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Forum Index : Solar : AC coupled SolarEdge GTI ?
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| petect Newbie  Joined: 03/02/2018 Location: United StatesPosts: 15 |
Hi ALL This is my first time here. Posting from Connecticut, USA. I’m a long way off, so I’ll press extra hard on the keys and hope I get through! I’ve been lurking here for a while and have been following the posts on using a GTI with an off grid pv system with interest. I think that there are some SolarEdge inverters in use in Au. So I’m hoping that someone here might be able to answer my questions. I want to set up a semi-off grid system with a critical circuit panel, a (possibly) a SloarEdge newer Wave inverter, an inverter to trick the SE inverter into thinking it is connected to the grid, and a very small <100 ah battery bank – AC coupling. I know that can be done. But here are my concerns. A GTI normally uses the grid as a buffer to push any extra (pv generated) power into and pull make-up power from, but the system I want to set up won’t have a grid connection and will only have the SMALL battery bank as a buffer. (1) Will the SE inverter be damaged if the pv power into it is more that the loads and batteries can use? (2) Will the SE inverter be damaged if the household loads are greater than the pv power into it can supply? I read an article by an installer that seemed to say that the SE optimizers will cut down on the power the modules supply to the inverter to protect it from (1) above. The boys at SE didn’t have an answer. Thanks in advance. Pete |
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LadyN Guru  Joined: 26/01/2019 Location: United StatesPosts: 408 |
Warpspeed has already designed and running such a system (actually a better one) and we are working on documenting it: https://www.thebackshed.com/forum/forum_posts.asp?TID=11086 Yes, Warpspeed's system dips into the grid for slack but never feeds "excess" output. From my reading, you can put limiters that will limit what you export to the grid on conventional GTIs. I am not sure if these limiters support 0 export but they do support antislanding if you are worried about safety. The downside of this is that it will not work when the grid is down - like any GTI OUT THERE, which is why a conventional GTI is useless to me and I am going to replicate Warpspeed's system. A GTI by design dips into the grid in this situation, otherwise it will be a pretty useless GTI. It would be cheaper to hookup a completely isolated MPPT inverter to the PV if a GTI could not pull this off. If you were trying to rewire a conventional GTI to behave as an inverter to a PV panel, the output regulation would likely be poor except the most bright cloudless days. |
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yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
Hi Pete, the way we used to do it in the old days was to let the GTI voltage climb until it hit its software limits and went through multiple shutdown and restart cycles. that only worked OK on large off grid systems where we had large loads and batteries to charge and using inverters that it was possible to set a lower maximum voltage, there would be an hour or so after lunch where the GTI would cycle on and off on a sunny day. Some of the inverters sold here have a 265,268,270 volt shutdown, it can play havoc with some appliances. Stage two was when Madness designed what I called the "artificial cloud". Basically a group of mosfets that throttled the DC before it gets to the GTI inverter. This was controlled with an arduino board that used battery voltage (or anything you want) as the trigger for adjusting the input to the GTI. I did play around with a transfer switch that isolated the off-grid system and run the house from the grid in the early hours of the morning to see if I could make a system with a tiny battery work. This was an attempt to reduce the capital outlay $$$ The problem with controlling the GTI based on battery voltage is that battery voltages move around a lot depending on the charge going in and the resistance to charge built up from the previous hours of charging, no two charge cycles are exactly the same. I get/got around this by using Lithium cells that never need to be fully charged, if I hit the target voltage at 92% it doesn't matter. They also have very low impedance so they can take some large charging currents without moving the voltage much1. On the low voltage trigger for the transfer switch there needs to be some delay and hysteresis programed in to stop things like a starting fridge cycling the grid on and off due to a short voltage droop. The DC optimisers are not something I have played with, the literature says they are fully self contained and work with other inverters, in theory they should be fine. I would find some and test them but I have one of the Electrodacus thermal controllers burning a hole in my workbench and that needs to get done so I dont have to cut firewood this winter.  I know, its all about me!if I assume you are doing lithium iron phosphate cells @ 52 volts and a 70% cycle you have around 3.7 kwh to play with after sunset, I have had a number of clients that consistently come in under 2.5 kwh for their overnight power usage. there is usually fridge, freezer, kettle, toaster, TV's, computers, routers but no cooking or heaters included. I'm confused, no wait... maybe I'm not... |
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| petect Newbie Joined: 03/02/2018 Location: United StatesPosts: 15 |
Hi yahoo2 It’s interesting how a post that's a year old can bubble up to the surface. Anyway, thanks for your reply. I’ve since built a diy 3.2K Enphase, Trina system. It has been running well for about 4 months now. The next phase of this “project” is to AC couple it so I can produce some power when the grid is down. BTW I’m curious if anyone has heard anything about the Enphase IQ8 they say is coming out later this year? The company moved a lot of its engineering to NZ a few years back, and I think that’s where it is being developed. Pete |
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| Warpspeed Guru Joined: 09/08/2007 Location: AustraliaPosts: 4406 |
As you already know, the basic problem with any grid tie inverter is that it just pushes out whatever power it can generate, without any regard for where that power has to go. It goes without saying that the grid tie inverter must be smaller than the main battery powered inverter, or the tail may wag the dog.... Two things can happen. The ac voltage will rise until the grid tie shuts itself down, or the battery voltage just rises and rises uncontrollably cooking the battery. There are sneaky devious ways around all this, but they will introduce extra complexity, and add more possible failure modes. Cheers, Tony. |
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| yahoo2 Guru Joined: 05/04/2011 Location: AustraliaPosts: 1166 |
Hi, I dont know a lot. the Enphase Ensemble system was shown to investors back in mid July 18 and some of the larger sellers got some hands on time in November. I do like the off-grid "battery-less mode" that is kinda cool. My tip is that we will see it released in Australia in the next 8-12 weeks and USA sometime shortly after. They may be doing some last minute tweaks to make the Ensemble system eligible for the Victorian and South Australian govt storage incentives. Up to $5k off the price is pretty tasty. To get the cash some of the schemes require the storage to be able to operate as a virtual powerplant and support the grid on the occasional day when demand response is called for. I personally think the EV vehicle to grid technology will plow over the top of this just through sheer numbers, cost and battery size. A drive-able UPS appeals to my sense of irony. I'm confused, no wait... maybe I'm not... |
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| petect Newbie Joined: 03/02/2018 Location: United StatesPosts: 15 |
Thanks Tony Yahoo IF The IQ8 delivers as promised, and it's affordable it will be very interesting. Tho not an ev, I've heard of people driving their Prius to their vacation cabin and plugging it in to power the cabin. Pete |
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