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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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To my surprise, shortly after Naomi Wu gave me a bit of fame for making good use of solar power, Allie Moore got really jealous of her...
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