Potential induced degradation (PID) is a phenomena that has only recently become a concern in the photovoltaic industry. PID impacts the ions of a solar cell and results in the degradation of the output of that cell.
PID can significantly reduce the power output of a photovoltaic (PV) module within the first year of operation, with power losses at the module level as high as 70% in the first 18 months. These module level losses can progress rapidly and become so severe that they affect the performance of an entire system.
Potential induced degradation is often seen as a defect with the module, but requires system level changes. The occurrence of PID depends mainly on the electrical configuration of the system and module design/construction. However, system voltage, temperature, humidity and irradiance also contribute to the occurrence of PID.
What causes PID?
It’s difficult to predict when PID will occur. As technical advisor for Munich Re’s Renewable Energy and New Technology (RENT) investment program, HSB is aware of a solar plant in Europe with modules from two different suppliers.
Although the designs of the systems in this facility are the same, potential induced degradation is only occurring in modules from Supplier A, but not Supplier B. We also know that PID is not occurring in other similarly designed facilities using modules from Supplier A.
PID occurs in systems where there is a high negative potential relative to earth. This happens if the negative pole of the inverter is ungrounded or in a bipolar configured system where the positive pole of the inverter is connected to the ground.
PID does not occur in grounded systems (Figure 1), where the negative pole of the inverter is grounded, or in systems less than or equal to 600V, which eliminates the high negative voltage potential that drives the PID phenomena.
Figure 1: Grounded PV system
Unfortunately, grounded systems are giving way to ungrounded and bipolar systems due to reduced costs and improved efficiency, and higher voltage systems are becoming more common due to recent legislation.
The effect will decrease along the string towards module 9- where the potential relative to ground is at or near zero. The other side of the string will have the highest positive potential in the module designated as Module 1+. In the positive part of the string PID does not occur.
Figure 3: Module layout for ungrounded system
In addition to the electrical configuration, there are several factors which can create the right conditions for PID to occur.
- Module design
- Quality control
What happens (or doesn’t happen) at the manufacturing facility plays a significant role in whether or not PID might occur in a given panel.
How to reduce PID
In photovoltaic plants with grounded electrical configurations, PID can be prevented reliably by grounding the negative pole of the inverter.
However, in systems susceptible to PID, it’s very hard to predict when and where PID might occur. HSB’s engineers recommend that a technical review for PID susceptibility be performed for any system where it can’t be confirmed that the negative pole of the inverter is grounded.
In cases where PID is possible, modules should be certified PID resistant by a reputable third-party organization such as PI Berlin, TÜV, or Underwriter Laboratories (UL). At this time there is not an approved industry standard for certifying modules for PID resistance.
However, the PV industry has created a series of tests, IEC62804, which is useful in measuring reliability. HSB engineers also recommend anti PID measures be implemented at the inverter until there is sufficient data to show that module certifications are effective.
Improper mitigation actions can result in equipment damage to the plant and may void some manufacturer warranties.
Mitigation strategy will vary from plant to plant and may include:
- Changes to the electrical configuration of the facility
- Replacement of modules
- Installation of additional equipment, or
- A combination of these
The process for detecting, confirming and mitigating against PID can take from several months to several years depending on the negotiations between the suppliers, operations and maintenance (O&M) providers, engineering, procurement and construction (EPC) contractors, owners and developers.
Many articles state that the effects from PID are reversible and that once the appropriate mitigation activities are implemented the modules will begin to recover, but this is not always the case. In precisely controlled laboratory conditions, some modules have shown improvement, but this depends on the severity of the power loss at the time recovery started.
Modules with slight power losses less than 10% generally recover, but those with severe power loss greater than 30% are not expected to fully recover. The strategy for resolving the effects of PID can have a significant impact on future plant performance.
There are currently two industry accepted mitigation options for PID:
- Charge equalizers, and
- High impedance grounding
The choice of which method to use depends on the system design, cost and compatibility. Any PID mitigation solution must be approved by the inverter OEM, module OEM and EPC contractor so as to not void the equipment or system warranties.
Option 1: Charge equalizers
A charge equalizer, such as the PV offset box can be used for transformerless inverters that can’t be grounded. Since in an ungrounded or bipolar system, PID occurs because of the high negative potential relative to the ground, the charge equalizer works by applying a reverse charge to the string at night.
The charge equalizer applies a high positive potential (inverse voltage) to the modules, discharging this high negative potential during the nighttime. This reverses the polarization effect which occurred during operation.
A charge equalizer will not allow for full recovery of the module. There may be a 5% permanent power loss at the module level. However, at the system level this will often become unnoticeable. This method has also shown success in regenerating most modules which have been affected by PID for an extended period of time. Severely affected modules, but those with power loss exceeding 50%, may not fully recover.
Option 2: High impedance grounding
A transformerless system can be grounded at the negative pole of the inverter via a high value resistor, (e.g. 22kOhm). This is considered high impedance grounding. Additional hardware has to be installed to provide for ground fault detection.
This method can reduce the voltage potential on the string, thereby stopping or preventing PID. Recovery of PID affected modules will not be as good as with a charge equalizer, however.
- HSB’s Solar Shortfall Coverage (product details)
- Insurance for when the sun doesn’t shine (video with John Stokes)
- Blackout implications of a solar eclipse (past blog post by climate experts at Munich Re)
© 2015 The Hartford Steam Boiler Inspection and Insurance Company. All rights reserved. This article is intended for information purposes only. All recommendations are general guidelines and are not intended to be exhaustive or complete, nor are they designed to replace information or instructions from the manufacturer of your equipment. Contact your equipment service representative or manufacturer with specific questions.