Grid tied solar system.

[Naptown]

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.

[posplayr]

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)

[posplayr]

It would be worth reading up on how Solaredge implements their zero feed in algorithm. It is done by monitoring current and adjust away from maximum power point on the panels (this is not chopping pv power up, the difference matters).

With a small dump load to act as the grid, and appropriate monitoring equipment, it might be possible to regulate an island this way.

This is more than just a different algorithm, they have added a feature to limit power to a maximum as measured from a separate power meter. It would be an easy matter to hack a RS485 message from a micro controller that is measuring output AC line voltage and commanding a power limit to achieve that output voltage regulation. Using the UPS's you now have an supply side control GTI Island. You could couple this with load dump as well.

http://www.solaredge.com/files/pdfs/products/se_feed_in_limitation_flyer.pdf

[posplayr]

It would be worth reading up on how Solaredge implements their zero feed in algorithm. It is done by monitoring current and adjust away from maximum power point on the panels (this is not chopping pv power up, the difference matters).

With a small dump load to act as the grid, and appropriate monitoring equipment, it might be possible to regulate an island this way.

This is more than just a different algorithm, they have added a feature to limit power to a maximum as measured from a separate power meter. It would be an easy matter to hack a RS485 message from a micro controller that is measuring output AC line voltage and commanding a power limit to achieve that output power regulation. Using the UPS's you now have an supply side control GTI Island. You could couple this with load dump as well.

http://www.solaredge.com/files/pdfs/products/se_feed_in_limitation_flyer.pdf

[sensij]

It would be worth reading up on how Solaredge implements their zero feed in algorithm. It is done by monitoring current and adjust away from maximum power point on the panels (this is not chopping pv power up, the difference matters).

With a small dump load to act as the grid, and appropriate monitoring equipment, it might be possible to regulate an island this way.

[posplayr]

An MPPT algorithm adjusts the PWM to the panels to obtain maximum power. Modifying the algorithm to push away from maximum power in an attempt to regulate output voltage seems theoretically possible to me, although some care would need to be made to make it responsive enough to handle a sharp drop in load.

http://www.instructables.com/id/ARDU...and-Algorithm/

Well yes of course but it has little to do with MPPT. The GTI is basically a programmable DC to AC convertor that have internal control loops based on sensed current and voltage (on both ends). In a standard GTI mode, it cannot regulate output voltage, as it would end up fighting the grid, so it has to float above the grid voltage. The AC power that the GTI pushed out is (with the operating efficiency) the same as the power coming in on the DC lines from the PV. For constant PV power, if line voltage goes up , then current must go down. This is closed loop control but only internal to the GTI; it does not control total power or output voltage. So it can only affect the AC amps for a given PV power in.

And as you note, some new software could be used to control based on new objectives. In the simplest view it is possible to sense the AC line voltage and control the PV power holding the AC voltage within spec and assuming constant loads the current will find it's own level. This is basically what a UPS with double conversion would do. None of this is impossible, just impractical, because the primary design features of the GTI inverter is that it changes from GTI mode to OFF mode off when the line voltage does away. I have little doubt that to get whatever approvals are required, that the GTI manufacturers had to demonstrate this type of fail safe to meet ENC! So there is only GTI and OFF, nothing else. It is purposely designed to have not only nothing else, but failsafe to NOTHING else. So to come in the fact and say, OK we are now going to design a fail safe GTI that has a software controlled UPS mode? You are now going to get into safety interlocks for mutually exclusive operation and the GTI can not accomplish that backfeeding a breaker at the bottom of a panel.

So while the GTI could perform the function with added UPS functionality, there is no way to implement it easily and keep the GTI failsafe "OFF mode". So for the most part until something radically changes in the nature of what a GTI is, any islanding of GTI will probably be by controls outside of GTI itself. Those two modes of control would both achieve line voltage regulation of the island. Load control (e.g. water heater load dump) of source control (e.g. shorting panels) or a combination of both are much more feasible without trying to change the GTI with it's fail safe. Load control would likely not have much impact on the GTI, while I could envision interoperability issues depending upon how much or fast you want to chop the PV power. I guess you could also do load control at the GTI input, but that that might be more difficult than panel control.

You would basically add the output voltage controller to this diagram along with a whole house transfer switch.

http://www.solarpaneltalk.com/showthread.php?22001-Grid-tied-solar-system&p=163439&viewfull=1#post163439

[posplayr]

An MPPT algorithm adjusts the PWM to the panels to obtain maximum power. Modifying the algorithm to push away from maximum power in an attempt to regulate output voltage seems theoretically possible to me, although some care would need to be made to make it responsive enough to handle a sharp drop in load.

http://www.instructables.com/id/ARDU...and-Algorithm/

Well yes of course but it has little to do with MPPT. The GTI is basically a programmable DC to AC convertor that have internal control loops based on sensed current and voltage (on both ends). In a standard GTI mode, it cannot regulate output voltage, as it would end up fighting the grid, so it has to float above the grid voltage. The AC power that the GTI pushed out is (with the operating efficiency) the same as the power coming in on the DC lines from the PV. For constant PV power, if line voltage goes up , then current must go down. This is closed loop control but only internal to the GTI; it does not control total power or output voltage. So it can only affect the AC amps for a given PV power in.

And as you note, some new software could be used to control based on new objectives. In the simplest view it is possible to sense the AC line voltage and control the PV power holding the AC voltage within spec and assuming constant loads the current will find it's own level. This is basically what a UPS with double conversion would do. None of this is impossible, just impractical, because the primary design features of the GTI inverter is that it changes from GTI mode to OFF mode off when the line voltage does away. I have little doubt that to get whatever approvals are required, that the GTI manufacturers had to demonstrate this type of fail safe to meet ENC! So there is only GTI and OFF, nothing else. It is purposely designed to have not only nothing else, but failsafe to NOTHING else. So to come in the fact and say, OK we are now going to design a fail safe GTI that has a software controlled UPS mode? You are now going to get into safety interlocks for mutually exclusive operation and the GTI can not accomplish that backfeeding a breaker at the bottom of a panel.

So while the GTI could perform the function with added UPS functionality, there is no way to implement it easily and keep the GTI failsafe "OFF mode". So for the most part until something radically changes in the nature of what a GTI is, any islanding of GTI will probably be by controls outside of GTI itself. Those two modes of control would both achieve line voltage regulation of the island. Load control (e.g. water heater load dump) of source control (e.g. shorting panels) or a combination of both are much more feasible without trying to change the GTI with it's fail safe. Load control would likely not have much impact on the GTI, while I could envision interoperability issues depending upon how much or fast you want to chop the PV power. I guess you could also do load control at the GTI input, but that that might be more difficult than panel control.

[sensij]

An MPPT algorithm adjusts the PWM to the panels to obtain maximum power. Modifying the algorithm to push away from maximum power in an attempt to regulate output voltage seems theoretically possible to me, although some care would need to be made to make it responsive enough to handle a sharp drop in load.

http://www.instructables.com/id/ARDUINO-SOLAR-CHARGE-CONTROLLER-Version-30/step39/Software-and-Algorithm/

[posplayr]

The easiest way to "remove a panel", is just short it out. A medium MOSFET with an opto coupler
and some doorbell wire could do that. Bruce Roe

So you guys have apparently thought about this; Are you talking about a closed loop panel switcher that monitors GTI inverter output voltage and adjusts the number of panels (i.e. PWM)?

It would control the Islanded house system running of the GTI inverter? With full double conversion UPS, it would fool the GTI and this panel Controller module would put a AC voltage loop around the whole GTI PV system so as loads go up, more panels are added, and vis versa.

The load control is somewhat more attractive as you can always remove load much easier which you cannot do with supply side control. Of course since we are going Blue Sky do a combo supply side /load dump controller. for GTI output voltage regulation. Only a few KW of UPS surge required.

Even this being the why not short the whole string out if you are going to PWM? And all bets are off if you have optimizers and I'm not sure what a Sunny boy will do if you start jacking with the input string voltage too much?

[posplayr]

...

[bcroe]

Yes of course a controller could be made that "effectively" removes panels. It would not be much different than the "optimizers" that are programmable DC to DC convertors. However, literally removing panels would only be practical if you had optimizers as you want to maintain the string architecture and avoid running a pair of wires from each panel to a central controller.

My point was that getting a UL certification and what ever other hoops have to be gone through for a safety critical element is going to beyond the funding scope for any DIY project.

The easiest way to "remove a panel", is just short it out. A medium MOSFET with an opto coupler
and some doorbell wire could do that. Bruce Roe

[posplayr]

A lot easier, IMHO, would be a controller that selectively removed panels from the array wiring when the GTI output went too high. Also avoids the need to dissipate the extra power.
The other common technique is to run the excess AC or DC to an opportunity load such as water heating. Unfortunately the controllers with that kind of control options are typically CCs rather than GTI installations.

Yes of course a controller could be made that "effectively" removes panels. It would not be much different than the "optimizers" that are programmable DC to DC convertors. However, literally removing panels would only be practical if you had optimizers as you want to maintain the string architecture and avoid running a pair of wires from each panel to a central controller.

My point was that getting a UL certification and what ever other hoops have to be gone through for a safety critical element is going to beyond the funding scope for any DIY project.

[inetdog]

Getting a UL approved load dump controller based on line voltage is another tick unless it exists. Recognize if it fails you whole house electrical system would likely overvoltage so you would want redundant systems or other means for high reliability.

A lot easier, IMHO, would be a controller that selectively removed panels from the array wiring when the GTI output went too high. Also avoids the need to dissipate the extra power.
The other common technique is to run the excess AC or DC to an opportunity load such as water heating. Unfortunately the controllers with that kind of control options are typically CCs rather than GTI installations.

[posplayr]

That would be great if all the smart people on here would reply to post like mine, but most dont like grid tied battery backup. So we must search the youtube professionals...Sad but true.

I went through a few variations a while back (in this thread) where the battery pack could be sized for surge demand rather than overnight power draw. In both cases I was assumed using UPS's for the surge capacity.

http://www.solarpaneltalk.com/showthread.php?22001-Grid-tied-solar-system&p=163369&viewfull=1#post163369

Of course you need some type of safety interlock, and if you want to power some portion of your Main Panel busbar rather than running extension cords you would want a whole house transfer switch and a capability to switch in a generator as well.

SMA SPS gives 1500 watts, but with two reduced size SB invertors you have two independent phases of 120V out each 1500 watts. Putting a 3KWatt UPS on each you could power your bus bar with two separate 120V legs.

I agree with you there is a gap between what he market provides and the demand/desires of at least some people. As you know now the GTI is not really designed for running autonomously. It is not impossible , they are just not made that way.
Part of my learning process was this post. You can see my edited comments echoed here.

http://www.solarpaneltalk.com/showthread.php?22001-Grid-tied-solar-system&p=163439&viewfull=1#post163439

The biggest issue beyond, tricking the GTI to output to a simulated GRID voltage is the fact that you would have no load control. The GTI does not try and control voltage and so as it tries to push the solar power to your island(you have heard it described as a current source), the line voltage in your house will rise. Now you need some type of load dump to skim off the excess preventing your electrical appliances from over voltaging. If you used a water heater element for load dump and had some other type of controls it is possible and even practical ;just need to work out some details.

With some know loads that you would want to run when the GRID shuts off (like swamp coolers and frig/freezers)it would not take as much of a water heat load dump to moderate the solar power output.

Getting a UL approved load dump controller based on line voltage is another tick unless it exists. Recognize if it fails you whole house electrical system would likely overvoltage so you would want redundant systems or other means for high reliability.

[North Texas]

We could spend all day wandering the web looking at neat stuff, but this is more of a forum, where folks interact with each other, a blind reference to a site where there are nnnyyyzzz is not really on topic. If there was a specific question, and you know of a specific answer (like we often refer folks to the Smartguage site where they lay out the math about how bad parallel batteries can be) that's much more in line with what we like to see.

That would be great if all the smart people on here would reply to post like mine, but most dont like grid tied battery backup. So we must search the youtube professionals...Sad but true.