Issue:
RS485 communication failure between Inverter and Smart logger. After normal reset, it apparently comes back to normal. But the communication failure persists in rainy seasons. One of the rainy days, thunder stroke on the connection point of PV plant damaging the Current Transformer which leaded to shutdown of the plant.
Analysis:
As the RS485 communication failure happens during rainy season, it could be the thunder strike affecting the communication between inverter and smart logger. There are several reasons: no terminal resistor 120 ohms at the end of the last device in the RS485 group, RS485 cable’s shielded is not grounded or PE is not properly installed or RS485 board is defective. Moreover, all the physical connections need to be checked both visually as it may have loose termination and software.
The second issue is thunder stroke on the grid PV connection point. And CT (Current Transformer) at that point was burnt due to that strike. Without proper lightening protection, it may damage the whole low voltage dependent devices along with damaging the whole plant. Huawei String inverter already has SPD on both output and string input built in. So, it has less chance to get damaged from Thunder stroke.
To eliminate and find the root causes, all possible causes need to consider from visual to software checkup.
Below is the list we had cross-checked against the existing installation of the affected plant:
1) Max num inverter in RS485 group: Not more than 30
2) Cable Length: 1000m (incase longer length>1000m, repeater needs to be used)
3) Connection type: Hand in hand
4) Cable type: Shielded twisted pair (CAT5e), Connector RJ45
5) Cable installation: through iron tube (must not run parallel along High Voltage Cable)
6) Grounding: All the Metal chassis need to ground along with inverter
7) Resistor: installing 120 Ohm across terminal A and B (the last device)
8) SPD installation: In case inverter and RS485 is outside (prone to Electromagnetic influence), low voltage SPD has to be installed.
9) Software: bit rate matched with smart logger
10) Network IP address, Inverter address
11) Low voltage power supply and connection (whether it has surge protection)
After visual check-up, we found:
1) Current Transformer (CT) at the Grid connection Point connected to PQ meter found burnt, PQ meter damaged and the Power supply UPS for the PQ meter found damaged too.
2) Smart logger 1000 found damaged.
3) RS485 twisted pair shielding is not connected with ground (PE).
4) All the low voltage communication equipment does not have SPD protection.
5) Low voltage Power supply is without SPD.
Root causes:
Over voltage due to thunderstorm near the plant is causing the communication failure. RS485 twisted pair Cables’ shield is not grounded.
As the Low voltage Power supply is without SPD, the voltage spike due to Thunder has damaged PQ meter, Smart logger 100 and UPS.
Solution:
All the equipment’s grounding has been established and confirmed. Low Voltage SPD type 3 has been installed to prevent over voltage from Switching and Thunderstorm at the Low Voltage supply side.
Lesson to take away:
If any devices are installed outside or prone to electromagnetic effect due to thunder strike, SPD type 3 needs to install at low voltage Power supply. And for the safety purpose all the equipment needs to have earthing(PE).
References:
1) HCS-Field-Smart PV Controller Training Materials V1.0
2) Huawei Sun2000-(50KTL,60KTL,65KTL)-M0 User Manual
3) Electrical Installation guide 2018, Schneider Electric
