Grid tied solar system.

[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!

[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.

[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.

[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.

[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]

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.

[SunEagle]

And since I can't edit... using hybrid equipment would be even more practical than the grid-tie / off grid system in parallel, but since the discussion is mostly around how to island with a grid-tie inverter, the "just use hybrid" solution is not interesting.

I wish I knew why I can edit on my work computer but not my home.

Both running windows 7 and both using Foxfire browser. I also log off from SPT on both computers and while I start or reboot my home laptop every day I do the same for my work Laptop about 1 a week.

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.

[sensij]

And since I can't edit... using hybrid equipment would be even more practical than the grid-tie / off grid system in parallel, but since the discussion is mostly around how to island with a grid-tie inverter, the "just use hybrid" solution is not interesting.

[sensij]

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

Agreed.. the most practical way to power an island would be to just bypass the grid-tie inverter completely and go through redundant off-grid equipment as you suggest, be it with batteries or a fat cap (with a hard interlock disconnecting the grid when making the transfer).

The rest of the discussion is somewhat rhetorical, but since SolarEdge has built in the ability to zero-backfeed with the grid-tie inverter, tapping into that logic in a way that would power an island is (to me) an interesting thought experiment. I don't need to build power conversion equipment, SolarEdge has already done that. The problem is figuring out the most responsive way to use the meter feedback control loop to regulate the PV power.

[bcroe]

This has some merit, but I would be worried about the relative communication cycle time of the RS485 and the response time required to prevent an overvoltage condition if a big load drops out. SolarEdge claims fast response time though, at least when using the embedded communication, so maybe that fear is misplaced.

Designing a circuit with a dump load behind a thermally protected MOV might buy enough time for the digital control loop to take action. With that, you could install the meter to watch the power going to the dump load and use the RS485 as SolarEdge intends, taking action to reduce the power generated whenever power at the meter is detected (See example two, 0 W feed-in, in this white paper). The non-linear nature of the MOV might screw that up though... the idea of faking the Pmeas with an independent control loop on voltage might be better.

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.

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

[posplayr]

This has some merit, but I would be worried about the relative communication cycle time of the RS485 and the response time required to prevent an overvoltage condition if a big load drops out. SolarEdge claims fast response time though, at least when using the embedded communication, so maybe that fear is misplaced.

Designing a circuit with a dump load behind a thermally protected MOV might buy enough time for the digital control loop to take action. With that, you could install the meter to watch the power going to the dump load and use the RS485 as SolarEdge intends, taking action to reduce the power generated whenever power at the meter is detected (See example two, 0 W feed-in, in this white paper). The non-linear nature of the MOV might screw that up though... the idea of faking the Pmeas with an independent control loop on voltage might be better.

Dynamic response is going to be an issue in any event. I would assume any response the solar edge has to faked power measurement is going to to be relatively slow. Unless Solar Edge actual design for voltage control with performance consistent with load variations, I would not rely be too much on it's dynamic response. On the other hand if there is a load cutout, the load dump to a water heater should be very quick and would keep from over voltaging. Same thing for increasing load. So in effect you can look as the load dump as a high bandwidth voltage controller taking up the slop left over from the pseudo power control loop of the "feed-in limitation" control.

RS485 throughputs should easily be able to hit 20-30Hz if not faster if you get the baud rate up to 512K or so. Closed loop bandwidths through the SolarEdge would then probably be limited by about 1/10 of that 2-3Hz. Not really high frequency. With direct control of load dump you should be able to get closer to 20-30 hz response as you only have to flip a switch based on a direct VAC measurement but even 1/2 cycle of 60 hz is 8msec. I can see needing some type of grid capacity to absorb load turn off transients especially(when voltage will rise) that you are just not going to get the dynamic response from the SolarEdge loop.

[sensij]

Posting is Fed up.

Basically what SolarEdge have done is added a programmable power limit to the GTI standard mode. Basically it will start to cut back if:

if(Pmeas-P_set>0)
DECREASE POWER
else
INCREASE POWER

It will limit cycle at P_set.

If you dynamically change Pmeas and pack it into a RS485 message

VAC_setpoint=120
Read(VACmeas)

Verr = ( VACmeas - VACsetpoint) // voltage control error
VTerr = dt*Verr+VTerr // Integral error
Pmeas = Kp*Verr + Ki* VTerr // PI control

send_485(Pmeas)

This has some merit, but I would be worried about the relative communication cycle time of the RS485 and the response time required to prevent an overvoltage condition if a big load drops out. SolarEdge claims fast response time though, at least when using the embedded communication, so maybe that fear is misplaced.

Designing a circuit with a dump load behind a thermally protected MOV might buy enough time for the digital control loop to take action. With that, you could install the meter to watch the power going to the dump load and use the RS485 as SolarEdge intends, taking action to reduce the power generated whenever power at the meter is detected (See example two, 0 W feed-in, in this white paper). The non-linear nature of the MOV might screw that up though... the idea of faking the Pmeas with an independent control loop on voltage might be better.

[posplayr]

I believe the entire system has to be approved for the intended use.
I can take UL listed products and use them in a way not consistent with approved use and fail.
An inspector may not pick up on this but in the event of a fire you can be sure the insurance company's engineer will

And yes, I agree. However, I would assume that approval would be for representative equipment and not specific combinations. The lynch pin is if Solaredge say that it is an approved use, which they would probably be more likely to have a proprietary system for doing this to sell systems and there is no reason that any GTI manufacturer not do to their own style.

I'm not really suggesting anybody go out and build one of these as a one off it is probably cost prohibitive to get it approved, and the respective manufactures probably already have plans for something similar already; just wait on it. The main reason they might not be motivated is the need for a whole house transfer and the typical expense to do that, what is their angle for trying to make money off of it. Maybe in the end it is just the flexibility that they provide to the consumer by adding value to the GTI that does it.

Bottom line this is not a technical problem, it is more market demand driven.

[Naptown]

Of course, that is what I have been saying is the biggest challenge. Although given the Solar edge feature, I can't help but think they (SE) are not easing into something like this. The GTI safety disconnect is not defeated, and a whole hose transfer would have to be implemented. As far as certification the rest should be pretty easy. You just have a RS485 device that plugs into the wall. That can't be that hard to certify.

I believe the entire system has to be approved for the intended use.
I can take UL listed products and use them in a way not consistent with approved use and fail.
An inspector may not pick up on this but in the event of a fire you can be sure the insurance company's engineer will

[posplayr]

Did not read everything
However a makeshift maguivered system will not pass code without a NRTL blessing.
Even if you use all NRTL listed equipment.
They are funny like that.

Of course, that is what I have been saying is the biggest challenge. Although given the Solar edge feature, I can't help but think they (SE) are not easing into something like this. The GTI safety disconnect is not defeated, and a whole hose transfer would have to be implemented. As far as certification the rest should be pretty easy. You just have a RS485 device that plugs into the wall. That can't be that hard to certify.