Need Help with reversed engineered AC Coupling...

[wpns]

Sorry, but this sounds like a real kludge, you are going to spend a lot of time and effort to make this work at the expense if some serious risk of everything blowing up.

First, why 3 phases? Unless you have 3-phase equipment, a single phase is much easier to balance loads and production across.

I’d bite the bullet, ignore everything you currently have, and do a clean-sheet design given your current loads and expected expansion. A micro-grid might really work best in the long term.

Then you can look at how to incorporate your existing equipment, or selling it off and buying the right stuff for the long term and you can focus on your Missionary & Charity work.

 

[Zwy]

I have 86kwh of batteries now... Also I believe the excess power that needs to be consumed is what the PV generates, not just the max inverter capacity right...?

Yes, that is correct. But when batteries are full, that power needs to go somewhere until the grid tie inverter is shut down.

 

[Zwy]

Sorry guys, been ill for a while. My largest battery inverter is 15kva. I also have 2 10 kva unit and a 7.5kva unit. I'm stressing I have multiple in case it's best to couple them in 3 phase mode as opposed to single phase mode

Do these inverters have AC coupling capability? Do these have frequency shift to shut down the grid tie inverter?

 

[wpns]

Do these have frequency shift to shut down the grid tie inverter?

frequency shift control and grid-tie don't really go together, unless you are running your own microgrid

 

[Zwy]

frequency shift control and grid-tie don't really go together, unless you are running your own microgrid

That is what I'm referring to. This is a proposed micro grid.

 

[RCinFLA]

Truth is I highly doubt I'll ever exceed 35kwh at any one-time.

When doing AC coupling you have to design for worse case conditions, or you likely end up with equipment damage.

If you are using battery powered inverters that cannot absorb maximum PV back feed excess power, your only alternatives is to provide a fast-reacting AC load dump that can absorb excess PV power for the few seconds it takes for battery powered hybrid inverters to shift frequency to cut back PV GT inverters output power or a cutout relay that opens the connection to GT inverters if excess power occurs.

The worst case situation is producing a lot of PV power, then a large AC load is switched off, leaving a sudden large amount of excess PV power to deal with. Other situation is producing a lot of PV power with a lot of power being backfed to grid, then grid suddenly drops out, again leaving a large amount of excess PV power to deal with.

 

[Partimewages]

@RCinFLA
Can you give some brief system type manufacturers that do well dealing with the situation? I understand overall that system design impacts this.

 

[RCinFLA]

@RCinFLA
Can you give some brief system type manufacturers that do well dealing with the situation? I understand overall that system design impacts this.

They all do different things to mitigate worse case situation.

SolArk highly recommends running GT inverter through gen AC input port with user setup change in gen port function. This gives them the ability to open the gen port pass-through relay to 'pull the plug' on GT inverter if they get into trouble with sudden excess PV power. I believe they also have ability to connect an AC load dump.

Xantrex prevents battery from being fully charged to hold a reserve for excess PV power dump capability.

HF hybrid inverters are more vulnerable to AC coupling sudden excess PV power compared to LF hybrid inverters.

If HF inverter is supplying AC power to supplement AC loads when sudden excess PV power occurs (from switching off a large AC load), the battery to HV DC converter must make a mode of operation change from forward power sourcing from battery to charging battery reverse power conversion. This takes a few milliseconds to make the operating mode switching which can allow excess PV power to overvoltage HV DC bus and cause instability in battery to HV DC converter. This can damage battery to HV DC power converter.

LF inverters like Xantrex XW are inherently bi-directional so sudden excess PV power will immediately be dumped to battery.

Pulling the plug on PV GT inverter with a relay is probably the safest approach. Hybrid inverter can then take the time it needs to make a frequency shift then re-enable pass-through relay to PV GT inverter. Once PV GT inverter re-activates, the hybrid inverter can walk down the frequency shift raising PV power to match demand.

 

[solar8484]

Sorry guys, been ill for a while. My largest battery inverter is 15kva. I also have 2 10 kva unit and a 7.5kva unit. I'm stressing I have multiple in case it's best to couple them in 3 phase mode as opposed to single phase mode

Unless the inverters are designed to be paralleled they are not relevant for the discussion here since their output power cannot be combined/synchronized in a single island grid with the large GTI.

 

[Israel Keys]

Hello everyone, really grateful for your answers. Sorry was ill for over 2 weeks and only now just on my feet. This has all been food for thought... I'll take a few days to chew and answer some of the further questions this has raised to see if this is worth trying to do...

I'm just a stubborn knucklehead so will probably try to make it work until my designs tell me it's impossible. But yes as one of you mentioned, might be best I focused on my actual assignment here and that's not to be a solar engineer

 

[wpns]

It is a fascinating Geek Trap however, which is why you’ve got so much engagement! Let us know how it comes out.

 

[Israel Keys]

Hello everyone, really grateful for your answers. Sorry was ill for over 2 weeks and only now just on my feet.

It is a fascinating Geek Trap however, which is why you’ve got so much engagement! Let us know how it comes out.

You're right my friend. There's a stubborn geek in me who is insisting on giving it a try... But the seller has just agreed to give me a refund if I ship it back at my expense (about 40 pounds)...

Decision time!!!

 

[Israel Keys]

They all do different things to mitigate worse case situation.

SolArk highly recommends running GT inverter through gen AC input port with user setup change in gen port function. This gives them the ability to open the gen port pass-through relay to 'pull the plug' on GT inverter if they get into trouble with sudden excess PV power. I believe they also have ability to connect an AC load dump.

Xantrex prevents battery from being fully charged to hold a reserve for excess PV power dump capability.

HF hybrid inverters are more vulnerable to AC coupling sudden excess PV power compared to LF hybrid inverters.

If HF inverter is supplying AC power to supplement AC loads when sudden excess PV power occurs (from switching off a large AC load), the battery to HV DC converter must make a mode of operation change from forward power sourcing from battery to charging battery reverse power conversion. This takes a few milliseconds to make the operating mode switching which can allow excess PV power to overvoltage HV DC bus and cause instability in battery to HV DC converter. This can damage battery to HV DC power converter.

LF inverters like Xantrex XW are inherently bi-directional so sudden excess PV power will immediately be dumped to battery.

Pulling the plug on PV GT inverter with a relay is probably the safest approach. Hybrid inverter can then take the time it needs to make a frequency shift then re-enable pass-through relay to PV GT inverter. Once PV GT inverter re-activates, the hybrid inverter can walk down the frequency shift raising PV power to match demand.

My Hybrid inverter is LF.

Also I'm that a way round this is to have different solar arrays for charging and load...? Then disable/block the grid export function so the GTI can NEVER feedback to the batteries...?

Also I can set up my battery bank to never charge above 90% (or 85%) in anticipation of those large load dumps. Will.probably increase the battery life anyway right?