Looking at the data logger the client notices that his PV system is generating less energy than the previous day, in spite of the sky being cloudless again. The summoned solar installer tests the PV system using the TRI-KA and sees in the characteristic curve that the voltage of one string is significantly lower than normal.
The solar installer inspects the system visually and finds a module heavily soiled by bird droppings. This soiling has resulted in the reduced output of the PV system.
So, what happened? If a larger area or complete cells are shaded, the circuit in the partial string is interrupted. Now one or more bypass diodes are activating. A characteristic curve analysis with the TRI-KA shows this as a typical bend in the voltage and output curve.
Since the bird droppings are partially shading several cells, the current travelling through the solar cell is reduced. If the shaded area is very large, no current is travelling through this solar cell and one or more bypass diodes are activated. The current is diverted via the bypass diode, and the solar module loses output. This becomes visible in the string voltage, while the current remains almost constant. The graph shows this output loss as the bend in the measured green (or the extrapolated red) curve.
After cleaning the concerned modules the system runs perfectly again.