Maximising power production of large PV systems: PID detection and mitigation

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Potential induced degradation (PID) is a degradation mechanism occurring in high voltage PV systems due to a large potential relative to the ground, and is dependent on the magnitude and polarity of the system. PID emerged as an issue of concern in the last decade due to the increase in utility-scale deployment of high voltage PV systems in the past few years, which are trending towards 1000-1500V systems. Consequently, the large electric potential of the PV system increased the modules’ susceptibility to PID. Though degradation due to high voltage stress was identified as early as 1978 at JPL, PID gained light in 2005 when Swanson identified degradation due to polarisation in SunPower’s modules. However, the issue is not yet been addressed by qualifications standards such as the IEC 61215 and IEC 61646, and a test method, TS IEC 62804, is being developed.

Cells affected by PID can have losses of 20-80% or even higher. Power output reduction of over 40% was observed in PV strings afflicted by PID, which affects the operations and financing of PV systems adversely. A survey conducted by PI Berlin reported PID in 20 power plants in Germany and a power plant with 12 strings showed 10-15% power loss in 39% of the strings; additionally, PID affected 41% of the modules in another 10.7MW plant in Spain. If unaddressed, this could result in significant revenue losses and additional PID mitigation costs over the years. It is essential to address the issue in the early stages to ensure durability and reliability of modules over their service life.

Factors contributing to PID

PID is caused mainly due to a large electric potential on the module, which results in a leakage current that migrates between the cell and the other components, resulting in power reduction. As Dr. Peter Hacke stated at the 2014 NREL PV Reliability Workshop, leakage current is not a metric for quality of the modules, but a parameter used as a detection mode in modules afflicted by PID.

There are numerous factors that contribute to PID at the environmental, system, module and cell levels. The occurrence of PID in modules can be due to one or a combination of some of these factors and the contributing factors might be different for different module technologies and areas of deployment.

High humidity and temperature are found to be the two most significant factors leading to PID. Various research experiments conducted at Fraunhofer ISE and NREL showed a strong dependence of PID on conditions of high humidity – beyond 60% relative humidity, coupled with high temperatures. Besides the environmental factors, a module’s design, glass and back sheet materials used, and cell properties can also result in PID-inducing mechanisms. Institutes such as Solon SE and PI-Berlin received a number of returned modules due to polarisation in the past five years, which indicates that the recent developments in module design and technology may be causing PID.

PID mechanism

A few different mechanisms can lead to PID, but not all of them are fully understood. Bauer et al. [8] found the combination of migration of sodium ions from the front cover to the solar cell along with EVA as the encapsulant and a silicon nitride anti-reflective coating (ARC) to be a common observance in modules affected by PID. It is hypothesised that during migration the charged ions reaching the cell surface accumulate there and contribute to an electric field which negates the passivation due to ARC, thereby increasing surface recombination and reducing the power output. The ions can also diffuse into the silicon causing inversion of the emitter region resulting in shunting of the cell.

Similarly, PID in some thin-film modules has also been associated with the migration of metal ions between the frame and the cell and significant degradation was observed in modules using sodium-containing substrates. However, further investigations are still ongoing to fully understand all aspects of mechanisms leading to PID.

Detection techniques for PID

With the significant decrease in capital cost for solar installations, the focus has shifted to the investments over a PV system’s lifetime, making it necessary to demonstrate a system’s reliability and bankability to investors. PID can drastically reduce a power plant’s performance if left undetected and consequently reduce the rate of return and profitability of the system.

Some commonly used detection techniques for PID are electroluminescence imaging (EL) and infrared imaging (IR) along with measurement of IV curves, which detect any drop in power and operating voltage. Light and dark IV can be used for detecting PID since affected cells would have lower module efficiency and operating voltage, and those severely affected would show reduced open circuit voltage as a result of shunting.

Conventionally, the modules would be dismantled and taken to a lab to conduct EL imaging and IV curve measurements but these can now be carried out in the field without uninstalling the modules. EL can be carried out on site using a CCD camera while applying a voltage bias to the module, but can only be conducted at night. IR imaging uses an IR camera while the modules are operating in the field, but PID might not be the sole cause of the hot spots and this method is not very accurate.  IV curve tracers that can be used in the field are also available through companies such as Solmetric and HT instruments, However, the process of testing each and every module can be time consuming and expensive. Therefore, testing modules for PID resistance before field installation where possible would prevent future detection and mitigation costs.

Predictive tests for susceptibility to PID

The main testing methods employed use an environmental chamber or the aluminum foil method to simulate PID in the lab. Experiments conducted at Fraunhofer CSE to compare the different testing methods for PID showed that using an environmental chamber, as specified in the standard, would result in more uniformity, control and reproducibility, with less variability. Some of the organisations offering PID testing based on different test methods are NREL, Fraunhofer, Intertek, TUV Rheinland, PI Berlin, and PV Evolution Labs. A brief summary of the tests being offered by a few labs has been provided in Table 1.

Organisation PID test [11]
TUV Rheinland 250C/-1000V/168 hours, aluminum foil method
PI Berlin 850C/85%RH/-1000V/ 48 hours
TUV Sud/ NREL/ IEC 62804 600C/85%RH/-1000V/96 hours in environmental chamber
Fraunhofer CSE 600C/85%RH/-1000V/96×3 hours in environmental chamber
Chemitox 600C/85%RH/-1000V/96 hours, module immersed in water

Table 1: PID tests offered by a selection of the main labs.

The tests offered by NREL and Fraunhofer CSE are based on the TS IEC 62804 test method. Tests are conducted at 60°C ± 2°C temperature and 85% ± 5% relative humidity conditions with a voltage bias of -1000V or the nameplate rated voltage, over a period of 96 hours, in an environmental chamber. The TS IEC 62804 uses a pass/fail criterion and a module passes the test if power degradation is less than 5%. This standard is based on the findings of various Round Robin tests, which revealed that 60°C and 85% relative humidity would be the most representative of actual conditions.

Thus, Fraunhofer CSE conducts PID tests based on the IEC 62804 and already tested various modules as a part of the PV Durability Initiative. The results from the first round of PVDI revealed that for most modules susceptibility of PID was detected during the first 50 hours, which is in accordance with the standard’s decision to test for 96 hours. However, the test was continued beyond the 96 hours and repeated two more times for a total duration of 288 hours, with interim characterisation, to increase the severity of the test and detect any late onset of PID in the modules. The rigorous testing ensured that the modules will work reliably over their lifetime and to prevent unexpected revenue losses.

PID mitigation

The industry has recognised PID as a potential threat and various solutions to mitigate and prevent PID are being developed. Companies like SMA have developed the PV Offset box, which is a commercially available technology to reverse the effect of PID since most cases of PID in silicon modules are reversible. Besides this, PID can also be mitigated at the panel level by using PID resistant encapsulants such as Enlight polyolefin encapsulant film and Ionomer films and chemically strengthened glass. These films were tested by organisations offering PID testing and proven to be resistant to PID. However, the phenomenon of PID was recently discovered and, currently, there is a lack of reliable and comparable data for PID in the field. Hence, the adequacy of PID mitigating technologies and test methods will be determined with time. Further research is required to understand the mechanism for PID time-scale for it to occur in the field, and the actual impact on large PV installations.

Electroluminescence images of a module before (left) and after (right) testing. © Fraunhofer CSE
PID mechanism in silicon solar cells. Image: Fraunhofer CSE
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