Raising the operating voltage of solar photovoltaic systems can reduce balance of system costs and improve the levelised cost of energy, but new research shows it also increases the risk of severe module degradation.
A new study titled ‘Degradation of photovoltaic modules at medium voltage with different types of encapsulants,’ examined potential induced degradation, or PID, in PERC photovoltaic modules at negative 1500 V, negative 3000 V and negative 6000 V for 96 hours. The negative 6000 V test is reported as the highest voltage examined in published literature.
The researchers found that degradation increased with the absolute value of the applied voltage. This was linked to changes in the electrical conductivity of module encapsulants under high electric fields. At medium voltage, polymer encapsulants may no longer behave according to Ohm’s law, with charge traps filling and allowing current to rise more rapidly as voltage increases.
Higher system voltages are attracting interest because they can reduce cabling and inverter costs. The study notes that moving from 1500 VDC to 3000 VDC could cut copper and aluminium consumption by 47% and reduce DC voltage drop power losses by 50% for a 2.5 MVA power plant. Trina Solar has also indicated overall capital expenditure savings of US$2.3 million for a 100 MWAC plant using 2000 V modules and inverters rather than 1500 VDC equipment.
The research tested four encapsulant materials at negative 3000 V: EVA, TPO, POE and ionomer. Modules using POE showed the lowest current voltage losses, confirming its value as a low conductivity barrier against PID. EVA and TPO showed higher degradation, likely because their lower threshold voltages allowed conductivity to increase more sharply under electrical stress. Ionomer, although a strong insulator at low voltage, also recorded substantial degradation.
The study found a strong correlation between maximum power loss and encapsulant conductivity on both the front and rear sides of PERC cells. Rear side losses were more strongly linked to conductivity, consistent with PID-p mechanisms involving electronic conductivity, while front side losses were associated mainly with PID-s and ionic conductivity.
A further finding was the first reported evidence of partial discharge degradation in photovoltaic modules, termed PID-d. This phenomenon occurred at the cell ribbon and produced electrical trees within the modules. The researchers warn that partial discharges must be avoided because they can lead to dielectric breakdown and say both encapsulant properties and module design will be important mitigation tools.
Modules exposed to negative 6000 V showed limited recovery after 20 hours of illumination at one sun intensity, indicating that severe PID damage at medium voltage may be difficult to reverse.
The authors conclude that the transition to medium voltage solar modules carries material risks, but that laboratory testing and careful encapsulant qualification can identify and quantify these risks before deployment. They recommend measuring encapsulant conductivity under realistic temperature, humidity and electric field conditions rather than relying only on low voltage measurements.
Link to the full paper HERE
Author: Bryan Groenendaal
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