Switch to Grid Due to Inverter Overload

[Aten]

One topic that seems to be rarely discussed is bypassing the inverter at overload. I understand ATS - that switching algorithm is based on battery voltage. My question is about infrequent overload (outgoing Amps) of the inverter for some minutes (not just a surge).

Will mentioned in one of his videos that the AIO units (his example MPP) switch to grid power in case of overload. How is this done with Tier-1 inverters such as Samlex or Victron?

Why would I want to do this? It's a simple utilization argument. Typically people are encouraged to choose a power inverter that is one step above of what the calculated need is. But what actually defines the "need" in an off-grid situation? Assume I hook up a bunch of devices that come on and off - fridge, microwave, toaster, laptop charging. The average is always much lower than the peak (which determines the size of the inverter).

Example: If I build a small off-grid system with 2kW solar to supplement (!) my grid power usage, and my average load is 1500W and once in a while I jump over 2000W for a few minutes per day, it makes no sense (to me) to buy a 3000W instead of a 2000W inverter, given its higher idle load and higher price. I will use grid power anyway when my ~2kW*4h=8kWh solar energy is used up. So switching over in those rare cases when I need say 2.7kW for a moment does not change my daily energy budget. On the contrary, I can hook up enough devices to make sure I actually use up the 8kWh without worrying about overload.

My question is: What is the best cost-effective way to allow the inverter or system to switch over to grid power in case of overload instead of shutting down the inverter? Maybe it's a silly question since I don't find much discussion of that sort (tell me if it is ;-) )

 

[Hedges]

Grid-tie is one way, assuming net-metering is allowed. Don't have a battery, just feed PV power into grid and draw what you need.

Same thing can be done with a battery inverter. My grid-tie inverters are on the output of my battery inverters (seen in my picture.)
If the grid goes down, it is disconnected and battery + PV powers the house.
Can alternatively be set up to never backfeed the grid. You run off battery/PV, connect to grid only if battery gets too low.

Other system are programmed to charge battery from PV some hours of the day, and feed battery to grid other hours.

All depends on what you're trying to accomplish, and how deep your pockets are.

 

[Aten]

It's not worthwhile to mess with the grid and the utility company for a small supplemental solar system. Plus it would be more expensive, so I could as well oversize the inverter for that money.

The fundamental question remains: How do the AIO (off-grid) units switch to the grid in an overload situation? And can it be done with individual components?

 

[Hedges]

To switch, a inverter needs to contain a relay capable of handling the largest load.
Simple and crude is just switch a relay from inverter output to load. And old square-wave UPS would probably do that. But it switches at time of grid failure rather than overload or low battery. AC is not synchronized at the time of switchover, not really an issue for most loads.

If you have a PV/battery inverter, where the inverter can handle your loads but battery gets low, battery charger powered by AC can kick in at a particular lower voltage. That would be an on-line UPS. It wouldn't help with loads in excess of inverter capability.

Sunny Island (battery inverter/charge not an all-in-one with SCC included) monitors grid AC, adjusts its output frequency until synchronized to grid, closes relay. If your load is 50A and you've set maximum 15A from grid, it provides the additional 35A from battery. If load is less than maximum available from grid, it recharges battery. It can operate from battery with separate SCC and connect to grid (or generator) when battery drops below a particular SoC.

There are also no-export configurations of grid-tied PV. PV panel feeds an inverter which puts current into the house AC wiring, but a monitor watches utility current/phase and output is adjusted so it never backfeeds the grid. I think Enphase offers that with no battery, also with battery (in which case expensive). Power Wall style "AC battery" products like the following from SMA also use batteries to achieve zero-export and store excess production for later (also expensive). They support larger loads than the 1500W you were thinking of, like 6kW and up to 10's of kWh.

Sunny Boy Storage 3.8-US / 5.0-US / 6.0-US | SMA America

Discover the flexible Home Energy Solution: The SMA Sunny Boy Storage 3.8-US / 5.0-US / 6.0-US battery inverter! ► Find out more about its benefits!

www.sma-america.com

 

[scottvanv]

To switch, a inverter needs to contain a relay capable of handling the largest load.
Simple and crude is just switch a relay from inverter output to load. And old square-wave UPS would probably do that. But it switches at time of grid failure rather than overload or low battery. AC is not synchronized at the time of switchover, not really an issue for most loads.

If you have a PV/battery inverter, where the inverter can handle your loads but battery gets low, battery charger powered by AC can kick in at a particular lower voltage. That would be an on-line UPS. It wouldn't help with loads in excess of inverter capability.

Sunny Island (battery inverter/charge not an all-in-one with SCC included) monitors grid AC, adjusts its output frequency until synchronized to grid, closes relay. If your load is 50A and you've set maximum 15A from grid, it provides the additional 35A from battery. If load is less than maximum available from grid, it recharges battery. It can operate from battery with separate SCC and connect to grid (or generator) when battery drops below a particular SoC.

There are also no-export configurations of grid-tied PV. PV panel feeds an inverter which puts current into the house AC wiring, but a monitor watches utility current/phase and output is adjusted so it never backfeeds the grid. I think Enphase offers that with no battery, also with battery (in which case expensive). Power Wall style "AC battery" products like the following from SMA also use batteries to achieve zero-export and store excess production for later (also expensive). They support larger loads than the 1500W you were thinking of, like 6kW and up to 10's of kWh.

Sunny Boy Storage 3.8-US / 5.0-US / 6.0-US | SMA America

Discover the flexible Home Energy Solution: The SMA Sunny Boy Storage 3.8-US / 5.0-US / 6.0-US battery inverter! ► Find out more about its benefits!

www.sma-america.com

@Hedges I have this exact same question, and I can't seem to quite discern an answer from what you've written. Let me try and ask this a different way: I sized at a macro-level based on kWH (80 kWH/day) and a peak of 1 hr of kW (9.96 kWH) during the worst day of summer, then assumed my peak was 9.96kW, then purchased an off-grid system of two MPP6548's rated at a total of 13kW, figuring the details from there would work themselves out (oops). As I've been working through the details, I surprisingly noticed that I need to feed my Inverters, THEN my Load from the grid, limiting my entire home's load to my Inverter's output, 54 Amps (13kW / 240V) even if it's night and my batteries are dead (this makes no sense to me, but, whatever). Monitoring actual whole home real-use AMPERAGE, I found my dryer, which with my AC and water heater also on, totals to 50A. Too close to be comfortable for me. Sigh. (Even though I'll rarely go over that 50A.) So here's the simple question: without purchasing different inverters, is there a device that I can put IN FRONT of my Inverters from the grid that will shunt anything over 60A (inverter max input) to my load, while concurrently keeping my inverters and their output functioning?

 

[Ampster]

The fundamental question remains: How do the AIO (off-grid) units switch to the grid in an overload situation? And can it be done with individual components?

One answer depends on the configuration of the AIO. My Skybox has a mode that prioritizes maximum independence with no sell to the grid, but I see moments when there is some slop and the grid is either used as a buffer or supports the temporary load that the inverter can not handle. I do not know if this is a switch or relay and I do not hear the sound of a relay closing but I see that slop on the screen. I suspect it has to do with subtle voltage differences and that is more likely controlled by firmware.

This may not answer your question about how it could be done with components. Conceptually some of the components needed in the inverter would have to measure voltage and control and vary voltage.

 

[Hedges]

My inverters can be told to allow up to 56A (or whatever lower figure I set) to come from grid and pass through. They will reduce charging or start inverting to stay below that.

Easier for you might be to rewire resistive loads like dryer element to draw less power. Dryer can run on 120V instead of 240V, will draw 1/4 the power half the current. Heating element will be on a larger percentage of time, and dryer can be run longer if necessary.

My inverter can signal based on battery SoC, and based on wanting more or less production/load to keep batteries charging and floating. But I don't think it has a signal to say it is loaded with excessive wattage. I think an added temperature sensor would provide that, within its 2.2kW to 7kW range of capability (which depends on temperature and time.)

 

[scottvanv]

My inverters can be told to allow up to 56A (or whatever lower figure I set) to come from grid and pass through. They will reduce charging or start inverting to stay below that.

Now this makes A LOT more sense to me than what mine are capable of ... looked for a similar setting. Noop. Sigh. This seems like a very strange, almost debilitating, limitation on my boxes.

My washer / dryer are 10 years old, so its time for their demise soon anyway ... looking at the heat pump versions ...

 

[Ampster]

My inverters can be told to allow up to 56A (or whatever lower figure I set) to come from grid and pass through.

Yes mine has a similar setting for the inverter to supply and the difference would come from the grid. My inverter can only supply 5kW but the circuit it is on can handle 14.000 kWs. I have never gotten close to either of those limits but presumably it is seamless.

 

[Ampster]

Noop. Sigh. This seems like a very strange, almost debilitating, limitation on my boxes.

That may be the difference between your boxes and the inverters that @Hedges and I have. Features cost money to program into firmware. That is why there also may be no simple answer to the remaining question posed by @Aten .

 

[scottvanv]

That may be the difference between your boxes and the inverters that @Hedges and I have. Features cost money to program into firmware. That is why there also may be no simple answer to the remaining question posed by @Aten .

What are y'all running? Maybe I'll swap out ... pretty sure I can still return them ... haven't even opened the boxes.

I do have 4 hyper-efficient mini-split ac units waiting to be installed at 2.5A(!!) each ... my central AC is pulling ~20A continuous, those mini-splits, which I had planned to run in parallel to the central as redundancy covering the extreme temp areas of the house, should pull the average power usage way down, but with just one on plus the central, will up the peak amperage. Sigh, again.

The heat pump clothes dryers (3-4A) take 2.5 hours to dry a load, according to Consumer Reports, and my wife would absolutely kill me. (I'm not sure if she EVER has the dryer off. Including warming PJs for the kids for bedtime. )

So ... for now ... unless I come up with something else, I think I'll be wiring only the dryer straight to the grid, then sending her the bill!!

 

[Ampster]

What are y'all running?

Outback Skybox per my signature.

 

[Hedges]

SMA Sunny Island 6048US. 4x, wired 2s2p
As shown in my avatar.

You can wire the dryer for 120V and put it on leg. Water heater on the other.

Old laundry machines are reliable. I've had a couple Miele washers, but they have electronics boards inside. When they suddenly won't turn on the motor or something, much bigger deal to debug and repair than swapping an electromechanical timer.

Heat pump ought to be able to make same heat with less power. Especially using dryer exhaust as heat source. But apparently performance wasn't an objective?

 

[scottvanv]

Outback Skybox per my signature.

Ah, yes, just saw your sig ...

 

[scottvanv]

@Ampster @Hedges Oh, man, neither of those are even close to what I could afford to attempt to go full off-grid. I could just buy a third MPP and wire it to handle basically only my AC or the dryer ... but that seems rather silly ... a third and fourth MPP would cost the same as buying a new washer / dryer, and would be total overkill to handle rarely used peak loads ...

Wouldn't pulling the power of the dryer double its drying time (?), so I'd be left with the same problem? And even though it's 10 years old, it has its fair share of circuitry on it ... was top end a decade ago ... and I've yet to dig into fixing an appliance much beyond a clogged dishwasher drain ... my wife usually just calls our washer dryer guy if something's wrong with them ...

As for the heat pump clothes dryers ... yes, you'd think so regarding the same heat with less power, but they ALL seem to take more time. Most A LOT more time, as in 2.5 hours for a typical load. The Miele dryers (1kW!) seem to be the fastest, tested at 94 minutes for a normal load (vs 70 for a typical regular full size). I might be able to talk my wife into that ...

I'm gonna sleep on this one ... thanks guys!!

 

[Hedges]

At about $5000 msrp each I would have set up just one with an autotransformer. It was a company's bankruptcy sale which made them so cheap I could assemble a ridiculously oversize system. But it still added up to kilobucks by the time I was done.

Full off-grid, you don't have the issue of dividing current between inverter and another source, except when running a generator. Some inverters suddenly load a generator, then disconnect again when frequency gets knocked out of range. You could have generator feed a battery charger to avoid that. Optional manual or automatic transfer switch so the loads are normally on inverter (up to you to avoid overload), but switch to generator when it is running if pulling power through inverter bypass relay is a problem.

For occasional heavy loads, like doing laundry (infrequently) for filling a water tank from well, generator can be started, and those loads wired to generator not inverter.

If a recent dryer resembles an old dryer but has electronic timer rather than mechanical rotating knob, the internals are probably similar as far as motor and heater go. Dryer is wired 240V, but motor and everything else is 120V. Does power cord have 4 pins, or 3? Dryers are usually sold without cord, and installer attaches one matching your electrical outlet. If 4-wire then 120V power cord with white neutral goes to dryer Neutral plus one of [Line1, Line2] "hot" on the dryer and black hot going to the other "hot" on dryer. If nothing happens, swap which of [Line1, Line2] gets connected to neutral. If 3-wire it could be more difficult. Old dryers used 3rd pin for both neutral (motor and timer return) and chassis ground, which I don't like.

On 120V, dryer will draw 1/4 the power. It has a thermostat in exhaust, so element cycles on and off to achieve desired temperature so not necessarily 4x as long. If washer spun it fairly dry, drying is reasonably quick. If soggy, takes longer.

Dryer may have two elements for low and high heat. Isolating one end of 2nd element would reduce heat to 1/2 power, better than 1/4 power and distributed across both phases of inverter. Or just put note on dryer asking wife to select "low".

Since you've already got working inverters (MPP) makes sense to figure out how to handle loads.
If you do literally have toaster and microwave, rig of a switch to enable one of two outlets at a time, and plug each of those heating loads into one.

 

[scottvanv]

On 120V, dryer will draw 1/4 the power. It has a thermostat in exhaust, so element cycles on and off to achieve desired temperature so not necessarily 4x as long. If washer spun it fairly dry, drying is reasonably quick. If soggy, takes longer.

Dryer may have two elements for low and high heat. Isolating one end of 2nd element would reduce heat to 1/2 power, better than 1/4 power and distributed across both phases of inverter. Or just put note on dryer asking wife to select "low".

Here's the dryer spec, not too sure I want to mess with it ... https://www.lg.com/us/products/documents/DLEX3360 DLGX3361 spec sheet.pdf

 

[scottvanv]

Since you've already got working inverters (MPP) makes sense to figure out how to handle loads.
If you do literally have toaster and microwave, rig of a switch to enable one of two outlets at a time, and plug each of those heating loads into one.

Yep. On another thread I was wondering why our 2500 sq ft home systems are sized at 150A or even 200A, when my average load was 41A on worst day of summer (prior to my super efficient hybrid water heater being put in). Now I understand that that's to handle peak loads ... AC plus Water Heater plus oven plus stove +++. Original objectives were: 1. Go full off-grid indefinitely, no change of lifestyle should the grid go down, 2. Pay wholesale rates for electricity over life of my system (total cost, including batteries, not including my time, cuz this is fun / kWH produced), 3. Keep grid available in case of solar system failure. These peak loads are killer to 1, and affect a bit of 2. So ... I'll have to think about this more. The dryer is used continuously, it's old ... pretty sure I can justify purchase of a new one outside of solar system econ math (unless you convince me I can pull a leg out from under it ) ... the other large, rarely used kitchen loads, also my garage tools ... I'll have to think about all that ..

 

[Hedges]

Dryer may or may not have an element that can easily be wired for 120V. Since gas model is 120V, likely motor and controls only use 120V.

May or may not have two elements (so one could be isolated). With so may temperature settings, could be single element and just electronic thermostat which turns it on/off. In that case, low setting would reduce average power consumption, but could be full power until it comes up to temperature when starting a load.

Try observing inverter wattage on low vs. high settings. One button cycles through them all, so could leave note reminding wife to select lower one (if that does reduce power draw.)

Otherwise prioritizing what appliances to run.
My first Miele washer was meant to work with a dryer where they took turns using heating elements, so could share a single 30A 240V outlet.
My dryers are still old low-tech. We'd like to recover exhaust heat to preheat incoming. I got an air-cooled condensing dryer for a rental unit (because no exterior vent) which they are satisfied with, but small and I think slow. Heat recovery comes with lint blockage issues. Ain't no such thing as a free lunch.

Yep. On another thread I was wondering why our 2500 sq ft home systems are sized at 150A or even 200A, when my average load was 41A on worst day of summer (prior to my super efficient hybrid water heater being put in).

Because we're Americans.
Used to be 30A fused. This is "progress".
Soon to be reverse when all gas appliances banned in California, and we find how much more efficient to use combustion for heat at point of use had been. Grid is going to have to support peak of all homes at once, which means paying for idle capacity the rest of the time.

I have 200A service, branching to a couple buildings.
Because I got a steal on Sunny Islands (although you have to spend money to save money), I used them as a higher current transfer switch. 100A circuit feeds through them to whole house (which is battery backed) and 10kW of PV backfeeds the grid or powers house during outages.
I need to set up some controls for automatic switching to gas not electric furnace when off-grid, and dryer heater and water heater only enabled if battery mostly full. With my 4-prong plug, that will just take a relay disconnecting the Line2 which feeds heating element, leaving motor and timer still operating.

For you (and me when grid down), priority switching to enable only some loads at a time is the way to go. Better utilization of generating capacity. For instance, when thermostat enables A/C (optionally also only if grid is down), interrupt power to water heater.

 

[Ampster]

I was wondering why our 2500 sq ft home systems are sized at 150A or even 200A

Be happy. It is the bus bar capacity of that bigger panel that will allow you to add more solar.