DC-INTF Alarm

When the inverter displays the alarm "DC-INTF" that means that a DC fault it being detected. This fault can be either internal or external so the troubleshooting steps below involve determining which one it is and how to go from there. 

1. Information that will help determine whether the fault is internal or external:

   1. How many times a day does the fault occur and at what time(s)? 

   2. How long was it after the system was installed before the first fault occurred?
   3. Are the PV string voltages and currents within the specs of the inverter? You can check the silver label on the left side of the inverter for these. 

2. Determine if there is a ground fault on one of the DC strings:

   1. Turn the inverter completely off first - AC and DC both
   2. If a battery is connected, pull out the battery fuses so that the battery is not connected to the DC bus
   3. Remove the PV strings with a technician screwdriver so that they are floating in free air - do this one string at a time
   4. With a multimeter, measure positive to ground and negative to ground for every string
      1. If there is stable voltage (not dropping) on one of the leads to ground then there is a ground fault on that line
      2. If there is no voltage to ground or the voltage drops to zero, then there is no ground fault on that string
      3. Also measure DC voltage to ground for the battery, these cables can be left landed if the fuses are out

3. If the system is using module-level rapid shutdown then troubleshooting will be trickier. 

   1. The best way to see if you have a bad MLRSD device causing the DC fault is to operate the system with just one string connected. 
   2. If the system does not throw the alarm, then swap out the strings and run it again. 
   3. If neither string on their own triggers the alarm but both strings together do, then the fault is likely internal. 
   4. If there is only one string with MLRSD, disconnect all of the MLRSD receivers and then run the inverter with full Voc. 
   5. If the system does not trigger the alarm when running without MLRSD, then one of the MLRSD receivers is defective. If the alarm does display when operating without MLRSD, the fault is likely internal provided that you have already measured the string voltages to ground. 
4. If there is second inverter installed next to the inverter having the faults, swap the PV strings, let the system run, and see if the other inverter also throws the alarm.

Technical Insight: Shading-Induced DC-INTF Alarms in Hybrid Inverters

In the field, the "DC-INTF" (DC Input Disturbance) alarm is often treated as a binary indicator of a physical fault—such as poor crimping, insulation failure, or ground leakage. However, recent field observations of the S6-Hybrid series suggest a secondary, transient cause: MPPT tracking instability triggered by severe string imbalance during partial shading.

The Mechanism of the Fault

When multiple strings are connected in parallel to a single MPPT input, the inverter expects those strings to operate at near-identical voltage levels.

1. The Trigger: In high-irradiance conditions, if an obstruction (e.g., a chimney or tree branch) suddenly shades a portion of one string, the bypass diodes in the affected modules activate to maintain current flow. This causes a discrete "step" drop in that string's voltage.

2. The Conflict: The parallel-connected, unshaded string continues to operate at a higher voltage. Because they are tied at the inverter’s DC input, the two strings are forced to reach a common operating voltage.

3. The Instability: Under certain conditions, the inverter’s MPPT algorithm may struggle to "find" a stable global maximum power point due to the distorted I-V (current-voltage) curve created by the mismatched strings.

4. The Alarm: This rapid oscillation or "hunting" by the MPPT, coupled with the potential for current back-feeding between the parallel strings, can create electrical noise or transient current spikes that the S6-Hybrid’s sensitive monitoring circuits interpret as an input disturbance—hence, the DC-INTF alarm.

Why This Is Different

Traditional troubleshooting focuses on steady-state faults. If your team is seeing this alarm only during specific times of day—correlating with known shadows—it confirms that the hardware is likely healthy, but the electrical architecture is being pushed beyond the tracking stability limits of the inverter.

Recommended Best Practices

If you suspect shading-related DC-INTF, consider the following diagnostic and mitigation steps:

• String Isolation Testing: During the time of day the fault occurs, isolate the strings. If the alarm clears when the "shaded" string is disconnected, you have confirmed that the fault is a result of the parallel mismatch rather than a permanent hardware failure.

• Reconfiguration: Where possible, move the shaded string to a dedicated MPPT input if the inverter layout allows. This isolates the shaded string, allowing the MPPT to track its unique I-V curve without affecting the unshaded strings.

• Optimizer Deployment: If the physical layout cannot be changed, consider adding DC optimizers to the affected string. This allows for module-level power management, preventing the voltage "step" drop that causes the parallel imbalance.

• Voltage Comparison: Use a data logger to compare the Vmp (Voltage at Maximum Power) of the two parallel strings during the shading event. A delta exceeding the inverter’s manufacturer-recommended threshold for parallel strings is a clear indicator of the source of the disturbance.

Note to the reader: This phenomenon highlights the importance of site analysis during the design phase. Even if an inverter is technically capable of handling multiple strings, the dynamic behavior of those strings under non-uniform irradiance is a critical factor in system stability.

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3. The system has an internal DC fault if the above steps have been performed and no external ground fault or faulty MLRSD receiver have been found to be the cause.