AC coupling questions.

solarvic

Anyone here have any experience ac coupling a gti to a Magnum inverter or have suggestions of different options?
GTI inverter= fronius ig-2000 maximum output 2000watts. Off grid inverter= Magnum MS-AE 4024. 630 amp forktruck battery. Magnum is set to shore power inverter turned off and feeds thru inverter taking enough power to charge battery and power my home. Should I connect the GTi 240 vac output along with the output side of the magnum or should I connect with the imput side of the magnum inverter. I think it probably doesn,t matter. I understand the gti will backfeed any extra power to the grid along with my other gti inverter that is grid connected. I never get close to the output to the grid I am approved for any ways. With my magnum set the way I explained the GTI would shut down. I can turn of the breaker from the grid and turn on my magnum and the gti can latch on to the magnum. Magnum engineer told me a 2000 watt GTI inverter is good for a 4000 watt magnum.
Solarvic

solar_dave

Re: AC coupling questions.

Assume you had less than the 2000 watt in load, how would the magnum react to being back fed?

solarvic

Re: AC coupling questions.

Why I am asking the questions. According to the drawing in the owners manual when you have the inverter set to shore power the relays for output and imput of magnum pass the power thru to service panel from grid. I think excess power from the gti should feed back to grid. The battery charger in the magnum takes out enough power from the grid to keep the battery charged whether you have the magnum turned on or off. If you have the inverter set to standby it will start inverting as soon as grid power is off. If you have inverter turned off it won,t invert . I figured the safest way to ac couple to the magnum is to have the magnum set to shore power and if the power goes off the GTI will stop inverting. I would then turn off the breaker that imputs power from the grid and then turn on the magnum. When the magnum starts inverting then the GTI should start inverting to my service panel along with the magnum. When i have the grid breaker shut off I would need a place to dump any excess power from the GTI that the battery or house load can,t use. I live in an area that has lots of cloudy weather so rarely would have the 2000watt from my GTI :Dsplarvic:D

SolaRevolution

Re: AC coupling questions.

Okay,
As I understand it-

The Scenerio:
-When the grid is on it is feeding through the Magnum inverter to the loads panel.
-The Magnum inverter/charger is off but AC power is feeding through the internal transfer switch to the loads panel.
-The grid tie inverter (Fronius) is connected to a breaker in loads panel.
If the grid fails, everything turns off.
With the grid off, (all breakers still on) if you turn on the Magnum inverter the loads panel will be hot. (The AC input side of the Magnum will be dead and no power will be able to back feed from the batteries or the loads panel.) After 5 minutes the grid tied inverter will "sell" to the loads panel. All available PV power will feed to the loads panel. If the PV power is less than the loads required power, the Magnum will provide additional current from the batteries to power the loads. If the PV is producing more power than the loads are consuming the PV power will feed back through the Magnum and onto the battery bus. The batteries will be receiving an unregulated charge current and the battery voltage will rise.

You will need to have a way to prevent the batteries from over charging.
This can be accomplished by:
-using a battery voltage controlled relay (with a hysteresis range) to disconnect the grid tie inverters AC connection to the loads panel when the battery voltage rises above a maximum setpoint. When the Battery voltage falls to the low "reconnect" voltage the grid tie inverter Ac circuit will be turned back on. This can provide "Bulk" charging only.

-using a diversion load controller can provide regulated charging but you will need to have a diversion circuit capable of handling 125% of the Grid tie inverters AC output. Not as easy as it sounds to find.

-using a battery voltage controlled relay (with a hysteresis range) to switch on a dedicated AC diversion load (ie. a space heater, air conditioner, etc...) which draws more than the Grid tie inverters AC output) when the battery voltage rises above a maximum setpoint. This will create a cycling condition where the dedicated load turns on, the battery voltage falls to the lower hysteresis setpoint, the dedicated diversion load turns off, the battery voltage rises again, repeat. Be careful what you use for your dedicated diversion load. It could be accidently un plugged or a thermostat could disable it.

Combinations of these meathods can be used.

I believe that code requires at least two ways to prevent overcharging the batteries. The second layer of protection could be the High Battery Voltage Disconnect (HVD) setting of the Magnum inverter could provide a failsafe and will make it obvious that something is wrong with the system if the Magnum, and loads panel, keeps shutting off at high voltage, turning back on when the voltage falls, after 5 minutes the grid tie inverter starts unregulated charging again, and repeat.

When the grid comes back on the Magnum will start it's programmed charging cycle afterwhich everything should go back to normal operation. You can then turn off the Magnum. There will be no need to turn on or off any AC breakers.

PS. If you want uninterrupted power you will need to leave the Magnum on or in a standby mode.

I've not yet coupled a Magnum so there may be something I am missing here. Check with Magnum for your specifics.

Alex

Cariboocoot

Re: AC coupling questions.

I'm looking at this carefully and wishing we could draw pictures within the software. Let's see if I've got the proposed set-up correct.

Main breaker -> service panel -> Magnum inverter -> GTI inverters

With Magnum set on "shore power" the AC should pass through the inverter, in either direction (confirm this with Magnum), allowing the GTI to back feed the service panel & utility grid and allowing the Magnum to keep the batteries up.

Grid goes down, turn Magnum on. GTI now back-feeds Magnum inverter.

Flaw: Magnum does not output power to AC IN when in invert mode.

Solution: sub-panel between Magnum and GTI for critical circuits.

Are we on the same page here?

BB.

Re: AC coupling questions.

Alex,

I think you have it all... If the Magum is left in "auto switch mode" (or whatever they call UPS type mode), it is very possible that upon loss of AC mains power, the inverter will "fail over" to off grid mode and the Grid Tied Inverter will not miss a beat and continue to supply AC Power--Only now it will be "mixed" with the Loads+Local Inverter (instead of Loads+Utility Main).

Using two relays (or solid state relays) in series with the GT to Inverter Load Breaker connection and two voltage controlled switches may be the "best" and least expensive way of "limiting" over charging of the battery bank.

I would be nice to find some temperature compensated voltage controllers (with hystorisus)--something like 14.5 volts relay opens, and 13.2 volts relay closes--that could also take bank voltage into account.

What would be really interesting is a PWM controller connected to a solid state relay (or thyristor--aka Lamp Dimmer circuit) on the AC side to an electric heater...

a) relay opens until 14.5 volts volts,
b) PWM starts to cycle close (turning on electric heater) until 14.7 volts for X hours.
c) After X hours, PWM duty cycle increases until 13.6 volts (float) voltage is reached.
d) reduce PWM cycling when battery voltage fails below 13.6 (to 13.0 volts)
e) Voltage falls below 12.7 volts for XX minutes, then start a) again.

And a second relay in line with GT inverter and voltage controlled switch... If >14.8 volts, open relay (shut down GT inverter).

-Bill

solarvic

Re: AC coupling questions.

My main service panel has a generator interlock kit installed. Set 1 way the grid feeds the service panel, locks out magnum inverter. Set other way locks out the grid to service panel then lets magnum feed thru from grid. My power load is low enough that magnum feedthru shore power is enough to power everything in my home. I also have the capability to operate independently from the grid if I want to let my other gti inverters feed the grid. Otherwise you got everything about right mark. The magnum battery charger can still charge battery with inverter turned off while shore power is feeding thru from grid. :Dsolarvic:D

solarvic

Re: AC coupling questions.

Thanks Alex and everyone else that responded. I feel a little more confident now. I found out for sure that the GTI inverter needs connected to the out put side of the Magnum inverter. The magnum can have shore power connected to to the imput side with it set on standby. Grid power will feed thru magnum and if grid power is interupted the relay shuts off from the grid and magnum will start inverting. That is suposed to be enough interuption for gti inverter to shut down. Then the gti inverter should reconnect with the magnum. I also found that while the grid is passing thru shore power that any extra power from gti inverter backfeeds to grid. Just the way I want it to work. If grid is off you need a dump load to get rid of any excess gti power after the batterys are charged. I have the MS4024 ae model. The MS4024 pae model has software that raises the cycles to 60.6 to shut off the gti inverter after batterys are charged. I probably won,t be doing this till after winter as I have to get the panels and cable buryed. Also ordered one of those new dc waterheating elements with builtin thermostat to use for a dump load and I am going to install it into my geospring waterheater. I disconnected the heating elements today to see if the waterheater will operate ok without the heating elements connected. So far it runs ok. So after I get all this done in a couple months I will post if it works as well as I hope. :Dsolarvic:D

balee123

Re: AC coupling questions.

Not sure if this should be posted in a separate thread, however, the question came to mind after reading here about this AC coupled system.

Theoretically, if one had:

- the AC coupled system mentioned in this thread
- a 24 V battery bank
- and only used electricity during the day during energy producing hours

could you get away with a very minimal sized battery bank, e.g. only 2, 12 volt batteries in series? In other words, if you never used the battery storage capacity (or very very little), can you get away with the most minimal of minimal battery banks?

This assumes you really never go much beyond the usage of what the grid tie produces.

BB.

Re: AC coupling questions.

The battery bank needs to be able to absorb 100% of rated output of the GT inverter/solar array, and it needs to be able to supply 100% of the rated load... So, which ever it higher, it works out to about 100 AH per 1kW @ 48 volt battery bank... For a 24 volt battery bank it would be 200 AH per 1kW of array/load.

If you did something that limited the absolute current "charging the battery bank" say an active load of heaters... Or were able to dump loads on the AC side when the sun went behind clouds--In theory, you could do it with a smaller battery bank.

But, in the end, why bother? If you do it "wrong"--You either end up with unreliable AC power and/or a damaged battery bank.

-Bill

solarvic

Re: AC coupling questions.

I have partly set up my ac coupled grid tied system. Post #3 and 4 is most relative to what I learned and did so far. I did everything in post 3 except I used a bigger inverter and more solar. Think I have too much solar now, 2920 listed watts. What happens is when I turn off the grid the GTI disconnects. I turn on the magnum 24 volt battery inverter and then the gti inverter will reconnect. The battery voltage will go to over 38 volt charge and gti inverter shuts off because of too high of voltage. Gti inverter will do this repratedly if you let it. The magnum inverter doesn,t shut off. My question is will the offgrid magnum make more than 240 vac when the battery voltage is so high? Another thing I tryed was turning on lots of load to see if the gti inverter would stay on and it still charged the battery too high of a voltage and shuts down again. Today it was very cloudy and my GTI was only making about 700 watts and everything seemed to work ok and battery didn,t overvoltage. I was taking more out of the battery than I put in. I wanted to exersize the battery as it hadn,t seen any use for awhile. I learned that when the inverter is set to shore power it lets power from gti inverter feed back thru to grid. So before I can really ac couple I have to find some way to keep the battery voltage down. I probably will use a pwm controler to control the battery voltage. Will let the grid equalize at night. Which voltage would I set the dump load for? I think most of the things I will learn by doing with a little direction from forum members. Solar revolution post was most helpful as he had some experience with ac coupling. Not as easy and it sounded for me at first. :Dsolarvic:D