helix approach. However, since the blades are mildly affected by the helix approach in general, choosing CLHC over conventional helix is not imperative.

The offshore wind energy sector has experienced remarkable growth in recent years, driven by increasing demand for renewable energy sources and technological advancements. As wind farms expand and turbine sizes increase, challenges associated with wake effects and turbine interactions become more pronounced. Therefore, wind farm control strategies have been developed to mitigate the negative effects of wakes, consequently enhancing overall power production or reducing fatigue loads. The helix approach has emerged as a promising method for controlling wake dynamics, shaping the wake into a helical pattern to increase wake mixing further downstream. In this context, phase synchronisation has emerged as an extension of the helix approach, simultaneously applying the helix on up- and downstream turbines. By leveraging the existing helical periodic components induced by the upstream turbine, phase synchronisation can initiate the helix at the downstream turbine with less pitch action. % while improving overall power performance. However, power is still compromised at the upstream turbine due to the execution of the helix. Consequently, the potential for improvement arises with the integration of phase synchronisation onto the meandering effect—a phenomenon often already present at the downstream turbine. This effect yields benefits similar to those of the helical wake concerning wake recovery, offering the potential to improve overall power performance without power loss at the upstream turbine. % This thesis has developed a phase synchronisation method to track the time-varying excitation frequency inherent to the meandering effect. Enhancements were made to a SOGI-PLL, enabling the extraction of phase, amplitude, and bias from periodic loads, which forms the basis of our closed-loop control strategy for the downstream wind turbine. The improvements led to the development of the ESOGI- and MSTOGI-HTan-QT2PLL methodologies, which address internal disturbances caused by offsets and enhance system stability, while accurately tracking of the phase during phase jumps and frequency steps and ramps. The methodologies were tested at different levels of fidelity. First, a low-order study validated the effectiveness of the proposed method during uncertain conditions, which showed better performance of the MSTOGI-HTan-QT2PLL methodology. Then, the results of the higher-fidelity experiments demonstrated that our method could successfully amplify the meandering effect without loss in power, while attenuation led to reduced fatigue loads with minimal power loss. These results show the potential of integrating phase synchronisation on the meandering effect.