Symptom:
During the grid-tied period of the project, the inverter displayed PV3 and PV5 input string exception alarm when the DC power was supplied. However, the inverter can be started and connected to the grid.
Cause analysis:
1) Power on the faulty inverter and connect it to the power grid. The inverter can work properly. The PV3 and PV5 input backfeed alarms are reported. The PV3 and PV5 input currents queried on the app are negative values, which are consistent with the inverter alarm information.
2) The currents of PV3 and PV5 read by the inverter are negative, but the current values measured by the ammeter and other routes are normal. If the total input currents of the three MPPT routes are the same, the PV4 route and PV6 route are faulty.
3)Check the DC-side port filter board and find that most of the Y capacitors on the filter board have cracks. Use a multimeter to measure that the capacitors are already open at this time. According to the information returned from the site and the weather, the problem may be caused by lightning strikes. The nominal withstand voltage of the Y capacitor on the DC port filter board of the Huawei inverter is 8000V, and the insulation voltage of the primary and secondary sides of the Hall sensor is 8000V. Therefore, the two components are damaged at the same time due to external high voltage. The external high voltage introduced to the inverter far exceeds the 8000V withstand voltage of the Y capacitor and Hall sensor.
As a result, the two components are damaged.
Conclusion:
The DC port filter board and DC current detection circuit are damaged due to high voltage on the DC side. The boost circuit and inverter circuit of the onsite inverter are normal.
Note: The lightning protection design of all electrical devices has its protection range. If the lightning strike exceeds the protection range, the lightning protection of the devices cannot be effectively protected. That is, the lightning protection is irrelevant to the product. The product meets the related quality requirements. Surge protectors are installed on both the DC and AC sides of the inverter. After lightning protection is performed, the main boards of the inverter are not affected, and only surge protection components work.
Model and capability of the SPD(surge protective device) on the DC side of the Huawei SUN2000-40KTL:
Model | PV20K510-MH |
Maximum continuous operating voltage Uc | 510VAC/820VDC |
Nominal discharge current In | 10kA (8/20μs) |
Maximum discharge current Imax | 20kA (8/20μs) |
Lightning protection capability of a single component
The following figure shows the principle of the DC surge protection solution.
Each MPPT (two PV strings) shares one surge protection module.
In the Huawei solution, the surge protection capability of a single DC+ or DC-cable to the PE is 10kA, and the total surge protection capability of two DC+ or DC-cables to the PE is 20kA. In Huawei string connection mode, two PV strings share the 20kA surge protection capability, which is more than four times that of the centralized solution.
