Different grid-tied inverters in one grid possible?

[Jordi]

Dear solar enthusiasts,

I have two solar grid-tied inverters;
#1 - 600W 24V grid-tied inverter for two 100W solar panels I have at the balcony.


#2 - 590W 12V grid-tied inverter with battery mode (adjustable discharge 60-250W without MPPT function) for a 12V lifepo4 battery I have and possibly a 12V panel that I also have.


I wonder at this moment about the possibility and risks of running both inverters simultaneously in the same house.

My understanding is that so long both supply a voltage higher than the grid voltage, the grid is able to receive all the current minimizing the impact of the inverters on each other.

My research on this forum and google concludes that in general, it should be possible, but some people issued warnings with regards to inverters lifespan. Would somebody like to express any opinion or advise on my doings?

Thanks in advance.

 

[Quattrohead]

Nope all horribly wrong. Keep them separate and DO NOT CONNECT to your house wiring under ANY circumstances.

 

[meetyg]

I'm not recommending anything, but I did do a test lately, with a 1000w 10-30v GTI (similar to your 24v model), trying to feed the grid while my DEYE 2000w Microinverter was working (connectedto solar panels). The 1kw model had a hard time keeping up with the higher voltage the microinverter had already brought up. It would stop after around 5 minutes.
Running the 1kw at night had no problems, other than excessive heat (ran from 24v Lifepo4 pack).

Some of the more professional and expensive GTI have can be paralleled, but I think they have some special communication going on for that.

 

[12VoltInstalls]

Some of the more professional and expensive GTI have can be paralleled, but I think they have some special communication going on for that.

The listings say they are stackable. They see the balanced grid “load”

The grid is a buffer of sorts: so massive that a little input from one of these devices would/should not be “seen” by other devices I’d imagine. Just like running multiple SCCs into a battery bank.

Input watts I would suspect is the only thing that really isn’t stable.

 

[Quattrohead]

Still a big fat NO.
As soon as anything gets out of phase you have an AC short circuit and things go BANG.

Many years ago, Dungeness nuclear power station were bringing a generator unit back online but somehow managed to connect it when out of phase. It ripped that !ucker clean out of the floor and caused millions in damage.

 

[Jordi]

Nope all horribly wrong. Keep them separate and DO NOT CONNECT to your house wiring under ANY circumstances.

Ok, thank you Quattro, I will stay in the conservative side. I will not use them together and If I still did, only in the night when the inverter connected to the panels is off.

 

[12VoltInstalls]

Still a big fat NO.
As soon as anything gets out of phase you have an AC short circuit

Are these units not phase sensitive and thereby self-balance themselves?

The manual for the one I’m running says it does that…

 

[Hedges]

I have two solar grid-tied inverters;

If they are actually "grid-tied" inverters, they can be connected to the grid no problem, and two of different brands are no problem. No different from you connecting one and your neighbor connecting the other.
The grid is a bottomless ocean of power, and nothing one inverter can do is likely to even be observable by the other (except for a few ripples on the surface.)

At least in the U.S., this should only be done with "UL-1741" listed inverters. That ensures they work properly and safely. Inverters which claim to be grid-tie but aren't "listed" to that standard are potentially a safety hazard for the lineman.

If two inverters are to be connected together, not to the grid, they need to be designed to work together, typically with a "paralleling kit".

If the inverters aren't specifically for grid-tie, then their output can't be tied to the grid. They would fight (and lose) as Quattro says.

 

[Jordi]

If they are actually "grid-tied" inverters, they can be connected to the grid no problem, and two of different brands are no problem. No different from you connecting one and your neighbor connecting the other.
The grid is a bottomless ocean of power, and nothing one inverter can do is likely to even be observable by the other (except for a few ripples on the surface.)

At least in the U.S., this should only be done with "UL-1741" listed inverters. That ensures they work properly and safely. Inverters which claim to be grid-tie but aren't "listed" to that standard are potentially a safety hazard for the lineman.

If two inverters are to be connected together, not to the grid, they need to be designed to work together, typically with a "paralleling kit".

If the inverters aren't specifically for grid-tie, then their output can't be tied to the grid. They would fight (and lose) as Quattro says.

Thank you for your comment. I so rationalized it this way too and the example of the neighbor makes it clear, specially If it is the neighbor next door being connected to the same grid.

The grid tied inverter produces a higher voltage to pump current to the grid. Depending on how much solar is receiving it may create a slightly higher voltage to push that more or less current into the grid. When two inverters are far from each other, this effect should be diluted by the cable losses and the grid itself, but maybe the difference here is that both inverters will be very close to each other (same house).

If they were professional inverters, then they may be more resilient to such effects.

I don't really need to connect them both together. For now I will stay conservative and maybe someday make an experiment connecting them both simultaneously for a few minutes.

 

[stienman]

Nope all horribly wrong. Keep them separate and DO NOT CONNECT to your house wiring under ANY circumstances.

You are suggesting that grid tied inverters should NOT be connected to the grid?

Still a big fat NO.
As soon as anything gets out of phase you have an AC short circuit and things go BANG.

Many years ago, Dungeness nuclear power station were bringing a generator unit back online but somehow managed to connect it when out of phase. It ripped that !ucker clean out of the floor and caused millions in damage.

That has nothing to do with this situation.

These devices are intended to connect to an active powered grid connection. They sense the phase of the grid, and when there's power input on the DC side, they convert it to AC in phase (well, slightly advancing the phase, technically), thereby pushing power onto the grid tied connection.

Putting two or more on the same grid will not be a problem - if they couldn't handle this, then they couldn't grid-tie in the first place.

If they work well separately, then they should work well together.

Make sure they are properly wired and fused, installed on a non-flammable surface, and maybe keep an eye on them for their first 24 hours of use, but beyond that this isn't some megawatt generator. If one or the other fails, then it would have failed by itself and not due to the other one anyway.

 

[Hedges]

I don't think a grid-tie inverter even thinks about producing voltage. It senses voltage (or makes its own internal sine wave, likely as data in microprocessor's memory, synchronized to grid) and it drives a sine-wave current through an inductor circuit.
The current is driven into grid, whatever voltage is required. Enough to overcome the wire resistance. I might see a couple volts rise on my AC wires when they are operating.

If they were professional inverters,

Unless home brew they are all commercially made somewhere, but the issue is whether they are safe.
Generators improperly connected while grid is down have electrocuted linemen. Or been burned up when grid returned, if they weren't overloaded trying to power the neighborhood.

Off-brand unlisted grid-tie inverters probably function most of the time and shut off if grid fails. But there may be situations where they keep operating and could be unsafe.

I've been able to buy either used or new-old-stock SMA inverters, and I've seen other brands, in the range of $500 and 5kW rating. There are models from 2.5kW to 10kW. Because they have UL-1741 listing I'm completely confident they will shut off and be safe. Compared to the 1kW $250 off-brands I've seen, I would go with used name-brand models.

As for non listed equipment, I think it is better to only get and use those which power an isolated circuit for protected loads, not backfeed the grid.

 

[RCinFLA]

If they come close to having proper anti-islanding capability, they should be stackable. Because these are questionable devices that are not legal and potentially dangerous, I would not bet much they have proper anti-islanding

A normal GT inverter attempts to wobble its phasing slightly to test for grid presence. The grid is a near immovable object when attempt is made to slew the driving phase by the grid tied inverter.

 

[Quattrohead]

OK I did not see they were grid tied, but I still would not do it with cheap shit. The risks are too big.

 

[Hedges]

OK I did not see they were grid tied, but I still would not do it with cheap shit. The risks are too big.

Possible risk is injury/death to utility lineman if they fail to shut off.

If paralleled with a much larger always-on load, I think that would crowbar them and prevent export.
For instance, if connected to the motor of an A/C or pool pump, or to the heating element of a furnace or water heater (after the control switch or thermostat), they would only operate when that load was powered, so would only offset some of the load, never export.
That is only true if there is nothing that can disconnect the load and leave inverter operating, for instance a thermal trip on the motor or over-temperature switch on the heating appliance.

This is something I've suggested for guerilla zero-export, even when using proper UL-listed inverters. If you want to ensure you never export, because don't have an agreement to do so and export would be detected by utility, just need to ensure a greater load is always connected.

 

[12VoltInstalls]

then they may be more resilient to such effects

The grid is a bottomless ocean of power, and nothing one inverter can do is likely to even be observable by the other (except for a few ripples on the surface.)

They will not be able to see each other above the weight of the grid. It’s just voltage.

The grid is a buffer of sorts: so massive that a little input from one of these devices would/should not be “seen” by other devices

 

[Jordi]

Answering a bit your comments.
First, I do have an agreement with my energy supplier to export electricity so doing that is not a problem.
Second, risks of using cheap Aliexpress grid-tied inverters (even individually) is probably there. They have certifications, either one or both claim to have the CE certification but I will put them on anti-flammable material and on a "safer" spot in the house.
Lastly, safety problems on feeding the grid I do not expect them. As soon as I disconnect my house from the grid; the DC current flow goes to 0 and the led lights on the inverter give "ac failure". Was the inverter protection failing, then the DC flow should not stop to keep feeding the passive drain of my house (eg. lights, fridge,...).

Thanks for the interesting discussion.

 

[Hedges]

Lastly, safety problems on feeding the grid I do not expect them. As soon as I disconnect my house from the grid; the DC current flow goes to 0 and the led lights on the inverter give "ac failure".

That will generally be the case.

With the prospect of millions of grid-tied inverters operating, and broken powerlines perhaps leaving connected precisely the load inverter can supply, there was the concern inverter could continue operating for an indeterminate amount of time.
Feeding resistive loads, that might never happen, because inverter won't send current during next phase unless something pulls voltage up. With motors or inductive/capacitive loads resonating, theoretically possible. Supposedly that has never been observed "in the wild" but has been carefully arranged on the lab bench. With millions operating, it might happen once and electrocute a lineman.

What active anti-islanding does is to try injecting a bit of current at the wrong time, out of phase with line voltage. If injected current causes voltage to rise, it's an island and not the utility grid feeding entire city, so inverter disconnects. In the US, this is part of UL-1741. There may be a similar certification in your location. We had to name our equipment when getting a permit, and it was looked up in the approved list. The permit was needed for utility approval of the hookup and net metering agreement, also for rebates offered at the time.

No lifespan issues for your inverters when using two.
Operating temperature (and humidity exposure) are what primarily impact lifespan.
Surges too - if you sometimes get lighting induced spikes on AC grid, then transient voltage suppressors might protect them.

You want more production than one can handle. If you have multiple roof angles, using two panel orientations with one inverter can increase hours of operation and kWh output. (shorter calendar life, maybe same kWh life.) Panels connected in series need to have same orientation. Strings of panels in parallel can be different orientations.

 

[12VoltInstalls]

there was the concern inverter could continue operating for an indeterminate amount of time.
Feeding resistive loads, that might never happen, because inverter won't send current during next phase unless something pulls voltage up

Thank you Hedges.
That was the first time I’ve heard the actual issues holistically discussed.
Most of the licensed ‘teachers’ I’ve had over the years said things- couldn’t explain- just said “do it like this” and seldom were able to answer scenario questions. They knew rules but not contexts.

 

[RCinFLA]

One of those units getting real certification by an agency is about as likely as getting approval on a male to male plug extension cord.

Even a real UL1741 approved GT inverter can take up to a couple of seconds to recognize grid has gone open before it shuts itself down.

If that unit is running and you pull plug from wall outlet there may be high AC voltage on exposed male plug prongs for some amount of time.

It would be safer if it detects neutral to ground prong connection but that would not be enough to get approval. I would not count on the unit doing this check and that function may cause GFI outlets to break open.

 

[Ampster]

I do have an agreement with my energy supplier to export electricity so doing that is not a problem.

Then I presume those devices are listed in your Net Energy Metering agreement? Can you confirm?