ac coupling

Principle

AC-coupling is the use of battery inverters and PV inverters in the same system. The different elements are connected together via the AC Lines.

                 

General system design

Studer-Innotec advises the following design rules for AC-coupling systems:
- The grid inverter power should be smaller than the battery inverter power

- For robustness of the system, use a mix of AC-coupling and DC-coupling

  • This allows black start of the units after and undervoltage of the battery

  • With AC-coupling, if the battery inverter is stopped, everything is stopped.

Simplified AC-coupling (no island mode) with AC flex

This case is for grid connected system with an existing grid inverter. A next3 can be added to increase solar quantity and add storage to the system.  This works with any kind of grid inverters and there are no special requirements on the grid inverter. The unit may not be compatible with frequency shift control, that is why it must be used with grid presence only and the grid inverter is disconnected when going to island mode. This prevents unwanted/uncontrolled charging of the battery. 

The connection/disconnection of the grid inverter is performed with the AC-flex relay with the proper programming given below.  

Setup

The AC-Flex must be configured as a load in the configuration wizard. Answer NO to question of genset presence.

Further settings are done later in dedicated menus accessible with the EXPERT user level. Change user level in the remote menu with the code 187382. 

The connection/disconnection in function of grid presence must be setup. Click on the AC-Flex part. And in configuration.


                 


                                    

Change for each phase (for 3 phased grid inverters), the operating mode to AUTO.   

                                  

And set the connection condition to “on-grid”. Add a small delay of a few seconds to be sure the connection to the grid is stabilized. 

                 


It is not a standard situation to have power coming from the AC-Load or AC-FLEX and this must be explicitly allowed with a setting, else an “back feeding” error will be raised. This is modified in the advanced settings of the AC-Loads.

                                    

                                    


And find the 0.1.6.26 in the list and set to YES.

                 

Example of use

All tests were performed at Studer factory with the Fronius Symo inverter.  
The Symo power production is monitored with the AC-flex measurement. The power flux on this connection are seen negative. 



The total consumption is the sum of the AC-flex and AC-Loads. The AC-loads alone can be seen on the dedicated screen of the portal.                 
Below 3 consecutive days with solar on AC-Flex in AC-coupling and on the MPPT entry.                  

Conclusions

The simplified AC-coupling as described in this document is possible with the next3 inverter. It is a first step toward a full AC-coupling coping with all the possible situations. Some improvement for the visualization (negative powers for production coming from AC-Flex) are possible and will be added with the regular update released for the next3.