NOTE: This document is a draft, and it has been written by someone who may not have the required knowledge and experience in both PV-systems and electrical engineering. Feel free to correct mistakes, or to delete nonsensical content.
There are multiple ways to structure the micro-grid, some of which were mentioned in the D1 Micro-grid meet-up at CCCamp2019. These different structures depend on the scale of the grid (in terms of power, distance, and availability), but also on safety requirements and existing regulations, which determine what can be done practically.
Very low voltage DC can not be carried very far, as the voltage drop implies a huge loss in efficiency. Multiple solutions were mentioned for this problem :
- High(er) Voltage DC, around ~300V : tricky to work with (expensive equipment required), and not very safe (DC is hard to break, especially at such voltages), and requires a licensed electrician
- Standard 230V AC : hard to synchronize properly, but already standardized. Sync problems can be avoided by only using unidirectional point-to-point links. The interconnection would present itself as a DC consumer to one side, and as a DC provider to the other. This way, standard, off-the-shelf equipment can be used.
- "Sneaker-grid" : carrying batteries using mechanical/human power
It may be useful to exchange data (supply, load, battery level, etc) between these subgrids. In that case, standard 230V AC would be practical, as it allows PLC using off-the-shelf equipment.
Short-distance interconnections can be done directly using DC current. Because of this, it may be useful to use a standard voltage. 42V was used at SHA2017, but 48V is a good candidate too (multiple of 12, lower than 60V). Even with DC interconnection there are lots of variations possible :
- The current flow can be bidirectional, or restricted to a single direction using diodes.
- The connection can be passive, or active (opening the circuit when required, based on loads/supplies on each side of the connection)
As these connections would not necessarily be point-to-point, it may be easier to conceptualize them as point-to-point links to a shared bus, which can be seen as battery. The diodes/switches would then be placed on the links to the shared bus. Designing and sizing this bus can be tricky (but very important) : ideally it needs to be able to carry the totality of the current supply, which could imply a big cable diameter. It may be possible to size the bus to less than the theoretical maximum current supply, but this would require some additional safety systems.
It should also be noted that the fault current calculation is more complex when there are multiple power sources
For active connections, it may be useful to exchange data between subgrids, for which a transmission method has to be determined. During the meeting, it was said that DC PLC is harder to do safely, as one would have to separate the signal from the relatively high DC currents.
A quick look on the internet suggests that DC PLC is also possible. This paper describes a DC PLC modem for PV cell monitoring.
The previous section covers short-distance subgrid interconnection, where each subgrid can be seen as a battery, but does not provide a solution for inter-grid PV-system interconnection (a concrete problem encountered by many at CCCamp 2019). Solutions to this problem were not discussed.
- A possible solution could be to avoid using batteries for each PV controller, and instead use batteries on the shared bus. This also makes the system "simpler", as the shared bus is already mentally modeled as a battery. This would require a load/supply management circuit at the batteries. Note that "power availability" cannot be solely determined from the actual load : if some batteries are charging, there is "available power", but it would not be apparent in the bus voltage. Because of this, this load/supply management circuit would need to communicate an "available power" metric to the possible loads/users. Also, this load/supply management circuit would have to be able to control the generators (PV systems) in order to avoid excess supply when batteries are full, unless this excess supply can always be used by some Power-to-X load. It may be possible to use the bus voltage to communicate this situation to the generators, but the load/supply management circuit may be more efficient if it has a precise idea of the state of all generators (independently from the loads), which cannot be communicated by the shared bus voltage. The disadvantage of this solution is that it requires an additional battery charging circuit (+ the load/supply management system), because the existing PV-systems battery charging circuits can not be made to work with this shared bus.