View Single Post
Old 12-04-19, 09:23 PM   #3
NiHaoMike
Supreme EcoRenovator
 
NiHaoMike's Avatar
 
Join Date: Oct 2008
Location: Austin, TX
Posts: 1,154
Thanks: 14
Thanked 257 Times in 241 Posts
Default

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.
__________________
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...
NiHaoMike is offline   Reply With Quote
The Following 2 Users Say Thank You to NiHaoMike For This Useful Post:
Daox (12-10-19), wlbryce (12-20-19)