What's and why's of PID!
12th February 2018
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
Figure 4 : Transformer PV inverter with galvanic isolation (Source: EE publisher)
Figure 5 : Transformer-less PV inverter (Source: EE publisher)
Figure 6 : Effect of thickness of ARC on PID (Source: Pingel S, et al.: Potential Induced Degradation of Solar Cells and Panels)
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
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