Will it work? Supplementing off grid inverter/battery with on grid PV inverter in one system.

[apctjb]

I have an existing 5KW grid connected PV system on my home; Growatt MIN 5000TL-X, 240 split phase. I am considering adding a 6KW Growatt SPF offgrid inverter/battery to provide backup power during power outages. The on grid and off grid inverters would both back feed breakers in the same main distribution panel.

During a utility power outage the main panel would disconnect from the grid (utility), and the offgrid inverter would connect to the main panel to provide backup power.

The question is what will happen if I leave the on grid inverter connected when the off grid inverter is powering the distribution panel? When the off-grid inverter is providing power to the distribution panel will the on grid inverter come back online providing supplemental power to the distribution panel reducing the load on the off grid inverter or will one or the inverters go into a fault condition (or worse)?

Easy enough to try, but before possibly damaging one or more inverters would appreciate hearing from others....

(PS: The 6kw off grid inverter is sufficient to power loads on this distribution panel)

 

[BentleyJ]

An Off-grid type of inverter cannot be used in the proposed configuration which is called AC Coupling.

The 6kW battery inverter would have to be grid interactive and UL1741SB compliant. This is necessary because the new inverter must perform 2 important functions.
1) Anti-islanding: Which means the new inverter must be inserted into the utility feed such that its internal transfer switch can disconnect from the grid so as not to back feed and potentially cause injury to a line worker. Alternatively some other type of disconnect or automatic transfer switch must be installed on the utility feed.
2) Frequency Shift Curtailment: The new inverter must be able to control and shut off the grid-tie inverter when solar production exceeds the house loads and the batteries are full. In this situation there is no were for grid-tie power to go. Alternatively, a "dump" load is connected so excess power is used on something like an electric water heater or load bank.

 

[12VoltInstalls]

If I’m reading the skeleton description correctly and applying it to your diagram I don’t think what you are trying to do won’t create smoke, for one.
Secondly, i see the switch (breaker? transfer switch?) from grid but not a description of where the UL1741 compliance takes place considering that if somehow it is configured to work successfully the grid-tie box (that is apparently battery-less operation) will interpret the off-grid box as grid power and not disconnect PV.

What you really need is a UL1741 gridtie AIO with a built in transfer switch, batteries, and potentially more panels.

The skeleton description lacks any disconnect between the two incohabitable devices.

 

[apctjb]

Thanks for the feedback. If we can put aside the issue of code compliance for the moment, and also the question of how the distribution panel will disconnect from the utility I am curious to know if it is likely that the grid tie inverter will come back on once the panel is energized by the stand alone off grid inverter, and if it does will this likely damage either the on grid or off grid inverter.

 

[cs1234]

Thanks for the feedback. If we can put aside the issue of code compliance for the moment, and also the question of how the distribution panel will disconnect from the utility I am curious to know if it is likely that the grid tie inverter will come back on once the panel is energized by the stand alone off grid inverter, and if it does will this likely damage either the on grid or off grid inverter.

Some grid tie inverters will detect some off grid inverters as an acceptable "utility" type connection and sync to their signal, some won't. It depends on both inverters.

From what I've gathered, LF off grid inverters are likely to handle the flow back of electricity into them better than HF inverters. In general though, the off grid inverter needs to be substantially larger in wattage handling than the expected power coming in from the grid tie inverter. Nobody on here recommends it, and it might not work very well.

However, if you use a "hybrid" inverter with ac coupling capabilities, it can control the regular grid tie inverter without any problems during a grid down or no grid at all situation. Think something like the EG4 18kwpv or Sol-Ark.

 

[timselectric]

I see smoke in your future.

 

[DIYrich]

The grid tied inverter will come back online when its grid connection is energized by the grid or off-grid inverter. And there is your problem. The grid inverter can push too much power to the panel and feed back the off-grid inverter.

Buy a hybrid inverter and move the pv to that.

 

[apctjb]

Great feedback, I love this forum. And yes a new EG or Sol Ark would be nice but is it really necessary. Shame to spend the time and money if a simpler solution is available.

Yes I understand that in practice it boils down to the specific inverters which if why I mentioned the brand/model and hoping someone has given this a try.

Specifically is the waveform of the Growatt 6KW SPF offgrid inverter clean enough to not trigger anti islanding protection of the ongrid Growatt MIN 5000TL-X so that it is able to synchronize and come online and assuming it does will the ongrid MIN 5000TL-X inverter throttle output to match load on the panel (most modern gridtie inverters can) so to not create an overvoltage situation on the panel that might damage loads or the off grid inverter?

 

[cs1234]

will the ongrid MIN 5000TL-X inverter throttle output to match load on the panel (most modern gridtie inverters can) so to not create an overvoltage situation on the panel that might damage loads or the off grid inverter?

I'm under the impression that *most* grid tie inverters just run full bore all the time with no CT's or monitoring of the main panel at all other than to drop off if they lose grid power. They will certainly throttle output if told to by frequency changes from the grid or an AC coupled hybrid inverter tells them to, it they meet UL1741sa(?), but by default they just go at it 100%.

The 5000TL-X is a battery storage inverter of some sort, so I have no idea what it does. It might have some type of CT's to monitor local load at the panel and only output for that. That would certainly make sense as you don't necessarily want to send battery stored power back to the grid 24/7.

 

[timselectric]

Grid-tied doesn't throttle.
It just runs wide open, all of the time.
The smoke will come from the off grid AIO. It's not designed to receive power through its output.

 

[apctjb]

Thanks, again I really appreciate the feedback. Regarding
Grid-tied doesn't throttle.
It just runs wide open, all of the time.

Posed the question to ChatGPT; Do modern residential grid tie inverters typically have the capability to throttle power output to match loads and prevent overvoltage conditions at the distribution panel? , below is the response

"Yes, most modern residential grid-tie photovoltaic (PV) inverters are designed with advanced control algorithms that allow them to modulate their power output to prevent overvoltage conditions and other potential grid issues. Here's how they generally handle these scenarios:

1. **Internal Voltage and Frequency Sensing**: Modern inverters have built-in voltage and frequency sensing circuits that continuously monitor the conditions at the point of connection. If the voltage rises beyond a certain threshold (which could be indicative of more power being fed into the system than being used), the inverter will automatically reduce its power output.

2. **Power Quality Management**: Besides just voltage, inverters often monitor power quality metrics like power factor, total harmonic distortion, and frequency. These factors can influence the inverter's decisions on power output.

3. **Reactive Power Control**: Many modern inverters can also control the reactive power output (VARs) in addition to the active power (Watts). By modulating reactive power, inverters can help manage voltage levels on the grid or within the home's electrical system.

4. **Anti-Islanding**: This is another crucial feature in grid-tie inverters. If the inverter detects that it's no longer connected to the main utility grid (often inferred from voltage and frequency anomalies), it will shut down. This is a safety measure to prevent power from being fed back into the grid when there might be utility workers repairing lines, thinking the lines are de-energized.

5. **No Need for External CTs for Basic Operation**: For their basic operation, including throttling power to prevent overvoltage, most grid-tie inverters do not require external current transformers (CTs) or additional voltage sensing devices. However, advanced features like consumption monitoring or zero-export configurations might require external CTs to measure the whole-home consumption.

6. **Zero Export and Load Consumption Matching**: Some advanced grid-tie inverters, especially when paired with home energy management systems, can adjust their power output to match the home's consumption precisely. This might be desirable in regions where exporting power to the grid is not allowed or not economically favorable. These configurations might use external CTs to measure home consumption and adjust inverter output accordingly."

 

[cs1234]

Grid-tied doesn't throttle.
It just runs wide open, all of the time.
The smoke will come from the off grid AIO. It's not designed to receive power through its output.

Don't some grid tied inverters throttle / turn off based on the frequency or other control signals? Coming from a control unit or an AC coupled inverter, or presumably from the power company itself if it has the ability to shift the frequency?

I could of sworn I saw on these forums somewhere, that the LF inverters may be more tolerant of backfeed on their AC output than their HF inverter brethren. Presumably, if you kept the output from the grid tie inverter and house loads somewhat close together, most of the feedback from the grid tie inverter should be at a higher voltage than the output from the off grid inverter and get used first.

 

[apctjb]

The 5000TL-X is a battery storage inverter of some sort, so I have no idea what it does. It might have some type of CT's to monitor local load at the panel and only output for that. That would certainly make sense as you don't necessarily want to send battery stored power back to the grid 24/7

Yes they make a version that can accept battery input, unfortunately mine does not. Thanks

 

[cs1234]

@apctjb

I don't think the automatic voltage rising detection stuff and other non CT (or other form of remote transmitter) based methods are likely to work fast enough to prevent blowing out a HF off grid inverter. Might work fast enough to prevent blowing out at LF one though.. maybe some of the other smart folks on here could weigh in.

 

[apctjb]

I could of sworn I saw on these forums somewhere, that the LF inverters may be more tolerant of backfeed on their AC output than their HF inverter brethren. Presumably, if you kept the output from the grid tie inverter and house loads somewhat close together, most of the feedback from the grid tie inverter should be at a higher voltage than the output from the off grid inverter and get used first.

I have no direct knowledge but suspect you are correct simply as a result of the greater capacitance of LF inverters.

 

[timselectric]

This is the problem.
Grid-tied inverters react to frequency shift.
The off grid AIO does not have the ability to frequency shift. (Not capable of AC coupling)
So, the AIO will present the source for the Grid-tied inverter. And the Grid-tied inverter will run wide open.
This is why the off grid AIO will get smoked.

 

[apctjb]

Grid-tied inverters react to frequency shift

Thanks,

My understanding up to now is that grid tie inverters use frequency shift to modulate power output and if frequency shifts outside of limits shut down to prevent anti islanding.

 

[DIYrich]

Thanks,

My understanding up to now is that grid tie inverters use frequency shift to modulate power output and if frequency shifts outside of limits shut down to prevent anti islanding.

Yes, but that is not your problem. The off grid may not be designed to frequency shift to modulate ac connected pv. In other words, what is controlling the grid tied inverter, not how can it be controlled.

 

[apctjb]

All great input, thanks.

Distilling it all down looks like there are 3 questions that need to be answered.

1) Is the AC output/waveform of the off grid inverter stable enough for the grid tie inverter to sync with and start back feeding. (I suspect it is)
2) Will the grid tie inverter modulate output to match load on the panel avoiding overvoltage at the panel? (Maybe, this model has the capability to limit power output)
3) If there is an overvoltage at the panel will the off grid inverter trip out on fault condition or be damaged because it can not handle the over voltage condition?

I am willing to give it a try but wish I knew the answer to #3. Don't really want to smoke the off grid inverter. If anyone knows if Growatt SPF have protection on the AC output side please chime in!

 

[timselectric]

1) Is the AC output/waveform of the off grid inverter stable enough for the grid tie inverter to sync with and start back feeding. (I suspect it is)

Yes

2) Will the grid tie inverter modulate output to match load on the panel avoiding overvoltage at the panel?

No

3) If there is an overvoltage at the panel will the off grid inverter trip out on fault condition or be damaged because it can not handle the over voltage condition?

Hopefully trip.
More than likely burn.

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