Re: Combining AC (mains) power sources

On the contrary inverters are remarkably efficient - around 95% or better - and big ones can do even better. Getting rid of waste heat is a pain, so perfectionism isn't the main motivation.

Electric cars are crawling with inverters, so there's huge market for some classes of inverters.

I think you might be able to repurpose one of those - look at openinverter.org Some of them require the control PCB replacing with something more useful, while some of them you can keep the existing control board and just speak CAN to them.

I'd guess you can likely program them for varying kinds of AC/DC power flow

- it's just a MOSFET H-bridge after all. The V2L/V2H/V2G (vehicle to load/home/grid) features of modern EVs mean those are able to push power both ways, and if you were able to couple two of those with an EV battery to buffer the outputs then it might work (ie onboard inverter one and two are 'charging' the battery, and onboard inverter three is 'discharging' the battery thinking it's in a V2H scenario).

Alternatively you could do something similar with solar+battery inverters - inverter 1+2 are charging the battery from mains, inverter 3 is discharging the battery at the same time.

I think you may be able to do it without a battery if you use a regular solar inverter as your DC to AC and a battery charger as your AC to DC. But needs careful control of the voltage as solar string voltages would expect to be higher than a typical 48V home battery.

Theo

Many of the off-grid hybrid solar inverters are designed to be paralleled up to six or twelve units. First example that comes to mind in your range is the EG4 Electronics 6000XP. It's one box with line power and solar MPPT inputs, a battery charger for 48V batteries, and a

Yes, they have a "Generator" input and can use that to both charge a battery and pass up to 6000W through to the inverter output, along with a line voltage battery charger input that can be connected to the grid but this does not pass through directly to the output. This is an off-grid system, not a grid-tie system, so the output cannot be connected to the grid. Look in the manual starting on page 22 for the input options. You may need to connect a small battery, or not - I've absorbed a lot of discussion on these units but have never wired one up myself. I knew what you say you wanted, and I'm pretty sure these will do that, but I can't give you the fine details and nuances.

If you want to use mains there may be serious legal obstacles: utilities are quite picky about devices that (at least potentially) can inject power to the grid.

With AC sources obvious condition is that they must be in sync. In small signal approximation each source has some voltage and some impedance. Voltages and impedances decide how power divides between devices. Using autotransformers you can adjust voltages to get more desirable power distribution. More modern solution could use inverter(s) effectively changing both voltage and impedance.

If you have control of devices you can tell them to adjust their voltage to get desired power distribution.

Considering your later examples, solar power naturally comes as DC. Batteries too. Wind turbines have speed of rotation depending on wind, so probably you need to convert to DC first (and then back to desired frequency). Classic engine-generator modules probably run engine at specific RPM to get desired frequency. Classic ones allowed independent requlation of voltage. I suspect that now some units use permanent magnets which probably do not allow requlation, so you would need autotransformer or inverter to adjust voltage.

If you combine sources as AC you need to look at stability of regulation, which probably means that you need to know characteristics of your sources.

The dynamotor motor generator sets of old were lucky to reach 70%~80% efficiency. I think modern electronic inverter technology is a more promising solution.

It does not matter how your device looks to the grid. What matters is how it look to utility lawyers ("there is galvanic connection between power source and the grid, illegal"). I suspect that in practice you are limited to commericial devices which have appropiate approvals. And your choice as a buyer will be limited by what utilities approve.

No, you did not understand. I mean inverter in series with power source (think of elecronic version of auttransformer), so inverter just needs to compensate for difference in voltages. Say, your source gives 110 V, but you need 120 V so that it "wins" with other source. In such case inverter needs to deliver 10 V (at source current), so its power is less than 10% of source power. Of course, this assumes reasonably well matched voltages.

Also, at high power it is resonable to build inverter with sections and activate number of sections matching load, so for rather large variation on load you can get close to maximal efficiency.

BTW: do you consider efficiency of wires?

Your problem is miniature version of problem that utilities need to solve. And classic solution is to essentially regulate sources. That is have requlators on generators and multitap transformers. If you can regulate sources, then you have clever solution. If your sources do not allow regulation, then you need to provide it externally (say via autotransformers or inverters). Some of your sources will need inverters anyway, so you can use them to provide regulation. Or you can use simple solution of convertiong everthing to DC and having big inverter whith multiple DC inputs and single AC output.

The Tesla Energy Gateway does the combining in the AC domain.

Power from the grid, battery inverter and solar inverter together with the house load all connect to the same bus.

There are a small number of operation modes:

In the most common configuration (called Time Based Control) at night the grid supplies all the power to the load. As solar power becomes available during the day the solar inverter syncs to the grid and supplants power taken from the grid. If there is excess solar available it then supplies the excess to the grid.

If the battery requires charging; excess solar power is directed to the battery. Excess power can also be directed to charge an electric vehicle.

Another selectable mode referred to as Self-consumption the system will avoid use of grid power at all and instead use battery power. I normally use this mode during the day but do not have enough guaranteed solar to use it year round. The battery inverters sync to the grid to be able to blend the power in this mode.

None if these controls requires any relays or contactors - it performed by varying the power of the battery inverter to either source or sink.

If the grid fails is is galvanically isolated from the system and the battery inverter becomes the reference for the solar inverters. The battery can either supply power if solar is unable to supply the house load or sink power if they need charging.

If the battery is fully charged there is then nowhere to dissipate the incoming solar power so the frequency of the battery inverter is raised to signal the solar inverters to curtail production.

The Tesla system does not provide for generator power to be merged into the load. To do so would require that it synchronizes the grid or battery inverter and there are very few (or maybe no) generators that can do that. Similarly wind turbines would need the same capability.

As a matter if interest most large wind turbines to go through a DC to AC conversion to provide flexibility in rotation speed. The couple of percent loss in efficiency is considered worth that.

Another mode that some energy management systems can provide is a "non-export" capability that allows solar power to be added together to grid power but never to the extent that power flows back to the grid. This can then then be used with Public utilities that don't allow export.

kw