[zaarn]

The grid isn't a fixed frequency. While on average the grid doesn't change phase, the reality is that the grid may be out of phase by several seconds most of the time.

When the grid comes back from a blackout, chances are that it browned out beforehand so your sync is to a low frequency and coming back it'll be high frequency because the grid wants to compensate for loads jumping back on.

Additionally the components to generate your own waveform are cheap, yes, but not that cheap, adding them to the microinverter would increase cost quite a bit.

And you'd still need a transfer switch because if you happen to be 180 degrees out of phase, which CAN HAPPEN then your panel will behave like a dead short at double grid voltage. The current flow will definitely exceed the maximum tolerances and the magic smoke goes out.

You will absolutely need a transfer switch just so you don't fry all your devices the moment the grid comes back. Even then, syncing to the grid is a rather delicate maneuver since the grid will be constantly changing phase and it'll be simpler to shut down all inverters, connect back and have it all run back up on the grid itself.

[08-15]

This sounds awfully complicated, near impossible. You'd need a circuit that disconnects from the grid once it detects a brown out or black out, then produces a near sine wave all by itself, then re-syncs with the grid once it comes back up and transfers back. And all that has to happen cleanly enough that even a computer connected to it doesn't glitch, truly a "delicate maneuver". No way this can be produced for less than a couple thousands of dollars!

It's called a line-interactive UPS. You can buy it for $200.

[zaarn]

Line-interactive gets around it by simply using a inverter for both grid power and battery power, thusly being able to produce it's own sine wave inside the house. You can also use an VFI those convert grid to DC and then plug the battery in there, then convert back. Same effect.

A line interactive UPS does not sync necessarily sync you to the grid, though the electronic will usually try to keep it in sync. It's not necessary here.

[namibj]

You don't really need a transfer switch, just a big relay to connect the grid and your internal wiring. Disconnect as soon as the grid drops out, and sync to the grid before re-engaging. It's not a full transfer switch you require, and should be in the price range of an resettable fuse of the same amperage rating. Just add a small solenoid to trigger the spring-loaded mechanism to disengage the switch, and use a slow-ish geared DC motor to re-load the spring and re-engage the connection. It should not be hard to enable the inverter to sync that way, and these fuses aren't expensive either. And if you cut the inverter along with only some of the circuits in your house from the rest, which are still connected to the grid, you can easily ensure that no high-power devices draw power and cause the inverter to either go into overload or have insufficient input power (solar/battery) to keep the output voltage on target.

[raverbashing]

> When the grid comes back from a blackout, chances are that it browned out beforehand

Not really, but depends on the state of the infrastructure, if it was really because of a power overload, yes, but most likely "your circuit" (which could be your street or your neighborhood) got shut off, in this case there shouldn't be much difference

Yes, a phase difference of 180 can definitely happen but I guess most electronics can survive a 1/60s (I'd say even 1/10) switching time, which is probably enough to have the grid take over.

(Or of course you could have the grid and solar charging batteries then your own high power inverter for your house but that of course would mean $$$$$$)

[zaarn]

A 1/10th switching time would be fine though if you change phase too much and suddenly some equipment might not like it.

And you'd still have to sync the inverter, having the inverter simply continue to run until it's back in sync with the grid, then just reconnect (as a previous comment wanted) is likely not an option for most consumers.

For that it would likely be cheaper to have a full DC stage as you mentioned.

[loa-in-backup]

It's knowledge that people in question are lacking

[namibj]

Most computers can't handle 100 ms power loss, unless you actively throttle power consumption as soon as you sense the loss, in order for the filter capacitors in the PSU to last that long.

[bestouff]

Most computers are laptops today.

[Doxin]

Or you have your inverters run a phase locked loop. When the grid is there it'll keep perfect sync, when it's not there it'll keep running. Switching back to grid power should be as easy as waiting to reconnect until your inverters are back in sync, which depending on the PLL design shouldn't take long.

[zaarn]

Waiting to be back in sync could take forever, plus you need a Sync Check Relay. Price of those is usually "contact sales team" and they don't work reliably if you don't need more than a couple dozen kW of power. (They're intended for 1MW+ installations).

Just shut it down, flip the switch and restart. Everything else will just be prohibitively expensive because it needs to be very safe.

If you get the phase wrong then you'll either reduce the lifetime of your components or the components explode after the nearest power plant tries to pump all available power into your poor inverter.

[Doxin]

> Waiting to be back in sync could take forever

Only if you design it to. The phase locked loop would "listen" to the mains frequency and slew to match. Slew speed is simply a design parameter you can set to any value.

[stephen_g]

A phase-locked loop would fairly quickly come back into sync with the reference frequency - that’s exactly what they’re designed to do.

I’m pretty sure companies like Victron Energy already make these kinds of systems - combination solar inverters and battery chargers that have transfer switches to be able to seamlessly switch to UPS mode when the grid drops out but can still export excess energy when it’s up.

[zaarn]

Of course these systems are made but they're just very expensive and usually for customers of theirs that don't play around with a couple 100W panels because they want to save a buck or two in the summer.

[namibj]

The relay you need is a normal breaker with a solenoid to trigger the spring and a small geared DC motor to re-engage/reset the breaker.

And that's for what you need to allow your inverter to handle this automatically (you might need another voltage sensing channel to sense the grid-side of this breaker).

[lostapathy]

If this is such a hard problem, how does the $150 UPS hooked up to my home computers pull it off???

[jrockway]

Your UPS doesn't save money by discharging its battery back to the grid when it's not in use.

Your UPS handles an order of magnitude (or more) less power than a whole-home solar installation.

None of these problems are intractable of course, but you are oversimplifying the problem a little.

[lostapathy]

I'd be willing to accept not discharging my solar system's battery to the grid when not in use, I can use it at night.

One can get a UPS affordably that will power 1500 watts of continuous power. Being able to supply just that much, or twice that much, from solar panels in a grid-down scenario would be tremendously useful, even if it's not enough power to fire up my welder.