## 2 Background
### Maintenance Records  
After a wind turbine stops due to alarms, manual inspections are arranged by maintenance personnel. The technicians investigate the turbine malfunction and document the specific details in maintenance records. Consequently, the fault type or tag that triggers the alarm is recorded in the maintenance records. Table 2 provides an example of a maintenance record. The record contains the turbine number, the start time and end time of maintenance activity, the actual faults, and the solutions to faults. However, not all faults can be found in the maintenance records. This is primarily because the wind turbine's main control system automatically handles certain alarms.  
To ensure the safety of wind turbine operation, the main control system responds to specific faults that trigger multiple alarms by performing different operations to eliminate them. The controlling principle is illustrated in Figure 1, wherein each alarm level corresponds to a particular severity of abnormality. When the alarm level is low, no operation is performed. When the alarm level is moderate, the wind turbine is restarted or reset. If the moderate-level alarm persists even after a restart or reset, the wind turbine is shut down. When the alarm level is high, the wind turbine is immediately shut down. After the shutdown, the main control system executes pre-set actions through the self-inspection function. If the alarms persist, manual maintenance is performed and the fault events are documented in the maintenance records.  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Turbine Number** & **Triggering Time** & **Alarm Types** & **Alarm Codes** & **Alarm Flags** & **Description** \\ \hline P01 & 2017/5/22 16:30:05 & Information & I2 & Start & The wind turbine is started \\ P01 & 2017/5/22 17:38:18 & Warning & A264 & Start & The first measuring point temperature of \\  & 2017/5/22 17:38:37 & Warning & A264 & End & The first measuring point temperature of \\  & 2017/5/22 17:38:51 & Fault & T21 & Start & The communication of the pitch system \\ P01 & 2017/5/22 17:38:52 & Information & I2 & End & The wind turbine is started \\ P01 & 2017/5/23 00:15:20 & Fault & T21 & End & The communication of the pitch system \\  & & & & & is an error \\ \hline \end{tabular}
\end{table}
Table 1: A sample of an alarm list.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Turbine Number** & **Start Time** & **End Time** & **Actual Faults** & **Solutions** \\ \hline P01 & 2016/12/21 & 2016/12/25 & A slip ring & Replace the \\  & 17:34:00 & 12:45:00 & is damaged & slip ring \\ \hline \end{tabular}
\end{table}
Table 2: Example of a record in the maintenance records.  
From the above, we can draw the following conclusions:  
* When a wind turbine is shut down due to alarms, manual maintenance will be performed. However, many alarms cannot cause a shutdown. Thus, the fault events that trigger these alarms are not available.
* Some alarms that can cause a shutdown are eliminated by the self-inspection function and thus have no recorded fault events.  
In addition, during the actual maintenance activities, due to the irregular work of the operator, some maintenance details are missing. Thus, more alarms have no available fault events. In this paper, we name these alarms the unlabeled data. On the contrary, the alarms that have available fault events are named the labeled data. The fewer labeled data make it harder to diagnose wind turbine faults. On the other hand, the unlabeled data are generally ignored by the existing studies. We will address both issues in this paper.