Gary Custer, PE
Introduction
The gradual deterioration of performance in some PV modules with crystalline Si cells, known as Potential Induced Degradation (PID), can lead to a loss of up to 30% or more after a few years. While some module manufacturers are developing new materials to combat PID, the trend towards three-phase systems and higher system voltages up to 1,500 V is currently worsening the issue. This article provides an overview of the PID effect.
PID Cause: Potential to Earth & Voltage
To comprehend PID, one must begin by defining voltage and potential. Electric potential refers to the voltage of a specific point in relation to a reference and zero point, usually the earth. Voltage, on the other hand, refers to the difference in potential between two arbitrary points. For instance, if point A has a potential of 380 V relative to the earth and point B has a potential of 430 V, the voltage between A and B is 50 V. PV modules usually provide around 40 V under standard conditions, but connecting them in series increases the array voltage significantly, resulting in a corresponding direct current that the inverter transforms into grid-compatible alternating current. The potential of the PV array is determined by the potential of the connected electrical grid and the inverter's design. Ideally, the PV array's positive and negative poles should be symmetrical to the neutral conductor's earthed potential. For example, if a module string's MPP voltage is 400 V, the PV module at the negative end has a potential of -200 V relative to the earth, while the module at the positive end of the string has a potential of +200 V. This potential can also be shifted to the negative side for certain transformerless inverters. However, having a positive or negative potential to the earth can have adverse effects depending on the module type, and PID is one of them.
When using galvanically isolating inverters, the range and polarity of the array potential can be freely determined. These inverters do not directly supply power to the electrical grid but use a magnetic coupling instead. This allows for the PV array to be grounded, resulting in the potential of the entire array being shifted to the positive or negative range. However, transformerless inverters cannot provide this option because they are connected to the electrical grid via electric conduction. If these inverters were grounded, it would lead to internal short circuits.
What is PID
This particular occurrence is exclusive to modules with cells composed of crystalline silicon. In such modules, a negative potential to the earth during operation results in an equally negative voltage between the PV module's cells and the aluminum frame, which is earthed, or grounded, for safety reasons. This phenomenon is more pronounced as the module approaches the negative pole of the PV array, where the potential (and voltage between cells and the aluminum frame) can surpass half of the array voltage. Consequently, electrons from the PV module's constituent materials may separate, follow this electric field, and exit via the aluminum frame. This generates an increasing charge (polarization) within the module, which alters its characteristic curve and consequently its power (as shown in Figure 1) unless remedial measures are implemented.
Figure 1: The characteristic curve of a PV module in the original state and during the degradation process A decreasing slope with a virtually unchanged open-circuit voltage and short-circuit current, while the maximum power (MPP) decreases by 30 percent or more, is typical.*
The PV module's electric charge is critical because of how solar cells operate. The photovoltaic effect is based on the use of two different semiconductor materials to create an internal electric field through charge exchange. This field prompts electrons freed by light energy to move from their original position and flow past the contacts as an electric current. Additional load carriers can significantly disrupt this process, resulting in a substantial loss of power. However, it has been observed that this polarization can be generally reversed. As a result, it is not an irreversible effect like corrosion or normal aging-related deterioration. The term "Potential Induced Degradation"* was first coined in a 2010 publication by Solon, a module manufacturer that scrutinized the phenomenon in great detail.
* J. Berghold et.al, Potential Induced Degradation of solar cells and panels, proceedings of the 25th EU PVSEC, 2010
Additional Information on PID
Previously, power losses resulting from PID were the exception rather than the norm. However, there are now increasing indications that many cell types display this failure pattern, unbeknownst to the manufacturer. The aforementioned Solon article identifies the major factors that influence PID susceptibility:
• Solar cells: The PV cells' structure influences PID via the charge carrier density of the silicon used and the chemical composition of the anti-glare coating.
• PV module: The materials used in the PV module also play a role, such as the laminating film that directly contacts the solar cells (usually EVA).
• System configuration: as described earlier, the maximum negative potential of the PV modules is critical. This depends on the module string length, inverter type, and potential earthing of the PV array.
• Time: The PID and resulting power loss are not immediately apparent; they develop over several months to a few years.