Using DC Solar Trailer as Rooftop Battery Backup

abilityonline

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Hedges: I was at a 'be prepared' (emergency prep) convention and a solar engineer had a both. Very friendly and knowledgeable guy willing to offer advise. He cautioned me on using the solar trailer on my current setup based on my GT solaredge inverters. According to him, the Sunny Island inverters are able to discern when power should be shed to home, but that the solaredge inverters don't have a ground, nor any way to communicate to the SI inverters. Because of that, I would need a very large battery bank to store excess storage because my PV on a Sunny day would continue to generate power and the Solaredge inverters are not intelligent enough to say 'stop, no more power, we're all full down here'. That wouldn't be an issue if the grid was accepting net zero backfeeding, but if the grid was down and backfeeding was shutoff, I could theoretically go boom in that setup or at a minimum throw a breaker. He said they way around it would be to get hybrid inverters in the place of my solaredge or a SunnyBoy setup that would easily communicate with the SI inverters. I couldn't tell if he just wanted to sell me on the new equipment and install fee or if he was right and I need to abandon my SolarEdge GT Inverters. Are you aware of the lack of communication between SolarEdge and SI?
 

Hedges

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There would be no communication between Sunny Island and Solar Edge except AC frequency. If SI wants less power it ramps up frequency.
If Solar Edge does the optional "frequency-watts" portion of UL-1741-SA then it should play nice. If it doesn't, it would just bang offline and back online every 5 minutes, which I imagine would cause wear of relays if nothing else. And cycle battery.
You want the frequency-watts feature for AC coupling (or similar behavior of earlier Sunny Boy inverters which predate that UL standard.)
 

abilityonline

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Hedges would this imply yes Solaredge does support frequency watts?

https://www.solaredge.com/us/products/power-optimizers#/

The UL 1741 SA certification ensures that inverters stay online and adapt their output to stabilize the grid during abnormal operation instead of disconnecting. This functionality allows improved grid resiliency and increased PV integration. Certified smart inverters offer anti-islanding protection, grid support functions, low and high voltage and frequency ride through, voltage variation mode, power factor control, power ramp-up during startup and normal operation, remote powering and curtailment, and frequency-watt and voltage-watt control.
 

Hedges

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Hedges

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I couldn't tell if he just wanted to sell me on the new equipment and install fee or if he was right and I need to abandon my SolarEdge GT Inverters.

Solar Edge has optimizers which also provide RSD. Replacing with another string inverter (e.g. Sunny Boy) would require a different RSD module, assuming that is required for your installation. An alternative would be replacing Solar Edge with StorEdge and its relatively expensive battery. For a system of modest battery size perhaps that would be the best overall cost.

Ideally, Solar Edge turns out to support frequency-watts and can play nice with any AC coupled battery inverter that uses frequency shift.

Sunny Island by default has a range of AC frequency set for the older Sunny Boys, which ramp down power from 100% at 61 Hz to 0% at 62 Hz. A UL-1741-SA frequency-watts inverter will simply ramp down sooner, I think from 60.5 Hz to 61 Hz (I don't follow the settings given in Solar Edge documentation.) That much is fine. But Sunny Island later drops to 59 Hz or below to get electromechanical clocks back on time. Maybe UL-1741-SA will remain continuously connected or maybe disconnects at low frequency. You could change SI minimum frequency if necessary so low-frequency disconnect doesn't happen.
 

abilityonline

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This was the correspondence with this solar engineer.
My question (I borrowed from you, not likely very well)
Speaking to knowledgeable individuals, it is my understanding that if Solar Edge does the optional "frequency-watts" portion of UL-1741-SA then it should play nice with the Sunny Islands.

Sunny Island by default has a range of AC frequency set for the older Sunny Boys, which ramp down power from 100% at 61 Hz to 0% at 62 Hz. A UL-1741-SA frequency-watts inverter will simply ramp down sooner, I think from 60.5 Hz to 61 Hz (I don't follow the settings given in Solar Edge documentation.) That much is fine. But Sunny Island later drops to 59 Hz or below to get electromechanical clocks back on time. Maybe UL-1741-SA will remain continuously connected or maybe disconnects at low frequency. You could change SI minimum frequency if necessary so low-frequency disconnect doesn't happen.

On page 9 of the attached pdf it talks about enabling Frequency Watt Configuration for SolarEdge inverters.

Curious about your experience with this advice.
His Response
The information you supplied refers to the ability of the inverter to deal with power factor issues (real vs imaginary power). It does not relate to the control issues we talked about in the ability of the battery inverter being able to control the output of the Soleredge inverters
 

Hedges

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His response was non-responsive.
The information I supplied relates to battery inverter controlling output of GT PV inverters that implement frequency-watts.

Some inverters may perform power-factor correction, useful for a facility with lots of motors. That's a separate issue.

If you've got instructions for enabling frequency-watts that applies to your model, it ought to work.
Unless, those instruction on page 9 were about power factor correction.
Power factor correction is something that happens at 60 Hz, either reshaping current waveform to make it more of a sine wave, or shifting phase of the waveform to align it with voltage waveform.

But it is hard to know for sure if something will work until you try it. Even documented features sometimes don't work (bugs, or feature got broken during an update.)
 

abilityonline

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Hedges: Does the SI 6048 need an Automatic Transfer Switch between the Main Service Panel and the Inverters? I thought I read somewhere that SI's could detect grid failure and switch on their own without a transfer switch.
 

Hedges

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Correct, the feature is built in.

No need for manual or ATS between grid-fed main panel and Sunny Island AC2 input. Unless, you also have a generator, and want to switch between grid/generator feeding Sunny Island.

Sunny Island has a relay inside (think of it as SPST) which can be closed to connect AC2 input to AC1 output if grid is good. If grid fails, the relay opens and Sunny Island powers the loads. later, if grid returns, Sunny Island monitors for 5 minutes before reconnecting.

You just put your protected loads on AC1. They will see a brief glitch when grid goes down, maybe 30ms.
But it is better to put only critical loads (e.g. alarm, telecom) directly on AC1, along with Sunny Boys. Add a "load-shed" relay from there to important loads, normally battery backed up but disconnected if battery gets too low.
 

abilityonline

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What if I have a generator as the backup to my batteries, but the generator only recharges the battery bank but doesn't directly feed the inverter. I'm assuming I'm still safe with no load transfer switch
 

timselectric

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What if I have a generator as the backup to my batteries, but the generator only recharges the battery bank but doesn't directly feed the inverter. I'm assuming I'm still safe with no load transfer switch
Yes, you can charge the batteries from multiple sources at the same time. Without affecting any other part of your system.
 

Hedges

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What if I have a generator as the backup to my batteries, but the generator only recharges the battery bank but doesn't directly feed the inverter. I'm assuming I'm still safe with no load transfer switch

So a generator powering a battery charger that recharges the battery bank?
That works, but Sunny Island is unaware of the current that battery charger delivers.

If there isn't one already, you should install a shunt on the battery cable between battery and Sunny Island. Sense wires from shunt go to Master Sunny Island.
Battery charger output goes through shunt to battery.
I think Midnight Classic (DC coupled SCC) also goes through shunt to battery if there is no data cable from Midnight Classic to Sunny Island. That is probably how the DC Solar trailer is delivered.

With Midnight Classic, it is also possible to have a data cable so Sunny Island is told about charge current, and can command charge voltage and charging phase. In that case, I think Midnight Classic goes direct to battery not through shunt.

This is (was?) the adapter for data bus to Midnight Classic. They may have discontinued it; the SMA protocol reportedly changed with recent firmware updates.

 

abilityonline

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Hedges: Sorry for ongoing questions. I keep getting different advice from different sources. One source is telling me that if I want to backfeed the grid (net metering) with my current PV setup (solaredge) but also want the power my trailer is generating to add to my netmetering balance, I need a transfer switch between my Sunny Islands and my Kubota 11kw generator. I was under the impression the generator was merely charging the batteries but I guess sending power back downline. If so I need a 50 amp transfer switch if and when battery power stops and generator starts. Does that sound correct to you? I would need a transfer switch between the Sunny Islands and the generator and the main power panel. If so, are you familiar with a good option that is bare bones? It doesn't need to do anything besides detect when the generator kicks on. Wondering if something like this would work.
https://www.amazon.com/Progressive-...b9fc1&pd_rd_wg=NyygW&pd_rd_i=B003VAWNVK&psc=1
 

Hedges

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The generator on DC Solar trailer would be feeding AC2 input, so Sunny Island acts as battery charger.

Sunny Island has only one input, so if you want to keep the generator connected and also connect grid (allowing DC coupled PV to backfeed grid), then you do need a transfer switch.
Make sure the transfer switch will disconnect one source, wait 5 seconds before connecting other source. Otherwise, inverter can get blown up backfeeding out of phase.
An alternative could be to wire generator to a battery charger, DC couple it.

I have not looked into automatic switches.
Manual switch is another possibility (make sure it has an off position, doesn't snap across automatically.)
I do use interlocked breaker panels. You could do that, an interlocked generator input on your utility panel, but you need to signal Sunny Island when generator is used so it doesn't backfeed. A relay closed by generator AC would be one way (stops backfeed when generator is running, even if connected to grid.)
 

abilityonline

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Hedges,

I'm getting closer to buying all my parts for this project. I keep getting conflicting advice about schematics. Can you please review this (pdf) and see if I'm nearly there? Page 1 represents the current Grid Tied install while Page 2 represents my proposed AC coupling add on with my DC Solar Trailer. One person has told me I need an Automatic Transfer Switch that only lets backfeeding from the generator if the grid is down, but if connected to the grid the generator can't compete with the grid. I need this for net metering credits when the grid is up. You'll see that in the page 2 diagram. One question I have is if I need an Automatic Transfer Switch as depicted on page 2 between the Main Service Panel/Sunny Islands and the critical loads panel. How close am I here? Thank you again.
 

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Hedges

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Automatic transfer switch so PV panels and critical loads either go to grid, or go to Sunny Island?
Instead, I would have PV and critical loads go to Sunny Island, which goes to grid. (manual interlocked breaker to bypass Sunny Island if down.)

400A utility service and 100A automatic transfer switch between generator and 50A RV plug? I didn't quite follow.
Normally one would have automatic transfer switch to select either grid or generator, but it has to handle full current (400A in this case).

What does generator supply presently?
 

abilityonline

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Thanks Hedges. So to be clear remove ATS between main panel and critical loads panel and install an interlock breaker that allows only the grid breaker to be on or the Sunny Island to be on, but not both at the same time. This would require a manual switch at the panel in case if grid failure. (Am I wrong to assume that an automatic transfer switch here would achieve the same result but not require me to physically swap power sources? But rather do it automatically? Thank you for the clarification.)

The Generator is 11kw Kubota Lowboy II. 50 amp. The trailer has (2) 50 amp outputs. I've seen some who have used both plugs to feed the critical loads panel, while others say to use only one. If I'm understanding correctly the ATS between the generator and the grid should be 400 amp per your recommendation.
 

Hedges

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An automatic transfer switch would accomplish same as interlocked breakers, moving critical loads from Sunny Island to Grid in the event Sunny island shuts down.
But you have to ensure switching stops in the "Off" position for 5 seconds before connecting to new source. Sunny Boy could be backfeeding one source (e.g. grid) and then if snapped instantly to other source (e.g. Sunny Island), it could be backfeeding out of phase. The 5 seconds ensures Sunny Boy shuts off. (Same needed if Sunny Island is switched between two sources.)

If only critical loads, not Sunny Boys, get switched, then any ATS would work without the 5 second requirement. Look for a quality one, not something cheap with a tiny relay that could arc and short its inputs (grid and Sunny Island) together.

Generator - Do you mean one that came with your DC Solar trailer? Or one you already had?

If you wanted to switch your main panel between 400A grid connection and 50A generator, you would need a 400A transfer switch. It needs that 5 second "Off" time, and a signaling contact to tell Sunny Island when generator is connected to stop backfeed.

You could use generator with transfer switch (or interlocked breakers) feeding Sunny Island, again with 5 second delay and signaling.

You could instead use the generator with automatic transfer switch (or interlocked breakers) to feed some other sub-panel that has no inverters backfeeding it.

You could have the generator feed a battery charger to charge batteries on trailer, through shunt, when Sunny Island requests generator start. Doing so would bypass the warm-up and cool-down time Sunny Island implements by holding off connecting. It would cause battery charger to charge any time Sunny Island exercises generator.

With grid + lots of PV, generator may rarely be needed, so could manually connect with breakers when required. If feeding Sunny island, use a signaling contact (could just be a relay closed by AC output of generator, which would stop backfeed whenever exercised.)
 
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