## 2 Materials and Methods  
### Data Relationship Modeling of the Wind Turbine Operating State  
#### 2.1.1 Data Analysis  
Wind turbine SCADA systems are data acquisition and monitoring control systems that include Computer, Communication, Control, and Sensing technologies (3C + S). SCADA systems sample and store many operational parameters during the operation process of a wind turbine, including wind speed, rotor speed, pitch angle, vibration acceleration, generator torque, and output power, at a set frequency. The data sampling and storage period is generally measured in minutes or seconds. An example of raw SCADA data with a sampling period of 1 s is shown in Table 1.  
Because a SCADA system records data during both normal operation and shutdown and startup processes, failure or abnormal operation of the wind turbine can also be detected. To improve the accuracy of the results, the raw data should be preprocessed, and only data from normal operation should be retained for analysis. In some cases, measurement reference error, such as pitch angle, or errors caused by measurement positions, such as with wind speed correction, should be corrected [21]. In other cases, specific data may be required for analysis based on the topic being investigated. For example, in this paper, the wind speed between the cut-in wind speed and the rated wind speed in the SCADA raw data must be analyzed; thus, the data describing the wind turbine pitch stage are removed, and the nonlinear effect of pitch change is not considered.  
A wind turbine converts wind energy into electric energy. Different wind speeds output different electric power, and different wind speed distributions also output different electric power distributions; these facts imply various functional relationships between SCADA parameter data and wind turbine health metrics. The factors that affect these functional relationships include both the wind turbine itself and external factors. There are two main factors in the system itself: on the one hand, the structural parameters of the wind turbine, including blade airfoil, wind wheel diameter, etc.; and on the other hand, the operating parameters related to the wind turbine control mode, such as the maximum power tracking control mode when the wind speed is lower than the rated wind speed and the constant power control mode when the wind speed is higher than the rated wind speed.  
To clarify the various relationships among the wind turbine SCADA data, this paper divides the parameter data into input and output parameters according to the operation parameters corresponding to SCADA data and the input/output relationships of the wind turbine system and its subsystems. This paper also establishes pairwise corresponding relationships between the parameters. For example, in terms of three parameters, such as wind speed, rotation speed, and output power, the parameter relationship includes: the relationship between the wind speed and rotation speed, where wind speed is the input parameter and rotation speed is the output parameter, which depends on the wind wheel subsystem and mainly includes the wind wheel, pitch control system, and yaw control system; the relationship between rotating speed and power, where the rotating speed is the input parameter and power is the output parameter, which depends on the transmission subsystem, electromagnetic subsystem, and power subsystem and includes components such as the transmission device, generator, converter, and transformer; the relationship between wind speed and power, where wind speed is the input parameter, and power is the output parameter, which depends on all parts of the wind turbine, including all subsystems.