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Schneider Gurus: Adding another Conext SW inverter on SW AC IN to increase output capacity of an off-grid system - Possible on a DC/AC coupled system?

GVSolar

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My system: Schneider Conext SW4024, 3kw DC coupled panels, 3kw Enphase AC coupled panels, 2 Conext MPPT 150/60 SCCs, Combox and SCP. Approx. 25kwh of batteries (20kwh of Gyll, EG4, Lifepower4 and 5kwh of refurbished BYD/TechDirect). Off-grid for 30+ years. Original SW4024 online since 2013.

Using a backup philosophy of 1 is none and 2 is 1, I’ve purchased an additional SW4024 as an emergency backup to my AC/DC coupled SW4024 setup.

Here’s the scenario: I’d like to add the backup SW4024 (to the AC input of my original inverter) in order to increase the output capacity of the system. Currently, the peak continuous output is 3500w; by adding the new unit I should be able to increase that to as much as 7000w continuous. Both inverters will be connected to the same 25kw battery bank.

However, I am also AC coupled (to 3kw of Enphase M215s). This additional AC input is used for mini-splits and car charging when sun is available. It adds as much as 3kw to my daylight AC output capacity – and charges my batteries when not consumed by loads. This has been working flawlessly - although passing clouds can and do scramble my usage. Because the M215s are of an older generation they don’t respond proportionately to frequency shifting – they only stop/start according to the frequency shift.

This AC coupling works well with one inverter, but with two? Since the new SW4024 would connect on AC input (and appear as the grid) the original SW will sync to that input – and theoretically the M215s will sync to that in turn.

New Proposed Setup:
New Backup Inverter: SW/B, Original Inverter: SW/A,
SW/B AC OUT -->AC IN of SW/A -- AC OUT -->MAINS<--- M215s. DC SCCs and both inverters connected to the same battery bank.

Questions:
  • Will the AC coupling work at all using two inverters? Magic Smoke is not a good option.
  • I'm assuming that both A&B should be set to AC couple. True?
  • Will the frequency shift from SW/B knock SW/A off-line? Perhaps adjust AC input acceptable parameters?
  • How would the new/old units be best programmed? AC Support? Load Shave? Battery Charge/ReCharge? Other settings?
  • Any neutral issues to be aware of?
Some light experimentation has shown me that any charge enable settings greatly affect AC coupling. btw: Schneider discontinued "stacking" support for the SW series some years ago. I have an old Trace SW4024 running my well shed which I’ve successfully used on my AC input before – but without the AC couple.

@400bird @wheisenburg @pvdude @BentleyJ @RCinFLA @JoeHam @Horsefly @GXMnow @Matruco

Many thanks in advance for your replies.
 
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Your proposal is interesting, I cannot offer any relevant theoretical or practical experience other than to speculate the AC coupling aspect of such a configuration just seems like its going to cause problems.

Given the same circumstances I would lean towards installing a separate second system. A 125A breaker panel is relatively inexpensive especially if its the same brand you already have. Move a few breakers over to the new system and share the battery but not the AC coupling. You could also tie the old and new panel together via breakers so both could be energized by either inverter in case of an inverter failure. Otherwise the connection between the 2 panels would be OFF for normal operation.
 
Schneider discontinued "stacking" support for the SW series some years ago.
This is the most frustrating part. It would work easily if parallel or stacking was still supported.

I'd run the "grid side" one in AC coupling mode.
Seems like the better option; the load side SW would allow backfeed to the through to the upstream SW and it would have the responsibility to control frequency and AC charging from your micro inverters.

Set the load side SW to grid support down to your minimum battery voltage, so it supports the loads under most circumstances.


But, as Bentley said, I have no real experience or reason to believe this is the right way to do it or that it would even work.
 
This is like using first B inverter as generator input to second A inverter.

This is a method to save idle power when A inverter can satisfy the loads.

You have to put up with lock time on second A inverter whenever the first B inverter is activated. The first B inverter will be phase/freq master when it is operating with A inverter locked on and slave to the first B inverter.

You have to inject AC coupled GT inverters to second A inverter output.

I don't recall Conext SW inverters having ability to sell to grid, which would mean AC coupling power will not make it pass the second A inverter into first B inverter.

If PV GT AC coupling doesn't make it pass second A inverter, then master B inverter will never detect excess PV power and since it is phase master it will never do freq shifting to cut back PV power.

I think injecting AC coupling between inverters would be risky. If first B inverter shuts down, the AC coupling may keep the AC input to second A inverter active for a while until the freq drifts out of range.

When a pass through relay is active, the inverter is sync'd and slave to its AC input.
 
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I don't recall Conext SW inverters having ability to sell to grid, which would mean AC coupling power will not make it pass the second A inverter into first B inverter.
I believe you are correct, the SW series can't do grid sell. But it has no function (or need) to stop something else (AC coupled PV) from selling to grid right through the SW. I don't think this will cause an issue. But, it wouldn't be the first time I was wrong.
 
I believe you are correct, the SW series can't do grid sell. But it has no function (or need) to stop something else (AC coupled PV) from selling to grid right through the SW. I don't think this will cause an issue. But, it wouldn't be the first time I was wrong.
You may be correct on the back feed. It has two CT sensors, one on inverter path and one on AC input path. I was thinking of XW with Gen input port that will block back feed on Gen port to prevent possible generator damage.

If it detects a negative AC input power flow, due to AC coupling back feed, it just prevents inverter section from sourcing any output power and only allows inverter section to possibly draw power for battery charging, if charging needed. Excess PV back feed is passed through AC input to first B inverter.

This also means second A inverter will not be any help in sucking up excess PV production, other than what it needs for legit battery charging, so first B inverter will have to absorb all PV over production. First B inverter is the grid surrogate.

If you ever need to hook up a generator, I would first open breakers to GT PV AC coupling just to be sure there is no chance of back feeding generator, although generator will likely not be stable enough to keep GT inverters on line.
 
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Your proposal is interesting, I cannot offer any relevant theoretical or practical experience other than to speculate the AC coupling aspect of such a configuration just seems like its going to cause problems.

Given the same circumstances I would lean towards installing a separate second system. A 125A breaker panel is relatively inexpensive especially if its the same brand you already have. Move a few breakers over to the new system and share the battery but not the AC coupling. You could also tie the old and new panel together via breakers so both could be energized by either inverter in case of an inverter failure. Otherwise the connection between the 2 panels would be OFF for normal operation.
Thank you. It's certainly a possibility, but it won't increase the overall system capacity - just for those added individual breakers. Some balancing issues might be involved. One other concern is overlapping neutral/ground bonds in an inter-connected dual system (I think). It may end up being the only option.
 
I believe you are correct, the SW series can't do grid sell. But it has no function (or need) to stop something else (AC coupled PV) from selling to grid right through the SW. I don't think this will cause an issue. But, it wouldn't be the first time I was wrong.
You may be correct on the back feed. It has two CT sensors, one on inverter path and one on AC input path. I was thinking of XW with Gen input port that will block back feed on Gen port to prevent possible generator damage.

If it detects a negative AC input power flow, due to AC coupling back feed, it just prevents inverter section from sourcing any output power and only allows inverter section to possibly draw power for battery charging, if charging needed. Excess PV back feed is passed through AC input to first B inverter.

This also means second A inverter will not be any help in sucking up excess PV production, other than what it needs for legit battery charging, so first B inverter will have to absorb all PV over production. First B inverter is the grid surrogate.

If you ever need to hook up a generator, I would first open breakers to GT PV AC coupling just to be sure there is no chance of back feeding generator, although generator will likely not be stable enough to keep GT inverters on line.
Once again, thanks @RCinFLA and @400bird

Proposed Setup:
New Backup Inverter: SW/B, Original Inverter: SW/A
SW/B AC OUT -->AC IN of SW/A - AC OUT -->MAINS<------ M215s. Both inverters connected to the same battery bank.

Here is what I am taking away from your comments:


1. It may be possible to do what I am suggesting.
2. SW/B would act as the grid and drive the “master” frequency.
3. SW/A can “sell/pass thru” back to SW/B.

Found this in SW Owners Guide: Rev H 1–13
Section on AC Couple Smart Charge

“The Conext SW itself cannot sell to the grid - it simply passes energy from the PV inverter to the grid. For this reason, the PV Inverter must be fully grid code compliant as it assumes responsibility for anti-islanding protection.”

4. SW/B would also have to be set to AC Couple in order to shift the “master” frequency and shut down micros at full charge.
5. The system should be open loop/voltage driven so that all inverters are working on the same page. The current working AC Coupled system is voltage driven.

Some more Questions:
1. Can AC Support settings allow INV B to carry most of the load with supplemental power only coming from INV A during “high load” situations?
2. How best to defeat the AC charging function so that SW/A doesn’t try to “charge” from SW/B? Based on my earlier test, I believe that the SW Chargers must be enabled to allow AC Coupling to work. Can Charge Block settings only disable the power flow from the grid (SW/B) or will it disable the AC Couple as well?

More from SW Owners Guide: Rev H 1–13:
“If PV production is lost for a prolonged period of time and the battery bank discharges below the Recharge Volts setting, the Conext SW will initiate a normal charge cycle and use power from the grid (SW/B) to charge the battery. For information on this setting, see “Configuring Basic Settings” on page 4–7. In all of the above modes of operation, only the PV Inverter exports energy to the grid.”

Any additional thoughts would be most welcome.
 
Some more Questions:
1. Can AC Support settings allow INV B to carry most of the load with supplemental power only coming from INV A during “high load” situations?
Does the SW support the breaker size or load shave settings? Either or some combo of those should get you there.
2. How best to defeat the AC charging function so that SW/A doesn’t try to “charge” from SW/B? Based on my earlier test, I believe that the SW Chargers must be enabled to allow AC Coupling to work. Can Charge Block settings only disable the power flow from the grid (SW/B) or will it disable the AC Couple as well?
Turn off the AC charger in inverter A? You don't need inverter A to be AC coupled right? That's happening in inverter B. A just needs to ride through the frequency changes.
More from SW Owners Guide: Rev H 1–13:
“If PV production is lost for a prolonged period of time and the battery bank discharges below the Recharge Volts setting, the Conext SW will initiate a normal charge cycle and use power from the grid (SW/B) to charge the battery. For information on this setting, see “Configuring Basic Settings” on page 4–7. In all of the above modes of operation, only the PV Inverter exports energy to the grid.”
It's probably wise to set different low voltage thresholds on each inverter to prevent that. It would create a pretty rough situation if it A did try to charge from B.
 
Andy's Off-Grid-Garage runs two inverters in series like your proposed arrangement. Victron Multiplus for second 'A' inverter has the ability to activate a generator if AC output load exceeds user setup power level. He uses the generator activation relay contact to enable the first 'B' Phoenix inverter as if it is a generator. The Multiplus is setup for load shaving with max AC input limit on Multiplus based on Phoenix inverter (acting generator) max output current.

As I previously mentioned, there is a few seconds delay for Multiplus to phase lock to the Phoenix inverter after AC input shows up on the Multiplus before Muliplus's pass-through relay closes, passing the Phoenix inverter power through the Multiplus. The Multiplus supplements the Phoenix power with its load shaving capability.

Advantage of this arrangement is you only have the extra inverter overhead battery consumption when you need the extra power. Disadvantage is you cannot instantly jump from low AC loads to high surge AC loads unless the two inverters are already running together.

I would add an extra user parallel Phoenix activation control switch (manual forced generator activation) to force both inverters active when expecting to use high startup surge devices like air conditioners.

With added AC coupled GT inverters on second AC output inverter, it may momentarily cause GT AC coupled inverters to drop when second 'A' output inverter is tweaking its phase to lock to first 'B' inverter. The GT's will reconnect after phasing settles down.

The Conext SW models have similar features of Victron Multiplus.

 
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