Author Topic: AC Coupling Control Issues  (Read 1129 times)

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Offline eraser3000

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AC Coupling Control Issues
« on: October 01, 2016, 07:50:24 PM »
Rant,

Anybody attempt to AC couple fairly large systems off grid?

Wow what a pain.  Who new you had to turn into a control engineer just to make everything play.

Does anybody know any rules of thumb for inverter/coupling ratios?

Just trying to get the conversation going with anyone who has experience.

Thanks


ila_rendered

Offline ClockmanFrance

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Re: AC Coupling Control Issues
« Reply #1 on: October 02, 2016, 03:53:53 AM »
SMA reckon for their 6kw inverter 11kW AC coupled.
For the 6kW OzInverter we reckon about 8kW. Its all about handling the back charging to the batteries.

How big is big?

We here are going to about 15kW for AC Coupling with PV.

We have 8kW DC coupled to the 48v 1300ah batteries and keep them in tip top condition, using Tristar diversion controllers for the Turbines and Tristar MPPT and a 200 Classic for the PV Trackers.

The Classic aux contacts are used as a fail safe trip out on the AC coupling GTI's at the 5 installations. So the classic goes to float charge and switches off the GTI's sequentially.

I purchased a very well known manufacturers Inverter, who insisted on lots and lots of expensive auxiliary control equipment to make it work, not mentioned in the Inverter specs/brochure. I sent it back, got my money back and made a simple, robust, and very cost effective OzInverter.

Battery charging. ....... The OzInverter will take GTI's AC coupled to the Inverters AC output and back feed it to the batteries. When the batteries start to fill they push back and the Inverters AC voltage rises slightly, say 12 volts above 230vac, and this is normally enough to trip out the GTI's. So my fail safe operated by the Classic never has the need to work.

I have a BigOzInverter 10kW to 15kW and can handle fantastic loads for short brief periods. And this will replace the present 6kW to 15kW OzInverter.

GTI's for me are relatively cheap, New Old stock or second hand discarded when the installation they were serving was upgraded. Mine are all under 3kW I use SMA SB's toroids, as I have internal codes for modding them.

Control equipment ....... once your locked in with all that expensive gear, the manufacturer seems to have you by the short and curlies.

All my control gear is under $3000 for a 23kW system.

Every time I have a nice glass of wine I raise a toast to 'Oztules', the man who lifted me out of manufacturers reliance.

Ps, As I understand it 'Oztules' is about to explore a simple, robust, and cost effective, Solar charge controller.

Offline eraser3000

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Re: AC Coupling Control Issues
« Reply #2 on: October 02, 2016, 08:29:57 AM »
Hey Clock man,

Thank you for your response.

Just a quick note were you able to see the attachment?

That should help explain the overall system.

When I'm done I'm hoping to have 75kw of solar installed and 55 of that I'm hope to have on GTIs.

Thanks

Offline DJ

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Re: AC Coupling Control Issues
« Reply #3 on: October 02, 2016, 09:13:39 AM »

How much are you paid for your grid feed per Kw?

Here you would need a lot of control equipment and a lot of approvals to back feed that much power. They are concerned about the grid voltage being raised too much and some other frequency issues.

I -think- the allowable limit is either 15 or 20 KW before things get real complicated.

Offline eraser3000

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Re: AC Coupling Control Issues
« Reply #4 on: October 02, 2016, 09:18:17 AM »
This is all off grid.

Offline DBCollen

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Re: AC Coupling Control Issues
« Reply #5 on: October 02, 2016, 09:36:09 AM »
How big is your battery bank? Pushing that much power around is going to require a very large battery bank to take advantage of that much PV.
Dustin.

Offline eraser3000

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Re: AC Coupling Control Issues
« Reply #6 on: October 02, 2016, 09:57:54 AM »
I have 2 banks around 15000ah 24v
And another smaller one. I think 1500ah at 48v.

Offline lighthunter

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Re: AC Coupling Control Issues
« Reply #7 on: October 02, 2016, 05:45:54 PM »
Wow, thats huge Eraser, I had my share of grief when i started out but now it works sweet
with no glitches (only 5kw).

Can you give us some specifics that you are having trouble with?

There are some GTIs that are more picky than others and some dont.
work at all. Delta solivia comes to mind. In some ways the bigger it is the better it should work
You may need to stagger the GTIs though so they dont all come online at the same time. Something like local network sequencing contactors to each GTI. Without sequencing all GTIs count off their 305 seconds and there is a lot of adjusting to do. Mine has another control watching the coupling voltage and it can throw dump loads on to keep voltage below the trip point. You could do that by changing the max voltage parameters of your grid ties so one by one they shut off as voltage rises too high. (provided you have access to parameters) just thoughts.
Cheers! LH
LH

Offline eraser3000

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Re: AC Coupling Control Issues
« Reply #8 on: October 02, 2016, 07:56:21 PM »
OK, on the Wind Turbine GTIs I now have them setup for a user defined grid type and 530V trip after 5 seconds 63 hertz after 5s and for lows 450v 5s and 57 hertz 5 seconds.
So my Under and Over volt alarms are gone pretty much, but I am getting a GRID interference alarm that is taking them offline.  I can't stagger them as they are both needed to handle the turbine and both have the wind curve loaded into them divided by two.  I have a group of SSRs that are driven by PID loops looking at AC current into the heater and as a setpoint I have the wind curve with interpolation same as inverters, plus or minus an offset base on battery voltage.

GRID interference occurs around 8000 watts.
Some pictures to help explain.

ila_rendered
Offset Screenshot

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Wind Curve for PID screenshot

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SSRs

Offline lighthunter

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Re: AC Coupling Control Issues
« Reply #9 on: October 15, 2016, 05:31:33 PM »
Hi Eraser, I didnt mean to abandon you here, nor did anyone else i'm sure. Your project is just.
so... soo, well larger than most of us have experience with. You must be Chief of the island in charge
of utilites.  :)

Wow that would be a fun system to play with. Guess it could also be a few headaches as well.
Have you found solution for the problem yet?
the grid interference fault you spoke of, does it only effect the two inverters for the turbine?

One thing i shouldve mentioned earlier, GTI's because of the government requirements actually skip cycles every second. Im not sure if its just one or maybe like 5. Every second they stop generating power and your battery inverter has to fill in while the GTI does error checking and if all goes well then generation continues for another second. I can see this happening on the DC side, causes the amp guage to flicker every second. On the AC side you dont notice it unless you use LED light bulbs then it drives you nuts. Fortunately i dont use lights much when the sun is shining.   ;D.

In your drawing i notice a fairly large load connected closer to large turbine GTI than battery inverter. Its harder for the battery inverter to regulate voltage this way unless the large 480 heater
load is wired directly back to the battery inverter. If the grid tie were fairly steady it wouldnt matter but keep in mind the power going to 480 heaters is halted every second from turbine and the skipped cycles have to come from the bank.

It has been my experience at least, that if you have load management based on incoming power then backfeeding capacity to batteries is less important than battery inverter capacity to handle all the loads that are switched on during maximum grid tie generation. This may not hold true with multiple grid ties unless they sync to each other. Thats an idea, if the turbine grid ties are synced and skip cycles at the same time, it may help to run them independently.
Just my 1ยข :)

LH
LH

Offline eraser3000

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Re: AC Coupling Control Issues
« Reply #10 on: October 25, 2016, 10:57:15 PM »
Thanks for the info, Its crazy to me that it works that way.  I have some parts coming an will update once I test everything.

Offline eraser3000

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Re: AC Coupling Control Issues
« Reply #11 on: January 17, 2017, 05:44:34 PM »
Yes I finally found a solution.
and yes I am king of the island.
ila_rendered
ila_rendered
ila_rendered

More info on new load: http://www.anotherpower.com/board/index.php/topic,1202.msg12795.html#msg12795

Offline lighthunter

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Re: AC Coupling Control Issues
« Reply #12 on: January 18, 2017, 09:51:33 PM »
Congratulations!!! did solution involve finding a finicky balance of many contrl setpoints? or did you find a major culprit to your problem? I still have to avoid incoming power above 3kw or gti can fault. At that point it gets diverted to water heater.

Am very impressed with your setup. We are so fortunate to have members like you and many others
who arent afraid to do things no one else has done.
LH

Offline eraser3000

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Re: AC Coupling Control Issues
« Reply #13 on: January 18, 2017, 09:58:42 PM »
Thanks for the kind words, mostly a "finicky balance of many contrl setpoints" Also dedicated controller for the SSR's was required and making a new boiler that could accept more heater elements.

I do like GTIs, fairly inexpensive, install anywhere, all shapes and sizes kind of equipment. 

Not much I like better then hot water in the cold winter months.  Its been awhile but you have a GTI with a brand I hadn't heard of I think what is it?

later

Offline lighthunter

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Re: AC Coupling Control Issues
« Reply #14 on: January 19, 2017, 05:35:30 AM »
you have a GTI with a brand I hadn't heard of I think what is it?

later

Its a Dow powerhouse 3.5k dual built by emerson.(only produced for a year obsolete now). Was only 40kwh on it when i got it and paid about 300usd shipped. I used to hate it but after working around its details it has been running perfectly! I managed to snag another this past year, a xantrex 3k with 44613kwh on it. (that might be some kind of record) works great also and paid even less. I agree grid ties are pretty nice. For these purposes it would even b better if they could be throttled somehow.

LH