3 Phase Microgrid DC Coupled to Grid-Tied Split Phase?

[natable]

I have a small cabinet-making business in MA/NH and am considering a property in NH. However, I will need 3-phase power (10-20kW loads & ~1MWhr/mo). 3-phase is about 1mi away and estimates are $90/ft.

I expect something similar to this: https://solar.se.com/us/wp-content/...ase-DC-Coupling-Triple-Inverter-System@1x.png but I've modified the diagram (see attached). The solution doesn't have to be Schneider, but they have a very good rating for short term peak loads.

The 3 phase XW Pro's would not be grid-tied (but, perhaps generator tied). The MPPT's (10-20kW PV) would charge a common battery (30-60kWhr) between the 3 phase XW's and 1 or 2 split phase XW's (12kVA). The split phase would be grid-tied for charging (winter) or sell-back (spring-fall).

I expect to phase the installation. 1st the grid-tied split phase and ~10kW solar, then 30kWhr battery, then the 3 phase, finally increasing battery / PV depending on my demand.

I would think there's a market of small rural manufacturing businesses that are just a little too far from 3-phase and want predictable OPEX.

How feasible/reasonable/reliable is it to DC-couple a 3-phase island (microgrid) with grid-tied split phase inverters? I don't want to invent or be a beta site but don't mind a new use of proven subsystems. The investment in a rural property and daily operation depends on a solid power system.

 

[timselectric]

I see no problems with this.

Edit: based only on the description.
Can't open the files on my tablet.

 

[natable]

Thanks @timselectric. That's where I'm landing too -- it's existing building blocks, but perhaps in a new configuration.

Here's a picture of the proposed schematic and conceptual look of what the power system could look like:

 

[teal95]

Sounds like exactly what I'm doing. Split phase system is done and I'm waiting on inspection. I have 20 kW of panels and 30 kWh of battery right now feeding a SolArk 15k. 3 phase setup will be EG4 6500ex. Should be able to do 19 kW of 220v with capability of adding another 3 for 38 kW of 3 phase. Right now it's feeding most of my house (6 strings of panels into 3 MPPT, shading and optimizers cause fighting which is hampering the output).

 

[Hedges]

I have 3x SI 5048US putting out 120/208Y, with master inverter taking in single phase 120V from grid. If grid is 125V, that passes through on L1, while L2 and L3 remain 120V (not ideal for motors or transformers.)

It can pass through up to 6.7kW in either direction, with battery buffering for higher power draw. With AC coupled GT PV, it can charge and pass back up to 6.7kW into the grid, but has no way to curtail more AC coupled power than that. If set up for grid as generator, then it wouldn't backfeed at all, and could support up to 30kW AC coupled PV. DC coupled PV can also be used.

Of course you could also DC couple (charge battery) from AC grid, including with additional inverters. If I did that, could be 2x or 4x SI on grid, and 3x SI for 3-phase island.

Are VFD available for your 10 ~ 20kW loads that accept single phase input? Should be more economical and versatile.

What voltage are your 3-phase tools? Step-up with transformer on output of inverter has some issues, but I have tricks to make typical transformers better behaved.

 

[teal95]

Single phase input VFD are limited to about 5 horsepower. At least that was my finding. And since I was going to have to have sufficient inverter power to feed everythinv anyways this seemed a good idea to me rather than making split phase and running it through a phase converter.

 

[natable]

Thanks @Hedges. My plan is similar -- run the shop on 120/208Y, but more conservatively with separate inverters. I appreciate that many of the members here are enthusiasts, experimenters, engineers who want to be on the leading edge of the inverter/charger wave. For a business, I'm a little more risk adverse.

The tools are typically 230V and can be wired for 460 (rotary screw air compressor, dust collector, paint booth exhaust fan, wide belt sander, etc), but only the biggest may have a transformer (ie. 5'x10' flat table CNC router & hold-down vacuum). I currently rent a shop (with equipment and 3-phase utility), but am putting a plan together for a move to independence.

For now, I have access to a 25hp rotary converter, but will want to take advantage of solar tax credits, shield peak draws from the utility (one of the many fees is a 30min peak kW charge), and fix my energy costs.

Regarding sizing, the 3-phase and split phase will have different capacities. 3-phase is a lot of short term, peak draws. Once I get some experience with the 3-phase draw, I'll size the battery and solar to harvest what I need for ~75% of the year. Then, I'd like to moderate the split phase. It'll likely be a 60A circuit on the 200A panel. I really like the idea of CT's on the mains with a CCS WattNode or equivalent feedback to the inverter to control when and how much power returns to the grid depending on SOC. In the summer, slowly sell back overnight and buy in the winter via charging (apparently NH utilities buy for about $0.40/dollar). Ideally, the split phase inverter/charger is only ~6kW. The trick will be leaving enough SOC headroom in the battery each morning, so that I can both run my shop and buffer self-consumption/sell-back below the split phase inverter's capacity.

 

[Hedges]

Some inverters will be able to handle the out of phase backfed current from transformer inductance. Others may have problems. I like to operate a transformer around 1/2 of rated voltage on an inverter; it doesn't go into saturation like it is designed to (cheap for grid use) and is much closer to ideal. Draws about 1/10th as much idle current. But best to avoid transformers if possible.

Starting motors draws inrush current about 5x rated running current. Or LRA, if that is on the nameplate. I think if you added a switch between 230V and 460V, a 3-phase motor would start at 1/2 the current (reduced torque), then switch to 230V winding configuration to run.

SolArk is popular and support is said to be good. Their 120/240V model can be wired for 120V and connected 3-phase. They also have a larger 3-phase model but that is higher voltage (both AC and battery.)

Midnight only recently offered inverters, but they designed many before at prior companies, and even though HF they have very high surge. Could be good for your application.