Understanding backfeed with critical load panel.

[drps10]

Am I thinking correctly?

I have a 18kpv, critical load panel and 1 indoor wall battery. I recently installed solar panels and was playing with the settings. At one point, I was backfeeding and I panicked a little bit. I have no agreement to do so and I don't want to raise any red flags. But I think I was ok and might implement backfeed up to 500w.

My CTs are on the feeder line (from feeder breaker) going into the inverter. The system sees me over producing solar and back-feed the grid, but really since my mains still are pulling 1kw constant, I am really not back-feeding the grid but really just powering the loads on the mains. Is this theory correct?

Is there a way to maybe implement 2 CT sensors and use the house as a solar dump? Example, have CT on the main wires and then CT on the grid tie line and use the difference? If you have an extra 2kw of solar (after battery is full and critical loads are powered) have the MAIN CT read that you are pulling 1.2kw of power on the main panel and allow the inverter "back-feed" house/feeder breaker that extra solar but don't go over. I'm sure you want a buffer incase a big load turns off, you have room to absorb the difference for a second.

Thoughts...

 

[Zapper77]

Am I thinking correctly?

I have a 18kpv, critical load panel and 1 indoor wall battery. I recently installed solar panels and was playing with the settings. At one point, I was backfeeding and I panicked a little bit. I have no agreement to do so and I don't want to raise any red flags. But I think I was ok and might implement backfeed up to 500w.

My CTs are on the feeder line (from feeder breaker) going into the inverter. The system sees me over producing solar and back-feed the grid, but really since my mains still are pulling 1kw constant, I am really not back-feeding the grid but really just powering the loads on the mains. Is this theory correct?

Is there a way to maybe implement 2 CT sensors and use the house as a solar dump? Example, have CT on the main wires and then CT on the grid tie line and use the difference? If you have an extra 2kw of solar (after battery is full and critical loads are powered) have the MAIN CT read that you are pulling 1.2kw of power on the main panel and allow the inverter "back-feed" house/feeder breaker that extra solar but don't go over. I'm sure you want a buffer incase a big load turns off, you have room to absorb the difference for a second.

Thoughts...

How did you settle on 500 watts? Thats 12 kWh per day.

 

[rogerheflin]

It is likely if you backfeed that the POCO will notice it (if they have non-spinning dial/meter less than 20 years old). They may or may not talk to you about it, they may or may not turn off your power. It all depends on how strict they are.

It does not matter the amount you try to limit it to. The issue is when a large load turns off(say a 6kw dryer) that 6kw will leak/backfeed until the electronics adjust settings and reduce the amount of power being back-feed.

 

[drps10]

It is likely if you backfeed that the POCO will notice it (if they have non-spinning dial/meter less than 20 years old). They may or may not talk to you about it, they may or may not turn off your power. It all depends on how strict they are.

It does not matter the amount you try to limit it to. The issue is when a large load turns off(say a 6kw dryer) that 6kw will leak/backfeed until the electronics adjust settings and reduce the amount of power being back-feed.

I have an old spinning dial, but they intend to upgrade soon. That is why I chose 500w or something low. That way when the dryer does turn off, there is still some other load to absorb it.

 

[drps10]

How did you settle on 500 watts? Thats 12 kWh per day.

Just throwing number out, but there is always something pulling power in my house. I usually use around 1-1.5kw /hr. Between the aerator and the radon, something is always running. I don't want them on the critical load panel, no need but would be nice to supplement their usage when available.

 

[TechRider]

That is correct. Sol-Ark has a different term for it but there's a setting that tries to match your incoming load. IE: You're pulling 1kwh from the poco on your main and you have your CTs around your mains. You can backfeed the "grid" from your inverter and cover your home loads. The hope is that you don't exceed that load.

 

[Citlalatonac]

What's your holdup on getting an interconnection agreement?

I know some PCOs have policies that absolutely torpedo any benefit to solar or plain ban them, but if it's not a huge cost to you it's the right thing to do as long as you have the potential to be feeding back to the grid.

The problem from their perspective is: if they are getting backfeed, they can't tell really tell if it's coming from a nice clean UL1741 inverter or some guy with a cheap generator who's come up with a brilliant plan to lower his power bill with diesel siphoned out of school buses.

 

[ricardocello]

My CTs are on the feeder line (from feeder breaker) going into the inverter. The system sees me over producing solar and back-feed the grid, but really since my mains still are pulling 1kw constant, I am really not back-feeding the grid but really just powering the loads on the mains. Is this theory correct?

Yes that is correct.

Some questions and thoughts to ponder (answering optional):

• What big loads do you have on the mains separate from the 18kpv?
• How do you know they are running at any given time?
• What the the largest load handled by the 18kpv?
• When the largest load on the 18kpv shuts off, instantaneously there will be a spike in negative wattage equal to the watts that were being used, and the negative wattage will be present for a second or two until the 18kpv can compensate.
• In order not to backfeed to the grid, this negative value must be compensated by other loads in the house that are still active.
• Just having a 1000W load on the mains side guarantees nothing, it just buys you a 1000W buffer for the negative wattage spike.

Is there a way to maybe implement 2 CT sensors and use the house as a solar dump? Example, have CT on the main wires and then CT on the grid tie line and use the difference?

In principle, yes, but the 18kpv isn't wired or programmed to do that, and never will be.

For example, I have an EM530 energy meter that measures grid backfeed independently with its own CTs.
But that information cannot be instantly used to compensate for grid backfeed, because nothing is infinitely fast.
It has to read out the backfeed current, send the info digitally over a cable, and a control loop would have to instantly start cutting back power.

If you have an extra 2kw of solar (after battery is full and critical loads are powered) have the MAIN CT read that you are pulling 1.2kw of power on the main panel and allow the inverter "back-feed" house/feeder breaker that extra solar but don't go over. I'm sure you want a buffer incase a big load turns off, you have room to absorb the difference for a second.

You are attempting to get to the root of the problem, which is great.
But it cannot be done fast enough to prevent backfeed. You might buy yourself a little time, that's all.
A bigger buffer certainly helps, but it still makes no guarantees.

 

[TechRider]

I think that's what he was getting at with the 500w limit. If he's always got a 1kw load and he limits the backfeed to 500w, he's essentially cutting his grid use in 1/2 during that period.

 

[ricardocello]

I think that's what he was getting at with the 500w limit. If he's always got a 1kw load and he limits the backfeed to 500w, he's essentially cutting his grid use in 1/2 during that period.

As long as he can guarantee the 1 kW load is always there, and his largest 18kpv load is less than that, yes.

 

[drps10]

Yes that is correct.

Some questions and thoughts to ponder (answering optional):

• What big loads do you have on the mains separate from the 18kpv?
• How do you know they are running at any given time?
• What the the largest load handled by the 18kpv?
• When the largest load on the 18kpv shuts off, instantaneously there will be a spike in negative wattage equal to the watts that were being used, and the negative wattage will be present for a second or two until the 18kpv can compensate.
• In order not to backfeed to the grid, this negative value must be compensated by other loads in the house that are still active.
• Just having a 1000W load on the mains side guarantees nothing, it just buys you a 1000W buffer for the negative wattage spike.

Thanks for the questions.
-I am running an Emporia on the main and the sub panel.
-My largest load on the 18kpv is my microwave. I know this setup is overkill, but I don't have the solar or more batteries to run big loads yet. This system was intended for critical load backup with a solar supplement. And to charge my electric commercial mower and future EV vehicle. I intend to put the HVAC mini split system on this load which avg 3kw when its 20*F out and 4kw when its 10*f, but that has variable compressor which helps with instance off situations.
-I don't know why they couldn't add another CT option and maybe program something like this. I know there is a delay with calculations and implementation where there could be a spike, but that would be on the user to understand that hybrid setup.
-I have not looked into selling back very hard. From what I did read, it's on a select basis and they give you credit.
First Energy in Ohio if anyone does have an agreement with them and can share some information about it.

 

[12VoltInstalls]

That is why I chose 500w or something low. That way when the dryer does turn off, there is still some other load to absorb it.

I don’t think that is a good idea.

usually use around 1-1.5kw /hr. Between the aerator and the radon, something is always running. I don't want them on the critical load panel, no need but would be nice to supplement their usage when available

system was intended for critical load backup with a solar supplement

it cannot be done fast enough to prevent backfeed

would be on the user to understand that hybrid setup

So if not just getting a netmetering agreement, why not just use the AIO as it was intended: run everything through it and use a bigger critical loads panel; disable backfeeding the grid in the programming, and add enough solar to cover your needs. You can set the battery usage with grid present to only use 20% (leaving the bulk of the capacity for grid down backup) and just go into bypass overnight.
Sidebar: you probably need a minimum ~20kWh of battery for that AIO inverter system

I think you are over complicating this.

 

[TechRider]

You'll still get backfeed if you do this. I had my settings up to 200w of continuous grid use on my AIO to prevent backfeed and I still sent about 1kWh/month.

 

[12VoltInstalls]

You'll still get backfeed if you do this. I had my settings up to 200w of continuous grid use on my AIO to prevent backfeed and I still sent about 1kWh/month.

So you can’t turn backfeeding Off?

 

[rogerheflin]

So you can’t turn backfeeding Off?

You must put it into off-grid mode to disable backfeed, and then solar is only used for the critical load panel.

Otherwise the inverter sees changes on the ct and attempts to keep it zeroed but the software and hardware can NEVER be fast enough to actually truly always keep it zeroed and have NO backfeed.

 

[12VoltInstalls]

then solar is only used for the critical load panel

So it won’t do the ‘normal’ bypass switching without backfeed, or did I read too much into that?

If solar charging only, but bypass capable, I would imagine that would be ideal.

 

[rogerheflin]

So it won’t do the ‘normal’ bypass switching without backfeed, or did I read too much into that?

If solar charging only, but bypass capable, I would imagine that would be ideal.

In off-grid mode either the load is being 100% fed by the inverter OR it is being 100% fed by the grid (controlled by the relays/transfer switch in the inverter). There is no grid boosting. I believe solar charging works in either case, but if fed by the grid mode all of the solar is going to the batteries, and the inverter is technically turned off. I believe switching from fed by grid to fed by inverter is slower than the take over in backfeed mode (when power is lost) where the relays do not have to be activated. When I have ran off-grid mode the quick switch from one mode to the other was noticed by my UPSes, but nothing else appeared to lose power for long enough to reset.

 

[12VoltInstalls]

There is no grid boosting

Which is different than ’pass-through.’
Yes, I’m trying to help the guy, yet I don’t know everything so was trying to establish a solution for him.

Pass-through is when you set a switchover point of a % of charge (sometimes that’s actual SOC, sometimes a voltage depending on how sophisticated the system is) for when the unit switches off the battery and sources power from the grid. (And every unit I’ve worked with lets you also opt to NOT charge from the grid, solar charging only)
If one’s priority is grid-down backup, I’d set the ‘bypass’ enable to be be about 80% of battery capacity so there’s some decent period of backup power available.
If one’s priority was killing the billing from the electwic crompanry then I would set it to go to bypass mode at like 15% of battery capacity to maximize use of self-generated power.
Grid boosting to me means meeting loads with battery and grid power when necessary.
I’m not arguing with you- I just don’t know everything and this thread has brought up questions in my head that I’d like to settle. Plus it may benefit OP.