What is PID? What are the causes of PID? How can we protect our solar panels from PID loses?
Potential Induced Degradation (PID) losses is a recent phenomenon observed by solar photovoltaic module industry. PID is observed due to movement of the ions from solar cells and results significant degradation in the solar cell power output. The degradation due to PID losses can be as high as 70% in the first year of operation. If PID losses are observed, this leads to significant reduction in the performance of solar panels. The occurrence of PID is mainly dependent on electrical configuration of solar PV system. However, it is also dependent on the module operating temperature, humidity operating voltage as well as radiance level on the solar panel.
There have been several research paper published on PID losses and many organizations have identified the causes of PID losses. PID losses in solar PV system occurs when there is a high negative potential relative to earth. It is observed that if the negative pole of the inverter is ungrounded or in a bipolar configuration where the positive pole of the inverter is connected to the ground, the PID phenomenon can be observed. PID phenomenon is not observed in the systems where the negative pole of the inverter is grounded. If the system voltage is also less than 600 V, there are less chances of happening of PID phenomenon. Following diagram highlights the grounding of solar PV systems and how negative grounding can reduce the chances of PID phenomenon in solar PV system.
FIGURE 1: GROUNDED PV SYSTEM
FIGURE 2: UNGROUNDED PV SYSTEM
As we can see in the following figure, PID effect is more severe in the modules which have highly negative potential with reference to ground. The effect increases in the string where the negative potential related to ground increases. As we can see the effect will increase along the string towards module 10- 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.
How to reduce PID?
PV industry has initiated the PID test for PV modules. This test is conducted as per IEC:62804. Some of the companies have also introduced the PID reduction devices such as SMA has introduced a PID box. However, that is to be introduced once the PID phenomenon is observed.
The PID mitigation strategy adopted by various system integrators are as follows:
- Changes to electrical configurations of modules
- Replacement of the modules
- Installation of additional equipment such as PV offset box
- Any combination of these items
Module degradation can be classified as reversible as well as irreversible phenomenon. However, the PID effect leads to irreversible reduction in the module performance. It is difficult to detect whether the PID is being observed in your solar modules or not. However, detecting module performance degradation against reduction through PID can take several months to observe.
Many companies have introduced additional devices by which they claim that they are able to reduce PID effect or reverse PID effect through appropriate mitigation measures such as negative grounding of inverter. However, it may not be always possible. Typically, if the modules losses are less than 10%, it is very difficult to justify whether these losses are because of PID or not. However, if the system losses are beyond 30%, then it gives an indication that the losses are happening because of the PID. The solar industry has introduced two PID mitigation measures which includes:
- Charge equalizer
- High impedance grounding
It is up to the system designer to choose which method is suitable to him. It is also dictated by the inverter suppliers.
It can be used for transformerless inverters that can’t be grounded. PID occurs in an ungrounded system because of the high negative potential relative to the ground, the charge equalizer works by applying a reverse charge to the string at night. It applies a high inverse voltage to the modules, discharging this high negative potential during the night, reverses the polarization effect which occurred during operation.
A charge equalizer will not allow for full recovery of the module, may be a 5% permanent power loss occur at the module level. However, at the system level, this becomes unnoticeable. This method shown success in regenerating most modules which have been affected by PID for an extended time. Severely affected modules, but those with power loss exceeding 50%, may not fully recover.
High impedance grounding
A transformerless system, via a high value resistor can be grounded at the negative pole. This is considered as high impedance grounding. Additionally, a hardware has to be installed to provide for ground fault detection.
This method helps in reduction of the voltage potential on the string, thereby preventing PID. Recovery of PID affected modules will not be as good as with a charge equalizer, however.