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Old 03-25-15, 06:48 AM   #1
stevehull
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Default Use of grid tie PV when grid is down

I understand that grid tie PV systems do not operate when the grid is down. Logical to prevent back feed into the system killing a worker of they are on the lines.

Apparently, the inverters, string or micro, sense 50 or 60 Hz current at 240 V and cut off the inverter if that is not present.

That said, can you turn the system off (transfer switch) turn on a small generator to "trick" the inverters into thinking the grid is still there?

I know that it would now be necessary to use a dump load to rid the system of excess power.

I can easily construct 240 V dump loads from oven electrical elements (2 kW each). Saw such a system when I was recently out of country. Problem was I did not speak the language and could not communicate with the builder (no translator present). The dump load was actually a set of hot water tanks with resistive elements.

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Old 03-25-15, 08:46 AM   #2
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Why not just run the generator on its own instead of coming up with a dump load system and potentially running an unloaded generator? An unloaded gas engine is an inefficient engine. Gas generators with an alternator directly producing power will likely not be clean enough of a power source to where the inverter will want to feed power. How would you plan to run this dump load, inverter, and generator to synchronize the loads, sounds like a PITA.

Why not just get a grid tied system with an inverter that is designed to produce its own sine when the grid is down like the Sunny Boy with the built-in Secure Power Supply?

SUNNY BOY 3000TL-US / 3800TL-US / 4000TL-US / 5000TL-US / 6000TL-US / 7000TL-US / 7700TL-US


That sort of solution would remove the need for a crazy solution like a generator. If you want to be sure it works without sunlight or running loads beyond the possibility allowed by the level of insolation provided at the time, you could get a small battery-backup type grid-tied system.

Think of your mission with doing this. Do you frequently have power outages that warrant the extra expense of batteries versus just running an appropriately sized generator for the loads needed in an outage? If you have a generator already and don't need individual MPPT on your solar system and a string inverter 3-7.7kw works for your application, the SMA-America Sunny Boy can provide you with a 1.5kw power source if enough sun is present to provide for your load without raising the cost of your solar system by much. In that case you can use your generator at night and pull power off PV during the day.

I considered this system but I have a partial shading issue and also really like the production I got this winter with a partial snow covered array with individual MPPT versus nil from a single MPPT string. I'm quite certain I'll produce more than I'll ever spend on gas for the generator in the edge case that I ever need it. Oddly my generator has only supported a load once in the 5 years I've had it and it hasn't even had a full tank run through it. Think critically about how often you'll need a generator when making the decision for a complicated weird dump load setup thing.
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Old 03-25-15, 09:19 AM   #3
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I have a small 60 Hz, 240 volt power supply (250 watts, sine wave) that is run off of a car battery. Would only need the PV system to run water well (for livestock) during the day, but it takes FAR more peak kW to start the pump.

Completely agree that running a generator is a complete waste.

Battery back-up is expensive and the 1.5 kW Sunny Boy back up power is not large enough to kick on the 2HP deep well water pump. Have already considered that.

It is actually quite easy to wire in a dump load!

Only really need power during the day to run water pumps on farm and it seems a waste to have ~ 10 kW of PV power just sitting there . . . . We lose power frequently in spring storms (one today forecast in central OK where I am located), so I am thinking ahead before I do physical install.

Good thoughts!


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Old 03-26-15, 11:20 AM   #4
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Steve, the frequency accuracy of a standard gasoline generator may not be good enough to turn on the grid tie. An inverter output generator might work, compare freq and voltage specs between generator and grid tie unit to be sure.

I have a foggy memory of xringer playing with something like this, IIRC his experiment ended with a dead grid tie inverter.
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Old 03-26-15, 11:36 AM   #5
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Foggy memory [kindof] verified:http://ecorenovator.org/forum/solar-...-inverter.html
Around page 4 bad things happened . . .
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Old 03-26-15, 12:40 PM   #6
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The micro generator is exactly 60.000 Hz and is used as a reference standard frequency for multi mega W power plants.



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