Grid tied solar system.

[bcroe]

In my simplistic way of thinking as long as the terminal voltage at the output of the solar inverter was within range then there would be no danger of over driving anything. This would require some sort of DC-AC sine wave generation (like in the UPS) but apparently this just some type of coupled current source without any regulation. Without a schematic it is hard to discuss. You don't happen to have anything representative do you?

You are getting good input from Inetdog. When a power supply is built, it typically is a voltage or a current
source (within practical limits). The PoCo is a voltage source, supplying whatever current the load conducts
while maintaining a stable voltage. For a GTI to supply the PoCo, it must be a current source, supplying
whatever power it has available as current into a stable voltage. A supply cannot be a voltage and a current
source at the same time; that operating point will be determined by the impedance of a load.

If a GTI were to change to output voltage reg at a defined point, it would cut off every time the line voltage
rose slightly. If the point was set way high, it would operate higher than the voltage desired when not PoCo
line connected. The only way to solve this is a method to change the inverter mode from one to the other,
which is not a feature generally available. Bruce Roe

[posplayr]

The problem is that when the standalone source is no longer actually supplying any power, and the GTI is able to produce more power than the load can absorb.
At that point the results will depend on just how the output voltage is regulated on the inverter or generator. In some cases the regulator will allow the voltage to rise to the point where the GTI cuts out. In other cases the circuitry of the standalone source will be damaged as it tries to reduce its output power below zero.

OK I guess we have to discuss this without a representative schematic.

I understand now that a GTI has no output voltage regulation. If it did it would have to maintain that voltage on top of any Grid line variation probably as measured through output current sensing. So it would basically be an AC current source with additional AC Voltage limits all synced to the prevailing AC voltage (i.e. grid ). From a practical standpoint if the voltage were to be limited it would be by series regulation v.s. shunt as the GTI would have to massive to shunt for example: 10Kwatts of solar if there was no load.

Apparently the Hybrid Inverter, essentially performs this function by generating a programmable DC voltage source (from high voltage solar DC) to charge a battery bank(the load) which are subsequently used to generate AC which presumable is regulated and terminal voltage adjusts to the load. This is like the DC to AC conversion in a dual conversion UPS.

It is not impossible, just not what GTIs do; a GTI can not regulate it's output based on load except the SB TL with SPS which does it on a limited basis (1500 watts per plug).

EDIT: I have been reading alot and I thought I read a comment that the SPS was a simple mod, to the existing GTI. That sort of flies in the face of the current discussion so I might not be remembering that correctly.

EDIT EDIT: As per the other discussion on potential Contingency GT implementations, you run into the NEC safety interlock requirements with the grid. That is the biggest impediment to having full solar output at the GTI terminals without the grid. Not impossible just messy without doing a complete house transfer switch. The dual SB SPS is looking better.

[posplayr]

You are getting good input from Inetdog. When a power supply is built, it typically is a voltage or a current
source (within practical limits). The PoCo is a voltage source, supplying whatever current the load conducts
while maintaining a stable voltage. For a GTI to supply the PoCo, it must be a current source, supplying
whatever power it has available as current into a stable voltage. A supply cannot be a voltage and a current
source at the same time; that operating point will be determined by the impedance of a load.

If a GTI were to change to output voltage reg at a defined point, it would cut off every time the line voltage
rose slightly. It the point was set way high, it would operate higher than the voltage desired when not PoCo
line connected. The only way to solve this is a method to change the inverter mode from one to the other,
which is not a feature generally available. Bruce Roe

Without characterizing the GTI as either current source or voltage source, it is just basic ohms law that if the grid AC voltage envelope is surging, the GTI AC output voltage envelop would have to float and follow just above that grid level to maintain a positive AC current flow into the grid. Within limits as you describe looks more like an AC current source with a floating output voltage that follows the grid undulations. There is no hard voltage control at the GTI terminals; with a grid you would never be able to do pull the grid voltage around with a GTI of any reasonable capacity.

[Mike90250]

.....
Apparently the Hybrid Inverter, essentially performs this function by generating a programmable DC voltage source (from high voltage solar DC) to charge a battery bank(the load) which are subsequently used to generate AC which presumable is regulated and terminal voltage adjusts to the load. This is like the DC to AC conversion in a dual conversion UPS......

Not really. The Hybrid Inverters generally do NOT have a solar PV charge controller. That is a separate box, to keep the batteries charged. They take DC from the battery bus, and invert it to AC. Some are rated to be able to back feed the grid, they have internal safety interlocks that pass UL testing. Some also sync with and work with generators when the solar is insufficient. As to the internal circiuts that manage the voltage and ability to supply AC amps, their design department can tell you more than I can.

[mmyler]

Check this guy and system out

go to youtube and search LDS Prepper. Look at his system he just put up. He is originally from Houston, moved to Idaho. He knows his stuff and has plenty of videos of his system.

[Mike90250]

go to youtube and search LDS Prepper. Look at his system he just put up. He is originally from Houston, moved to Idaho. He knows his stuff and has plenty of videos of his system.

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.

[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.

[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.

[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]

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.

[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]

...

[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?

[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]

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.