# Energy Efficiency Improvement in Reconfigurable Photovoltaic Systems: An Evaluation of Team Systems
## 5 Laboratory Equipment Characterization
### Transient States during Systems Switching  
In contrast to theoretical assumptions, a certain transient time is required for inverters to attain a stable state following any change. Considering the transient state in the used inverters after switching in the measurement and control system helps to ignore temporary values and prevent unintended system switching.  
Figure 10 displays in blue and red traces the AC powers delivered by the two used inverters along a switch cycle. The switch position is indicated by the yellow trace and it is in accordance with the position indicated in Figure 7. A low level indicates position NC and a high level indicates NO position.  
The two used SAS are programmed with a 220 W maximum power point and at \(t=0\) s only the inverter 1 (blue trace) is connected to the SAS. As can be seen, the inverter control system needs a period to perform some internal tests, the grid synchronization, and start proper operation.  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & \(\mathbf{k_{0}}\) & \(\mathbf{k_{1}}\) & \(\mathbf{k_{2}}\) \\ \hline Inverter 1 & 0.027444 & 0.096693 & 1.004568 \\ Inverter 2 & 0.026962 & 0.110187 & 0.999356 \\ Merged data & 0.027626 & 0.105882 & 1.000143 \\ \hline \hline \end{tabular}
\end{table}
Table 1: \(k\) coefficients comparison according to three datasets.  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & \(\mathbf{P_{ULG}}\) (W) & \(\mathbf{P_{UD}}\) (W) \\ \hline Inverter 1 & 53.76 & 107.52 \\ Inverter 2 & 53.43 & 106.85 \\ Merged data & 54.06 & 108.12 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Power thresholds obtained from the three datasets obtained.  
Figure 9: Efficiency curve characterization of the used inverters.  
At \(t\) = 200 s the switch is activated and the PV generator is divided between the two inverters. The output power of inverter 1 (blue trace) decreases in half and the controller of inverter 2 starts the turn-on cycle. After the initialization period, the output power of inverter 2 (red trace) increases and reaches the proper value, practically equal to the inverter 1 output power. As can be deduced from the obtained data, the transient period needed by the inverters to start the proper operation is about 95 s.  
Finally, at \(t\) = 400 s, the switch is deactivated and inverter 2 becomes inactive. All the PVG is connected to inverter 1 and the transitory operation in this case is very short because it only depends on the operation of the implemented MPPT algorithm.  
The transient previously described during the switch-on cycle has two main consequences that are necessary to consider:  
* Multiple switching around the power thresholds.  
Figure 11 shows multiple switching within a short period. To address this issue, defining a margin or hysteresis band around the power threshold is proposed. In this regard, instead of using a specific value, the switching logic considers an upper limit and a lower limit. For instance, in our case, the lower power is \(P_{UD\textit{low}}\) = 95 W and the higher one is \(P_{UD\textit{high}}\) = 110 W. The results presented in the next sections provide evidence that this method is indeed effective and useful in preventing multiple switching operations around the power threshold levels.  
Figure 11: Multiple switching operation around the threshold level \(P_{UD}\).  
Figure 10: Output power of the two used inverters along a one switch cycle.  
* Power losses during the switching operation.  
As can be seen in Figure 10, during the 95 s needed by one inverter to start the proper operation, some losses of power appear. In this period, only one inverter sends energy to the grid and the power lost is equal to \(P_{UD}/2\) or \(P_{UG}\).  
As is presented previously, the tests performed use a threshold practically equal to \(P_{UD}=110\) W, and, as a consequence, the energy losses (\(E_{lost}\)) in each switch turn-on operation can be estimated using Equation (10).  
\[E_{lost}=\frac{P_{UD}}{2}.t=\frac{110\text{ W}}{2}.95\text{ s}=5225\text{ kJ}\approx 1.5\text{ Wh} \tag{10}\]