This articles serves to explain the common issues that arise when installing systems with module-level rapid shutdown (MLRSD). The most notorious and troublesome problems are cross-talk and arc fault circuit interruption (AFCI) detection.
Cross-talk is what happens when multiple PLC transmitter signals "cross" over the DC lines and "talk" to receivers of neighboring inverters that do not belong to it. When cross-talk happens, the system could experience true or false arc-faults, system shutdowns, and even receiver failures. There are a few ways to avoid cross-talk when installing MLRSD:
1. Only use one transmitter per system. This is difficult or impossible for larger systems where one transmitter and core cannot accommodate all of the DC strings.
2. Therefore, if only a few inverters are being installed then it is recommended to install inverters without internal MLRSD transmitters and install one external transmitter instead.
3. If multiple transmitters must be used, the DC runs between the PV and the inverters must be kept physically separated by a minimum of 3 feet. Cross-talk happens when the PLC signal "jumps" to a line belonging to different inverter and transmitter pair.
***If you are planning to install more than one transmitter at the same site please review the Cross-Talk Application Note for the best way to design the system so that cross-talk does not happen. It is also recommended to read up on any material provided by the MLRSD manufacturer regarding cross-talk.***
Tigo's cross-talk info can be found by clicking here or seeing the attached PDF.
SunSpec's MLRSD best practice's can be found by clicking here or seeing the attached PDF. Pages 16-17 and 19-20 are what you should review.
When MLRSD is installed and AFCI is enabled, there can be false arc-faults. The reason for this is that the PLC signal can be detected as a false arc-fault if there is line noise or an abnormality with the one of the receivers/transmitters. Cross-talk can also cause false arc-faults to occur. Number one is to make sure there is no cross-talk, which will only happen if there are multiple transmitters on site. Number two is to make sure that the right transmitter is being used with the receivers being installed. Tigo receivers are not to be used with a SunSpec transmitter; SunSpec receivers are not to be used with a Tigo transmitter. Even though all of these devices can "do SunSpec", the poll rates may not be aligned, and that can cause false arc-faults.