Grid tied solar system.

[tehan]

One tricky aspect of off-grid operation you are missing is that you have to support, at least for the US market, loads that are unbalanced across phases. Even the SolarEdge off-grid solution, which is mostly implemented in software, requires new hardware for this.

I suspect all of the major GTI manufacturers will launch off-grid products in the near future. Fronius, SolarEdge and Enphase have already made announcements and I would be astonished if SMA is not working on something. It will be interesting to see if they can add off-grid capability without adding too much to unit cost.

[bcroe]

One tricky aspect of off-grid operation you are missing is that you have to support, at least for the US market, loads that are unbalanced across phases. Even the SolarEdge off-grid solution, which is mostly implemented in software, requires new hardware for this.

I suspect all of the major GTI manufacturers will launch off-grid products in the near future. Fronius, SolarEdge and Enphase have already made announcements and I would be astonished if SMA is not working on something. It will be interesting to see if they can add off-grid capability without adding too much to unit cost.

You would need an autotransformer to deal with unbalanced loading, another issue. Those
use some power and hurt the power factor, unless there is an electronic version now.

The control feedback has already been done for inverters, but making the same equipment
do both that and GTI is a problem. The safety issue has to be solved. How is the switch over
going to be set up? I'd expect them to continue to use a different set of output terminals for
the 2 functions, leaving the transfer function to be solved by local switching/interlock regs.
All that is costly, too costly to include in every GTI built. So it would be a special model, can
they sell enough of those to justify building them?

And finally just how well is this going to be accepted, given it will die without warning for a
cloud coming by, then try to restart multiple times until it passes? Bruce Roe

[tehan]

The safety issue is best solved, as you suggest, by having two outputs. That increases installation complexity/cost, but it doesn't really add much cost to the inverter. I'm not sure what you mean by your cloud comment but I think it's a given that any off-grid capable inverter will use a battery. As to how much demand for this stuff there is, who knows? But the battery guys sure seem to think it is meaningful.

[inetdog]

There has to be something that the SPT Forum Software is either not recognizing on my home computer or is somehow stopping some functionality on it.

I can't even post with spaces between paragraphs on the home one.

This class of problem has been reported over the last few months, and is under investigation. But since, as you noted, there seems to be some interaction with the specific user agent (browser) it is very difficult to isolate the underlying problem.
I have occasionally been forced to copy the posted text before opening the Edit window so that I can paste it in as a starting point. I have not seen a problem with the buttons disappearing though.

The Mobile version of the forum site is significantly different from the desktop version, so that may be playing a role too.

[sensij]

The safety issue is best solved, as you suggest, by having two outputs. That increases installation complexity/cost, but it doesn't really add much cost to the inverter. I'm not sure what you mean by your cloud comment but I think it's a given that any off-grid capable inverter will use a battery. As to how much demand for this stuff there is, who knows? But the battery guys sure seem to think it is meaningful.

SMA is selling plenty of inverters on the promise of the SPS, even without a battery to buffer the clouds. However, any serious market for the off-grid / zero-feed-in tech would see the weakness of that approach. It does seem unlikely the SolarEdge's existing method of relying on remote feedback would ever be considered safe enough to truly protect against grid feed-in, even with whatever "failsafes" they claim to have designed in.

[sensij]

One tricky aspect of off-grid operation you are missing is that you have to support, at least for the US market, loads that are unbalanced across phases. Even the SolarEdge off-grid solution, which is mostly implemented in software, requires new hardware for this.

You would need an autotransformer to deal with unbalanced loading, another issue. Those
use some power and hurt the power factor, unless there is an electronic version now.

Thanks!

[Naptown]

On the solar edge website they show it paired with a tesla battery.

[tehan]

It does seem unlikely the SolarEdge's existing method of relying on remote feedback would ever be considered safe enough to truly protect against grid feed-in, even with whatever "failsafes" they claim to have designed in.

Agreed. I'm fairly sure The SolarEdge "zero feed-in" solution is an economic promise rather than a technical one. By this I mean it will try very hard not to feed in, but it may. Just as one example, they don't have an autotransformer so if your load is unbalanced across the phases then it will feed in on one of the phases.

[posplayr]

If I wanted to generate some island AC direct from panels, I might get a 48VDC
input sine inverter. Arrange my panels to deliver Vmp at a lower point in the
inverter range, hopefully Voc would also be in range, or a bit more (series)
circuitry would be needed to limit V applied to the inverter. With a big DC cap
across the inverter input, it ought to run until the load exceeds what the panels
can deliver at that moment, and it will shut down. Guess it will oscillate at that
point, what did you expect?

Using higher voltage panels, use an MPPT controller designed for a 48V battery.
In other words, build a classic panels-MPPT-battery-inverter system but replace
the battery with a fat cap. Bruce Roe

I see Sensji answered, but I will add to the rhetorical dialog. This immediate discussion and the one that perplexes most people when they first encounter GTI solar systems is why can't it (the GTI) operate when the PowerCo power is off? The questions has several facets, some of which are being explored and discussed. My point has been that there is no insurmountable technical issue to making a GTI or the basic elements of a GTI operate in an Island Mode, but if you feel there is one please illuminate the discussion.

I will point out as Sensji has that your description of a "hybrid conversion", is pretty much irrelevant to the discussion as your are describing a system that will provide power when the sun is not shining, when we are exploring how to make a GTI provide power when the sun is shining but with PowerCo power off. Perhaps, your suggestion is even a tad absurd as most hybrid systems are much more expensive that a GTI, I doubt anybody would "augment" a GTI with a hybrid system. The only sane thing would be to just build a hybrid systems, but that is downright idiotic to suggest a hybrid system when you just want a GTI to Island during daylight hours. Don't let me put words into your mouth but that is how I see your comments. In fact the functionality of GTI islanding is exactly the capability that the marketplace as a whole has largely ignored except is certain specific cases such as SB SPS. So this discussion is to explore the reasons for this gap, and what the technical, legal and business factors the revolve around the capability.

I would ask if either of you have actually built some power converters? I think
the whole idea is just this side of impossible, and all this discussion is entirely
retorical.

Since you think it is impossible, you might learn something from the discussion. Obviously reading the posts is no guarantee of comprehension, but I would welcome any cogent technical issues or comments you might want to bring up.

As far as my design experience building a power convertor, no I have never built anything as I presume you mean. However what you fail to realize is that that does not matter.

[SunEagle]

I see Sensji answered, but I will add to the rhetorical dialog. This immediate discussion and the one that perplexes most people when they first encounter GTI solar systems is why can't it (the GTI) operate when the PowerCo power is off? The questions has several facets, some of which are being explored and discussed. My point has been that there is no insurmountable technical issue to making a GTI or the basic elements of a GTI operate in an Island Mode, but if you feel there is one please illuminate the discussion.

I will point out as Sensji has that your description of a "hybrid conversion", is pretty much irrelevant to the discussion as your are describing a system that will provide power when the sun is not shining, when we are exploring how to make a GTI provide power when the sun is shining but with PowerCo power off. Perhaps, your suggestion is even a tad absurd as most hybrid systems are much more expensive that a GTI, I doubt anybody would "augment" a GTI with a hybrid system. The only sane thing would be to just build a hybrid systems, but that is downright idiotic to suggest a hybrid system when you just want a GTI to Island during daylight hours. Don't let me put words into your mouth but that is how I see your comments. In fact the functionality of GTI islanding is exactly the capability that the marketplace as a whole has largely ignored except is certain specific cases such as SB SPS. So this discussion is to explore the reasons for this gap, and what the technical, legal and business factors the revolve around the capability.

Since you think it is impossible, you might learn something from the discussion. Obviously reading the posts is no guarantee of comprehension, but I would welcome any cogent technical issues or comments you might want to bring up.

As far as my design experience building a power convertor, no I have never built anything as I presume you mean. However what you fail to realize is that that does not matter.

I am pretty sure a GTI inverter that works 100% in "Island mode" is possible. The question is how much will it cost to design & build, is the price really worth it and will the POCO's allow one to be connected to "their" power distribution system.

The anti island fail safe is a demand of the POCO and while SMA has built their SPS version inverter I am sure they needed to comply with a lot of regulations put on them by the POCO's to make it safe for the Line Man to work on a "dead" grid.

[Naptown]

I requested info from solar edge.
I suspect highly that this is very similar to a sunny island.

[inetdog]

The anti island fail safe is a demand of the POCO

More specifically, it is a requirement of the UL listing for non-hybrid grid tie inverters. And even hybrid GTIs when connected to the grid rather than to a non-POCO source such as a local generator.
POCO demands that the connected inverters are UL listed in the appropriate category.

[sensij]

I see Sensji answered, but I will add to the rhetorical dialog. This immediate discussion and the one that perplexes most people when they first encounter GTI solar systems is why can't it (the GTI) operate when the PowerCo power is off? The questions has several facets, some of which are being explored and discussed. My point has been that there is no insurmountable technical issue to making a GTI or the basic elements of a GTI operate in an Island Mode, but if you feel there is one please illuminate the discussion.

I will point out as Sensji has that your description of a "hybrid conversion", is pretty much irrelevant to the discussion as your are describing a system that will provide power when the sun is not shining, when we are exploring how to make a GTI provide power when the sun is shining but with PowerCo power off.

I don't understand your comment, maybe you've misunderstood the definition of a hybrid inverter? It is grid interactive, but also provides power when the grid is down. Isn't that what you were looking for? See the Outback Radian series, Conext SW, Xantrex, Magnum, etc.

Most of the weeds we've been walking through are are related to ways of taking a non-hybrid GTI and making it work without the grid. While interesting, I don't think it is much more than an engineering experiment, with a high probability of generating smoke while the voltage regulator / feedback mechanism is developed.

One other grid-down implementation to keep in mind uses Enphase Microinverters, along with a hybrid inverter. With this approach, you can avoid buying a separate MPPT charge controller in the typical hybrid setup.

[posplayr]

Agreed. I'm fairly sure The SolarEdge "zero feed-in" solution is an economic promise rather than a technical one. By this I mean it will try very hard not to feed in, but it may. Just as one example, they don't have an autotransformer so if your load is unbalanced across the phases then it will feed in on one of the phases.

It will readily admit to having forgotten most of what I learned about 3 phase power, so I did a little reading about unbalanced loads with the goal of trying to determine what would be different between a GTI operating normally and "Islanded" GTI. Basically is seems as if the standard GTI has no strict output voltage control and so load imbalance which would create voltage imbalance or even POCO line to line voltage imbalance is irrelevant to the GTI. Basically it does not want to nor need to control voltage unbalanced or not. The GTI does not care about voltage so there is no need for autotransformers on it's output. If voltage control is required, then the "extra hardware" you describe is required.

In an Islanded mode, the GTI is still operated in the same way without internal output voltage regulation. So something has to setup output voltages. This is now where the story diverges. There are two cases: 1.) Either the GTI adopts another mode with output voltage regulation or 2.) an external voltage reference POCO surrogate comes into play. In case #1 a separate set of output are most likely as SB SPS is a prime example and already exists. or in case #2 a UPS as shown in the my figure is incorporated. I might mention that in Case #1, the system will still need surge capacity that the PV system will not provide and so incorporation of a line interactive UPS is required at a minimum.

In case #2 I have suggested a Double conversion 240V UPS to serve as the POCO surrogate and voltage reference. Unless I'm mistaken a 240V UPS is capable of dealing with any "normal" load imbalance that might be expected. Given that being the case, there is little reason for the GTI to do anything else but regulate on total power. Granted in an unbalanced situation the power in the two phases might be unbalanced but the sum is what your be managed not the individual powers. As you have suggested this is a weak (i.e. slow ) type of control, however I assume it is still at least Type 1 control. The ability of the system respond to fast demand changes is going to be largely a function of the UPS performance. At some point though that limited capacity will have issues with too much power being pushed into the system by the solar and so this is where the PV power control would come into play. A slow loop to throttle back the power coming in from the GTI system is necessary. Basically the it seems the logical architecture for Islanding GTI is to incorporate output voltage control beyond the standard UPS capability to add GTI power control and load control.

Until such a beast is actually designed (perhaps Musk is working on something like this) the closed thing for a DIY is a 240V server UPS and a micro controller controlling a load dump and RS485 power control to the SolarEdge GTI.

Having described all of this, I don't see where load imbalance is an issue unless the double conversion UPS cannot support some extreme level of imbalance. I have not gone through the thought process here of using a line interactive UPS on a separate output, but it is entirely possible as already exists although limited to 1500 W as in the case of the SB SPS. Under case #2, the whole problem is addresses and the entire GTI power output is available because of it and I see that as actually preferable. A total system solution needs to deal with the demand and surge requirements which dictate some battery storage as well as GTI output voltage regulation. Case #2 does that and just happens to deal with load imbalance as well so there is really no reason to add anything else other than what has been mentioned.

[tehan]

It will readily admit to having forgotten most of what I learned about 3 phase power, so I did a little reading about unbalanced loads with the goal of trying to determine what would be different between a GTI operating normally and "Islanded" GTI.

Not for the first time, you have written a lengthy post when you have no idea what you are talking about. This issue has nothing to do with 3 phase power: it is related to split phase systems used in the US. You seem like a smart guy and you make some interesting comments, but you need to turn your "dickishness" setting down to say 70 because its current level is more than most of us can stand.