solar in combination with backup generator

[electrolust]

I don't know what I'm talking about so bear with me and correct my assumptions where necessary.

Residential PV cannot generate power when grid is down. This is to prevent danger to line workers. When I say cannot generate of course I mean cannot backfeed power to the grid. The only 2 solutions often adamantly stated are

a. Turn off the PV entirely when line/grid power is gone.
b. ATS or ATS-like switch that locks out grid but thus allows PV and/or battery power to remain online. (micro-grid or islanding)

So far so good?

Now if you add in a [whole house] backup generator, the genset and PV cannot coexist, because the PV will backfeed power to the genset, which is bad for the genset. So there seem to be 2 solutions for that configuration as well.

1. Turn off PV and run only on generator. Typical solution seems to be putting the PV power before the meter genset ATS grid input. The genset therefore never sees the PV power during an outage. A grid outage is seen by the PV and the genset together. So this is like the first solution above, but your topology is more limited. The limitation doesn't seem to be too onerous. This setup would work with any PV system.

2. Double ATS. PV must be fed ahead of the genset like #1. When the grid goes down, you transition to PV using the PV's ATS. When PV/battery depletes, only then does the genset ATS senses loss of "grid" power and kicks in. This has various issues but also seems to be able to work with any PV that already can island itself. The main issue I see is if you are drawing more load than the PV+battery can supply. The genset ATS isn't going to detect this condition. Seems that would be pretty annoying.

Now then,

per Enphase their system can allow PV and generator power to coexist, and not only share load but primarily use PV and only require supplemental power from the genset. See attached. To operate this way, their design requires pairing with their micro-inverters as well as their smart controller. That tells me that during grid outage they are instructing the inverters to do something different than normal. Also note the minimum genset capacity, even though the delivered power is limited to far less than the required output capacity.

When sharing PV power with grid power, this works because (AIUI) the line resistance of the grid is so high relative to local load, so first your loads consume PV power and second, grid power. If your load is less than what's supplied by PV the rest will be backfed to the grid. If your load is more than what is supplied by PV, you draw the excess from the grid. This must be order of magnitude difference in impedance, I suppose.

I am thinking something similar could be going on with their genset control scheme? Like if they imposed high impedance on the genset line input, the PV power wouldn't backfeed (or minimally so, hence why they a) qualify specific generators and b) require a minimum generation capacity which must have an influence) to the genset.

They limit the usable generator power to 48A but don't make any mention of PV power limitation. It's easy to have a PV system with more capacity than the 48A. So where would they shed that load? Or is that part of the inverter control requirement? They don't describe that the household load must also be limited to 48A, in fact they imply that the maximum is combined PV + 48A. So it isn't simply/only high impedance on the generator input, because then excess PV power would still be backfed to the genset, same as with grid.

So how do folks think this might work?

This generator capability was only released this year. I did a cursory look through their patents and didn't find anything that seems to be this.

[NiHaoMike]

They just reconfigure the system into "zero export" mode.

[David Hess]

When sharing PV power with grid power, this works because (AIUI) the line resistance of the grid is so high relative to local load, so first your loads consume PV power and second, grid power. If your load is less than what's supplied by PV the rest will be backfed to the grid. If your load is more than what is supplied by PV, you draw the excess from the grid. This must be order of magnitude difference in impedance, I suppose.

You are on the right track but the grid impedance is very low, which is how it regulates the voltage at each load.  A 200 amp change in load might only result in a 2 volt change in line voltage making the source impedance only 10 milliohms.

The low impedance means that the grid-tie inverter can monitor voltage (and frequency) to determine how much current to produce and disconnect if power is lost.

To operate this way, their design requires pairing with their micro-inverters as well as their smart controller. That tells me that during grid outage they are instructing the inverters to do something different than normal. Also note the minimum genset capacity, even though the delivered power is limited to far less than the required output capacity.

They probably have to configure the inverters to accept a wider variation in line voltage and longer timeouts because the generator has a higher output impedance.  So the inverters then take on more responsibility for regulating the voltage and frequency.