## 4 Analyses
### Q3: What Is the Recommended Aggregation Frequency?
* The steepest drops were associated with wind speed and its direction, such that even short aggregation periods had less than 50% of the data below the 0.1 and 0.025 IQR error threshold respectively. Electrical signals, grid frequency in particular, also had clear drops in accordance with the results obtained in Section 4.1.3. * Within the temperature signals, the ones related to the gearbox showed significant information losses at 600 s resolution. More than 25 to 30% of the data were above the 0.1 IQR error threshold. * The inflexion point, i.e., the point of the strongest change in the slope, of the elbow curves indicates the aggregation period above which most of the information in the signal was lost. It can be seen that for most signals this inflexion point, for this specific error threshold, lay between 100 and 200 s.
* The higher the tolerable error was, the longer the optimal aggregation periods size could be, as the inflexion point moved towards larger values. This was explicitly shown by the rotor speed and grid frequency signals. Discussion  
Figure 12 is a proposed method for choosing the ideal resolution for turbine signals. It allows to determine the optimal resolution of a signal with the definition of a maximum error \(\Lambda_{\text{max}}\) and the minimum percentage \(P_{\text{min}}\) of data that should not exceed this limit. The inflexion point that is visible for most curves determines the aggregation period, above which most of the information and details of the dynamic of the signals are lost. This allows to determine a sweet spot for SCADA data storage, allowing to reduce memory footprint of the data without excessive compromises on data quality. Moreover, the comparison of the profiles of the various curves allows to determine differences in signal dynamics. Signals with inflexion points at low aggregation periods are characterized by faster dynamics. Wind speed, wind direction, and grid frequency have a drop for aggregation periods below 10 to 100 s, after which the rate of information loss with respect to the length of the aggregation period remains almost constant and greatly reduced. Accordingly, most short-term information is contained on very short timescales. Voltage and current measurements have their inflexion point at lower resolutions between 100 and 300 s, depending on the threshold set for the maximum tolerable aggregation error. Other signals, namely temperatures have a different behavior. Instead of elbow curves they show a more linear decay or even no decay at all such as for the main bearing, whose percentage of data below the lowest error threshold, i.e., [0,0.025 IQR] is not lower than 95% even for a 600 s aggregation period. The choice of the acceptable error \(\Lambda_{\text{max}}\) and the percentage \(P_{\text{min}}\) are highly dependent on the usage of the data as well as the economics of collecting, storing, and processing high volumes of data. Choosing a more restrictive error threshold moves the inflexion point towards lower values of the aggregation period size, but increases the amount of memory necessary to store information. Nevertheless, the proposed methodology allows to take informed decisions on the strategy to store and aggregate SCADA operating data. Moreover, insights concerning the dynamics of the different signals can be inferred by studying the profiles of the signal curves. Figure 12: Relation between temporal resolution of the aggregated signal and percentage \(P[\Delta t]\) of data below a given error thresholds. Multiple error limits \(\Lambda_{\text{max}}\) are represented with different colors, see legend. The scale of the y-axis is not fixed to magnify changes in signals. A black horizontal line is drawn for a value of 80% of the data to improve readability and comparison of the different plots. Labelling of the axes was omitted for a less overloaded presentation: The x-axes denote the time resolution \(\Delta t\), the y-axes the data percentage \(P\) of data inside \([0,\Lambda_{\text{max}}]\).