These days, solar panels installed in large sites are suffering through newly observed phenomenon called Potential Induced Degradation (PID). Today, I will be briefly discussing about PID effect. P.I.D. directly affects energy production of the plant. This is the reason why it has attracted lot of attention by investors and plant developers in last 2-3 years.
What is PID?
Potential induced degradation is the end-effect of PID formation and P.I.D. recovery process which takes place at solar modules connected at high voltage negative potential side of the string.
PID adversely affects the performance due to combined effects of the following factors:
- System voltage
- Glass surface conductivity
- Encapsulant (EVA) conductivity
- Anti-reflection coating (ARC) property
- Interface property
In solar power plants, solar modules are connected in series to form a string. Many such strings are connected in parallel before connecting to inverter. Such as series connection of modules form high voltage in a string. High potential difference between cell and module frame forces sodium ions from glass to diffuse through encapsulate and accumulate on cell surface. This rises cell shunting which decreases shunt resistance and affects fill factor. Formation of leakage current path is one of the key element in the occurrence of P.I.D. Leakage current path can be formed between;
- Glass and cell surface (I1)
- Cell and glass and encapsulant interface (I2)
- Encapsulant and cell (I3)
- Back sheet and cell(I4)
Leakage current pathways I1 to I4 are shown in fig below.
Occurrence of PID
If positive DC terminal of string is grounded, module at extreme other end will have negative potential. If negative DC terminal is grounded, module at extreme other end will have positive potential. It should be noted that PID effect only occurs in negative high voltage side of string.
If inverter is using transformer for isolation, either one of the DC terminal can be grounded. If transformer less inverters are used, grounding one of the terminal will create short circuit fault. Hence, floating systems are preferred in case of transformer less inverters.
Systems with higher string voltage are prone to PID. Nowadays, systems with maximum 1000V or in some cases, 1500V are designed for ultra-mega power plants. Such systems are prone to PID phenomenon.
Interestingly, high potential difference is not the only parameter which causes P.I.D. High temperature and high humidity also accelerate the effect. However, plant temperature and humidity are beyond human control hence, most of the literature speaks about design modifications in maximum system voltage.
Generally, Potential Induced Degradation does not occur in residential solar systems as most of the residential solar installations have maximum voltage of 300 Volts. Though large residential complex may have higher system voltages.
As mentioned earlier, PID is the after-effect of P.I.D. formation and P.I.D. recovery process. When there is good surface conductivity on the glass, the leakage current will initiate the P.I.D. formation process. Glass surface conductivity may increase due to dew, rain drops, or even due to soiling. Dust particles in soiling act as storage matrix for humidity. As the day moves on, due to irradiation and increased atmospheric temperature (hence increased module temperature), surface conductivity reduces and limits the P.I.D. formation (as leakage current reduces). In the night when module temperature is less and there is no high voltage (due to unavailability of sun), resistance of leakage current path is high. Accumulated Sodium ions recombine with the glass material and PID effect is recovered. PID recovery is prominent in daytime (high temperature and high irradiation) than that of night time.
How to detect PID?
There are various methods to identify P.I.D. in the plant. In most of the tests, modules are required to be disconnected from plant to test for PID. Following are the methods which can be used to detect PID in the plant.
- IV Curve plotting
Reduction in shunt resistance will reflect in the reduced Pmax and Voc in the IV curve.
- Electroluminescence Imaging
Cells near frame are easily affected by P.I.D. Darker cells will be observed in EL image if the module has PID.
- Operating voltage measurement
This method takes minimum 2 or more moths to reach to the meaningful conclusion.
- String voltage comparison
Though string voltage comparison is not rigid way to make comments on occurrence of P.I.D.but a good method to initiate the detailed analysis of modules connected to affected strings.
Solutions for PID:
PID can be reversed if detected in earlier stages. If we apply reverse high voltage supply to affected modules during night time, then P.I.D. recovery process can be accelerated. Once P.I.D. recovery dominates P.I.D. formation, the overall degradation phenomenon can be avoided. However, if P.I.D. is detected in later stages, it is not possible to reverse back the effect as permanent electro-chemical corrosion takes place in the module. To avoid this, periodic (generally annually) PID check is recommended to plant developers.
Image References:  J. A. del Cueto and S. R. Rummel, "Degradation of Photovoltaic Modules Under High Voltage Stress in the Field," in Reliability of Photovoltaic Cells, Modules, Components, and Systems III: SPIE Conference, San Diego, CA, 2010.  ‘What is P.I.D.?’, http://pidbull.com/what-is-pid/