How do you combine 2 AC power sources?

Question

To combine 2 DC power sources, you can diode OR them both together. You can't just directly connect them together because their voltage outputs may differ ever-so-slightly, leading to imbalanced currents as well, causing one generator to try and kill the other by driving current into it.

To passively balance the two DC sources, you can add 2 small (small resistance, high power) resistors, one in series with each of the loads, before or after the diode, before the junction of the two.

To actively balance the two DC sources you can use a microcontroller and control logic to PWM the side delivering more current until it delivers identical current to the side delivering less.

But...how the heck do you combine two AC power sources? It seems to me you'd have to synchronize the AC waveforms somehow. Let's look at solar panels on your roof. I hear some microinverters require "5 minutes to synchronize with your house mains power", meaning somehow, they are doing this. But how do you combine the two? If you put one AC wave from the house at 110V RMS on top of another AC wave from the solar panel at 110 V RMS, you'd double the amplitude to an RMS of 220 V, which seems wrong. If you put them out-of-phase you've just neutralized the AC mains power in your house, not added to it....so I don't get it. I have no idea how this works, how to approach the problem, or even how to conceptualize or think about the problem.

Sort of adjacently-related:

1. https://physics.stackexchange.com/questions/515204/how-does-an-ac-circuit-get-anything-done
2. How do you determine the direction of AC current? (ex: whether your solar-powered house is drawing from the city power grid or adding to the grid)

Thought experiment (added 24 Oct. 2022)

From this answer:

The reason you don't get 220 V is that the incoming AC supply and the inverter(s) are wired in parallel, not series.

I think there's more to it than this. One way or another, the solar power needs to feed the grid. That requires a voltage potential into the grid.

As a thought experiment, if you put two 10 V voltage regulators in parallel, and they each output exactly 10 V, then the results will be 10 V. If the total current draw on that voltage output is 1 A, then each regulator will deliver 0.5 A, again, because they are exactly balanced in voltage in our hypothetical example. Now, to try to get one of them to deliver let's say 0.9 A and the other to deliver 0.1 A, you'd have to try to change their voltage output settings to make the 0.9 A one have lower resistance and a higher set-point on its voltage output.

So, to feed the AC grid I think you'd have to do the same: set a set-point slightly greater than the Vrms of the grid. I'd need to think this through some more and analyze it. I'll save that for future work another day. This feels like a multi-day problem.

Answer 1

You have answered your own question. The AC sources MUST be synchronized before merging using fuses plus low-ohm resistors and high-current inductors (to allow slight shifts in phase and voltage). FIRST you must verify with a 2 channel oscilloscope that they are in phase, and the same voltage and current rating. The fuses or breakers sourcing each AC supply MUST have the same rating.

This means that separate AC sources need to come from the same breaker panel to have any chance of being in phase. The other option (mentioned by Peter Bennett) is expensive grid-tied inverters that will sync to the grid first before connecting to it. This allows multiple sources to feed a common grid, but this is expensive application-specific hardware.

If NOT in phase another safe option is to convert both to DC, then use synchronous inverters or just sum the DC currents to drive a bigger inverter ( which will require its own high-current breaker on the outputs). This is a place where caution must dominate your decisions, or things may go BANG and you get hurt.

You CAN dependably connect 2 AC sources IF they are synchronous or can be made synchronous in a given time period. It is assumed both sources are at about the same voltage level and can supply matching currents from sources of similar impedance. Else you cannot just merge 2 AC sources without validating the requirements I just mentioned.

Unless you NEED to have a single source of high AC current (such as running a high-powered arc welder) you are better off with lower current AC sources with safe and legit-sized breakers. Breakers ONLY stop wires from catching fire, if they have correct ampere ratings.

If a AC source is weak and dynamic such as solar cells piped through an inverter, they cannot be merged with conventional AC source that is stable. Dynamic(and uncontrolled)AC sources cannot be summed together due to backfeed and driver issues, but DC sources can.

Answer 2

^{simulate this circuit – Schematic created using CircuitLab}

The reason you don't get 220V is that the incoming AC supply and the inverter(s) are wired in parallel, not series. If your supply is 110V 60Hz, then the inverter will also put out 110V 60Hz, synchronised to the supply.

Most of the time the inverters spend synchronizing is actually self-testing and then checking that the incoming AC supply is stable and within specification.