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Old 12-03-19, 10:39 PM   #1
NiHaoMike
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Default PV inverter from Prius inverter

I'm looking to build a bigger solar power system, with thermal storage. Since going full grid tie is a bit involved, I'm planning to make it "zero export" with limited battery storage instead. Looking at off the shelf inverters, so far I have only found one (Sol-Ark) with full "zero export" support using external sensors. (The rest that claim such functionality merely have the capability to power loads on the output side from DC power even if AC power is available, without backfeeding power to the "input". They are completely unaware of loads on the input side.) At over $6000 for just the inverter, it costs more than I plan to spend on the solar panels (~38c/W in bulk) themselves.

Therefore, I looked into repurposing a Prius inverter for solar power use. Having found a few going for just over $100 (far less than the cost of building a comparable power stage from scratch), that's certainly a great value for something that can very easily power the whole house. It would take more parts to make a whole PV inverter, of course, but even after adding the capacitor bank and filter inductors (the other expensive components), there's still a lot of room to stay within my $1000 budget, probably more like $500. (That's for the completed inverter unit itself, not including the battery bank or other system components.)

Looking at what the EV community has done, it looks like the inverter, with the stock control board removed, just accepts a bunch of logic level PWM signals and some enable signals. In other words, pretty much exactly what a FPGA board can be programmed to output. It even allows independent control of the high and low side IGBTs, good for enforcing a certain current direction.
https://openinverter.org/forum/viewtopic.php?f=14&t=51

Basically, I plan to use two phases of MG2 for interfacing to the two hots and the third phase for a dedicated 120V UPS output. (The first two phases could also serve as UPS outputs with the addition of a contactor to disconnect them from the grid.) Neutral will be connected to the center tap of the capacitor bank, so each phase will be switching between +-200V or so. All 3 phases of MG1 will be used to run the thermal storage compressor, eliminating the need for a separate VFD. The boost converter can be used to interface to a battery pack that is less than the 340V or so needed to get 240V output. The DC/DC converter will also come in handy for low voltage output.

The control side of things will involve a current sensor installed inside the breaker box, transmitting the waveforms in real time to the inverter using PLC. The inverter responds to load current by sourcing current to offset the load current. (That possible since FPGAs can respond very fast, far faster than any microcontroller can.) It will not export power to the grid, thereby avoiding the hassles with going full grid tie.

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Old 12-04-19, 10:19 AM   #2
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By zero export do you mean that the power will be automatically switched between the inverters and the grid as needed? I was wondering how long it would take for people to start doing this type of mod. What do you think the chances are of actually getting this project permitted? Regardless, it looks like lots of fun. I'll be watching.
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Old 12-04-19, 09:23 PM   #3
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No, the inverter works by sourcing a current to offset the load current, more like "blending" than "switching". For example, if the load is drawing 10A, the inverter will supply 10A to offset it. (In practice, it will be more like 9.9A to prevent accidental export due to inaccuracies.) Or if there's a 60A load but the inverter can only supply 40A at that time, it will supply that 40A and cut down grid power use to 1/3. That is done seamlessly, so it's basically grid tie without the hassle.

The inverter connects to the house wiring using a 240V plug so it will be considered an appliance as opposed to building wiring. The solar panels will be on a frame anchored to the ground so they will not be part of the house either. The UPS output will (at least at first) only serve loads in the same room and won't interface to building wiring. (And most likely will stay that way, since for my use, it would be pretty easy to move all or most of the critical loads close to the inverter.)

To prevent the disconnected plug from being a shock hazard, it will have various circuit integrity checks including voltage/frequency checking and neutral-ground continuity checking, opening the input contactor if a fault is detected. (The inverter will also lose the PLC signal if unplugged, which while not intended to be a safety mechanism, will be interpreted as no load current.) That said, US plugs aren't exactly the safest power connector in existence - pull them out a little and there will be the full mains voltage on the exposed pins. (Yes, that's even true for 240V US plugs!) Another option is to use a "doubly safe connector" where both and plug and socket are insulated against touch when disconnected, for example Anderson Powerpole.

The first part of the project would be building the current sensor, then I can more or less capture power data on a cycle by cycle basis and run simulations to determine how much current the inverter will have to supply to offset X% of usage. I can also capture the PLC signal using a high speed ADC card connected to a coupling circuit and see if there is any particularly problematic interference. (Since the inverter uses a FPGA for low level control, I could install some Tiffany Yep DSP tricks in it in order to improve tolerance to interference.)

The PLC signal I'm planning to implement is a pair of FM carriers at around a few hundred kHz, one for each phase, with the current signal companded to improve performance at low current. There will also be pilot tones to allow proper scaling of the waveforms and instant detection of a SNR too low to give a trustworthy signal. The pilot tones could also be frequency shifted in order to digitally encode data such as a more accurate power measurement signal although that might be adding a lot of complexity for little additional gain. (I'll build a test circuit and do some measurements to decide if I want to go that route.) I decided to make the current sensor analog since it's pretty difficult to do it in digital without adding a lot of latency - a Wifi link is on the order of a few ms of latency while the inverter needs to react to the current waveform in a few hundred us for it to work properly.
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Old 12-07-19, 05:25 PM   #4
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NiHaoMike,
That sounds like a great project. I cant do that in my neck of the woods because anything that touches the grid is subject to their inspection and approval. They are concerned a lineman might get electrocuted. I got around that by keeping all my generated power completely separated including neutrals and grounds. Keep us posted on your progress.
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Old 12-07-19, 07:30 PM   #5
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The nice part about not exporting to the grid is that it avoids the problems with exporting to the grid. In practice, the chances of a grid tie inverter powered from a varying source like solar panels staying within voltage and frequency limits without the help of the grid while powering varying electronic loads is very close to zero. With the "zero export" inverter programmed to only source power up to the point where there would still be a (small) draw from the grid, it would be impossible for it to continue operating without the help of the grid.

BTW, as for commercial inverters that can do "zero export" to loads on the output, internally many if not most of them use a "line interactive" architecture that works basically the same as an inverter that can "zero export" to loads on the "input", only difference is that the current sensors are inside the unit rather than outside. There are a few that use a "dual conversion" design but those are designed to primarily operate as a UPS and many of those are incapable of exporting power to the input at all. (Those are mostly used in commercial environments where the load is great enough that having excess power to export basically never happens.)
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Old 12-20-19, 03:06 PM   #6
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NiHaoMike that is a great project. The high voltage of the Tesla inverter always scared me off because of the expensive cost of the HV components (and HIGH VOLTAGE DC). HV solar components are available, but its limited to more commercial type gear.

Low voltage systems are just easer to build. But, if you can manage to build it would usher in a great use of the Tesla Inverter.

On a side note I use a Schnieder XW 6848 to do exactly what you are wanting to do. My Electrical Coop makes interconnection way to expensive to do. So I have the XW set to Grid Interactive Net-Zero and set the SELL amps to Zero. It will then use my battery power, and excess solar to power all my loads until I hit a DOD that I set. The inverter will pass through extra energy if I go over the inverters rated output. I know that both the XW and SW can do this.

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Old 12-20-19, 06:16 PM   #7
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Quote:
Originally Posted by wlbryce View Post
On a side note I use a Schnieder XW 6848 to do exactly what you are wanting to do. My Electrical Coop makes interconnection way to expensive to do. So I have the XW set to Grid Interactive Net-Zero and set the SELL amps to Zero. It will then use my battery power, and excess solar to power all my loads until I hit a DOD that I set. The inverter will pass through extra energy if I go over the inverters rated output. I know that both the XW and SW can do this.
I have looked at that one, but unless something has changed, it can only "zero export" to loads on the output side, not the input. I have also watched a review on it and the owner criticizes that the settings are not flexible enough (e.g. can't set the low voltage cutout high enough) despite the hardware being perfectly capable of it.
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Old 12-20-19, 07:22 PM   #8
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It can "export" on the input but that would be "net metering" you can set it to export by setting the "sell" amps. so if you know the load amps you can do that. But, if that load drops it will export to the grid. On the Plus side it is 1741 certified so if the grid goes down you will not backfeed.

As to the net Zero setpoint. On the most current firmware version I can set the battery DOD from 0 to 100% so its not a issue. You have full range of when the inverter goes from Net-zero to grid use.

Maybe I don't understand what your really wanting to achieve in regards to powering loads on the input side.

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Old 12-20-19, 11:03 PM   #9
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Basically, the inverter plugs into the existing breaker box via a 240V outlet with a dedicated breaker. It then powers loads on that breaker box but not backfeed the grid, which is done using sensors installed inside the breaker box to detect the current from the grid. Thus far, I'm only aware of Sol-Ark having that feature.

It's also worth mentioning that the reviewer of the XW has reconfigured his system to run in off grid mode most of the time, since with an unbalanced load, that could cause an export condition on one of the phases in "zero export" grid tie mode! (For example, put 2kW of load on one phase and since it is only capable of sourcing to both phases at once, it will source 1kW to each phase, with one phase exporting 1kW to the grid and the other phase importing 1kW from the grid.) The design I'm working on will be able to adjust the phase currents independently, avoiding that problem.
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Old 12-21-19, 05:26 PM   #10
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Quote:
Originally Posted by NiHaoMike View Post
I have looked at that one, but unless something has changed, it can only "zero export" to loads on the output side, not the input. I have also watched a review on it and the owner criticizes that the settings are not flexible enough (e.g. can't set the low voltage cutout high enough) despite the hardware being perfectly capable of it.
With my xw+ 5546, I just installed it in between my meter and my main load panel in my house and Iím using it as a whole house transfer switch. Then all of my loads are on the load side of the inverter. As long as you are DC coupling to your battery bank and XW plus will do exactly what you want to do if you can put it between the grid and your loads it has a 60 amp breaker and a 75 amp rated transfer switch in it.

The only reason I have to keep my net metering with my utility is the fact that I am AC coupled. I have a SolarEdge inverter that back feeds my main home panel and When the grid is up the XW plus will not control the backflow to the grid. So any extra solar power just flows right back out to my utility. if the grid is down the extra solar will go into the battery bank. The XW plus is also smart enough that once the batteries get full in an off grid/ grid down AC coupled scenario it will start to frequency shift up enough to knock the grid tie inverter off-line.

I was told by several Schneider engineers that the new XW Pro has a lot more export related features than the old XW pluses. they told me that the reason the software was more limited on the XW plus Was that were totally out of memory space.

You seem to know control wiring a lot better than I do but I have my xW plus paired with the new Canex gateway device which has all sorts of useful features if you were a software/automation geek. They donít offer a lot of features for programming ....... 😎. but it has the pin out for:
12v Digital signals
two can buss circuits
two iso RS 485 circuits
0 to 10 V analog inputs
0-20 mv imputes
And two sets of relay Nc, no and ground pins

They claim in the manual that it is mostly for battery BMS hook ups.... but the device has usb and an ST card slots, networking and it looks to me like you could probably use it to do all sorts of cool things.....

I would love to see somebody figure out a way of using the device to mod an xw+. The gateway is also a master configure tool for the XW plus. I can Wi-Fi login and change all the master settings on the XW plus.

The XW plus is monitoring power current and frequency flows on both AC lines 1 and two. It can also detect power coming in on both of its AC imports the second one is usually designed for a generator but you can prioritize to if you wanted to do things a little differently. It is also monitoring battery voltage and temperature .so it has a full picture of the power flow and would be able to take a mod bus power meter input signal if you wanted to have remote sensing.


It would be nice to set up some sort of PLC in her face or modify the program so that you could start a manual charging up the battery bank based on current flows go into the grid and in a sense Self consume all your AC coupled power.


I think a Xw+ Would do something similar to what you want and as long as you are do DC coupling. the software is all built in to prioritize self consumption of solar and export limited amounts back to the grid four loads on the load side of the unit. if you want to get into programming you could probably tweak the firmware or add remote current sensing and figure out a way of on the fly changing the sell amp setting to match your loads on the import side.

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