feeding DC into a PV grid connect inverter

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acmotor
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feeding DC into a PV grid connect inverter

Post by acmotor » Fri, 15 Nov 2019, 15:32

Does anyone have experience with feeding DC (not from PVs) directly into a PV grid connect inverter ?

e.g. DC from a power source (generator, power supply, battery) within the rated DC input voltage range of the GCI and current limited (or not) to GCI current rating.

I see that some MPPT trackers expect a source impedance similar to a PV otherwise they shut down on fault.

PV source impedance (actually just resistance ?) being (at underside of MPV) around R=(Voc-Vmp)/Imp e.g. (41-37)/5 = 0.8ohm and over Imp being around 41/5 = 8.2ohm

Reason is that I wish to add some other DC injection via AC (GCI) to my off grid inverter system that has both AC (GCI) and DC(battery) coupled PVs.

There is the concern of DC isolation with non isolated GCIs.

Comments ?
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antiscab
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Re: feeding DC into a PV grid connect inverter

Post by antiscab » Fri, 15 Nov 2019, 16:39

It's worked every time I have tried connecting a battery to a normal grid tie inverter

The inverter starts at open circuit and just ramps up input current till it either finds MPPT or runs into it's current limit
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acmotor
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Re: feeding DC into a PV grid connect inverter

Post by acmotor » Fri, 15 Nov 2019, 22:49

good.

What sort of DC voltages did you use ?

I expect running to DC input current limit is one way of controlling things.
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antiscab
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Re: feeding DC into a PV grid connect inverter

Post by antiscab » Sat, 16 Nov 2019, 10:00

acmotor wrote:
Fri, 15 Nov 2019, 22:49
What sort of DC voltages did you use ?

I expect running to DC input current limit is one way of controlling things.
I have used 140vdc and 320vdc

you could use a modern GCI that supports export limiting, as another way of controlling things

That would facilitate the use of a modest sized standalone inverter (say an old UPS of around 2kw) with a large GCI for doing heavy loads (say a popular 8kw unit) and just set a 0 export limit, or even a minimum 1kw import limit if the UPS doesn't function well with momentary reverse current (not sure how adjustable they are)
Matt
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acmotor
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Re: feeding DC into a PV grid connect inverter

Post by acmotor » Sat, 16 Nov 2019, 12:07

My offgrid system runs a 6kW (18kW surge) low frequency inverter with 3kW and 2.5kW GCIs connected to output (so AC coupled).
Also a HAWT that helps, on occasion via a Windy Boy GCI.
Battery bank charges by backfeed through inverter as it is also a charger when CGI pushes. Works a treat. Inverter and GCIs all happy.
My BMS via picaxe removes AC coupling from CGIs when battery has been charged so they don't push to their AC voltage limit around 264V when battery charge current drops.
This keeps AC grid at inverter output voltage of 245V and stops Lithiums being floated by inverter. Coupling returns at 95%SOC.

Tried HF inverter. More efficient and lower self consumption but just can't handle the turn on surge of 3kW aircond and welder etc.
There was no backfeed through the HF inverter I tried, though some (Victron and SMA sunny island handle charging via GCI)

At this stage I am looking at feeding DC to another CGI so I can charge EV, run aircond and kettle etc. and charge main battery from a generator or another battery pack.
So you are telling me that the GCI works fine with source impedances lower than PVs ? I don't want to find out the hard way.
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antiscab
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Re: feeding DC into a PV grid connect inverter

Post by antiscab » Sat, 16 Nov 2019, 14:00

acmotor wrote:
Sat, 16 Nov 2019, 12:07
So you are telling me that the GCI works fine with source impedances lower than PVs ? I don't want to find out the hard way.
All the ones I have tried have worked
Make sure to pre-charge before connecting though

how big a GCI are you thinking of?
I would just use a cheap second hand one thats no longer CEC listed off gumtree
Matt
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Re: feeding DC into a PV grid connect inverter

Post by acmotor » Sat, 16 Nov 2019, 16:12

I can go for a 2.5kW GCI with output power limit and DC current limit as you note. A Delta Solivia 2.5 is to hand.
spec is 3.1kW max PV in but sofware parameter limited to 2.5kW AC output.
18A input DC current limit. GCI Starts around 100V but would need 150V to develop 2.5kW AC output (eff. around 95%)

If I feed this from a 3.5kVA 240VAC generator, via a 35A 800V bridge rectifier on H/S and some filter cap (1,000uF) with a precharge resistor/switch and or 20A NTC before the cap how would that go ? (pf not great without an inductor)

edit: This would have the GCI working at around 330V, well within its range.
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antiscab
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Re: feeding DC into a PV grid connect inverter

Post by antiscab » Sat, 16 Nov 2019, 19:02

It should work, though I wonder how the inverter will be with the ripple.

Only one way to find out
Matt
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antiscab
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Re: feeding DC into a PV grid connect inverter

Post by antiscab » Sat, 16 Nov 2019, 19:03

Couldn't just synchronise the gen set to the inverter and have it direct online?
Matt
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acmotor
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Re: feeding DC into a PV grid connect inverter

Post by acmotor » Sun, 17 Nov 2019, 07:52

Thinking about that .....

Inverter genset with no apparent option to sync, or at least maintain sync as there will be two independent frequency sources.
Honda and some others have a proprietary 'between gensets' sync system. Might research that.
Could take DC direct from HF 3 phase rectifier inside genset if accessible.
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Re: feeding DC into a PV grid connect inverter

Post by acmotor » Sun, 17 Nov 2019, 13:29

Interesting...
Honda EU inverter generator will sync with an existing power source if power source is present first.
https://www.youtube.com/watch?v=VxvhU03EuD0
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Re: feeding DC into a PV grid connect inverter

Post by acmotor » Sun, 17 Nov 2019, 15:27

Looked at some more Honda EU generator info then tried an EU10 (1kW) genset direct in parallel to house offgrid inverter.

It works fine... with the proviso of care with islanding safety and plug gender etc.

Sequence used..
connect genset to operating house offgrid inverter first (via an energy meter for info)
start genset (don't start genset before connecting to operating house offgrid inverter otherwise it won't start up synced)
genset comes up synced and pushes on house offgrid inverter if the house offgrid inverter voltage drops below genset nominal output voltage
genset 239VAC inverter 245VAC so initially the genset did not push
added a variac in line to make small voltage adjust (genset voltage stepped up a few volts)
output VA from genset can then be adjusted by fine trim of variac. I set at around 850VA and ran for an hour. (genset is rated at 1000VA max, 900VA cont. current limits at 4.3A)

Looking around on youtube it appears this may be workable with other inverter generators. Careful experiment maybe. Risk of genset inverter damage.

Good idea antiscab to consider generator direct connect !
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acmotor
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Re: feeding DC into a PV grid connect inverter

Post by acmotor » Sun, 17 Nov 2019, 20:28

Tried a few inverter gensets.
Only the Honda EU range with parallel connectors work (safety shielded banana plugs that are just in parallel with 3 pin sockets).
Other brands with parallel connectors are likely to work. The more basic (older) inverter gensets don't support AC sync.
Mickey Blue and Gentrax 3.5kVA for instance were tested.

I did use an older inverter genset connected to the Honda and started first before Honda. That works. Honda syncs and comes up in parallel.
If the older genset is stopped then restarted while still connected to the running Honda, then one or both gensets trip lock on overload and must be stopped to clear.
This is probably not kind to the silicon and I would avoid it, but works.

All nice to know.

Now back to my original task of DC to GCI...
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