CSLB C-46 Solar ContractorPhotovoltaic System InstallationHard
A solar technician is performing an open-circuit voltage (Voc) test on a PV array string. The measured Voc is significantly lower than the expected value based on the module specifications and temperature. Which of the following is the MOST likely cause for this discrepancy?
- AA bypass diode within one of the modules has failed shorted.
- BThe array is experiencing partial shading.
- CThe ambient temperature is much lower than expected.
- DThe pyranometer used for irradiance measurement is inaccurate.
Show answer & explanationAnswer & explanation
Correct answer: A. A bypass diode within one of the modules has failed shorted.
If a bypass diode fails in a 'shorted' state, it essentially bypasses (shorts out) the cells it's protecting, reducing the total voltage contribution of that section of the module, leading to a lower overall string Voc. Partial shading primarily affects current (Imp) more significantly than Voc, and lower temperatures would increase Voc, not decrease it. An inaccurate pyranometer affects power measurements, not direct Voc readings.
Why the other options are wrong
- B. Partial shading primarily reduces current (Imp) and power, with a less significant impact on Voc.
- C. Lower ambient temperatures actually cause the open-circuit voltage of PV modules to increase, not decrease.
- D. A pyranometer measures irradiance, which is used to calculate expected current and power, not directly open-circuit voltage.
Bypass Diode Failure (Shorted)
A bypass diode failing in a short-circuit condition, effectively bypassing a portion of the PV module cells and reducing the module's output voltage.
- Reduces module voltage contribution.
- Can lead to reduced string Voc and power output.
- Often difficult to detect without IV curve tracing or individual module testing.
Memory trick: Low Voc? Check for 'Bypassed Blocks' or bad connections!