Author Topic: "Tricking" a GTI?  (Read 793 times)

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petect

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"Tricking" a GTI?
« on: January 18, 2018, 03:43:40 PM »
Hi All
It seems like one of the hot topics in the PV world is setting up systems so that they will work when the grid is down, and the sun is out. One method of doing this is AC coupling, in which, after appropriate safety measures, a sine wave is provided to the inverter at the grid connections. The inverter ‘sees” the 60 Hz signal, syncs to it and does its thing just as though it is connected to the grid. A transfer switch is usually used to connect the GTI to a “critical loads” panel and disconnect it from the grid. The companies that make retrofit kits for doing this say that a fairly small pure sine wave inverter and battery will do the trick.

So, for a 240V GTI,  how small of an inverter could I use if I were to go this rout – 100W, 50W, smaller?

Going in the opposite direction – and here’s where it might get crazy  :-X  Suppose a bigger inverter is used – maybe 1500W.  Assuming that 1500W is overkill for “tricking” the GTI. Could it also be connected to the critical loads panel to supply a little more grunt for start-up loads? Since the GTI is synced with it they should get along together – yes, no, crazy?
Thanks in advance
Pete

mab

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Re: "Tricking" a GTI?
« Reply #1 on: January 18, 2018, 04:41:08 PM »
Quote
Going in the opposite direction – and here’s where it might get crazy  :-X  Suppose a bigger inverter is used – maybe 1500W.  Assuming that 1500W is overkill for “tricking” the GTI. Could it also be connected to the critical loads panel to supply a little more grunt for start-up loads? Since the GTI is synced with it they should get along together – yes, no, crazy?
Thanks in advance

Not crazy at all - probably less crazy than using a small one just to trick it. It needs to be a 'low frequency' type pure sine inverter (the type with a big transformer inside), the battery will support the start up loads, but the GTI will backfeed the inverter and charge the batteries through the inverter, so the inverter needs to be big enough to handle the output of the GTI - unless you are sure there is enough load to use the power, and you will need some way of handling the excess power once the batteries are fully charged otherwise you may overcharge the batteries. I have a GTI connected to my off grid inverter system this way to get power from the hydrogenerator back to my batteries - it's called ac coupling.

I've not tried using a small PSinverter  - nor a high frequency two stage inverter - so I can't advise on what might happen if you did.


clockmanFRA

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Re: "Tricking" a GTI?
« Reply #2 on: January 19, 2018, 02:10:07 AM »
Hi petect,

GTI's have in built codes that follow a particular Nation/Country specifications.

 Most need to see a good HZ and a good stable AC voltage that is between certain voltages.

 But there is a time lag where the GTI has to see good parameters before it will start working, and begin AC Coupling.

The time lag for start up can be from just 10 seconds to several minutes, so your batteries must take the load for that short time.

The below is my usual AC Coupling comments.
   

I do the OzInverter, 6kW-15kW, 48vdc to 230vac Pure sine wave 50HZ. Its taken 3 years to get to a published kit design that is now perfected and now operating all over the World.

The below relates to the OzInverter only.

With 'Oztules' help, one of my criteria was to allow the OzInverter to back charge from its 230vac Mini Grid when standard GTI's are connected into that created Mini grid and therefore charge the batteries. And to handle DC sources directly charging the batteries ie, DC coupling at the same time.

However their are parameters that must be met.

1. Max GTI input should not exceed the OzInverter upper working output. Its an H bridge design so voltage can go either way.

2. Basically ......... We can control the GTI's 4 ways,
 A. ..... We use the AC Voltage push back as the batteries charge, this higher AC trips out the GTI's, however the latest OzControl board No12 has a rock steady AC output voltage.
 B ....... We use a DC comparator on the batteries to trip out, simple AC disconnect, the GTI's when the batteries are at a charged state.
 C ........ We control the DC on the DC input side of the GTI's before it goes into the GTI, again comparing the batteries voltage state of charge.
 D ..... We can connect in dump loads or diverting loads, so no RE generation is lost.

(Note, we do not use the HZ increase to back off the OzInverter, this method is foolish and leads to domestic appliances failing).

3. It is preferred to use smallish GTIs rather than one big one, as the smaller ones can be switched in and out sequencially.

4. The Ozinverter must stand alone and be the master, and importantly, it must have sufficient batteries to take the loads asked for the short time before the GTI's kick in. Ie for the 6kW OzInverter a minumum of 500ah of batteries is satisfactory.

At present the LF GTI's all work well, and work is progressing on the HF types of GTI, although there are serious spike issues with HF types.

Other folk are working on a Inverter design that connects to a GTI with minimum batteries. "The design shows great promise".

MPPT DC chargers are very expensive, so AC Coupling using used/second hand GTI's where most have MPPT built in are very cost effective.

PLEASE REMEMBER, High Voltage is dangerous, please seek a competent persons advice with GTI's and Inverters.
« Last Edit: January 19, 2018, 02:23:55 AM by clockmanFRA »
Everything is possible, just give me time.

OzInverter man. Normandy France.

3off Hugh P's 3.7m Wind T's (9 years).  .. 5kW PV on 3 Trackers, (5 yrs) .. 9kW PV AC coupled Used/SH GTI's, on my OzInverter created Grid, and back charging with AC Coupling to the OzInverter to my 48v 1300ah batteries.

petect

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Re: "Tricking" a GTI?
« Reply #3 on: January 20, 2018, 10:09:16 AM »
Hi mab and clockmanFRA
Thank you for your replies. They have been very helpful, but I still can’t quite figure things out. So hopefully if I explain things better, you or others might be able to fill in a few more pieces of the puzzle.
Where I live the power goes out several times a year for ½ day up to 12 days (twice in the last several years). With the typical grid tied pv system the GTI shuts down when the grid goes down. The longer grid shut downs are storm related and are usually followed by several days of sunny weather. I want to be able to use my 3.2K of pv modules during that time – sun up / grid down.
My “critical” loads are fairly small – small fridge, freezer, furnace blower, a few led lights, battery charging,  etc. I’ve managed in the past using battery powered lights and a generator with  a manual transfer, switch for safety reasons. The problem is, when the generator is needed most, fuel is very hard to get.
So I have to make a decision about how to set up my pv system. I bought the modules, but nothing else. So far my half-baked plan is to set up a critical loads panel with pv and generator inputs. Critical load panel to be isolated from the grid with a transfer switch. Pv and generator isolated from the CL panel in a similar manner. I don’t mind the wait time requirement before GTIs restart as I usually wait several hours before firing up my generator. 
I see hybrid and add-on kits popping up on the market. They are well beyond what I can afford. The AC coupled systems seem to work and would require fewer additions to what would be a regular grid tied system. That’s why my original questions about tricking a GTI.  I would also probably use a stand alone battery charger as I have a couple LA batteries for now, and would like to switch to LiFePo4 later. I think the right stand alone charger will also help me regulate the charge rate better to avoid cooking my small battery bank.
Confused? Pm me and I’ll send you a bottle of aspirin.   ::)
Thanks again
Pete

clockmanFRA

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Re: "Tricking" a GTI?
« Reply #4 on: January 21, 2018, 04:50:09 AM »
Okay. I will go back to basics, so please excuse me if I sound simplistic.

GTI's are slaves, they need to see and synchronise with an existing AC voltage and correct HZ. Either from your Main Grid utilities company or a stand alone Inverter say the OzInverter and its created Mini Grid.

That AC voltage and HZ  should be stable  and not sag or go out of range of the GTI's internal settings.

Once the GTI is putting into the Grid, it pushes up the Grid voltage a little and hence your Power supply meter starts going backwards. ie your pushing into the localised community with Main utilities Grid, or your pushing the OzInverter backwards which turns the AC voltage from your GTI and Mini Grid into raw DC voltage at about 60v into charging your batteries. (my previous post shows how we control that).

The GTI's are designed so that above a certain voltage, this varies across the World, the GTI shuts it self down and awaits suitable AC voltage to restart.
With the Main Utilities Grid this happens a fair bit when its sunny and there are several PV GTI systems on the consumer side of the Utilities power Transformer and no one is using up that localised power.

At present you need some sort of machine to create a stable AC Voltage and correct HZ, and here I use the OzInverter. The 48v DC battery bank get the Inverter going and producing a MINI GRID that the GTI wants to see.

The GTI's turn on, and that Mini Grid starts using up the power from the GTI's, and backs away from using the 48v DC batteries, and then starts pushing DC back into the batteries, that's if you have spare power from your GTI's.

So theoretically the GTI's do all the work and the OzInverter just gives a stable reference AC voltage and HZ reference.  However, in real practical terms to keep the GTI's on board, the OzInverter must be able to handle many loads instantaneously backwards and forwards without the AC voltage sagging or going high, hence those parameters I have set in my previous posting.
 Sunshine is just not consistent, its remarkable to see so much fluctuation even on a clear bright sunny day.

At Present there are some whizz kids out there working on a small Inverter that can instantly match loads and excess to the GTI's output, without the need for the main Mini Grid Inverter to be so robust as the OzInverter.  As I said, 'this concept shows great potential', and I await the results with interest.


So at present you need 2 systems of AC Grid for your GTI to work on each.

Get some good change over switches, ON OFF ON, at your side of your consumer/switch board unit, and have your Main utilities Grid on one side and Your own Mini Grid on the Other. see photo.

We have many Main utilities Grid EDF outages here, and I only use the Grid at night time as its a cheap rate and more cost effective than installing a decent Generator.

That's a big used/second hand Krauss & Namier, fleebay, big 90amp change over switch, or known as a generator Change over switch. All poles are switched and there must be an OFF central position. Earthing here in this country is a Long copper earth rod at each consumer unit.

This system can be automated with AC contactors.

I trust this helps.




« Last Edit: January 21, 2018, 05:05:50 AM by clockmanFRA »
Everything is possible, just give me time.

OzInverter man. Normandy France.

3off Hugh P's 3.7m Wind T's (9 years).  .. 5kW PV on 3 Trackers, (5 yrs) .. 9kW PV AC coupled Used/SH GTI's, on my OzInverter created Grid, and back charging with AC Coupling to the OzInverter to my 48v 1300ah batteries.

petect

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Re: "Tricking" a GTI?
« Reply #5 on: January 21, 2018, 10:26:39 AM »
clockmanFRA    Thank you VERY MUCH for your detailed post. It helps a LOT!
For me "simplistic" is good, as it helps me understand why something works, not just how to do it.
Best of luck with your Ozinverter venture. I've been following your progress even though I don't understand most of it.
Pete