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I confused my micro inverters by connecting two panels

jcdesenhaf

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Aug 28, 2021
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IT seems that sharing the DC output of two panels (in parallel) confuses one of the two panel-mounted microinverters when grid tied.

Solution ideas?

Here's the scene:

I own an 11 panel grid-tied system Sunpower X21-345 with SOLARBRIDGE (Enphase) Type E microinverters under each panel. The microinverters are daisy-chained delivering 240 to the panel and safely switch off when there's no load (power out situation)

With rolling blackouts being common here, I've successfully tapped one panel's DC line through a Y-fitting to charge two 200AH Renogy batteries through a Victron SmartSolar MPPT 50 Amp Charge Controller. The DC line is fused, switchable, 6AGW, and safely routed. Add a 3KW 24v sine inverter, and it works great to run the fridge, computers, internet, whatever on a completely independent circuit when the power's out. Easy.

Then, I got greedy.

I connected a second panel's DC line, in parallel, to my DC feed up at the panels. In testing, It worked great, delivering 500 watts at 10 amps while also delivering power to the grid.

Nice, huh? Only when the power is out though.

It seems that connecting two panels' DC lines in parallel confuses (reduces output of) one of the microinverters when grid-connected. This is with no other DC load (my charge controller is disconnected). It takes about 10 minutes for it to happen, but it is consistent.

How do I know this? Well, I just finished setting up network access to my Sunpower PVS6 system and am capturing the stats for each of the inverters for the last few days.

When I have two panels sharing the same DC line (in parallel), one of the two microinverters involved drops its amps down to about 22% of its normal output. This is with my charge controller and any DC load disconnected.

Any ideas of ways around this? Or am I destined to run a second pair of 6AWG through the house, or climb the roof to connect the second panel for every power outage?

All ideas appreciated.
 

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Very bad idea to start out. Any and all MPPT controllers sweep the voltage range from open circuit to near short circuit to find the MPP......the maximum power point. Microinverters do have a MPPT front end.

This will happen so quick that even a very high end dvm such as a Fluke cannot see the sweep, you will need a oscilloscope or a 25 megasample/second front end DVM to even see the sweep...not even remotely cheap.

What you are attempting is paralleling two controllers in the microinverters with the controller in the battery MPPT unit
When one controller sweeps it will shut down the other controllers down due to the changing voltages and the two or more controllers will fight each other for control.

My $500 Fluke 87V DVM cannot see the sweep on my MidNite Classic 150 controller but my $200 60 mhz bench scope sees the sweep very clearly.

So dont try to pursue this further......its just not practical......and probably past your understanding of electricity
(not being judgemental....my honest opinion)


So what to do .........add a smaller array that is dedicated to the MPPT controller that feeds the battery bank.

Leave the grid tie system alone....
 
200Ah Renogy batteries- Pb or Li?

If lead acid 300W from a separate array will keep them happy. If lithium 400- or 500W will be robust for your situation (both are a wildhat guess) and with the latter you could use more of the backup but dollarwise since you’re already engridded that makes less sense to me.

There are other inefficiency-ridden ways to glean from your gridtie without much upfront - or spendy with your installers and a ‘powerwall’ (sic) could be had. But for $250 of panels and another $75?-ish in parts/cables I’d probably go for 300W of panels.
Nevertheless, if borrowing watts from the gridtie’s been working
amendment
may well pass this on to persons in close proximity to this equiptment
(So read following post)
 
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The other possibly very dangerous situation which is created by this attempt to fool the grid tie system is the fact that no panel microinverter is isolated from the AC output buss (i.e. the power grid) by a isolation transformer. This will result in spikes from the AC grid being present on the solar panel.

This will result in grid voltages being applied to the off grid controller. They were never designed to handle this feed back voltage from the microinverter and it will present a dangerous situation in the off grid controller. Solar panels do not produce the spikes in voltages that commonly occur on AC grid networks so the off grid controller may well by damaged by this spike in energy damaging the circuitry and pass this spike in voltages on to circuits that were not designed for this “AC noise” and may well pass this on to persons in close proximity to this equiptment

FACT........A lightning strike in the Sierra Nevada mountains on the power grid can and will send a 8,000 volt spike all the way across the state blowing out non protected gear in San Francisco. Not if and or but, it has happened and it will happen again and again.

FACT.........All DVM (quality) (digital volt meters) have an IEC 1010 rating.....if you need to measure the voltage at the entrance of your home, shop, or whatever you need to use a meter that has an IEC 1010 cat 4 rating. Anything less will be blown out by the 8,000 volt spike. A device that has a IEC 1010 cat 4 rating does have protective devices that protect not only the meter but the user.

FACT........Absolutely no solar controller has a IEC 1010 protections due to the fact that no solar panel is capable of generating a 8,000 volt spike Unless someone drops a nuke at close range in which case you will have much bigger things to worry about

The only safe way to do this double duty is to completely disconnect the microinverters from the solar panels that would be shunted to the off grid controller and connect those panels only to the off grid controller

NOW THE RIOT ACT........If in fact that you do have a system failure that does result in personnel or equipment damage your insurance will be invalidated by the fact that none of the equipment involved will have a UL listing to do this, your insurance company is off the hook. All insurance companies can and do require that you must comply with the NEC , this includes the way that you interconnect devices. Even if your microinverters are UL approved (they are or they could not be marketed), the UL listing is for the “intended purpose” which does not include such an installation. They can and will refuse to compensate you for your loss.

a few abbreviations.... IEC International Electrical Commission
NEC National Electrical Code

Again i state that what is proposed by the OP will not work correctly and is very dangerous and a hazard.

Ask the manufacturer of the microinverters it they approve this connection
Ask the manufacturer of the off grid controller if they approve this connection
Ask the building inspector if they approve this connection
Ask your insurance company if their policy will cover a non approved connection scheme

The answers will be NO,NO,NO,NO
 
The other possibly very dangerous situation which is created by this attempt to fool the grid tie system is the fact that no panel microinverter is isolated from the AC output buss (i.e. the power grid) by a isolation transformer. This will result in spikes from the AC grid being present on the solar panel.

This will result in grid voltages being applied to the off grid controller. They were never designed to handle this feed back voltage from the microinverter and it will present a dangerous situation in the off grid controller. Solar panels do not produce the spikes in voltages that commonly occur on AC grid networks so the off grid controller may well by damaged by this spike in energy damaging the circuitry and pass this spike in voltages on to circuits that were not designed for this “AC noise” and may well pass this on to persons in close proximity to this equiptment

Just coming back to check on this thread. Thank you @Tecnodave for the insights. I'm going to write off the lost blood from running that 6AGW line and go an entirely different route. I've already lost an uninsurable home to wildfire and don't want to go there again.

Next research is to see if there is a 'powerwall' like solution (charger, transfer switch, inverter) that can safely coordinate the AC output of the rooftop array, a larger battery bank. I see there are a few threads about that so will monitor those for now.
 
If I read the first post correctly. This also forces the two micro inverters into parallel inputs. Which is not good either.
 
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