Adding large capacity battery backup to newly installed grid-tie solar system in lieu of generator?

[solar8484]

I thought the IQ8 marketing material specifically says it has a much faster microcontroller/lower latency/finer grain control loop controlling the inverter output vs IQ7, to handle load matching.

This is also used as an explanation for why their system needs a particular minimum ratio of IQ8 to IQ7 microinverters in some configs.

I mean, sure, take with a grain of salt, I guess.

iQ8 is certainly faster but not that relevant to the AC coupling issues being discussed. The main AC coupling issue in an island grid with iQ8's is the lack of full initial sync and power ramp up with the grid-forming inverter (GFI). It happens well before there is excess power beyond the load consumption or the charging power limit of the GFI. The likely cause is due to the iQ8's grid disturbance injections (anti-islanding, sunlight only backup testing, etc.). My speculation is about when Enphase home batteries (e.g. iQ10) are paired with iQ8 micros where proprietary comms are likely used to disable iQ8's grid disturbance injections in the island grid and curtail power without frequency shifting. Such scheme would allow for much more stable island grid and no annoying side effects of frequency shifting (e.g. flickers, clock drift, etc.). BTW, Tesla Powerwall works in a similar manner when paired with Tesla PV inverters. I suspect PV inverter makers are simply leveraging the out of band power control functions they developed to comply with UL1741SB. It's just too bad that the comms interfaces are not made accessible to owners. If this trend continues then AC coupling with third-party GFI (e.g. Schneider, etc.) will likely get worse.

 

[Hedges]

It could be simply turns off active anti-islanding as Solar suggested. Then microinverters respond to frequency-watts but don't upset each other.
Other GT PV inverters couple better using Rule 21. Maybe just not having so many testing the grid.

Sunny Island also switches (older) Sunny Boy GT PV to offgrid mode, no anti-islanding and also looser voltage and frequency limits. It does that with proprietary RS-485 signaling. The Sunny Boy can also be configured for offgrid all the time, in which case no signaling is needed.

SMA America recommended in a video that offgrid be used when behind the Sunny Island battery inverter. They did not announce that in writing (so far as I have seen) and did not explain why. Must also have some issue.

 

[mwk1000]

i went with fronius they also have fast modbus control of thier inverters from a smartmeter for zero export i have 30kw diy batteries on a solark 15k and a DC generator on the same bus the gen will auto start if battery voltage is 48 and prevent complete discharge shutdown of the solark i will let you know how it goes ac connecting the fronius i have not tied it in yet and may wind up on the load side if the solark wants to shut of acpv when batteries are full i suspect some capacitor bank gadget would be a huge help in off grid ac coupled to eat up short bursts when the dryer stops have not found it yet ….. ? fronios also has a euro based ohmpilot as a dump load maybe a us version of that would also be made to work as a place to pick up transient surges ??

 

[Hedges]

A capacitor on battery bank can only store power by changing voltage (which battery will also see, also taking current.)
If you put a larger capacitor on SolArk's HV DC bus, that could do it.

A dump load on same DC bus could also eat bursts, but unlike caps would not return the power. Dump load on AC, enabled by DC bus voltage, would be another implementation. A circuit would need to enable dump load only at voltage higher than normal regulation.

 

[solar8484]

The non-US PV inverter makers are much more open with their power control interfaces. SolarEdge and Hoymiles also provide info and access to their power control interfaces. I wonder if it's due to cultural or regulatory differences from the US. It seems the US companies think (or at least want the investors to think) they can be the "Apple of solar" by pursuing walled gardens of proprietary products.

 

[wheisenburg]

If the grid forming inverter does not have enough load to support the operation of the GT inverters it will not lower the frequency to the point that the GT inverters will turn on. It works that way with any GT inverter, not just Enphase.

In my testing with the Schneider, the inverter always starts up at 60 HZ regardless of the charge level on the battery, so you are wrong about the issue being caused by the initial inverter frequency. If the PV inverters actually do turn on they will ramp up slowly until the Schneider sends them a 1 second 62 HZ pulse. This will turn them off as it should. The problem is getting the inverters to start up in the first place. Part of the impedance or voltage rise from the Schneider inverter is going to depend on how much current the battery can accept. Double the battery means double the current. Anyone doing AC coupling will tell you it works better with more batteries. It seems that when ALL the strings are connected none of the inverters start at all. I'm not running a scope on the system. I also don't have a way to monitor the PLC activity, but it does seem that Enphase has a way to measure how much current can be pushed into the Schneider inverter without raising the voltage back to the PV inverters too much.

So my assumption is that fewer PV inverters, more batteries, or batteries at a lower state of charge will all help to get the inverters to turn themselves on. The IQ8s, because of their Day light backup capability, have additional logic in them that is not in the IQ7s. This logic makes them more difficult to use for AC coupling. Enphase actually advices that some of their "Off Grid" profiles will not work with IQ8s. Of course I'm stuck with the darn things since I assumed the "latest greatest" would be better. It turns out that for AC coupling, the IQ7s are actually better.

I am still waiting to find even one person out there that actually been able to add a hybrid inverter and batteries to an IQ8 system and has it working well. If anyone out there is planning on getting Enphase IQ8s now and adding AC coupled batteries later, they might want to think again. Don't believe a sales person because they say inverter XYZ does AC coupled. Unless they are are willing to give you a guarantee in writing that the Batteries, the PV inverters and the Hybrid inverters you are planning to use will actually work together don't buy it assuming it will work.

 

[wheisenburg]

I look forward to your test results. Did you mention that Schneider informed you that Enphase is working on a fix for iQ8 AC coupling?

I heard this from someone that I believe is reliable. I have no current information on ETA or if this is even still being pursued.

 

[wheisenburg]

The non-US PV inverter makers are much more open with their power control interfaces. SolarEdge and Hoymiles also provide info and access to their power control interfaces. I wonder if it's due to cultural or regulatory differences from the US. It seems the US companies think (or at least want the investors to think) they can be the "Apple of solar" by pursuing walled gardens of proprietary products.

I think a desire to sell way overpriced batteries may be interfering with some companies making their PV inverters play nice, but I think there are also issues with hybrid inverter companies selling half baked poorly thought out solutions that work better on paper than in reality.

 

[Ampster]

In my testing with the Schneider, the inverter always starts up at 60 HZ regardless of the charge level on the battery, so you are wrong about the issue being caused by the initial inverter frequency.

That is not the case with my SolArk so we have different issue. Also the more I read about IQ8s the more I believe what you and others are saying about some of their AC coupling issues. I have IQ7s so I am still focusing on the SolArk as the issue and we are in agreement that the more batteries the better the chances.

 

[wheisenburg]

I assume the proprietory communications is much faster that frequency Watt which has latency built into it because of the UL and CA Rule 21 standards

I agree. It is why Enphase is able to do backup with no battery. That would seem to be even more difficult to control than AC coupling to a hybrid inverter. At least there you would have a battery to even out the peaks and valleys a little.

I think the big issue here for most people trying to add a hybrid inverter to an existing Grid tied PV system, is that there is no integration and testing being done by any of these manufacturers. The hybrid manufactures should have a "Supported PV Inverters" list. The PV manufacturers should have a "Supported Hybrid Inverters" list.

 

[zanydroid]

I wonder why no one has hopped on the thread talking about AC coupling bench test results with IQ8 connected in a super controlled topology with external factors removed and easy reconfig of things like number of IQ’s per branch and solar to battery ratio

I guess not that many people are incentivized to do it, compared to bench testing the other microinverters (that also happen to be more DIY friendly and AC coupling friendly).

 

[wheisenburg]

I wonder why no one has hopped on the thread talking about AC coupling bench test results with IQ8 connected in a super controlled topology with external factors removed and easy reconfig of things like number of IQ’s per branch and solar to battery ratio

Great idea, let my break out my oscilloscope, current and voltage meters, monitoring software and a couple of spreadsheets. I'll get back to you.

 

[zanydroid]

Great idea, let my break out my oscilloscope, current and voltage meters, monitoring software and a couple of spreadsheets. I'll get back to you.

Do you have some spare IQ8s, Combiner, Envoy, hybrid inverter, and space on your property to prop up some solar panels? ?

 

[Johneb]

If you go battery, use dump load and excess energy to heat pool/water.