How to solve String Current Backfeed problem

Introduction

In this case study, we examine a scenario where DC backfeed was discovered. This article outlines the symptoms observed, the analysis conducted to pinpoint the issue, and offers practical recommendations to prevent such occurrences.

Issue description

The inverter is working, but the DC current back feed alarm is generated.

• Turn off the DC switch of the inverter and use a clamp meter to measure the DC input current. The current is still high. The current of the two PV strings connected to the first MPPT is about 7 A. The PV string current in other MPPTs ranges from 0.1 A to 0.7 A.
  
  
  
  

• When the DC switch is turned off, a PV string 1 reverse alarm (string current backfeed) is generated. When the DC switch is turned on, the inverter is connected to the grid properly.

• Wait for the current value to be small (below 0.5A), remove PV string 2 from the first route, and measure that PV string 1 has no current and the inverter does not generate an alarm.

• The damaged PV module is connected to the inverter. The PV module has been burnt twice before. This time, the PV module is connected to the first MPPT of the inverter.

Diagnostic tools

To solve the issue, you will need the following instruments:

• DC voltmeter up to the maximum system voltage range:

• DC clamp amperemeter;

Troubleshooting steps

Step 1: Turn off the DC switch of the inverter and wait until the current in the string decreases to 0.5 Amps or less.

Step 2: Disconnect both strings from the inverter and measure the DC voltage on both strings and compare them. If both strings have significant voltage differences, please check if the strings are not shaded.

Step 3: If there is no shade on the string, check if the PV panel quantity and PV panel type are the same for both strings.

Step 4: If possible, change the string connection terminal on the inverter to the empty MPP-trackers.

Conclusion

After the DC switch is turned off, the two PV strings connected to the same MPPT are still connected in parallel. In this case, two voltage sources are connected in parallel. When the two voltage sources have different electromotive forces, a circulating current is generated. For example, two batteries with different electromotive forces are connected in parallel. The battery with lower voltage is charged with a low voltage.

Two PV strings must use the same number of PV modules and the same PV module parameters under the same MPPT to prevent voltage difference between the two PV strings under the same MPPT and generate the current between PV strings.

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