The PID insulation tester (TOS7210S) is designed based on the insulation resistance tester (TOS7200) to carry out the evaluation of the PID (Potential Induced Degradation) effect of the PV module precisely and efficiently. Being equipped with the output ability of 2000 V and the ammeter with nA resolution as well as a polarity switching function, the TOS7210S is also applicable not only to the PID evaluation but also the evaluation of the insulators that requires a high sensitivity of measurement. The tester is equipped with the panel memory that is externally accessible and RS232C interface as standard; it can be flexibly compatible with the automated system.
The PID effect is a phenomenon that the amount of power generation by a cell remarkably decreases when high voltage is applied between the solar cell and the frame for long hours. It is supposed that the higher the applied voltage is and/or the higher and more humid the environment is, the further deterioration accelerates. For example, the potential difference in the string becomes extremely higher when the number of sheets that are connected in series increases even if the output voltage of the crystalline silicon solar cell module is just tens of volts. On the other hand, the grounding methods of the PCS (Power Conditioning System) that connects to the system as an AC power supply varies according to the type of PCS. In recent years, the transformerless method, where the input side is in the floating method (the negative polarity is not connected to the ground) is increasing. In this case, a high potential difference occurs between the cell and the ground. Regarding the crystalline silicon solar cell module, it is proved that the cell having a higher negative electric potential than that of the frame (ground) easily causes the PID effect. (See Fig. 1) The PV modules are currently managed with the maximum system of voltage as 600 V in Japan and 1000 V in Europe. There is a market trend that the maximum voltage of the commercial mega solar system is raised from the perspective of reducing the number of the strings and the total number of the PCS, and improving the efficiency of power generation.
Figure 2 shows the simulation of a crystalline silicon solar cell module being exposed to a high potential difference. It is considered that, in the PID effect of the crystalline silicon solar cell module, the sodium ion in the white tempered plate glass moves to the side of the cell and then causes the deterioration. (The PID effect of the thin film solar cell module is also confirmed, however, the mechanism of the deterioration is different from that of the crystalline silicon solar cell module.) The cause of the PID effect is currently being tested with and researched by various research organizations.