Micro inverters

Bluedog225 · September 2021

Good morning,

I’d appreciate some help understanding my options regarding micro inverters and an off grid system.

I’m interested in a large array (13kw), micro inverters (to minimize risk of DC arc), a central charger/inverter, and a battery. With the option to add battery capacity later as needed.

The part I’m having trouble with is understanding how to “trick” the micro inverters into producing. Alternatively, I’d like “dumb” or non-grid interactive microinverters. But all the focus seems to be on enphase.

Any advice on what to research or areas I need to study would be appreciated. I seem to be coming up empty in this area except for buying a complete (and expensive) Enphase ecosystem.

Thanks
Tom

BB. · September 2021

More or less, the battery based Off Grid (or hybrid inverter) system sets up the 120/240 VAC micro grid voltage and frequency. To the Grid Tied inverters, it just looks like connecting to a normal utility "Grid".

"Dumb" GT inverters simply look for good voltage and frequency (many GT Inverters in the US are 240 VAC units, possibly with a Neutral connection to verify the AC "network" is good). So--Your base Off Grid/hybrid inverter needs to typically be a 120/240 VAC split phase type.

The older Off Grid (PSW/TSW type) inverters could be forced to absorb power from a GT inverter and charge the battery bank--The issue being since the OG inverter was never designed to "regulate" charging of the battery bank this way, you needed to setup a "dump load" system to keep the batteries from over charging, or a controller that turns off the GT Inverters (relay) to cut their power and stop battery charging.

The newer systems--The OG inverter will cause the line frequency to vary from 60 Hz (100% power accepted from GT inverters) to (as and example) 61/59Hz to indicated no current is needed from GT Inverters (I believe the frequency shifts both high and low frequency to keep the average frequency at 60Hz for clocks and timing circuits).

So the "smart" GT Inverters that are setup for this micro grid operation (programmed to respond to frequency for output power control) accept the varying frequency feedback to help manage battery charging/other AC loads.

Another method with older "dumb" GT inverters was the OG inverter goes to +/- 1.0 Hz and "knocks" the GT inverters off line (5 minute timeout typically). This is known as "bang bang" control (max power then off--Vs proportional control which varies the power as needed).

While "micro grid" systems are out there... You need to make sure it all "plays nicely" together. SMA has their system. Finding another system--You have to figure out the mix and match of hardware/programming/needs to ensure it all works together.

https://www.solar-electric.com/sma-sunny-island-6048-us-inverter.html

A local electrical/power engineer that has successfully implemented such a system can be a big help.

Going back to your needs--Why a micro grid system vs a standard OG/Hybrid inverter with DC solar charging? Do you need the distributed networking?

-Bill

Bluedog225 · September 2021

Thanks. Very helpful.

I’m curious, if an micro inverter is smart enough to shut down, what happens to the panel energy? Is it just dissipated as heat at the panel level?

Regarding a standard inverter DC input, my concerns are mostly fire (DC arc) and long term safety as the wires degrade over time. I’ve got a lot invested in my cabin and the idea of higher voltage DC lines running around the roof and down to the basement makes me uncomfortable. Seems like a valid concern for a bigger system over 20-30 years.

Interestingly, LG sells an integrated micro inverter.

https://www.lg.com/us/business/solar-panels/lg-lg375m1c-a6

I’ll need to dig around to see how they are controlled. Nice to have some choices.

Thanks again.

mike95490 · September 2021

< 600VDC has not been much of an issue, and new gear is being rated to 1,000VDC

Your HV-DC can be run into conduit and protected that way.

Bluedog225 · September 2021

I guess that’s true. I could run it all in plastic conduit from the panel the the sol-arc (or whatever). I’d feel a lot better about that versus worrying/waiting for the insulation to fray, crack, or get chewed up by a rat. It would be a lot less expensive than jumping through all the hoops of trying to use micro-inverters.

Though I’m still going to put some more research into the IQ8s. I’m curious how they address the issue of dissipating the unused energy.

BB. · September 2021

Standard Crystalline solar panels (the type we mostly use)--When the output is switched off, the solar cell rises over the 0.5-0.7 volts of a normal diode junction, and then current flows internally in the cell. At that point, energy that would have been transported off the panel into the charge controller/etc. is now just "burned off" inside the cell. So, the panel now has ~15-20% energy that would have been transported off is now heating the panel... Overall, not an issue.

There is at least one panel chemistry/construction that does not like being in full sun without a load limiting the Vmp-cell/panel voltage. It will (after a week or two in sun) degrade the panel. If the panels are stored outside, as I recall, they are supposed to be shorted while unused.

These panels are typically use in large Utility Scale solar power stations--And not available/sold to residential installations.

Adding hardware/electronics (especially on the roof) is always a big question mark. More parts in thermally stressful location, more chance of failures.

And it is not just short circuits that are an Arc Fault issue. It can also happen when connectors fail (pulled crimp, pulled connection, corrosion, etc.)... Anything over ~12 VDC is a risk for creating a sustained arc. Two 12 volt car batteries in series make a pretty good emergency arc welder power source.

Example of the difference between AC and DC arcs (at ~220 VAC) using a knife switch.

https://www.youtube.com/watch?v=Zez2r1RPpWY

-Bill

Bluedog225 · September 2021

That’s great info. It sounds like panel heating when the micro inverter shuts down production is not a real issue in most cases. I also get the impression that concerns about dc arc may be well placed.

Thanks

Dave Angelini · September 2021

For offgrid, it is really a poor idea to use electronics outside a protected structure. Really Poor Idea !

It does keep me in business

RCinFLA · September 2021

Bluedog225 said:
The part I’m having trouble with is understanding how to “trick” the micro inverters into producing. Alternatively, I’d like “dumb” or non-grid interactive microinverters.

All grid tied inverter (that comply with UL1741 spec) have 'anti-islanding' function. Just a name for it shuts off quickly when grid is not there. One of the requirements for grid is the frequency tolerance. Power grid if very precise within 0.15 Hz. So any variance outside limit will be detected by GT inverter causing it to shut down.

An AC coupled battery based hybrid inverter used to create a micro-grid plays the role of a real grid for off grid operation. Issue comes into play if there is too much generated power. First level is to use excess PV power, not consumed by AC loads, for battery charging. When battery is full and still too much power being generated the GT inverter power output must be reduced or shut off. The hybrid inverter moves it frequency to be outside of grid freq specs causing GT inverters to shut down. As mentioned there is a middle ground where GT inverter reduces its output depending on how far off the freq is. This is a little trickier as the GT inverter must still obey the UL1741 specs for shutdown anti-islanding.

There can be an issue if battery based hybrid inverter, including its battery size, is too small with a very large GT inverter power array. GT inverters verify grid presence by attempting to wiggle its output phasing lock. The real grid is an immovable object so will resists any attempt by GT inverter to shift its phase. For a micro grid hybrid inverter it must be strong enough, including battery size, to resistance this grid testing by a strong high power GT inverter.

You may have a lot of AC loads, everything fine on consuming generated power, then a large AC load is switched off. There might be a few thousand excess watts that must be dealt with. It will take a few seconds for hybrid inverter to detect the situation, move its freq, and PV GT inverters recognize and shut down their power. The batteries must absorb this excess power for the few seconds. If batteries are too small their voltage may jump up beyond inverter input limit causing a total system shutdown. A mistake is made by folks thinking they only need a small AH battery because they have so much PV power available.

PV panels can be unloaded (or shorted). PV panels are in range of 20% efficient for solar to electrical conversion, so theoretically they have to dissipate about 20% more heating if left unloaded. This is not significant. There will be more variance in panel heating based on how close the panels are mounted to roof, color of roof, or how much the wind is blowing.

Micro inverters

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