Solutions of the Potential Induced Degradation (PID) Effect
2022-03-28 14:45
PV module is the most expensive part of the entire PV system, module failure directly affects the power generation profit of the whole project lifetime. While module failures such as shattering, hidden cracks, and hot spots are irreversible, the PID effect of a module is a failure that affects power generation efficiency but can be prevented and repaired.
PID Effect
The PID effect of the module is that the module is under the condition of high voltage for a long time so that there is leakage current between the glass and packaging materials, and a large number of electrons gather on the surface of the module, which makes the passivation effect on the surface of the battery deteriorate, and the filling factor, open circuit voltage and short circuit current of the module decrease resulting in the module performance degradation. In extreme cases, the PID effect will lead to more than 50% power loss of the module and affect the power output of the whole string.
Figure 1 Electroluminescence display
Causes of the PID effect
The external causes of module PID effect are mainly related to the humidity, temperature, pH, and ion pollution level of the environment, installation and construction process; the internal causes are mainly related to the system voltage of the module, packaging material, and cell manufacturing process.
External factors leading to module PID are mainly related to the environment of the project. In the high temperature and high humidity environment, steam gets into the module, which increases the conductivity of the module packaging material and causes the PID effect; in the acid or alkali and ion-polluted environment, chemical reactions occur in the module glass and frame area, causing ion precipitation, which also increases the conductivity and causes the leakage current; in the process of module transport, installation, operation, and maintenance, the factors that impact the close combination of the module packaging material and the module glass are also the reason for PID effect.
For the internal factors leading to the PID effect, the EVA (Ethylene-vinyl acetate) packaging material of the module can not completely block steam and easily decompose in water to produce free ions causing the PID effect. compared with the EVA material, steam blocking and anti-deteriorating performance of POE (Polyolefin elastomers) packaging material is more stable. POE material's steam transmission rate is only 1/8 of EVA material, but the cost is much higher than EVA material; in the module manufacturing process, the uneven cell doping leads to uneven cell resistance, resulting in PID effect; compared to micro-inverters, to match the string inverter system which voltage above 1000V, the higher potential difference is more likely to cause PID effect.
Inverter solutions for the PID effect
The inverter, as the key equipment in the PV system, is also capable of preventing and repairing the PID effect of the module from the electrical system side.
On the system side, the grounding of the inverter and the position of the modules in the array determines whether the cells and modules are subject to positive or negative voltage bias. If all the modules between the middle module and the negative output of the inverter are under negative bias, the PID effect will be more obvious the closer the modules are to the output; while all the modules between the middle module and the positive output of the inverter are under positive bias, the PID effect won't be obvious.
1. Inverter's Anti-PID Function
GoodWe inverters adopt the solution of AC voltage equivalent DC level lifting. The inverter has a built-in anti-PID module that compensates for the voltage difference between PV- and PE by lifting the equivalent DC level of the three-phase AC voltage and adding a positive voltage bias between the equivalent DC level of the three-phase AC voltage and PE. Since the voltage of the negative pole to the neutral point of the DC side is half of the PV string voltage output, the potential between PV- and PE can be raised above 0V when the increased positive bias between the three-phase AC equivalent DC level and PE is greater than 1/2 Udc.
GoodWe's Anti-PID is full-time prevention, and the prevention work signal can be displayed in the background during the prevention operation, and the Anti-PID function can also be turned off by software.
2. Inverter's PID Recovery Function
The GoodWe PID repair solution uses a positive bias voltage adding measure with a module built into the inverter. This module converts the AC power obtained from the internal AC side of the inverter into equivalent high voltage DC power to generate a positive voltage of about 700V between PV and PE when the inverter is not operating at night. This fixes the PID effect of the module by flowing the impurity electrons from the PV cell back to the glass panel.
GoodWe PID repair solution starts the built-in PID repair module when the string voltage is detected under 50V, and the PID module stops working when the inverter is working normally to reduce the loss of the whole system. The PID repair module function cannot be shut down by software, but it has corresponding over-voltage and over-current protection devices, and the inverter shuts down the string output when ground insulation, over-voltage, and over-current are detected.
Options for prevention and recovery
Since the PID effect of the module is mainly influenced by environmental factors, the actual construction environment of the project should be considered when selecting the function of PID prevention and repair. The selection recommendations for prevention and repair are shown below.
Project Conditions |
PID Recovery |
Anti-PID |
Newly built coastal projects |
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Dry Environment |
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Argo-photovoltaic |
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Retrofit existed projects |
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Color Steel Tile projects |
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Desert |
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Salt Mist/Acidic and alkaline environment |
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Conclusion
In the whole life cycle of the PV system, it is necessary to focus on the cost of the project, safety of system operation, and power generation revenue. Under the current trend of parity price and subsidy cancellation, how to keep the power plant revenue is a problem that every constructor needs to face. For the obvious system failure, it can be solved by strengthening the production management and improving the operation and maintenance level. But like PID effects such as invisible failure, system selection configuration, and other aspects of PID prevention and repair issues need to be fully considered in the module production, construction and installation environment which should be to stop the loss in time and protect the safe operation of the system.
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