# Machine Learning-Based Monitoring of DC-DC Converters in Photovoltaic Applications
## 3 Results
### First Selection of the Measurements  
The first step of the prognostic procedure is the selection of the most significant measurements to obtain a correct evaluation of the converter status. Since the DC source corresponds to a photovoltaic panel, the input voltage and current are highly variable and depend on the incident solar irradiance and the temperature of the panel. This means that variations in measured quantities can be introduced due to changes in environmental  
Figure 5: Set-up of the output layer and coding of the classes.  
conditions. If these measurements are used as inputs to the monitoring system, classification errors may occur. To avoid this issue, measurements on the component's quantities are first normalized against input quantities (which are in general known due to their use in almost any MPPT algorithm). Among the normalized quantities, the ones with lower sensitivity towards irradiance and temperature are chosen as inputs for the prognostic classification.  
Therefore, the best choice of measurements includes all voltages and currents with low sensitivity compared to the input ones.  
As said before, the dataset used to train the neural network-based classifier contains measurements of ripples and mean values. Initially, all currents and voltages on the passive components are taken into consideration, and their variations with respect to the irradiance and temperature are graphically evaluated through the Simulink model. The operating points considered for this simulation are extracted from [37] and represent common situations with a realistic relationship between irradiance and working temperature. Table 5 summarizes these working points.  
Starting from a common situation characterized by an irradiance of 1000 W/m\({}^{2}\) and a working temperature of 55 \({}^{\circ}\)C, and considering the fixed grid voltage of 48 V, the maximum power point is obtained with a duty cycle of 0.6. This working condition is chosen as the starting point to evaluate the effects of changes in environmental conditions. Therefore, the duty-cycle is set at 0.6, and the changes in voltages and currents across the passive components are analyzed by moving to the three operating conditions presented in Table 5. Since the environmental situation changes but the duty-cycle is kept constant, the three working points shown in Figure 6 are obtained. These three points indicate three different pairs of input voltage and current. As previously said, to correctly choose the quantities to be measured during the monitoring procedure, the sensitivity of all voltages and currents with respect to these changes is studied.  
Figure 6: Variations of the working point and variations of the input current and voltage.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Operating Point** & **Irradiance (W/m\({}^{2}\))** & **Temperature (\({}^{\circ}\)C)** \\ \hline A & 400 & 15 \\ B & 800 & 45 \\ C & 1200 & 65 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Operating conditions.  
It should be noted that the duty-cycle value used in this paper is not mandatory and that several methods can be used to choose it. In this case, the starting point is a condition of maximum power transfer, and this value of D is maintained. Once this parameter has been chosen, it is necessary to keep it constant during the generation of all the samples belonging to the dataset matrix.  
In this paper, the sensitivity analysis is performed graphically by using the Simulink model described above.  
Analyzing the simulation results, it can be observed that the voltage ripples on the passive components and the average values of the currents exhibit a low level of sensitivity with respect to the irradiance and temperature variations. For this reason, the average values of the inductor currents and the ripples of the capacitor voltages are selected as possible measurements. Figure 7a-d presents the approximately constant behavior of these quantities with respect to the changes in input current and voltage.