Multiple grid-tie (non-micro) inverters

Question

This is a question that just occurred to me. Assume you have multiple grid-tie solar PV inverters connected to your home load center. What would happen if, while in operation, you were to open the main circuit breaker?

The inverters are monitoring the AC side to detect loss of grid power to shutdown. But wouldn't each inverter see the other's output as grid power and continue operating?

Obviously, this is not a problem with micro-inverters as there are generally multiple on a house. Is it a problem with full-sized inverters? If not, how is this solved (or how do micro-inverters solve this)?

In case it matters, I'm specifically asking about US split-phase 240 volt systems.

Answer 1

Yes, your grid tie systems could see each other as a grid that is still running, but ONLY if they are putting enough power out to keep the voltage up. If the grid is down, and still connected, it is unlikely that you would be putting enough power out to keep the grid up, so if the grid is connected they should see it go down and act appropriately.

If the grid is disconnected, each unit should find the other is unwilling to accept any current, if they are not under load, on the other hand, one unloaded unit may prop up the loaded unit. In theory they should try to help each other, within the limits of available power. If they can't find a happy balance, one or both will shut-down using whatever fail safe they have.

It is possible that one will not like the quality of the power from the other, especially if the sine waye isn't as pure as they need for syncing up. In that case they should simply shut down.

However no matter what, you should discuss your intended design with the manufacturer, to be sure they don't have a problem with what you are trying to do. There may be some additional equipment, connections or precautions to be taken.

I have found grid tie systems to be incredibly intelligent, and reliable. I used them when I was building a critical lab that needed quality power, and I used them without solar inputs, rather I had one set being used to charge battery from the grid, and generator as backup, with a second set that simply ran off the battery. Clearly I had both grid-tie AND battery storage capabilities, I know many newer grid-tie systems don't support battery. No matter the issues around any power outage, my battery backed power source was solid with zero interruptions for 15 years straight. All maintenance, including periodic refresh of battery, installing new generators, replacing pole transformers, and even weeks long power outages were all handled by the system gracefully with no issues. I was using Trace inverters, purchased around 1996. Trace has changed names, and has newer models now, but they were so darned flexible, and so much cheaper than the more traditional systems for lab/datacenter power... They had three inputs, two AC inputs and one DC input and I could current limit each input to match the source capability. the DC and City connections were bi-directional, and the generator input was input only, at least that is how the software worked. In practice I had inter-tie functionality turned off, except when I was doing off-grid experiments that mimicked your scenario.

I did play with them when I first bought them, just to answer questions like yours, and I was very impressed with their capabilities. If I was building a power plant again today, I would very likely try to use the same type of equipment. Of course you have to be an engineer to know what you are doing, and know the electrical code well enough to stay safe.

Will your systems work as well as the systems I used? Who knows? So your mileage will vary. Not all systems have the same features that the Trace units had in 1996.

Answer 2

In my limited understanding (I ended up not putting in an off-grid system but did a lot of research first) odds are excellent that they will see the effect (voltage spike, or drop, likely frequency bobble) of opening the breaker as reason enough to go off-line, and then they won't see a grid and will stay off-line. Grid-tie systems are quite picky about the quality of the grid and will go offline if there's any doubt about it; when it comes back, they will monitor it for a while before connecting.

There may be other mechanisms by which they would notice - if there's no grid connection to absorb excess power (perhaps, perhaps not) and supply power shortfalls (here, they will see the voltage drop below tolerance once and promptly go off-line, for certain) as supply & loads vary.

Functionally, grid-tie means you need some other source of back-up power, as you are tied to the grid including when it's down, even if the sun is shining. Off grid is a different animal with different issues (batteries, battery maintenance, battery life...) In most places with grid access, the grid is reliable enough that the overhead of an off-grid setup is not an economically viable choice.