Single diode / substring

Definition

A panel's cells are usually divided into three substrings, each protected by a bypass diode. When a substring cannot produce current, or its diode is shorted, the current flows through the diode and the energy from that substring is lost. This class describes a panel in which a single substring has dropped out.

Signs in the imagery

In the thermal image one lengthwise band of the panel, a substring, is uniformly warm with sharp boundaries, while the rest of the panel is at normal temperature. The junction box area may also look warm.

Possible causes

• A shorted bypass diode
• A broken ribbon or cell interconnect in the substring
• Shade or heavy dirt covering the substring for long periods
• A contact problem inside the junction box

Effect on production and safety

The output of the dropped substring is lost and the panel's power falls accordingly. Because the diode carries current continuously, the junction box heats up and the diode's life is shortened. A second diode dropping out on the same panel increases the loss quickly.

Recommended action

First check the RGB image for shade or dirt covering the substring. If there is no such cause, inspect the panel in the field, check the junction box and diode, and replace the panel if needed. At the flight after the repair, confirm that the finding closes on the same panel record.