The main components of the source selector are two Opto Triacs wired in a SPDT (Single Pole, Dual Throw) switch fashion (Blue and Red boxes).
In case both Triacs are ON at the same time, a source paralleling condition occurs. This means that the Inverter output and the Mains are connected together. This would create a severe short circuit because the inverter is not synchronized with the grid. The result in a damaged inverter.
To avoid source paralleling, an interlock circuit (pink box) is placed between the processor and the triacs. This circuit based on CMOS logic inhibits simultaneous activation of both triacs.
The synchro optocoupler (yellow box) generates a pulse each time the Output voltage crosses zero. This happens 100 times per second in Europe and 120 times per second in 60 Hz networks.
The microcontroller board receives the switching commands through a CAN bus. In my System, these commands are provided by the BMS (Battery Monitoring System).
A three position switch controls the operation mode:
- Automatic mode (middle position) : the selected switch is controlled by the CAN bus
- Solar mode: the Solar Opto Triac is always on, regardless of the CAN bus commands
- Grid mode: the Grid Opto Triac is always on, regardless of the CAN bus commands
Three LEDs indicate the operation mode and the selected source.
A manual override Circuit Beaker is usefull in case of failure of the box.
About the Microcontroller Board:
I have chosen an Embedded Artist AOAA board. This board is really fantastic and I was sure, at the beginning that it would have enough I/Os and power to do the job. Of course, this board is overkill and people ask me if such and such board would do the job as well:
1 - Any Arduino or Atmega based board can do the job
2 - Any PIC micro can do the job
3 - A Raspberry Pi or any linux based board is definitely NOT a good choice for this application