A critical case in relations with the operation of an inverter SUN2000-60KTL-M0, where the case has been reported as an issue outlined as a malfunction in the operation of DC switch #2, where all its related MPPTs were out of service due to the generation of a warning alarm indicating a back-feed current linked with the PV strings connected to the MPPTs associated with the mentioned DC switch -alarm name “String Current Back-feed”- which in result, the normal operation of the MPPTs on the DC switch #2 side are halted and a consequence of a total power loss around 15 kw per day.
The inverter SUN2000-60KTL-M0 has an exact number of 6 MPPTs, where each MPPT has two inputs. In our case, the inverter was connected to 8 PV strings distributed along the inverter’s connection points where the connection of the 8 strings is accomplished through MPPT 1,2,3 and 4. The inverter operation status would become normal and returns back to the “Operating” working mode without any warning alarms once that any PV strings connected to any MPPTs associated with DC switch #2 (that is MPPT 4,5 and 6) is removed, which in our case, two full strings connected to MPPT #4 shall be removed.
In order to give a technical explanation behind the breakdown of the DC switch and all its related MPPTs, we need to analyse the problem by first referring to the inverter’s circuit structure and investigating the possible cause behind the generation of the warning alarm “String Current Back-feed”.
Inverter's Structure; The DC-AC converter unit of the inverter is linked to a power circuit where the connection of DC input takes place. This circuit is divided further into two separate coordinates, where each network consists of a DC switch with its related MPPTs, where a number of three MPPTs are associated with each individual DC switch and each MPPT has two inputs. The inverter can verify if the operation requirements from the DC input side complies with the operating standards and efficient operation through its input current check circuit.
As the issue is attributed with a back-feed current, this gives an indication of a voltage miss match between two parallel PV strings connected to the MPPT. This incident usually occurs when the number of PV modules connected to a certain string is less than the number of PV modules connected to the adjacent string, all of which are connected in parallel with the same MPPT. Another reason is when the open circuit voltage of one of the PV strings is different than the other string due to shading. A site visit was paid in order to perform measurements to narrow down to the actual cause behind the interruption of the inverter’s operation. The first step was to perform DC measurements to ensure that parallel strings have the same end voltage, which in results, all parallel PV strings connected to any individual MPPT of the inverter have the same end voltage as well as the same number of PV modules per each string. This has led into the conclusion, that the generation of the warning alarm has nothing to do with a voltage miss match between parallel PV strings, and further investigation must be taken in order to relate the warning alarm with the actual cause that has led to the generation of back-feed current.
The following figure is a screenshot taken from FusionSolar APP indicating the alarm warning along with the possible cause:
Upon the commissioning step where the inverter shall be turned on, during this step the inverter is considered to be in "stand by" working mode and the inverter continuously performs status check and enters the "Operating mode" once the operating requirements are met. According to the information provided by the site engineers and contractors working on the project, the inverter has detected an abnormal insulation resistance during the environmental status check, hence right before the operation of the inverter. Abnormal insulation resistance is usually caused due to damaged insulation of the DC input cables, short circuit or some PV panels might have been damaged which was then confirmed by the site engineers that a huge damage has occurred at the DC input cables with one of the PV strings connected to MPPT #4 as a consequence of an accident that occurred at the site. Unfortunately, this incident was neglected by personnel workers and was not rectified promptly and quickly.
Due to the disregarding of the low insulation resistance warning, the faulty string has caused a damage to the MPPT which is connected to (MPPT#4) and the whole MPPTs related to the switch where the faulty MPPT is connected to, had become defective and suffered as they are all assigned to the same switching coordinates. This kind of fault shall be rectified on the field quickly as its consequence would be a flow of high faulty current that would destroy the MPPT and the DC switch at any instance.
The Generation of Current Back-feed Alarm; The explanation behind the generation of the alarm is as follows: after the inverter was set to be commissioned and the warning of low insulation resistance was neglected, the threshold value of low insulation resistance was not reached. However, since there is indeed a low insulation resistance fault which would cause a leakage current to occur, only string current back-feed alarm would be generated, and after the low insulation resistance value caused the damage of the inverter's associated MPPTs linked to the DC switch #2, a generation of circulated currents have been flowing through the inverter's input. This is the cause of the generated warning alarm under the name "String Current Back-feed". All of this is due to an issue related to a low insulation resistance which has caused fault current to flow into the MPPT and hence, destroying the entire DC switch and then promoting the creation of circulating currents known as back-feed current, the inverter has generated the warning alarm and the MPPT#4 was completly out of service and is not generating any output power. By then, the DC switch was already being damaged since such faults shall be rectified on the field very quickly in order to prevent a permanent damage to the inverter’s internal circuit.
Amid the establishment of PV strings with the inverter, the positive or negative terminals of PV strings may be short-circuited to ground on the off chance that cables are not properly insulated. In this case, an AC or DC fault currents (short circuit) may occur and harm the inverter, hence the installer must be cautious when it comes to the installation of the inverter.
During the installation of any PV project accompanied with electrical connection to the inverter(s), the installer must follow the safety instructions carefully and ensure that all cables and electrical connections meet the operation requirements before ahead of final commissioning, especially in cases related to the insulation of equipment. This can be verified at the end of installation by using high voltage insulation testers for insulation measurements in order to prevent any dangerous accidents that may harm the inverter and interrupt its operation. It is also important to mention that damaging to DC input cables which are the main reason behind the breakdown of the insulation material might occur during project insulation when not complying to the product installation regulations, therefore, project managers must ensure proper training for personnel workers in such engineering tasks.
