## 2 Reverse Bias Characteristic of a PV Module Due to Voltage Mismatch Conditions
### The Failure of Bypass Diodes  
In PV plants, the case of failure of bypass diodes have been reported which causes the output loss and reliability of the PV system [15; 16]. Moreover, the plant diagnosis results for thermal images of the short circuit fault of the bypass diode are reported as Figure 6[17]. The causes of the failure of bypass diodes are Electrostatic Discharge (ESD) and thermal runaway. ESD refers to very large current flowing to bypass diodes in a flash by very high voltage pulse, and it is related to surge current by nearby lightning strikes. Thermal runaway is widely known as a cause of the failure of general diodes, and diodes are destroyed by continued heat and current [18]. It causes the bypass diode to fail, and the junction of the bypass diode is damaged. Bypass diodes with damaged junction lose their switching characteristics and are in a short-circuit state with connected solar cells [19]. Figure 4: Reverse voltage and current due to the number of operation of bypass diodes by shading. Figure 5: I–V characteristics of a PV module due to the number of operations of bypass diodes by shading. In this paragraph, an experiment was conducted to confirm the electrical characteristics, simulating the short-circuit fault of the bypass diode. The failure of the bypass diode is generally a short-circuit fault, such that the diode was artificially damaged by applying a voltage above the reverse breakdown voltage of the diode The failure of bypass diodes was inserted into the PV module to measure the open-circuit voltage, through which current flows, while changing the number of failures of the bypass diodes. During the measurement, the irradiance of the light source device was set to 1000 W/m\({}^{2}\), and the experimental results are shown in Figure 7. Unlike the previous non-uniformity of irradiance and the operation of bypass diodes, it can be seen that the open-circuit voltage, through which reverse current flows, are significantly reduced. Compared to the case where all of the bypass diodes are normal, the voltage decreased by about 10-12 V, as the number of failures of the bypass diode increased. The reduced voltage is about 12 V, multiplied by the number of connected solar cells (20 ea) per bypass diode and the voltage per solar cell (0.6-0.65 V). Therefore, no output voltage occurs in the solar cells that is connected to the failure of the bypass diode. In addition, as the number of failures of the bypass diode increased, the reverse current increased linearly. This is because the current flows to the failure of the bypass diode (=conductor with a small resistance), rather than to solar cells, resulting in a resistance-shaped curve [20]. Comparing the I-V curves of the PV module with the normal bypass diode and the failure of the bypass diode, shown in Figure 8, the open-circuit voltage of the PV module with a fault bypass diode was reduced by 1/3. The failure of the bypass diodes causes them to lose their reverse characteristics, so when a forward voltage is applied to the PV module (bypass diodes in a reverse state), current can flow into the failure of the bypass diode. At the same time, the solar cells connected to the fault bypass diode constitute a short-circuit condition, and their generated voltage is about 0 V, which reduces the overall voltage of the PV module [21]. The experimental results show that, if all of the bypass diodes in the PV module fail, the open-circuit voltage is reduced by up to 90%, so the reverse current will likely flow into PV modules with the failure of the bypass diode. Figure 6: IR (Infrared) images on short-circuit fault of the bypass diode in field. Figure 7: Reverse voltage and current due to the failure of bypass diodes. PV Arrays of Electrical Characteristics Analysis under Voltage Mismatch Conditions Using Matlab-Simulink  
In the previous chapter, the voltage and current, when the reverse current began to flow due to voltage mismatch conditions in PV modules, were measured and analyzed.