###### Abstract  
The growing significance of wind energy in supplying renewable electricity underlines the increasing importance of wind turbine efficiency. Hybrid towers, integrating steel and pre-stressed concrete in a stacked structure, address traditional limitations in nacelle height but face new vulnerabilities, exemplified by a collapse in September 2021. This highlights the crucial need for continuous monitoring, particularly of the tower structure's tendons. This study introduces acoustic emission monitoring as a novel approach for the early detection of wire breaks within the highly stressed tendons of hybrid towers. The investigations described focus on evaluating the suitability of this method for the specific use case and developing a generalized monitoring approach. Accordingly, background noise in an operating wind turbine tower was recorded and analyzed over a year-long operational period. Correlation analyses of these data unveiled intricate relationships between operational parameters and noise levels, with wind speed, rotor speed, and blade pitch angle exerting influence. Laboratory experiments were conducted on a full-scale specimen, and wire breaks were artificially provoked to characterize the damage signal and assess its attenuation in relevant structural components. The experimental results were integrated into a stochastic model to determine feasible sensor distances, aiming for a 90% probability of detection at a 95% confidence level. Low attenuation along the tendon was identified, enabling reliable detection over significant distances. Nevertheless, practical considerations suggest a focus on tendon anchorages, with the potential for grouped monitoring in specific areas to optimize sensor deployment. The study proposes a sensor network configuration to enhance the safety and reliability of wind turbine structures. a 2025 15 2164 17 February 2025 1 1  
The efficiency and performance of wind turbines depend on various factors, with tower height having a decided influence. Traditional steel construction reached its limits due to the rings' limited maximum diameter, consequently leading to the development of hybrid towers. In this construction method, the lower part of the tower consists of pre-stressed precast concrete elements with highly stressed tendons. Typically, these tendons are internally routed inside the tower and contribute significantly to the stability and load-bearing capacity of the structure. This enables higher tower heights and, hence, greater hub heights. However, on 29 September 2021, a hybrid tower collapsed on a wind farm near Haltern am See [1]. On analysis, the pre-stressed concrete part of the tower was identified as a weakness, resulting in the decision to dismantle similar turbines, thus incurring significant financial losses. Even though the tendons were not directly responsible, such incidents emphasize the need for continuous condition monitoring of wind turbines. While previous research predominantly concentrated on monitoring machine components and rotor blades [2; 3; 4; 5], as these are considered more susceptible to damage than the tower structures and foundations, this case underlines the need to detect damage to the tower structure as well. Damage to the support structure can rapidly lead to complete failure of the turbine. Vibration analyses based on strain gauges and acceleration or inclination sensors have been investigated to monitor the tower structure during operation. These analyses are frequently supported by detailed numerical models or experimental investigations in wind tunnels [6; 7; 8; 9; 10; 11]. However, certain types of damage, such as damage to tendons, cannot be recognized early using these methods. Due to high dynamic loads, individual pre-stressing wires in the tendons and their anchoring can fail while remaining undetected, posing the potential risk of complete loss of the load-bearing capacity of the tower structure.