A critical case where A Sun2000 12KTL-M0 inverter that has two MPPT and four inputs which was connected to three DC strings was generating alarm ID 2002 DC arc fault with a cause ID 1. A site visit was paid to check on the connectors on the inverter side as well as the DC side. It was found that many connectors that were used on site are not compatible. Which was the first diagnosis of the issue that the incompatibility of the DC connectors was creating a loose connection which led to generate the DC arc alarm. The client was advised to change all the incompatible DC connectors and so he did.
On the other hand, after changing all the DC connectors on site to be compatible the same alarm has been generating frequently. Therefore, it was advised to check all the DC cables for any damages that might happened due to sharp edges or cable ageing and that was also not detectable on that level.
In order to track down the faulty string a trial and error method was used. In order to apply that we need to understand the cause ID of the fault which is described in Table 1 Alarm description as well as the string distribution among the MPPTs which are as follow:
Table 1 Alarm description
| Alarm ID | Alarm Name | Alarm Severity | Possible Cause | Trouble shooting |
| 2002 | DC Arc Fault | Major | The PV string power cable arcs or is in poor contact. Cause ID 1 = PV1 and PV2 Cause ID 2= PV3 and PV4 | Check whether the string cables arc or are in poor contact. |
MPPT 1: two strings were applied string A and B
MPPT 2: one string was applied string C
Because the alarm cause ID was 1 it was determined that the issue is existed on either string A or String B or both of them. To narrow it down; string A was connected to MPPT 1 and String B was connected to MPPT 2. The inverter was turned on and the inverter generated the same alarm with the cause ID of 1. This means that the string A was faulty. To double check the methodology, string B was connected to MPPT 1 and string A was connected to MPPT 2 and the expected result that the same alarm would be generated with a different cause ID which is 2. Surprisingly that was not the case. The cause ID of the alarm was ID 1 as well, which indicates that the fault exists on string B. To test again string B was connected to MPPT 1 and string C which is considered a healthy string is connected to MPPT 2. The inverter then was turned on and the same alarm was generated with Cause ID of 2; in which it indicates that string C is faulty.
The randomness of the alarm cause ID it leads us to one of the two conclusions which are:
1. The inverter itself is faulty
2. The inverter is detecting a faulty Arc
To explain the faulty Arc fault, we need to understand the behavior of the Arc itself and how could it be detected within the inverter. First of all, the DC Arc behaves as a high frequency white noise signal concentrate mainly between 10-100kHz. Where the Arc detection method measures AC component of DC current flowing through PV cable and analyze the frequency spectrum of measured value. In this case we need to look for any sort of disturbance that may generates this AC component in the DC current and indicates to the inverter that there might be an Arc fault. For that we have the following assumptions:
1. High frequency switching of power electronics device in power optimizer also interferes arc fault detection.
2. Common mode noise from adjacent connected inverter in parallel
Taking into consideration the above two assumption an SLD was required to be supported from the client side to show the connection of any parallel device operating. The SLD shows that there is an Air conditioning unit connected in parallel to the inverter which was the main cause of the interference of the Arc fault detection. To test that, we monitored the inverter in cases were the Air conditioning unit was turned off which in this case the alarm wasn’t triggered and in the other case were the Air conditioning unit was turned on which in this case the alarm was triggered.
In conclusion the alarm was triggered because of high frequency switching of power electronic devices connected in parallel to the inverter where it generated a false Arc fault alarm. As a solution it was suggested to turn off the AFCI feature to avoid any further generation loss until the issue with the AC connection is rectified.
To summarize what should be taken into consideration when looking into an Arc Fault:
1. Incompatible DC connectors
2. Broken DC connectors or PV cables.
3. Poor welding in the junction box of the PV module
4. False Arc fault detection.
