Efficient gearbox monitoring is vital for improving fault detection, enhancing design, and reducing operation and maintenance (O&M) costs, particularly for offshore wind turbines. This paper introduces an innovative approach using high-resolution distributed fiber-optic sensing (DFOS) based on optical frequency domain reflectometry (OFDR) to measure gearbox strain in real time. By bonding a single optical fiber around the full circumference of the outer surface of a 2.152 m diameter ring gear of the first planetary stage in a 3.75 MW wind turbine gearbox, we measured circumferential strain from planetary gear passage every 2.6 mm around the ring gear under different input torque levels. Our results show accurate identification of planet gear locations in real time and rotation speed (10.42 revolutions per minute), with a strong linear correlation (R²= 0.9997) between applied torque and measured strain across all 2500 measured locations. Strain variations of approximately 200 microstrains were observed on the ring gear exterior from each (planet passage or planet-ring gear mesh) event, providing granular insights into mechanical behavior and load distribution. Additionally, DFOS also showed its potential for detecting temperature variations during operation, indicating its potential for concurrently monitoring thermal and mechanical anomalies. This study represents the first application of continuous DFOS to a full-scale wind turbine gearbox. The approach offers a scalable and practical solution for early fault detection and the support of design validation and, when combined with modeling, can lead to a more durable and optimized design.

Accurate knowledge of the mechanical loads of wind turbine gearboxes has become essential in modern, highly loaded gearbox designs, as maintaining or even improving gearbox reliability with increasing torque density demands is proving to be challenging. Unfortunately, the traditional method of measuring dynamic mechanical torque using strain gauges placed on the outer surface of a rotating shaft and transmitting the resulting signal is unsuitable for serial deployment due to technical and economic constraints. An alternative method based on fiber-optic strain sensors placed on the stationary outer surface of the gearbox ring gear has been proposed. Like shaft torsion, the radial deformation of the ring gear is proportionate to the rotor torque. Placing the sensors on a stationary component is a cost-effective alternative for serial implementation because the need for complex and expensive data transfer via wireless transmission or a slip ring is eliminated. In this paper, we present the results of an extensive field experiment conducted to evaluate the torque measurement accuracy of this novel sensing solution installed on the gearbox of a Gamesa G97 2-MW wind turbine at the National Renewable Energy Laboratory’s Flatirons Campus. Torque measurements derived from fiber-optic strain sensors placed on the ring gear of the planetary stage are compared to conventional torque measurements from strain gauges placed on the main shaft. Two different torque estimation data processing methods were evaluated, with the method based on operational deflection shapes providing the most accurate results with an average normalized root mean square error below 0.7% for a load revolution distribution analysis. The effect of operating conditions on the torque estimate was also investigated, and the third planet-passing operational deflection shape was found to be the least sensitive to nontorque load-related effects. The fiber-optic strain sensors’ successful operation during the complete test campaign has demonstrated a robust and accurate solution for fleet-wide enhanced gearbox remaining useful life estimation.

The mesh load factor, K 3, describes how loads are shared between planet gears and has become one of the key design challenges in modern wind turbine gearboxes. Planet load sharing directly impacts tooth root stresses, a critical driver of torque density and gearbox reliability. Experimental evaluation of K 3 is typically performed from sun gear tooth root strain gauge measurements, which are complex. Furthermore, such measurements can only provide an average value of load sharing. The present study describes an alternative method to evaluate the mesh load factor in wind turbine gearboxes based on fiber-optic strain sensors installed on the outer surface of the fixed ring gear. We present the results of an extensive measurement campaign to evaluate this novel sensing solution installed on the input planetary stage of a 2-MW wind turbine gearbox at the National Renewable Energy Laboratory's Flatirons Campus (Colorado, USA). The number of strain sensors on the ring gear was selected as an integer multiple of the number of planets, which has enabled an instantaneous evaluation of the mesh load factor. The effect of operating conditions on the planet load-sharing behavior of the gearbox has been investigated. The mesh load factor measured for operating conditions close to rated was below 1.05, well below IEC 61400-4 standard requirements.

The significant increase in rotor diameters seen in modern wind turbines has pushed gearbox manufacturers to introduce technological innovations to increase the torque density of current designs. Driven by the need to lower the cost of energy from wind and size limitations imposed by logistic constraints in onshore wind, a trend has emerged to increase the number of planetary stages and the number of planet gears per stage. One of the main challenges of next-generation gearbox designs is sharing the load evenly between a high number of planets. This paper presents an experimental evaluation of the mesh load factor of a modern 6MW wind turbine gearbox with five planets in the first planetary stage. Results from the traditional method, based on tooth root strain gauges, and from strain measurements in the outer surface of the ring gear are described and assessed. Both experimental approaches have yielded lower mesh load factor values than the default values required in the standard "Design requirements for wind turbine gearboxes"IEC 61400-4. Since the mesh load factor is used for gear rating and sizing, a lower value allows a more optimized gearbox design, which leads to a significant improvement in torque density and cost.

Wind turbine technology has seen remarkable advancements in the last decades. Most notably, the rated power and size of wind turbines have grown considerably to reduce the cost of energy from wind. The increase in rotor diameters has pushed gearbox manufacturers to introduce multiple technological innovations to boost the torque density of current designs. One of the critical challenges of next-generation gearbox designs is to optimize structural components and gears. Complex models are needed to predict the gearbox components' load-carrying capacity and fatigue life. These tools need to be demonstrated and validated through experimental evaluation. Through physical testing, this study evaluates the structural calculation models used for a modern 6MW wind turbine gearbox. The measurement system is composed of fifty-four fiber Bragg gratings. Optical strain sensors have been used because they offer a higher signal-to-noise ratio, are immune to electromagnetic interference, and allow a more straightforward installation than conventional electrical strain gauges. A good correlation between the structural models and the test results in a full-scale back-to-back test bench has been achieved. This enables studying the effect of a range of design parameters through simulations. Hence, without the need to carry out physical testing for each design. Increasing the confidence in structural models through experimental data leads to more optimized gearbox designs and significant improvements in torque density and overall cost of the gearbox.