What's and why's of PID!
Already in our previous article "Common problems in PV plant - Part 2", we had discussed about the major PV module specific problems. However we felt necessary that out of all those mentioned problems Potential Induced Degradation (better known as PID) needs to be explained in detail to our readers.This is primarily because PID while not initially visible can cause sudden death of the affected module (as evident from Figure 1). Additionally, PID also leads to formation of daisy chain reaction where the surrounding modules (in the string) are (almost equally) affected. This article in addition to basic introduction would also explain the readers the causes of PID, its effect and the corrective measures which needs to be taken for preventing (or at least reducing) PID to enhance the life time of the module (and the power plant). =
Figure 1 : Typical failure scenario of crystalline solar modules (Source: IEA-PVPS)
A solar module (mono or multi crystalline) as we know can generate a maximum voltage output of 30 to 40 Volts. However, in a megawatt (MW) scale power plant the solar modules, in order to meet the starting voltage requirement of the inverter are connected in series.Ideally for a string generating the voltage output of say 500V, it is expected that the string remains balanced by generating positive and negative voltages of - 250V. However for few inverter topologies (discussed in detail in later part of the article), the voltage is shifted more towards the negative side say to +100V and -400V. Considering the safety feature, the entire string (of all the modules) is grounded to earth. This negative potential (of -400V) and the grounding (of the entire string) form the prerequisite for PID. While in operation the grounded module with the most negative potential (say module number 20 in Figure 2) is more prone to PID. In such modules, there is a high potential difference between the cells and the Aluminium metal frame of the module. Such negative charge at the cell forces the positive ions from glass (say sodium) to migrate into the module and sit onto solar cell. This primarily causes accumulation of positive ions on solar cell leading to recombination of electrons (known as surface recombination). This phenomenon leads to PID (or more commonly known as PID-s). Additionally,the high potential difference also leads the electron to loosen from the materials of module and flow out (known as leakage current) into the ground via the frame (Figure 3). This effect leads to what is known as PID-p.Both PID-p and PID-s may or may not be reversible in nature. Thin film modules also observe PID, where in due to electro chemical reactions the top transparent conducting oxide (TCO) layer is corroded. Such PID is strictly irreversible in nature. This article in general deals with both PID-p and PID-s (for crystalline module) effect and they are both in general termed as PID.
Figure 2 : Effect of PID on modules connected in a typical string Electroluminescence (EL) image (Source: Waaree Energies)
Figure 3 : Leakage current in PID (Source: Pingel S, etal.:Potential Induced Degradation of Solar Cells and Panel)
Causes of PID
- System level (Inverter topology): As we had mentioned above the inverter topology plays an important role in PID. The two basic inverter topology i.e. transformer and transformer-less inverter are discussed here. An inverter in general convert the DC input power to grid compatible AC output power. A transformer inverter as the name suggest has an inbuilt transformer in conjunction with the inverter which enables boost the voltage in order to meet grid requirement. Such transformer inverters come with galvanic isolation which means that the electrical circuits are separated from each other (in order to avoid stray currents). Such technique allows the negative pole of the module to be earthed. This ensures that the negative pole stays at ground potential (or 0V) which completely eradicates PID (Figure 4). However due to cost constraints, the PV market uses transformer-less inverters where there is direct connection between the grid and the inverter. Such direct connection negates the purpose of negative pole grounding which if done, would lead to short circuiting. Hence, in such inverters the negative pole is not at ground potential (Figure 5) and become active spot for PID.
Figure 4 : Transformer PV inverter with galvanic isolation (Source: EE publisher)
Figure 5 : Transformer-less PV inverter (Source: EE publisher)
- Module and Cell level factors: The module and cell level factors usually deals with the materials in use. The choice of glass, encapsulant used (in solar module) and the Anti-Reflective Coating (ARC) coating (on solar cell) are all found to affect and/or enhance the effect of PID. The front glass which contains more soda lime (primarily sodium) content is known to enhance the effect of PID. The most commonly used encapsulant Ethyl Vinyl Acetate (EVA) contains acetic acid which in combination with moisture (and high elevated temperature) enhances the metal ions deposition from the glass to cell surface. The ARC while enhancing the light falling on solar cell may also contribute to cause PID. This coating is made by various processes and the used material SiNxhas different ratios of Si and N in each of this process. As evident from Figure 6 below, the ratio of Si to N (and hence its refractive index) in conjunction with the thickness of the ARC coating and the process usedaffects the amount of PID in the cell.
Figure 6 : Effect of thickness of ARC on PID (Source: Pingel S, et al.: Potential Induced Degradation of Solar Cells and Panels)
- Environment factors:The environmental factors unlike the above two factor cannot not be controlled but can just be observed. The worst of all the environmental factors enhancing PID is moisture and elevated temperature. The moisture seeps into the module causing increment in leakage current in the module. This process is enhanced at elevated temperaturesleading to PID in such modules.
Effect of PID
While PID is such a complex process with multiple factors affecting and enhancing it, its effect is simple i.e. reduction in power output of the module and reduce its life. Enhanced by time ageing, PID affects the Voc and the fill factor of the module (Figure 7). Such affect leads to reduction in power output from the module (and hence the entire string) to about 30%. Additionally as the time passes by added reduction in the power output can be observed. At enhanced temperatures, reduction in power output as high as around 90% can be observed.
Figure 7 : Effect of PID on module output (Source: Schuetze, et al, Laboratory Study of Potential Induced Degradation of Silicon Photovoltaic Modules)
Corrective measures of PID
- System level: As we had mentioned above on the system level, PID can be avoided by using transformer PV inverter. However, if we use transformer-less inverter various options are available in the market such as PV offset box by SMA, Zigzag-connected Chopper Converter by Omron, etc. When after sunset if the array voltage drops below a threshold value, the offset box raises the voltage of the entire string (to a higher positive value (as shown in Figure 8 ) with respect to earth). This reverses the polarization effect which had occurred in the module (and the string) during operation.
- Cell and Module level: We can avoid PID (partly or fully) by changing the raw materials we use. For glass, we could use glass with low sodium content. One option in consideration is to shift to quartz based glass instead of soda lime glass. In encapsulant, we could shift from EVA to Poly Olefin Elastomer (POE), an encapsulant believed to reduce and even limit PID. For cells, the thickness of ARC and the ratio of Si:N can be closely monitored. Additionally, homogenous deposition of ARC on solar cell can ensure that the solar cell is PID free.
- Environment factors: The best and the only option available so far for environmental based factors is to get the modules tested as per the testing standards. International Electrotechnical Commission (or more commonly IEC) has come up with a testing certificate IEC TS 62804-1. This test prescribes the modules to be tested at elevated temperature (at 85 °C) and Relative Humidity of above 85% for 96 hours in a special chamber. The module's performance is then tested to check the effect of stresses inducedin the module.
Figure 8 : Positive voltage shift during night (Source: Waaree Energies)
Waaree uses standard materials which are certified to be PID free. Additionally, our modules are certified by the IEC agency to be PID free. Also, we do extended testing above the specified test standard to ensure the reliability of our product under extreme climatic conditions. The reader can view all the certifications we have here.
Let us all pledge to make solar energy the primary source of energy in the near future.
RAHE ROSHAN HUMARA NATION