Potential Induced Degradation (PID) may happen with PV systems connected to transformer-less inverters. Smarter power electronics used in recent inverter design models make the need for transformers unnecessary as the voltage is produced directly from the power electronics. The benefits are increased efficiency and lower costs.

However, these new fully electronic transformers have a floating ground. This means that the negative pole is no longer grounded as it is with traditional systems. The string voltage will ‘float’ around the ground, for example in a 800 Vdc string, the positive pole will be at +400 Vdc and the negative at -400 Vdc to the ground. The aluminum frame of the module being connected to the ground, there is a large negative potential (Static electric difference) between the cells on the negative side and the frame. This negative potential impedes the free flow of electrons in the PV cell and reduces its efficiency.   The cells are connected in a series and thus this phenomenon affects the entire string. ( Full details here)   The reduction of performance gradually increases over time to reach up to 20%.

There are various methods to address PID:

  • Install ‘PID-free’ or ‘PID-resistant’ PV modules. However, no commercially available PV modules are 100% PID free.
  • Install grounded inverters. However, these are more expensive and less efficient.
  • Use a PV offset system that reverses the polarity overnight and ‘discharges’ the negative potential. (also expensive)
  • Use frameless (glass-on-glass) PV modules. Without aluminum frame, the issue don’t exist.

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Other performance factors

  1. GHI to POA ratio
  2. Far shading
  3. Inter-shading
  4. IAM
  5. Soiling
  6. Snow
  7. Low irradiance
  8. Temperature
  9. Module quality
  10. Mismatch
  11. Inverter efficiency
  12. Power clipping
  13. Wiring
  14. Transformer
  15. Dispatch
  16. PID
  17. Light soaking
  18. Aging