## 3 Results and Discussion
### Equipment and Procedure for the Analysis with Four Chassis  
In the tests of the previous phase, the quality of a distributed capture system with cDAQ was analysed when the TSN standard was used as the synchronisation medium. The response of two of the four channels of the same Sigma-Delta converter of the ADS1274 24-bit chip integrated into the NI-9242 modules was taken as a reference. This converter was analysed in detail by \"[PERSON] et al. [25]\" as a proposal for a distributed measurement system. With the reference of this high-precision capture system, the results were analysed with the AD7655 16-bit converter integrated into the NI-9215 modules. In the first phase, the level of synchronism was analysed with equipment located at a maximum distance of 10 m.  
In the second phase, the tests were carried out in two consecutive stages with the distribution shown in Figure 4 and the two types of Sigma-Delta and SAR converters, as indicated, were integrated in the NI-9242 and NI-9215 modules. The study was carried out with the aim of observing the behaviour of the distributed capture system in conditions very similar to those that will be found in the photovoltaic plant; the results were compared to assess whether it is acceptable to use the lower-cost NI-9215s, guaranteeing the level of synchronism required of the capture system in the synchro-phasor standard, instead of the NI-9242s with TSN synchronism and with an Ethernet cable length of up to 75 m. Category 6 Ethernet cables and an ASUS RT-AC66U router with access speeds of 100/1000 MB were used.  
Figure 13: Probability density for SAR converters.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Level of** & **Same** & **Different Slots** & **Different** & **Different** \\
**Uncertainty** & **Slot** & & **Chassis-2 m** & **Chassis-10 m** \\ \hline \multicolumn{5}{c}{24-bit Sigma-Delta converter} \\ \hline (5\%) & 20 ns & 35 ns & 57 ns & 85 ns \\ Degrees & 0.36 \(\times\) 10\({}^{-3}\) & 0.63 \(\times\) 10\({}^{-3}\) & 1.0260 \(\times\) 10\({}^{-3}\) & 1.5300 \(\times\) 10\({}^{-3}\) \\ \hline \multicolumn{5}{c}{16-bit SAR converter} \\ \hline (5\%) & 38 ns & 48 ns & 63 ns & 68 ns \\ Degrees & 0.684 \(\times\) 10\({}^{-3}\) & 0.864 \(\times\) 10\({}^{-4}\) & 1.134 \(\times\) 10\({}^{-3}\) & 1.224 \(\times\) 10\({}^{-3}\) \\ \hline \hline \end{tabular}
\end{table}
Table 5: Comparison of uncertainty levels for Sigma-Delta and SAR converters.  
In these tests, measurements were carried out in three different configurations with four chassis, as shown in Table 2. In all cases, they were connected to a mains supply with a Vrms = 230 V and a frequency of 50 Hz. The NI-9215 modules used LEM LV-25 active transducers and the tests were carried out in two stages. In the first stage, the analogue channels were connected to two LV-25 transducers to observe the possible delays associated with the sensors, and in the second stage, the two inputs were connected to a single LV-25 LEM output.  
This set of tests will be compared with the results obtained in Section 3.4, which was dedicated to Sigma-Delta converters under the same test conditions but without using the LV-25 LEMs because they integrate the high precision VMN221 dividers from CADDOCK inc.  
Figures 14 and 15 show a picture of LV-25 LEMs and VMN221 high-precision resistor networks integrated into a NI-9242 module.  
The tests were performed consecutively with two, three, and four cDAQ-9185 chassis, and an in-line topology with 50 m, 75 m, and 10 m. Ethernet connection was chosen, as shown in Figure 16, at one end connected to the ASUS RT-AC66U router and at the other end connected to the cRIO-9054, acting as the TSN master.  
Figure 14: External module with three LEM LV-25s.  
Figure 15: NI-9242 module with four VMN221s from CADDOCK.