Looking for a specific type of power conversion device

[DrDeke]

Greetings,

I am looking for a power conversion device with the following characteristics:

• Accepts 120 or 240 V single-phase AC input power from the electric utility
• Accepts DC input power from a PV panel or string of panels (with or without external MPPT)
• Provides 120 or 240 V single-phase AC output power
• As much of the output power as possible is derived from the DC input, with the AC input being used to make up the shortfall (if any)
• Never, under any circumstances, not even for a short period of time, attempts to feed any power back out through its AC input terminals

I would imagine that any such device would need to be contain a rectifier, inverter, and some kind of power-handling/control circuitry to make the DC bus powering the inverter draw power preferentially from the PV input (as opposed to from the rectifier). But I have only limited knowledge of power conversion systems, and I think any architecture that meets the requirements above would work for the application I have in mind.

I don't know whether there are any commercially-produced devices that do what I need and, if there are, I don't know what they are called (and thus what to search for). Any ideas or advice will be appreciated!

[NiHaoMike]

Look up "zero export inverter". A few even work using external sensors so you can have it feed the whole house, without exporting to the grid.

[DrDeke]

That would probably "work", but I would prefer to avoid paralleling this device with the utility at all.

[NiHaoMike]

Then look for the most common kind of zero export inverter that has a mains input and a mains output. Most don't even have the capability to zero export to loads on the input side. (One of the few exceptions to that is Sol-Ark.)

[Faringdon]

i dont believe what you want exists....but its easy to make out of the bits.

For a start, you want an inverter that can shovel 120-240VAC into your  wires.......they are offtheshelf items...........so you get one of them......look at what DC voltage it requires.........then you simply make a converter which converts mains to that DC voltage...and  another converter that converts the solar output voltage to that DC voltage ......

So then you just need some way of switching between the two.

This could be very simple......just switch on the solar one and see if it can provide the power (ie, the output voltage of it doesnt drop out).....if it drops out....then switch to the mains converter and use that for 5 mins......after 5 mins......try the solar one to see if the suns come out yet......etc etc.

Also, i think you should go for MPPT....solar panels are high Z and it'll mess up the power transfer if you dont.

...Of the 2 converters that are feeding the inverter with DC.....(one from panel, one from mains).....you can actually just have them both feeding the inverter input together..and just enable one,  and disable the other, and then vice versa......perhaps sense the current of the solar one...and if not enough current is flowing...disable it, then enable the mains one......then after say 5 mins repeat...etc etc.

A micro or Aruino sounds handy for doing the small bit of logic....Arduino is nice and simple to learn...youtube.

[f4eru]

I would prefer to avoid paralleling this device with the utility at all.

Why ?
it's technically, and economically the best solution.

[DrDeke]

I would prefer to avoid paralleling this device with the utility at all.

Why ?
it's technically, and economically the best solution.

I agree it is definitely the best solution both technically and economically. However, the electric utility here prohibits it unless you go through an expensive and time-consuming application/permitting/inspection process. And they have smart meters here which will send an alert message to the utility if it detects reverse power flow. And of course it will do this even if the reverse power flow was unintentional.

The scale of the system I would like to build is small enough that DIYing it seems like the only semi-economically-reasonable approach.

[DrDeke]

Then look for the most common kind of zero export inverter that has a mains input and a mains output

That seems like it might do exactly what I want! Do you have any makes/models in mind? I have not run across any like that so far.

[DrDeke]

So then you just need some way of switching between the two.

This could be very simple......just switch on the solar one and see if it can provide the power (ie, the output voltage of it doesnt drop out).....if it drops out....then switch to the mains converter and use that for 5 mins......after 5 mins......try the solar one to see if the suns come out yet......etc etc.

This would be a nice, relatively simple approach except that I envision the load always or almost-always being greater than the available solar generation here. So, I would need some way to make both DC sources share the load instead of the much simpler switching back and forth approach.

[NiHaoMike]

That seems like it might do exactly what I want! Do you have any makes/models in mind? I have not run across any like that so far.

The Schneider XW Pro is one of several out there, although it needs a battery and external charge controller to work. 48V of EV modules should be enough to act as a buffer.
https://solar.se.com/us/en/product/xw-pro-120-240v/

Or just look up "zero export inverter". The ones that have backup capability usually can only zero export to loads on the output. Might want to ask on a solar power group, should be able to find someone with actual experience to know which ones work best.

If you see a mention of HECO Rule 14H, that most likely means it can support zero export. (Double check since I think the zero export was a recent addition to the rule and there could be inverters built to an earlier version of the rule which did not require zero export capability.)