Isolating generator from solar system

electrofelon

Senior Member
I am not a power electronics engineer, but I dont think there is much difference between a stand alone inverter and a GTI other than programming. I suspect the anti islanding requirements and having to incorporate transfer equipment and, in the case of paralleling with a generator, some sort of industry standard generator-inverter control protocol is why it doesn't exist.
 

jaggedben

Senior Member
SMA doesn't really count because only ~1000W or so is available from the PV system when it is running off grid no matter how large it is. I don't know about Trace, but I still submit that if were easy and/or cheap to run a PV system off grid without storage and supply up to its full capacity on demand, it would be widely available. It's not like no one wants it. If you know how to do it, do it; it will make you very wealthy.
The two issues are really the same issue; they are about the ability (or lack thereof) of a grid tied PV system to supply power on demand in response to a load. The problem with all three scenarios you propose is that the home load is not a static quantity; it could conceivably be zero. A voltage source like a generator has no problem tracking the load but a current source like a grid tied inverter (at the present state of the art) is a different animal.
A few points:

Nobody was talking about having a grid-tied PV inverter 'supply its full capacity on demand' in an offgrid scenario. The discussion was almost precisely the opposite; it was about whether a grid-tied PV inverter can be re-programmed to supply less than its full capacity in order to follow the load. Specifically the question was about whether it could load follow along with another generator that's doing the same to fill gaps in the PV inverter's ability to supply demand.

We have examples of inverters that can switch from grid-tied current sources to load-following voltage sources in seconds or less. SolarEdge StorEdge, Powerwall, Enphase IQ8, and Outback can all do that in one way or another. They are not different animals, they are the same animal that behaves differently in different scenarios. So we know that the core components are not hardware-constrained to be current sources. It's clearly about controlling things properly and not conflicting with other generators.

Notably, I don't know of any yet-released PV inverters without storagethat can swiftly switch from current source to voltage source, although the long delayed Enphase IQ8 is touted to be such. But I think that may show that the engineering problem with grid tied PV inverters switching to a voltage-source mode has less to do with the grid-tied part and more to do with the PV part. After all, PV cells are not a stable source of power and that, rather than inverter limitations, has always been the main issue with using them off-grid without storage.

A good question for Enphase right now (or in a year or whenever they start supporting generators in their Ensemble system as promised) is whether you need any storage when the micro-grid islands, or whether you can just have IQ8s on the roof with PV panels and run them along with a generator. I suspect they'll need some minimal storage as a buffer to absorb power by charging if the load suddenly drops, but I don't really know, maybe not.

(BTW, when I mentioned SMA I wasn't talking about the secure power supply, I was referring to their whole Sunny Island multi-cluster system from years further back.)
 
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synchro

Senior Member
It seems like the most effective way for PV to support generators would be if they were "inverter generators" so that the engine could throttle down and reduce RPM when supplemented by PV output. I would expect PV power variation to be slow enough that engine RPM could be varied quickly enough to maintain required power levels. Many available portable inverter generators can be paralleled, and so I expect the same could be done with an inverter generator and PV inverters if they were designed to do so. Even if a system integrating an inverter generator and PV without storage was made workable, having storage could allow such features as boosting power for peak loads and allowing the engine to shut off when loads are light, as well as storing excess PV production beyond the immediate demand. As was mentioned before, from a strictly economic viewpoint it would be difficult to justify any of this vs. a common generator if the outages were infrequent with a relatively short duration.

By the way I did find a few academic papers about inverters that could support both current mode and a voltage mode for micro-grid islands, but not from industry probably because of proprietary concerns.
 

ggunn

PE (Electrical), NABCEP certified
A few points:

Nobody was talking about having a grid-tied PV inverter 'supply up to [my edit - ggunn] its full capacity on demand' in an offgrid scenario.
You apparently missed my "up to" in the above; I said that to differentiate it from the SMA inverters that will supply only a fraction of the PV system's output when the grid is down.
 
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