Decision on Inverter

Hedges

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Also forgot to mention, I have a single phase 15KW (240V, 63A) connection, but need 10KW of 3-phase intermittently for the workshop. The batteries help here since I can use an off-grid 3-phase inverter and not risk blowing the main breaker if my wife decides to make a cup of tea (which is a 3KW kettle) while I'm using the machines in the workshop. All of the 3-phase hybrid inverters (and single-phase paralleled hybrid inverters) that I've seen require a 3-phase main connection unless they are in off-grid mode.

SMA and Victron battery inverters can be connected as either split-phase or 3-phase.

I think I've tested that a pair of Sunny Island with 120V single phase coming in from grid or generator can produce 120/240V split-phase, so would also do 120/208Y. Each 120V inverter has its own relay to connect input. The relay is 56A max so 120V x 56A would be the maximum power it could be fed by the grid. As 3-phase, it could supply 3x that with the balance coming from battery.

What in the shop do you want to power with 3-phase? A VFD can rectify 240VAC and deliver 3-phase. I have that on a 2 HP pool pump. One issue I saw as that if disconnected from the grid, the harmonics or poor PF of the VFD upset a transformerless inverter. A transformer type should be OK. (I had transformer type Sunny Island 6048US and Sunny Boy SWR2500U, also transformerless 10000TLUS operating.
 

HateGrid

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SMA and Victron battery inverters can be connected as either split-phase or 3-phase.
Yes, the SMA and Victron systems are nice, but the dealers here want a fortune. I was originally looking at Voltronic (MPPSolar, etc) and Sacolar despite the low 93% DC-to-AC efficiency, but I have had problems getting the proper certificates for the local authorities. The local inspector wants AS4777 (grid-connect) certificates even for off-grid systems which is what led me to Deye since their hybrid inverters are AS4777 certified. I request the certificate for the paperwork last week, but haven't received a reply as of today.

What in the shop do you want to power with 3-phase? A VFD can rectify 240VAC and deliver 3-phase.
The VFD suggestion is a great one for running motor loads. I have one for a 240V 60Hz air compressor that ran hot on the 240V 50 Hz supply here.

Unfortunately, my load is a vertical CNC milling machine with a 10KW spindle. Ironically, everything except for the coolant pump is driven by VFDs, so it will take the provided 3-phase, rectify it to a DC bus, and then convert it into 3-phase VF for the spindle and servo motors. In theory, everything running off the VFDs could be fed single phase, but the VFDs are designed for 3 phase and do not have enough power storage to work off of single phase power.

I have that on a 2 HP pool pump. One issue I saw as that if disconnected from the grid, the harmonics or poor PF of the VFD upset a transformerless inverter. A transformer type should be OK. (I had transformer type Sunny Island 6048US and Sunny Boy SWR2500U, also transformerless 10000TLUS operating.
VFDs rectify the AC to DC which tends to pull the most current at the peak of the voltage. Small VFDs don't have any power factor correction since they assume that the supply capability is at least 5x to 10x the VFD capability, which isn't the case when running from an inverter. You may be able to add a line reactor to solve this if it concerns you. I have a pool pump that I'll run from the inverter as well, so give me 6 months and I may have some recommendations :p
 

Hedges

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I saw the smaller VFD up to 2HP supported single phase or 3-phase, larger wanted 3-phase.
Off-brands had higher horsepower for 3-phase.

Many 3-phase inverter solutions (Sunny Island and Sunny Boy) are a bunch of single-phase inverters. I'd rather see that done with a 3-phase inverter because it wouldn't have to draw an 60 Hz ripple; power delivery of the three phases is a constant.

I did put a reactor on the output of VFD before motor because I heard of them cooking motors. I through of adding one for my GT inverter upset issue, but am instead putting in transformer-type inverters. That one pump is my only VFD load, small compared to system wattage. I've had it about 15 years but only installed grid-backup battery system in the past year.

Sounds like you're in a different market. We have Sunny Islands being liquidated due to DC Solar bankruptcy (they got US $1 billion investment and bought lots of hardware.) If you have to pay retail, three Sunny Island would be $12k to $15k.

Rotary 3-phase converter? Kick over a 3-phase motor and connect it to single-phase, then connect your mill? Add more capacitors to a VFD? PV direct to VFD rail (or maybe through an MPPT regulator)? Have to watch max voltage of caps. A PWM regulator, or connect to caps through diodes (typical VFD front end) and short out PV whenever voltage rises above a threshold?
 

HateGrid

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Sounds like you're in a different market. We have Sunny Islands being liquidated due to DC Solar bankruptcy (they got US $1 billion investment and bought lots of hardware.) If you have to pay retail, three Sunny Island would be $12k to $15k.
Yes, I'm in rural New Zealand. Typically one can buy stuff at recommended retail pricing (RRP) if it is on sale.

Rotary 3-phase converter? Kick over a 3-phase motor and connect it to single-phase, then connect your mill? Add more capacitors to a VFD? PV direct to VFD rail (or maybe through an MPPT regulator)? Have to watch max voltage of caps. A PWM regulator, or connect to caps through diodes (typical VFD front end) and short out PV whenever voltage rises above a threshold?
I had the CNC working on a 3-phase rotary converter at my previous house, but I couldn't take it with me. The mains connection at my new place is only 63A and the rotary phase converter needs a 60A circuit, so that is a problem. Buying a new one of reasonable quality is still more than solar inverters and a battery anyway. A VFD is basically the front end of an inverter without the low-pass filter on the output and no support for unbalanced loads and should only be used for a single motor or resistive load. Multiple loads would be a recipe for disaster. I don't like disasters, especially when they destroy my toys (and potentially invalidate my insurance).

In an ideal world, I would get an ultra-efficient, ultra-reliable 20KW 3-phase unit to run both the workshop and house and balance the house loads over the three phases. However, power export[1] to a single-phase grid isn't an option on any of the 3-phase hybrid inverters that I've seen (they all require a 3-phase grid). In addition, the house is only single phase, so that would require a rewire to balance the load. So, single phase it is. The single-phase hybrid inverters that I have found so far typically are limited 8KW max. AC power and around the same for solar[2]. I'll have almost 14KW of solar power, so I would need two single-phase hybrid inverters in parallel to handle the full PV power (my max export power is 10KW, so an PV power is the limiting factor here).

Back to the three-phase portion. I can use an off-grid 3-phase inverter and run off of the batteries or use three single-phase inverters paralleled for 3-phase. Since getting repair parts may take weeks and learning two different inverters is twice the work, my preference[3] is to use the same hybrid inverter that I use for the house for the 3-phase (just running in off-grid mode). That way, if there is a failure on the house system, I can swap over parts from the workshop system and just not use the workshop equipment instead of telling the family not to do something. I might be able to get a deal for 5x inverters as well, but that's just icing on the cake.

[1] This assumes I can get the spot power export, otherwise I'll get $0.08/KWh for power exported and pay $0.27/KWh for power used during the day and $0.18/KWh for power used at night. At $0.08/KWh for export, I would be better off storing the power and only topping up the batteries at night.

[2] Note that I am excluding anything that uses > 48V battery systems since anything higher could be declared as high risk and the entire DC system needs to be inspected and verified and I'm not allowed to do any work on it.

[3] The idle power of the inverters and extra wiring may cause the parallel approach to be undesirable
 
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Hedges

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New Zealand uses 230V, 50 Hz. You have 63A service.
Sunny Island for your market is 230V, has 50A relay to pass grid power in or PV power out, and generates 6kW from battery at 25 degrees C.



Consider if you had three Sunny Island, configured for 3-phase. It generates 230/400Y with 120 degrees between them. Assuming that model continues to run with only one grid phase input ...
This would deliver 18kW for the 3-phase shop (from battery), or 6 kW single-phase for the house, or any division between them. Surge rating to go higher for a limited time.

The one inverter connected to grid can charge battery with up to 115A (about 6.5 kW) to be used by the other two, so should support up to 10 kW continuous single to 3-phase conversion off grid power. DC coupled PV would boost that (but may or may not be available for export to grid - I haven't read up on this model). AC coupled PV would boost it but only if on the other phases that don't feed grid.

Whatever PV you AC coupled on the phase that does have grid, can be exported to grid, up to 11.5 kW.

Check with SMA to see if my understanding is correct - that a 3-phase Sunny Island system could be connected to single-phase grid.
 

HateGrid

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Consider if you had three Sunny Island, configured for 3-phase. It generates 230/400Y with 120 degrees between them.
Correct. I always forget that the official voltage is 230V since the 80's, but the actual voltage coming out the wall is almost always right around 240V/415Y.

Assuming that model continues to run with only one grid phase input ...
Yes, that is the big question. There is no electrical reason I can think of why it wouldn't work -- it is normally just a firmware issue where it isn't a supported configuration. If any of the systems can do it, it would likely be the SMA systems since they separate out the PV invert (Sunny Boy) and the storage inverter (Sunny Island). I'll see if I can get an answer out of them on the configuration and also check with Deye to see if their new 3-phase hybrid units could support a single phase grid. The only issue I have with SMA is that their stuff is expensive here. The Sunny Island 8's are US$5,570. I did find a package shipped from Europe with two Sunny Boy 6.0, three Sunny Island 8's, and the energy meter for US$15,990, but that is still more than the other options.
 

Hedges

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I would expect master SI connected to grid to work, since the two slaves would follow master for timing of their phases. As you said, so long as firmware does let each independently monitor grid and connect. I think it did with my 6048US on 120/240V split phase.

If the cost is something you can cover, just buy SMA. Very good stuff.

18 kW of Sunny Boy is not going to be compatible with grid-tie through a single phase. Approximately 11,500W is the most that can go through a single 50A 230V relay. While on-grid Sunny Island can't frequency shift to curtail.
That amount of PV between neutral and the on-grid phase should work.
I have PV arrays of multiple orientations. That reduces peak wattage and spreads it out over the day, so more total watts of PV can be used.

Slave SI would draw from battery to maintain voltage on their phases, and I think master would supply the current by charging battery, so should remain at float. I think, and thinking lead-acid. If using lithium, BMS tells SI what it wants - I suppose SI master calculates sum of charge/discharge currents on phases and adjusts its charge current accordingly. As SMA about the imbalanced grid connection, PV, and load, and tell us what you learn.

You could have additional DC coupled PV.
You could have additional AC coupled PV, but connect to the other two phases by relay only when off grid. Each SI can manage up to 12,000W of PV when off-grid, so 36kW would be supported.

With the new model transformerless Sunny Boys, you may have to filter the VFD harmonics.
You may be able to use older models.
 

HateGrid

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While on-grid Sunny Island can't frequency shift to curtail.
Ah, that is a key point to keep in mind while piecing together the system.

I've emailed Deye / Sunsynk. Will get to SMA once I have a diagram for them.

For those that are graphically oriented, here are some diagrams showing some of the things I discussed earlier. Some details may be slightly different as the design evolves.

Conceptual design​

2021-02-02 Solar Electrical Diagram - Conceptual.png

3x Single-Phase Inverters​

With individual hybrid inverters (8KW or greater) working in parallel (RS-485 / CAN communication cables not shown), this is the minimal setup as long as the inverters support hybrid mode on one phase and off-grid on the others. Lux Power actually has this configuration in their documentation for their 5K hybrid inverter, but it only outputs 4KW in off-grid mode, so this arrangement would require 6x 5K inverters.

2021-02-02 Solar Electrical Diagram - 3 single phase inverters - 8KW export limit.png

2x 3-phase Inverters​

Here is a 3-phase hybrid system. It depends upon the inverter supporting unbalanced phases. There are not many advantages here compared with the 3x single-phase inverters other than taking up less space and potentially a slight decrease in idle power. The 3-phase inverters are available in higher-power configurations, but the higher power ones step up the battery voltage as well to reduce power losses, but I would much rather stick with 48V for regulatory reasons.
2021-02-02 Solar Electrical Diagram - 3 single phase inverters - 8KW export limit.png
 

Hedges

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[While on-grid Sunny Island can't frequency shift to curtail.] Ah, that is a key point to keep in mind while piecing together the system.

I've emailed Deye / Sunsynk. Will get to SMA once I have a diagram for them.

For those that are graphically oriented, here are some diagrams showing some of the things I discussed earlier. Some details may be slightly different as the design evolves.

The 10kW export limit is further restriction beyond 15 kW of utility service voltage/current.

I'm not sure, but 6.6kW from 3-phase AC coupled GT inverter might let 2.2kW flow to grid from the leg on same phase as grid, 2.2kW each on other two phases would get drawn down to battery by two Sunny Island on those phases, 4.4 kW would go from battery to grid through Sunny Island on that phase (total 6.6kW fed to grid). Additional PV could be DC coupled to the battery.

Better (if you have unlimited PV) might be a 10 kW GT inverter feeding the grid, and 3-phase Sunny Island set to draw from grid but zero export (I think that can be done, disconnect from grid whenever PV production exceeds consumption + charging). This would allow up to 36 kW PV on Sunny Island.

Your ideal system would have current transformer monitoring export to grid, loads both directly on grid connection and on protected load side of battery inverters, and communication to curtail PV production which doesn't rely on frequency shift. I don't know what brand/model would be best for that. There are zero-export and peak-shaving products for consumers, like Tesla Power Wall and SMA Sunny Boy Storage (has data bus to curtail PV). There are probably 3-phase commercial products too. Having only one phase from grid makes yours more difficult.
 

HateGrid

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Your ideal system would have current transformer monitoring export to grid, loads both directly on grid connection and on protected load side of battery inverters, and communication to curtail PV production which doesn't rely on frequency shift. I don't know what brand/model would be best for that.
Yeah, I keep coming back to the Deye inverter as an all-in-one solution. The generator port is configurable to be a generator, load dump, or micro-grid (e.g. microinverters or another inverter). Both the grid connection and the generator connections apparently have current transformers built-in and can be configured for zero-export. If there are non-essential loads connected to the grid connection, an external current transformer can be added to control the grid export (not shown on any of my diagrams).

1612303788163.png
I'll likely configure the pool as the load dump since I can run the circulation pump which is 0.800KW and a 3KW heat pump.

Concerns I have are if I can configure the max grid export value and the load dump configuration during operation over wifi or RS-485. If I can't then, I can't take advantage of spot pricing very well. Looking over historical pricing, the spot price for power varies from around $0.05/KWh to $0.40/KWh. It is fixed over a 30-minute time block. Average is roughly $0.17/KWh with the $0.40/KWh being rare opportunities to sell or rare wallet-emptying events. The battery will cost somewhere on the order or $0.05/KWH when I amortize the purchase cost over its lifetime.

If the spot price is high enough, I'll sell all solar (and potentially some battery) and recharge the battery from the grid at night. As long as I predict the future price, this is perfect. What could go wrong?
 
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Hedges

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Good that you've put a dollar figure on power from battery.
Even if you can't manage export according to real-time pricing, the $0.17 average is good enough. But is export based on surplus PV (not draining battery) and are spot prices high or low when you have sunshine? Our retail rates used to be peak in the afternoon, now they are lowest up until 3:00 PM and highest 4:00 to 9:00 PM.
If you end up spending a nickel for battery, plus a nickel for PV, plus extra costs, and sell for a nickel, you lose.

Loads, you can shut off during the rare opportunity to empty your wallet.
 

HateGrid

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The rates fluctuate a lot - take a look at the following chart for the local substation. The cost is NZ$ per MWh, so just add a decimal point to the front to converter to KWh - e.g. $300/MWh is $0.30/KWh.

1612309779681.png

I would have to do some simulations to see what a good algorithm would be. As a starting set of rules, something like this may be a good start and then iterate on it.
  • When PV is producing power:
    • charge batteries from PV
    • when batteries are at 90%, start load-dump loads
    • export any remaining power
    • if export price goes above 2x the night rate, turn off load dumps to maximize exported power
    • [rare] if export price goes above 3x the night rate, export power from batteries down to 50% charge
  • When no PV power:
    • Power loads from batteries. If batteries are below 10%, power house from grid.
    • [rare] If power is 0.5x the night rate, charge the batteries up to 75%
    • [rare] If power is 3x the night rate, export power from batteries down to 50% charge
Time of year and weather will likely change some of the decisions.

. . . but all of this is a fun future problem. First I need to get the design finalized, approved, purchased, and installed. I'm still building the workshop that this goes in and on, too.
 

HateGrid

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Our retail rates used to be peak in the afternoon, now they are lowest up until 3:00 PM and highest 4:00 to 9:00 PM.
That is a fascinating shift as people install solar. During winter, power seems to peak at 9:30 AM and 6 PM and the cheap rates are 3 AM to 5 AM. I don't get enough solar to meet demand during winter, so I'll be a net buyer in the winter and will try to time the buying for that 3 AM To 5 AM time.

Summertime seems to shift the peak times to 7 AM and 7 PM which I presume is cooking and potentially air conditioning.
 

Sanwizard

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Thanks for suggestions . I looked at the link. The HP 12.000 watt unit might not supply enough continuous power and the 15kw and 18 kw would be good especially the 18 kw unit but they require a 4 unit order minimum. I believe you may be thinking of a different Growatt inverter. the 1200dvm says it is a low frequency inverter type and it weighs 75kg or about 150lbs which sure sounds like it is and it also claims to be able to start two 4ton HVAC units with 36kw peak but not high enough pv watts or voltage input.

Surely 3 LV5048s at 15,000 watts continuous and 30,000 watts peak would start the first stage of a 5 ton unit that draws about 17 amps or 4,000 watts running at most. They should provide a short peak of 125 amps and the 2 LV6548 s should peak 108 amps or so . I could also put a soft start capacitor on it if needed. The 240 v fan portion of it is inverter controlled. I know they are not known for starting reserve power but they would provide about 3- 4 times the rmax running amps continuous. Really want all in one though I also can charge different set of batteries with MPP MG 5048 now I got for dedicated west panels several hundred feet from unit use 400 volt of panels open circuit.
I think the new LVX6048( not LV6548) has a 450V PV input. Two of those may do the trick.
 
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