CSLB C-46 Solar ContractorPhotovoltaic System InstallationHard
A solar technician is inspecting a PV array and notices a significant drop in power output from one string. Upon further investigation, they find that a portion of the string's modules is heavily shaded by a newly grown tree. How does this partial shading primarily affect the string's overall current (Isc)?
- AThe Isc of the entire string will increase due to the shaded modules acting as a short circuit.
- BThe Isc of the entire string will decrease to the level of the lowest current-producing (shaded) module.
- CThe Isc of the string will fluctuate rapidly as the shaded modules attempt to draw current from the unshaded modules.
- DThe Isc of the string remains largely unaffected, as voltage is the primary factor reduced by shading.
Show answer & explanationAnswer & explanation
Correct answer: B. The Isc of the entire string will decrease to the level of the lowest current-producing (shaded) module.
In a series string, the current is limited by the weakest link. When modules are shaded, their ability to produce current (Isc) is significantly reduced. Because all modules in a series string must carry the same current, the entire string's current will drop to match the current output of the lowest-producing (most heavily shaded) module in that string.
Why the other options are wrong
- A. Shaded modules reduce current, not increase it, and do not act as a short circuit in a way that increases string Isc.
- C. While there might be some minor fluctuations, the primary effect is a sustained reduction in current, not rapid fluctuation due to current draw.
- D. Both voltage and current are affected by shading, but current is particularly limited in series strings due to the 'weakest link' effect.
PV String Current Reduction (Shading)
In a series PV string, partial shading on one or more modules will reduce the total string current (Isc) to the level of the lowest current-producing module in that string.
- Series circuits are current-limited by the weakest element.
- Shading drastically reduces module current output.
- Bypass diodes can mitigate this by shunting current around shaded modules.
Memory trick: Series current is like a chain; the weakest link (shade) breaks the gain.