What does PID mean?

During the normal operation of a pv module, impacted by the weather conditions (changes in temperature and humidity, mainly) of the site, they suffer a known degradation which can be estimated. The problem arise when this degradation goes further than expected. Within the factros that can cause such abnormal accelerated degradation of the pv module, I would like to make special mention to the well know PID effect (potential induced degradation). PID effect is a degradation due to a mix of different factors: weather conditions (temperature and humidity), array design (systme voltage, topology and module position within the string) and pv module design and materials used. Such degradation is characterized by a power reduction up to 30% (or even more), affecting the I/V characteristic and therefore the P/V curve.

PID effect provoques an electrical leakage current to flow from the frame to the cells in an order of magnitude of nanoamps accumulating positive charge over the cell surface affecting electrically and in a long term physically the cells. Modules based in P-type wafers, this effect will occur in the modules installed in the most negative part of the string and as commented above, among other factors, it will depend on the drop voltage between the cells and the frame (the bigger the difference the highest the risk) and the dielectric quality of the materials used in the pv module. There are different ways to detect the PID effect: through thermal or EL pictures or obtaining the I/V curve of the pv module, among others.

As per the commented here above we could classify the reasons driving the modules to suffer of PID in two main groups: external factors to the module as the weather conditions or the design of the installation and pv module design factors (dielectric quality, distance between the frame and the cells, ARC,..). In the last group the lamination itself plays an essential role.

In order to minimize or mitigate the PID effect we could act at installation level taking into account that we can not do anything to avoid the weather conditions.

One of the solutions adopted when the installation is already done or the pv module is not prepared for such phenomenon is to connect one pole to ground. In a floating system (I would say that 99% of the grid connected installation are of this type where no pole is connected to ground) the distribution of the drop voltage between the cells and the frame or ground is represented by the red line in the chart below. In this example, for an identic dielectric or isolation level for every module connected in the string of 23 pv modules and for an installation with a system voltage of 800V, if we could measure the voltage between the cells (any pole in an module) of the pv module installed in the most negative part of the string we would see a voltage of -Vsystem voltage/2, which is -400V. As long as we go thorugh the string towards the positive pole, the voltage will be propportionally approaching to 0V, being reached in the center of the string or central pv module in the string (position 11) and from this one the voltage will become positive until it reaches +400V in the most postive module in the string. According to this, for cells based in P-type wafers ( the vast majority in the market) the current can potentially flow from the frame to the cells and therefore degrading the pv modules located in the negative part of the string. PV modules "seeing" a positive voltage vs the frame or ground (from the center point of the string to the most positive part of it) will never be affected by this phenomenon. Therefore in order to avoid a leakage current flowing from the frame to the cells, a connection of the negative pole of the string to ground is made (represented by the green line over the x-axis). For cells based on N-type wafers, the positive pole is connected to ground (represented by the green line below the x-axis). This type of connection has a potential risk of electrocution in case there is an isolation issue in another part of the string since we will be closing the circuit through ground. For this cases, a fuse is installed in the connection of the negative pole to ground acting in case of isolation fault. Another factor to consider in this type of connection is the inverter: with a pole connectd to ground, a transformerless inverter can not be used but always is better to refer to the inverter manufacturer guidelines and recommendation.