## 4 Reverse Current Measurement of PV Arrays in Field  
To verify the simulation results, an experiment was conducted to measure the reverse current in actual PV systems. The test site includes a PV system with a capacity of 50 kW, and the test was conducted on four PV strings. Each string consists of 15 PV modules, connected in a series, and fuses are installed in place of the blocking diodes on the \(+\) and \(-\) poles of the string. The inverter was stopped for the experiment. The purpose of this experiment is to check the effect of bypass diode operation and the short-circuit fault on the circuit. A black sheet that simulates the operation of the bypass diode under shading is used, as shown in Figure 11. The failure of the bypass diode is simulated by shorting the PV modules using cables, as shown in Figure 11. Reverse currents were measured on one string of the PV arrays, in which the voltage mismatch occurred. Under each experimental condition, the number of PV modules with a turn-on bypass diode and short state was increased to three. The irradiance was measured during the experiment, and the back-surface temperature of the PV module was also measured. The irradiance and temperature were measured for comparison with the simulation results. The experimental results are as follows. First, the measured reverse currents ranged from \(-0.01\) to \(0\) A on the turn-on bypass diodes, and, for the PV modules, with the operation of bypass diodes raised \begin{table}
\begin{tabular}{c c c c} \hline \hline
**A Number of PV** & \multicolumn{3}{c}{**Parallel Channel Number**} \\ \cline{2-4}
**Modules** & **2 Circuit** & **3 Circuit** & **4 Circuit** \\ \hline
1 & \(-2.64\) A & \(-3.69\) A & \(-4.24\) A \\
2 & \(-5.25\) A & \(-7.74\) A & \(-9.12\) A \\
3 & \(-7.44\) A & \(-11.88\) A & \(-14.47\) A \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The reverse current due to the failure of the bypass diode, as shown by simulation. Figure 11: Experimental conditions: (**a**) Operation of the bypass diode, and (**b**) failure of the bypass diode. to three, the result was the same. This result shows that reverse current hardly flows into the string with voltage mismatch, when the bypass diode is in regular operation. Next, the results in the case of the failure of bypass diodes are in Table 3. The measured reverse currents were \(-3.76\) A, \(-6.41\) A, and \(-12.03\) A, as the number of PV modules increased to one, two, and three, respectively. At the time of measurement, the irradiance and temperature of the back surface of the PV module were \(712\) W/m\({}^{2}\) and \(29.12\)\({}^{\circ}\)C for one PV module, \(530\) W/m\({}^{2}\) and \(29.41\)\({}^{\circ}\)C for two PV modules, and \(706\) W/m\({}^{2}\) and \(28.96\)\({}^{\circ}\)C for three PV modules. In the simulation results using measured environmental conditions, the reverse currents were \(3.32\) A, \(-6.43\) A, and \(-11.30\) A, as the number of PV modules increased to one, two, and three, respectively. Comparing the simulated values with the measured values, similar results were obtained with \(0.44\) A, \(0.02\) A, and \(0.73\) A differences, respectively. The experimental results show that the failure of the bypass diode causes a potential difference between PV strings, and reverse current flows into the low-potential string. From the simulation and measurement results, it was confirmed that the current flowed by the failure of bypass diodes (=potential difference) in a PV system situation where there is no blocking diode and the inverter failure. In these conditions, the flow of the current is shown in Figure 12. In normal operation, the inverter of the PV system operates and output current of PV arrays flows into inverter. If an inverter fails, the output current of PV arrays does not flow into the inverter because the electric circuit is disconnected. Thus, the PV arrays connected in parallel are in a closed loop. If there is no potential difference between PV arrays, the current rarely flows into any PV string.