How does Powerwall control solar inverters

[pgrovetom1]

I'm having 2 Powerwalls installed and was wondering if anyone here knows how the Powerwalls and their Gateway control/manage my existing 2 Sunnyboy 7000 Inverters? Solar Grid-Tie inverters are designed as current sources since they push all the power the panels can produce out into the PG&E grid and don't have the ability to be throttled. Tesla is pairing each of my Sunnyboy 7000s with one Powerwall 2. When in backup mode with the grid disconnected, the inverters produce their maximum power which is used to power my house and charge the batteries. But when the batteries are fully charged and the inverter power cannot be used, they must be shut down "somehow". The Powerwalls or the Gateway must have some mechanism to fool the inverters into shutting down.

As far as I can see on the schematic they gave me, it does not appear the Powerwalls are able to disconnect each Sunnyboy inverter using the UL1741 shutdown. The only trick I can think of would be if the Powerwalls shift the frequency of the AC off 60Hz so the Sunnyboy solar inverter sees this as a fault and shuts down. I cannot see any other AC "inband" method ( voltage, frequency, wave-shape etc..) and the Sunnyboys don't offer and other shutdown method.

Current source inverters raise their voltage gradually and slightly above the grid voltage such that their current is forced out into the grid. But if there is no grid or power "sink" and the power cannot be used, it will keep raising the voltage until it goes into a fault. I cannot see how Tesla can be sure that "any" grid tie inverter as a current source will behave properly when there is no ability to sink the current into the home or battery charging. Does anyone know how the solar inverters are managed/controlled once charging is complete and there is nowhere to use the power?

 

[Ampster]

UL 1741 includes protocols for frequency shift to modulate the Grid Tie Inverters. That is what the PowerWall uses also.

 

[power.saver]

To elaborate further, when the batteries approach being full, the Gateway begins to raise the frequency until the inverter shuts down. This typically happens at 60.5Hz, but may be higher if your inverters are configured to allow a higher frequency. Once the batteries power the house for a while and are no longer full, the frequency returns to 60Hz and the inverters come back on line. This process repeats until the grid outage is over.

 

[pgrovetom1]

Thanks. That makes me wonder? Is anyone aware of any good DIY efforts to truly build a Powerwall for backup purposes now that PG&E will be shutting off power in California in the Autumn fire season regularly. It would be difficult to duplicate the fancy Powerwall software APP but it should be reasonable to build a straightforward power loss backup system controller using Python on a Raspberry Pi, one or 2 Tesla $1000 5.2KWh on Ebay, a commercial automatic transfer switch, a large frequency adjustable sine wave inverter and a battery BMS and solar charge controller. The tricky part is whether it can be done code compliant. A commercial transfer switch helps to assure proper disconnect. Tesla is compliant. I wonder what it took? Anybody seen anything like this?

 

[Ampster]

Yes the tricky part is the code and UL rating. I bought an Outback Skybox for $6,000 and powered it with used Nissan Leaf modules. Not as robust as a PowerWall but it gets me through the PG&E power shutoffs. It is UL 1741 compliant.

 

[reddy]

I watched my installer configure my Powerwalls at startup. One of the fields was a pick list of various inverters, which I assumed loaded the protocol for inverter shutdown. In my case, it means shifting the frequency off 60hz.

I had a real world test a few weeks after installation when a bad windstorm knocked our grid power offline for 30 hours. When the Powerwall batteries reached about 97% the inverters shut off, and we went on battery power for several minutes, then the inverters restarted. It was rinse and repeat on shutdown / startup until sundown.

Here's what it looked like that day.

 

[pgrovetom1]

It would be difficult to duplicate the fancy Powerwall software APP but it should be reasonable to build a straightforward power loss backup system controller using Python on a Raspberry Pi, a few Tesla $1000 5.2KWh batteries on Ebay, a commercial automatic transfer switch, a large frequency adjustable sine wave inverter and a battery BMS and solar charge controller. The tricky part is whether it can be done code compliant.

I have a Telecom Engineering background but the part that is daunting is whether it can be done code compliant. It would seem that duplicating Tesla's architecture with a commercial automatic transfer switch based disconnect gateway run by the Raspberry Pi communicating with battery modules built around the available 5.4KWh Tesla batteries on Ebay would make sense. The battery modules could be run by Raspberry Pi's all networked. Each battery module would require a commercial BMS, charge controller appropriate for the Tesla battery and frequency agile pure sine wave inverter. The frequency control over the inverters allows for shutting off the solar inverters when the batteries are fully charged. The UL1741 has a requirement that solar grid tie inverters shut down when the "grid" frequency drops below 59.3Hz. This can be used by the software to shut down the solar grid tie inverters once the batteries are charged and turn them back on when charging is needed.

I haven't given this much thought and was wondering if anyone had actually done a workable code compliant system. It seems feasible. It should be possible to build it cheaper than the $1100 gateway plus $6700 per 14KWh PW2 plus expensive installation. I see a 4 module 85KWh Tesla battery for $1250 on EBAY. If its accurate, just one of those 4 modules should exceed a 14KWh PW. Otherwise one could utilize 2-3 of the $1000 5.4KWh modules. I'm curious if anyone has actually built one or investigated the code issues and components. Hopefully a code compliant commercial automatic transfer switch would go a long way to meeting code requirements. It would disconnect the home and alert the software to bring up a battery powered master inverter in the 5-7KW range. It would bring up the house and prime the solar inverters to begin startup. The charge controller could be run off grid power or off backup mode solar power. The home would be powered by all the inverters slaved off the main frequency agile master inverter. When the software detected the batteries were fully charged, the frequency output of the voltage mode master inverter would be dropped to 59 Hz. That would cause the UL1741 current mode solar inverters to drop off. The Raspberry Pi's provide easy to use WiFi which could be used to monitor the system via a simple Web interface in the home on a PC. When the power came back on, the ATS would alert the software and the master and any slave system inverters would shut off till next time. Its a bit more complicated than that but should be feasible.

 

[power.saver]

You might find this thread interesting... Building my own pseudo(Powerwall)

 

[pgrovetom1]

This is fabulous. I would like to see a final design description. It went through changes. It's just what I was thinking about. Can you message members to ask him for the final design and results?