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Old 02-07-16, 11:45 AM   #1
stevehull
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Default Problem with back up generator and PV

Long time readers know of my interest in putting up some 15 kW of roof mounted PV in a grid tied configuration here on the farm. That plan is to use EnPhase microinverters.

These inverters automatically shut off if there is loss of grid power. This protects line workers from PV initiated shocks during grid power restoration.

Other readers also know of our recent experiences in an ice storm where we were out of power for several days and I ran the farm on my tractor power take off 16 kW 240 V AC generator. I have a manual transfer switch that I first flipped to prevent back feed of the grid.

All OK . . .

But what happens if I combined the two? Assume the PV system is attached to my farm electrical infrastructure AND I have a Generac generator (or the generic equivalent) that senses grid power loss and turns on to provide generator power within a couple minutes of power loss.

First, the Generac flips an transfer switch and isolates the farm wiring from the grid. The PV system is now shut off as the inverters do not see grid power.

Then, the Generac turns on and provides the now isolated farm electrical system with power. This unit has a voltage modulator and provides only the power required for the farm needs - up to the rated capacity of that generator. If the farm uses only 2 kW it will provide that - if you need 10 kW if will provide that, up to the rated power of the generator.

Here is the problem . . . The PV system inverters now detect AC power and turn ON! If in mid day, the PV will turn on full blast, potentially pushing some 15 kW into my farm electrical system all the while the Generac is also pushing power in.

Something is clearly going to fry!

I am sure I am not the only one that has dealt with this. The simple solution is that you must manually disconnect the PV system FIRST before the generator turns on. That would entail shutting OFF the automatic Generac grid power detection circuit.

Or . . . you can put the PV feed ahead of the transfer switch so that it remains connected to the grid. When the transfer switch goes off, the PV system is attached to the grid and not to the generator.

Or, I could create a huge "dump load" using oven electrical resistance elements together with detection circuitry.

Thoughts from those of you that have PV and a generator?

Bottom line question from an EE standpoint. Is a PV system a current source, a voltage source or a power source? I surmise it is a mix of the three . . .


Steve

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Old 02-07-16, 02:11 PM   #2
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The PV should work like any power source and it will only output what you need.

It's a power source. Supplies voltage which is stable and current varying with load.

I'm not sure if the generator and solar will play nicely together. Someone else may chime in.
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Old 02-07-16, 03:22 PM   #3
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Because of how gens make power, basically a motor in reverse. So giving it more power than it is producing is going to fry things. THOUGH it depends on your gen. Some sense the power and shut it off. But either way you don't want to run your tractor with your solar panels. (Another project entirely I'm sure. lol)
You need a buffer, like batteries. With batteries you can feed the grid with solar, and use it during an outage. If they can't keep up then plug in the gen and charge them back up. Tesla has some awesome batteries that last a long long time and are very efficient.
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Old 02-07-16, 11:24 PM   #4
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Until Enphase gets around to creating a Power Wall, I'd go with:
"you can put the PV feed ahead of the transfer switch so that it remains connected to the grid. When the transfer switch goes off, the PV system is attached to the grid and not to the generator."

There is no down side to having the PV attached to the grid when the grid is down. I really prefer the simplicity of leaving the PV attached to the grid when the grid is down. I also expect this is how the NEC likes things and expects things to be wired as well.

With a 10kW generator, this isn't like the scenario I've read others contemplating where they want to insert a 300-500W pure sine wave inverter to trick the enphase inverters into waking up to create a micro-grid with a battery charger system that charges the battery used to drive the PSW inverter used to wake up the system, plus the home's base load.

Your 10kW generator setup is literally like taking two generators and attaching them together, or attaching your generator to the grid while the grid is actually live. Something will go, and it will NOT be inexpensive to repair what goes. Keep it simple, keep the PV on the grid side of the automatic transfer switch so it stays with the grid when the generator is running.

In vague theory, the micro inverters adding to the generator AC output should rapidly go outside the stored voltage profile and automatically trip the enphase over-voltage disconnect. Reading through the latest Enphase Envoy setup manual today, there were multiple faults described where one could have to reset a logged fault in the micro inverters which locked them into a solid red light disconnected condition. I'd certainly hate to have to call Enphase and say my entire array is locked up with a red light fault on every micro inverter.
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Old 02-09-16, 10:10 AM   #5
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The solar installer I originally consulted was gung ho for enphase, as I am too. He was against generator and pv at the same time for similar reasons of frying an expensive investment. Outback might have something in a gridtie hybrid system. I think you'll end up needing batteries though. It seems like enphase might work with the signal of another battery powered inverter, like faking the grid.
It probably makes the most investment sense to reap the savings of gridtie micro inverters and spend it on diesel fuel when you need it.
In a previous house I built I installed a couple of emergency generator only outlets that were powered only when needed. It wouldn't be too hard to do the same powered by solar and battery. Battery could be charged by generator too. With your well pump and freezer load you'll need a good sized generator no matter what. Can you downsize your generator needs by moving everything else to grid tie battery hybrid?
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Old 02-11-16, 12:10 PM   #6
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I have read about connecting grid tie and stand alone inverters.
In almost every case it didn't end well.
I fully expect a isolated micro grid being run by a generator to have the same result.

The thing with gird tie inverters is they are made to put out as much power as possible, they do not produce power on demand.
I would expect the kind of inverter you need to exist. I would not expect them to be cheap, they may have to be custom ordered or made.
The problem with custom stuff or special order stuff is if you need another one now, the turn around time is weeks and sometimes months no matter how much you need it.

The cheapest off the shelf solution I can think of that we know will work is run everything that produces power at a common DC voltage. Take the solar panels plus generator with transformer rectifier running at a common voltage to a big power inverter or inverters that are designed to be stacked and work together.
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Old 02-16-16, 06:38 AM   #7
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Doesn't the micro inverters lock in phase (60 cycles) with the grid? What would be the difference if the phase is sampled from the grid or the generator? As long as the voltages match (output of the micro inverters and the generator) there shouldn't be an issue..

If the two sources are in phase, the PV would only make the load lighter on the generator. Both would be supplying wattage and assist each other. Both sources should control their own voltage. Might need to tweak the generator output voltage to match the micro inverters but the phase would be generated by the generator..

My concern would be the grid detection of the generator and definite isolation with grid failure..
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Old 02-17-16, 08:14 AM   #8
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All enphase warranties are voided with when you use a generator to fake a grid. So keep that in mind if want to try that.
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Old 02-17-16, 09:56 AM   #9
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Did a fair amount of research on this and sadly a PV array is a power source that does NOT modulate with load. Unlike an internal coupled generator, that matches load with power, a PV system must have the grid, a "dump load" (or batteries) to suck up power in excess of what is needed.

Ok - then. I need to build a switch that is energized by the grid and when the grid goes down, the switch trips and stays off until it is manually reset.

A 240 V dPsT switch is easy, as when power goes off the relay shuts down and the contacts open. But when the grid (or generator) comes on, then it reconnects the power to the PV system.

If you have a generator powering the system, and if attached to a PV system, then something will give. It could be that all the EnPhase microinverters turn off and play dead. The other is a bad (read expensive) way . . .

Anyone know of a switch like this? Think of a circuit breaker, that when energized, stays off until you manually reset it.


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Old 02-29-16, 02:24 PM   #10
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They use a switch like that on big cabinet shop tablesaws to prevent unintentional start ups from power failure. I think they use a contactor relay with 240vac coil.

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