Inverter Underproduction / No Production (Causes and Solutions for Negative Current and Power under the Same MPPT)

1. Inconsistent Number of PV Modules per String

• Cause:
  When multiple strings are connected to the same MPPT and the number of photovoltaic (PV) modules varies between strings, the resulting difference in open-circuit voltages causes the higher-voltage strings to backfeed into the lower-voltage ones. This can lead to negative current and negative power issues.

• Inspection Method:
  Check whether each string connected to the MPPT where negative current is detected has the same number of PV modules.

• Solution:
  Reconfigure the PV array on-site to ensure that each string under the same MPPT has an equal number of PV modules.

2. Mismatched PV Module Brands or Models

• Cause:
  Connecting different brands or models of PV modules under the same MPPT can result in mismatched open-circuit voltages. This may cause higher-voltage strings to backfeed into lower-voltage ones, leading to negative current and negative power.

• Inspection Method:
  Verify whether the PV modules connected to each string on the affected MPPT are of the same brand and model.

• Solution:
  Reallocate PV modules so that each MPPT only connects to strings using the same brand and model. This ensures consistent open-circuit voltage across all strings.

3. Shading on One or More Strings

• Cause:
  If one string under a shared MPPT is partially shaded, its voltage will drop below that of unshaded strings. This voltage difference can cause current to backfeed from the higher-voltage strings into the shaded one, resulting in negative current and negative power.

• Inspection Method:
  Check for shading on any of the PV strings connected to the MPPT.

• Solution:
  Remove or minimize shading where possible, or consider separating shaded and unshaded strings across different MPPTs.