Upstream ATS for Hybrid Inverter?

[Shimmy]

I am planning on adding a hybrid inverter to my system and am trying to decide if an automatic transfer switch to bypass the inverter "output" in normal operation is a good idea or not. I have a 200A service and am looking at a 6-8kW inverter with an internal 60A transfer switch. Currently our house can peak at about 70A (5-second average) when on-grid and not watching things.

Any arguments pro or con?

[edit] Post 10 has a schematic of what I understand my options to be.

 

[BentleyJ]

Based on trying to make the system as simple as possible without compromising performance or safety, first thing to do would be to determine if there is a way to reduce your 70A peak to 60A or less with some efficiency improvements. If for example that amperage peak is generated by inrush current from an air conditioning compressor, a soft starter can reduce that by 70%. Just that one change alone could make the difference.
Bottom line: Any electricity saving upgrades you do to get under that 60A threshold will pay for themselves over time and eliminate the cost of adding a transfer switch.

 

[Shimmy]

@BentleyJ The 70A peak happens when I am charging the car to maximize self-consumption and my wife starts the oven or dryer. Technically, with the grid-tied microinverters downstream and the grid upstream of the inverter the transfer switch itself is only seeing half the load, but that isn't really a safe way to operate. I don't want to interlock loads when on-grid, that defeats the benefits of being on the grid in the first place.

If I reduce my maximum EV charging current from ~20A to ~10A that is a significant hit in functionality on that end.

I guess it gets me back to the question... is the added complexity a big deal or just added cost? Will the added complexity make it harder to operate-- will interaction between the ATS and inverter cause problems when it switches to off-grid?

 

[newbostonconst]

A 200A ATS is like a couple grand installed and the system isn't going to act smooth like it does now when the power goes out. Don't know what unit you have but can you stack another hybrid inverter? I have friend that had to do this exact thing because he didn't have the current through one unit. I know Outback and Sol-Ark can be stacked.

Are you grid tied? Can you move the dryer to the grid side? Dryers don't use that much yearly energy? Might be the cheapest option.

 

[Shimmy]

A 200A ATS is like a couple grand installed and the system isn't going to act smooth like it does now when the power goes out. Don't know what unit you have but can you stack another hybrid inverter? I have friend that had to do this exact thing because he didn't have the current through one unit. I know Outback and Sol-Ark can be stacked.

Are you grid tied? Can you move the dryer to the grid side? Dryers don't use that much yearly energy? Might be the cheapest option.

The delta for me is under $1k. I have a 6-disconnect meter/main panel right now that feeds two sub-panels, three mini-splits, and the PV. If I don't go with the 200A ATS I end up needing two panels and re-wire a number of existing loads as you suggest. Whole house backup (at least in theory-- with good load management) is more in line with what I would prefer to do.

The ATS also makes it easy to cheaply hook up a portable generator if I ever need to-- although there are other ways to achieve the same thing.

The stackable inverters I was looking at were still limited by the rating of one transfer switch. I will do more digging on that.

Thanks!

 

[newbostonconst]

Each time you stack an additional it is additive. so if you when from one unit to two the current capacity should double.

Most hybrid inverters also have a generator input spot to make generator hookup easy.

The ATS is likely over a couple second transfer time. You could just put a off grid invertor on the main input to the the ATS and when the inverter battery goes dead the ATS switches to grid till the batteries charged back up.....but I think you may have some funny switching when bigger loads come on and cause the inverter to shutdown and then turn back on again and off and on and off....

Not sure how you plant to balance 3 inputs (inverter, generator, grid) into a 2 input ATS.....Hybrid inverter does this nicely.

 

[Shimmy]

Each time you stack an additional it is additive. so if you when from one unit to two the current capacity should double.
...
Not sure how you plant to balance 3 inputs (inverter, generator, grid) into a 2 input ATS.....Hybrid inverter does this nicely.

The stacking adds inverter capacity, but the bypass contactors do not always add, or the system requires an external bypass contactor for UL.

My original thought was to have a 48V battery bank around 30-40kWh with a grid-tie microinverter that would export a continuous 1kW outside of daylight hours, an off-grid inverter that would establish a voltage reference for my existing PV microinverters and AC-couple the system. I was going to have a separate charger or charger array that I staged up and down based on available capacity. I would be able to switch some chargers over to a small portable generator if I needed to. This arrangement would need an ATS, since the off-grid inverter needs to be isolated electrically.

After doing some reading, I then came across the hybrid inverter and its tradeoffs-- cheaper, easier, but only one DC<->AC converter. There are ways to work around the limitations, but it adds cost and complexity back in.

My strategy for the generator was to be able to manually hook it up to an L14-50 inlet connector to the ATS that would normally be used for the inverter output, essentially bypassing everything.

 

[timselectric]

I'm getting confused.
Are you talking about having 2 ATS"s?
One before and one after the hybrid inverter?

 

[timselectric]

I don't think that you want the generator and micro inverters, connected at the same time. Unless, the generator is in front of the hybrid inverter.

 

[Shimmy]

Starting to understand a little more about the logic/wisdom provided. Attached is what I see as my three options for configuration. The ATS seems to give me the least stress solution to supporting everything under a "normal" outage while I ensure I don't overload the inverter if the EV is charging at a high rate (>6kW) and the dryer, water heater, or oven is running (each 4.5kW). Any further wisdom on why I should consider something other than my "Option 3?"

 

[Dave21]

Starting to understand a little more about the logic/wisdom provided. Attached is what I see as my three options for configuration. The ATS seems to give me the least stress solution to supporting everything under a "normal" outage while I ensure I don't overload the inverter if the EV is charging at a high rate (>6kW) and the dryer, water heater, or oven is running (each 4.5kW). Any further wisdom on why I should consider something other than my "Option 3?"View attachment 88517

I have similar setup as your option 3. Except that I have PV connected to hybrid inverter via separate DC distribution panel (DC breakers, PV disconnectors, surge protection). Next, beside of the ultimate transfer switch connecting all the whole house electricity between grid and output of hybrid inverter I have also granular control of major loads - hot water accumulation, heat pump (HVAC system), whirlpool, washing machine, dishwasher and others. Both washing machine and dishwasher (AEG in my case) remembers in which state they are and they continue after resuming their power connection. Thus, my system proactively monitors load of the inverters and controls major loads dynamically (contemporaneity of power consumption). This helped me to power the whole home with inverters not having capacity to support sum of all my loads. There is no issue during transfer from inverter output to grid. But on the other direction you have to be careful, I've already burnt IGBTs and MOSFETs in the inverter when switching load close to the inverter maximum without these granular control. I've repaired inverter main board several times but it was big pain to de-solder and replace many components to fix shorted components and MOSFET drivers. Thus, I ended with this granular control.

 

[timselectric]

This is sounding more complicated than it probably needs to be. Without knowing what you actually have currently. And, what you want to end up with. It's impossible to even attempt to offer suggestions.

 

[Dave21]

This is sounding more complicated than it probably needs to be. Without knowing what you actually have currently. And, what you want to end up with. It's impossible to even attempt to offer suggestions.

My problem was that transfer switch switches total load in few milliseconds without considering what is the current load at that moment. Switch from grid to inverters' output when the total load is close to the max output of your inverters resulted into destroy of internal inverter components.

 

[Shimmy]

This is sounding more complicated than it probably needs to be. Without knowing what you actually have currently. And, what you want to end up with. It's impossible to even attempt to offer suggestions.

Currently I have an Enphase M215 system with 33 panels for a capacity of ~7kW. My daily energy consumption is about 35kWh, but can be throttled down to about 24kWh for a day or two. My peak electrical demand without any load management is about 70A over a 5-minute interval-- dominated by 30A of car charging and ~28A of dryer loads. Supporting that would require at least a 90A circuit breaker. There are other loads that tend to be non-coincidental with that-- the oven and water heater (primary water heating is solar), along with a 2-ton mini-split.

My goal is to be able to run in an off-grid mode (nearly) indefinitely in the event of a major utility outage, so hanging loads directly off utility is not an option, nor is strandinig my PV capacity. I need a means to charge the batteries via portable generator, but that isn't hard to achieve.

The viable options today appear to be a transfer switch or grossly oversizing the inverter-- at least 12 kW stacked capacity. Manually backfeeding (Option 2) wouldn't really save me any money, but operationally as @Dave21 alluded it might be a better solution to avoid overloading the inverter.

 

[timselectric]

You need a hybrid inverter, that can AC couple with your existing Enphase system. Sol-ark can, and I believe Outback also can. Sol-ark has a separate generator input and can start it as needed.
It will automatically choose between its DC solar inputs, the Enphase solar, the generator, and grid.
Depending on how you prioritize it all in the settings.
Should be able to get by with the 8k model.
(I really wish that there were better options, so that I didn't have to recommend Sol-ark)
Not sure what the capabilities are for the Outback.
Maybe, I should do some investigating.

 

[Shimmy]

You need a hybrid inverter, that can AC couple with your existing Enphase system.

That part I get. The challenge I am really having is if the hybrid limitation on internal isolation relays necessitates a bypass ATS, or if I am really better off just grossly oversizing the hybrid inverter for the sake of simplicity. If I grossly oversize the system though, I end up with much lower efficiency.

The one challenge I do see is the single AC-DC converter in the hybrid unit, which makes some operating modes a little more complicated. Easy enough to add a separate charger to address most of that, but at what point do you just build up a system from parts in order to get what you want?

Depending on how you prioritize it all in the settings.

The real art of the issue.

 

[timselectric]

The 8k , should be more than enough, unless you expect to feed full loads from the grid. If so, then go with the 12k. It has 63 amps of pass through capability. And would probably handle the 70 amps for 5 minutes. The next option would be the new 15k , with 200a pass through. Not sure if it has been released, yet. If not, it should be any day now.

 

[timselectric]

Personally, I want the Deye 16k. It's what Sol-ark is rebranding, as "their" 15k. But, some people aren't comfortable with adding a transformer for split-phase power.

 

[Shimmy]

Personally, I want the Deye 16k. It's what Sol-ark is rebranding, as "their" 15k. But, some people aren't comfortable with adding a transformer for split-phase power.

While I love transformers, the system efficiency starts to really suffer as we add all these components.

My average load (without storage) is 1kW, excluding charging (~7kWh/day); it is pretty even throughout the day. If I want to self-consume 18kWh via battery per day my math shows a best-case loss of 3kWh/day. Adding in a transformer sized for the 200A peak is going to be another 500Wh/day minimum. I guess if I just add 3-4 extra panels it is covered, but it is still really inefficient.

 

[timselectric]

While I love transformers, the system efficiency starts to really suffer as we add all these components.

My average load (without storage) is 1kW, excluding charging (~7kWh/day); it is pretty even throughout the day. If I want to self-consume 18kWh via battery per day my math shows a best-case loss of 3kWh/day. Adding in a transformer sized for the 200A peak is going to be another 500Wh/day minimum. I guess if I just add 3-4 extra panels it is covered, but it is still really inefficient.

I'm referring to a small (5 or 10kva) auto transformer.
That's all that is needed.